**Marine Ecology Lecture - Primary Production Pt. 1 — Transcript & Summary | SozAI**
Source: https://sozai.app/transcript/marine-ecology-primary-production-pt1/

Lecture on marine primary production covering photosynthesis, chemosynthesis, and their ecological importance in marine ecosystems.

## Key Takeaways

- Primary production is fundamental to life, producing organic matter and oxygen.
- Marine ecosystems rely heavily on microscopic and simple photosynthetic organisms.
- Bottom-up control means ecosystem health depends on primary producer productivity.
- Photosynthesis and chemosynthesis are two main pathways of primary production.
- Oxygen measurements are a practical way to estimate primary production rates.

## What the video covers

- Primary production is the process of making organic matter from inorganic molecules by primary producers.
- Organic matter includes carbohydrates, lipids, proteins, and nucleic acids, all based on carbon frameworks.
- Primary production occurs via photosynthesis using light energy or chemosynthesis using chemical energy.
- Primary producers form the base of food chains, supporting higher trophic levels through bottom-up control.
- Photosynthesis produces organic matter and oxygen, both essential for life and ecological balance.
- Marine primary producers include phytoplankton, cyanobacteria, and various algae such as red, green, and brown algae.
- Chemosynthesis occurs in deep-sea environments like hydrothermal vents where light is absent.
- Oxygen production during photosynthesis can be used as a proxy to measure primary production.
- Different types of light affect photosynthesis differently, influencing marine plant productivity.
- The lecture emphasizes the ecological significance of primary production in sustaining marine food webs.

## Chapters

1. 00:00 Introduction to Primary Production
2. 04:23 Bottom-Up Control in Food Chains
3. 08:22 Photosynthetic Organisms and Marine Plants
4. 12:22 Evolution and Adaptations of Land Plants
5. 15:35 Marine Primary Producers and Nutrient Environments
6. 19:31 Measuring Primary Production Using Oxygen
7. 26:33 Light and Photosynthesis in Marine Environments
8. 36:02 Light Spectrum Effects on Photosynthesis

Answers

## Questions about this video

What is primary production in marine ecology?

Primary production is the process by which primary producers like phytoplankton convert inorganic molecules into organic matter using energy from light or chemicals.

Why is oxygen important in primary production?

Oxygen is a byproduct of photosynthetic primary production and is essential for the respiration of higher life forms, making it crucial for ecosystem health.

How do photosynthesis and chemosynthesis differ?

Photosynthesis uses sunlight as an energy source to produce organic matter, while chemosynthesis uses chemical energy from compounds like hydrogen sulfide, often in deep-sea environments.

## Full Transcript — Download SRT & Markdown

00:02

Speaker A

Hi. This lecture is about primary production. This is being recorded for my Florida Gulf Coast University marine ecology class, but anybody who's interested in primary production, particularly primary production in the marine environments, might find it interesting. So, what is production? Production is a process of making stuff. What is produced when we're talking about primary production? In primary production, the stuff that is made is organic matter, the material of life, and it is made from inorganic matter, small simple molecules, and it is made by organisms that we call primary producers. More detail on this will be on the next slide.

00:22

Speaker A

So, organic matter is the stuff that is made in the process of primary production, and organic matter is a set of infinite variety of complex molecules that all share a carbon framework. So, these complex molecules that we call organic matter include classes of molecules like carbohydrates, lipids, which are fats, proteins, nucleic acids like DNA and RNA, and any of the complex large molecules that make up living things. They're all what we call organic matter.

00:44

Speaker A

So, carbon is obviously the main framework of these molecules, but they also include many other types of atoms like hydrogen, oxygen, nitrogen, phosphorus, and sulfur, and other elements as well can be found in some organic molecules. So, these large complex molecules don't come out of nowhere. They're stitched together from smaller, simpler molecules, which we call inorganic matter. So, some examples of the small simple molecules that are assembled into larger more complex molecules include carbon dioxide, water, ammonia, phosphate, and other simple molecules that are stitched together by these organisms called primary producers into the large complex molecules of life.

00:55

Speaker A

The process of stitching these small molecules together, if it uses light as the energy source to accomplish this, is called photosynthesis. And if it uses chemical energy to fuel the synthesis, it's called chemosynthesis.

01:12

Speaker A

The reason we care about primary production is because it's the ultimate source of all the organic matter on Earth, including the organic matter that comprises our own human bodies. So, the primary producers are able to assemble this organic matter from the raw inorganic materials, the tiny inorganic molecules, whereas organisms that are higher up the food chain like humans, we don't make organic matter from scratch. We get our organic matter from the things that we eat. And the organic molecules are therefore transferred up the food chain from the base of the food chain, which is the primary producers, also known as autotrophs, up to the consumers, also known as the heterotrophs.

01:28

Speaker A

So, that's one of the reasons that we care about primary production. The other reason that we care about primary production is because one of the byproducts of primary production, byproduct of photosynthetic primary production, is oxygen. So, as organic matter is being produced by these plants, they're also making oxygen. And oxygen is super important for all higher life forms which use oxygen in their metabolic processes. We require oxygen to break down our food and to use energy. So, we need oxygen as well. So, we need the living material that's made by the primary producers and we also need the oxygen that's made by the primary producers.

01:48

Speaker A

So, in ecology there's this term called bottom-up control and it refers to the bottom of the food chain. The bottom of the food chain is the primary producers, the plants in a photosynthesizing space food chain. So, the idea of bottom-up control means that what's happening higher up in the food chain is totally dependent on what's happening at the bottom of the food chain.

02:07

Speaker A

So, here are two example food chains, the one on the left from a terrestrial environment and the one on the right from a marine environment. Both of these food chains begin with primary producers, large multicellular plants in the terrestrial environment and microscopic algae in the marine environment, but in both cases the food chain works its way up from these primary producers ultimately to top predators.

02:22

Speaker A

And how many hawks you can have or how many sharks you can have in the ecosystem ultimately depends on the primary productivity at the base of the food chain. So, really everything depends on the primary producers.

02:38

Speaker A

We're not going to go into great depth about the cellular and molecular mechanisms of primary production, but there are some things that I would like to remind you of. If you've taken a general biology course, you probably learned in more detail the cycles and chemical reactions that are occurring during the process of photosynthesis, but I'm just going to cover the very basics here. And the very basics are that the raw ingredients that are being combined to form the basic type of organic matter in primary production are carbon dioxide and water. And carbon dioxide and water are stitched together with the power of sunlight to create organic molecules with this approximate ratio of carbon, hydrogen, and oxygen. And there's also a byproduct created, which is oxygen gas. So, these two products of primary production are organic matter and oxygen. This is going to come into play later because there's a proportionality between the amount of organic matter produced and the amount of oxygen produced. So, sometimes if you want to measure primary production, you actually just measure the amount of oxygen that was produced and you can relate that to the amount of organic matter that was produced. That's why it's important to know the chemical equations for this.

02:55

Speaker A

In chemosynthesis, this type of primary production that does not use light as the energy source, but instead uses energy-rich compounds like hydrogen sulfide as the energy source, some of the chemical ingredients are a little bit different, but carbon dioxide is still one of the raw ingredients, and organic matter, of course, is still the product because primary production is the production of organic matter.

03:14

Speaker A

Photosynthesis happens in the sunlit parts of the Earth like the surface waters of the ocean and the terrestrial environment. These organisms here are phytoplankton, microscopic single-celled plant-like organisms that live in the ocean. And in the picture for chemosynthesis, we're looking at a deep-sea hydrothermal vent environment, where energy-rich chemicals burble out of the seafloor, and chemosynthetic bacteria harness those energy-rich chemicals to do their production of organic matter, and the other organisms in that food chain eat those chemosynthetic microorganisms. So, the common theme for these two sort of different types of primary production, the light-based one and the purely chemical one, is that an energy source is harnessed in order to synthesize complex molecules from simple ones. So, you're taking simple molecules and an energy source and doing these chemical reactions to turn them into complex molecules that we call organic matter.

03:25

Speaker A

So, throughout this lecture, when I'm talking about photosynthesis, I'll use the word plants to refer to any photosynthetic organisms. And this is a little bit informal because really, technically speaking, plant means just a certain class of complex photosynthetic organisms that have true roots, leaves, seeds, and vascular tissues. And there are some true plants in the oceans and shallow waters along the edges of the ocean. But most of the plant-like organisms in the ocean are actually evolutionarily more primitive. They include prokaryotic organisms like cyanobacteria and relatively simple eukaryotic organisms like single-celled eukaryotic algae and multicellular eukaryotic algae that we call seaweeds, such as the red algae Rhodophyta, green algae Chlorophyta, and brown algae Phaeophyta. So, lots of organisms that are s

03:38

Speaker A

matter is being produced by these plants, they're also making oxygen. And oxygen is super important for uh all higher life forms which uh use oxygen in their metabolic processes. Uh we require oxygen to break down our food and to um

03:56

Speaker A

use energy. So, we need oxygen as well. So, we need the living material that's made by the primary producers and we also need the oxygen that's made by the primary producers.

04:08

Speaker A

So, in ecology there's this term called bottom-up control and it refers to the bottom of the food chain. The bottom of the food chain is the primary producers, the plants in a photosynthesizing space food chain. So, the idea of bottom-up

04:23

Speaker A

control means that what's happening higher up in the food chain is totally dependent on what's happening at the bottom of the food chain.

04:33

Speaker A

So, here's two example food chains, the one on the left from a terrestrial environment and the one on the right from a marine environment. Both of these food chains begin with primary producers, large multicellular plant in the terrestrial environment and microscopic

04:49

Speaker A

algae in the marine environment, but in both cases the food chain works its way up from these primary producers to ultimately to top predators.

05:01

Speaker A

And how many hawks you can have or how many sharks you can have in the ecosystem ultimately depends on the primary productivity at the base of the food chain. So, really everything depends on the primary producers.

05:17

Speaker A

We're not going to go into great depth about the cellular and molecular mechanisms of primary production, but there are some things that I would like to remind you of. If you've taken a general biology course, you probably learned in more detail the

05:33

Speaker A

cycles and chemical reactions that are occurring during the process of photosynthesis, but I'm just going to cover the very basics here. And the very basics are that the The raw ingredients that are being combined to form the basic type of organic matter in primary

05:52

Speaker A

production are carbon dioxide and water. And carbon dioxide and water are stitched together with the power of sunlight to create um organic molecules uh with this approximate ratio of carbon, hydrogen, and oxygen. And there's also a byproduct created, which is oxygen gas. So, the

06:12

Speaker A

these two products of primary production are organic matter and oxygen. Uh this is going to come into play later because there's a proportionality between the amount of organic matter produced and the amount of oxygen produced. So, sometimes if you want to measure primary

06:26

Speaker A

production, you actually just measure the amount of oxygen that was produced and you can relate that to the amount of organic matter that was produced. That's why it's important to know the chemical equations for this.

06:36

Speaker A

Uh in chemosynthesis, this type of primary production that does not use light as the energy source, but instead uses energy-rich compounds like hydrogen sulfide as the energy source, uh some of the chemical ingredients are a little bit different, but um carbon dioxide is

06:50

Speaker A

still one of the raw ingredients, and organic matter, of course, is still the product because primary production is the production of organic matter.

06:57

Speaker A

Uh photosynthesis happens in the sunlit parts of the Earth like the surface waters of the ocean and the terrestrial environment. Um these organisms here are phytoplankton, microscopic single-celled plant-like organisms that live in the ocean. Um and in the picture for

07:13

Speaker A

chemosynthesis, we're looking at a deep-sea hydrothermal vent environment, where energy-rich chemicals burble out of the seafloor, and um chemosynthetic bacteria harness those energy-rich chemicals to do their um production of organic matter, and the other organisms in that food chain eat

07:32

Speaker A

those chemosynthetic microorganisms. So, the common theme for these two sort of different types of uh primary production, the light-based one and the uh purely chemical one, is that an energy source is harnessed in order to synthesize complex molecules from simple

07:50

Speaker A

ones. So, you're taking simple molecules and an energy source and um doing these chemical reactions to turn them into complex molecules that we call organic matter.

08:03

Speaker A

So, throughout this lecture, when I'm talking about photosynthesis, I'll use the word plants to refer to any photosynthetic organisms. And this is uh a little bit informal because really, technically speaking, plant means just a a certain class of uh com- complex uh

08:22

Speaker A

photosynthetic organisms that have true roots, leaves, seeds, and and vascular tissues. Uh and there are some true plants in the oceans and shallow waters along the edges of the ocean. Um but most of the uh plant-like organisms in

08:35

Speaker A

the ocean are actually evolutionarily more primitive. Uh they include prokaryotic organisms like cyanobacteria and relatively simple eukaryotic organisms like single-celled eukaryotic algae and multicellular eukaryotic algae that we call seaweeds, such as the red algae Rhodophyta, green algae Chlorophyta, and brown algae

08:56

Speaker A

Phaeophyta. Um so, lots of organisms that are sort of in different locations on the evolutionary tree, but which all do photosynthesis and for which for the purposes of um presenting the information, I'm I'm just going to refer to them all as plants,

09:11

Speaker A

even though that's not technically true. So um plants have been around since not too long after the very first life formed.

09:22

Speaker A

The very first life uh that arose on Earth through the process of abiogenesis um was uh non-photosynthetic. It was chemosynthetic. So, it chemical energy was the original energy source for the production of organic matter by the very first organisms in Earth's history. But,

09:40

Speaker A

uh as early as 3 and 1/2 billion years ago, we think that photosynthesis had evolved. The first forms of photosynthesis were more primitive than the modern kind, less efficient. Uh they did not produce oxygen as a byproduct of

09:54

Speaker A

that early photosynthesis. Uh an oxygenic photosynthesis, the type that's most common today, arose about 2 and 1/2 billion years ago. Uh and we think that ancestors of today's cyanobacteria were the the first organisms to do oxygenic photosynthesis. When that type of

10:11

Speaker A

photosynthesis evolved, it really changed the atmosphere of the Earth as uh the oxygen that was produced transformed the the atmosphere and actually made it really difficult for some existing types of life to survive.

10:24

Speaker A

Uh but eventually life did adapt and and now many types of advanced life use the oxygen and uh Um so, that was a big big change early in Earth's history. Uh about 1.6 billion years ago, the single-celled algae sort

10:40

Speaker A

of had an evolutionary leap where they went from being those simple prokaryotic forms to being more complex eukaryotic algae. We think that one of the ways that the eukaryotic algae evolved was through endosymbiosis. So, there were these um

10:54

Speaker A

simple uh prokaryotic algae that got engulfed by eukaryotic cells. Um and then instead of being digested, they actually developed a symbiotic relationship where they lived inside the uh cells that had sort of engulfed them.

11:13

Speaker A

And then eventually, they fused into one organism. And and this happened uh this endosymbiosis kind of thing happened multiple times in the evolution of algae. And and it makes the evolutionary tree of the plants and algae pretty complicated um because it's not like the

11:27

Speaker A

branches are just separating. There's some of the branches that were coming back together through endosymbiosis.

11:34

Speaker A

Anyway um by 600 million years ago, which was around the time that the first uh complex multicellular animals were developing, we were also getting multicellular algae. So, complex seaweeds were leaving their fossils behind by 600 million years ago. And by

11:51

Speaker A

475 million years ago, some of those seaweeds from the green seaweeds group were developing uh features that allowed them to survive being out of the water for a little bit. Uh so, plants began to colonize the land and they evolved from

12:07

Speaker A

uh this particular type of algae called the green algae. Uh and plants over the millions of years developed more adaptations to live in and reproduce on land, uh seeds and pollen, roots, and and complex veins and leaves that

12:22

Speaker A

allowed them to expand on the land. And one of the features that actually didn't evolve until relatively recently in land plants was flowers. Um but flowers were really a great adaptation that helped plants uh evolve and adapt faster,

12:36

Speaker A

reproduce efficiently, and some of the plants that have recolonized the ocean like the sea grasses and mangroves are actually descended from land-based flowering plants.

12:47

Speaker A

So, whether plants are living in the ocean or on land, they all need some of the same basic ingredients to survive.

12:54

Speaker A

They all need sunlight as their energy source, they all need water and carbon dioxide as the raw inorganic molecules for their photosynthesis, and they need nutrients, uh chemicals like nitrogen and phosphorus to form some of the organic molecules that have nitrogen and

13:11

Speaker A

phosphorus in them. Um so, what differs between the land plants and the ocean plants is where they get those four things. So, uh on land, the nutrients and water tend to be concentrated beneath the soil, and so plants have these root structures

13:28

Speaker A

uh that suck up the nutrients and water from the soil and then the above ground structure of the land plants is very different from the below ground structure because the above ground structure is adapted to capturing sunlight and carbon dioxide. In the

13:40

Speaker A

ocean, they're absorbing everything from all around them, so they don't need to have this differentiated two-part body structure. They just absorb the nutrients and carbon dioxide and water from all around them.

13:53

Speaker A

But one of the limitations for water plants is that only the surface of the layer of the water down to a certain depth has enough light for the plants to grow in. So the plants need some way to

14:08

Speaker A

stay near the surface and usually that's by being really small so they don't sink rapidly because if this these plants sank to where there's not enough light, then they would die.

14:21

Speaker A

So there are many different types of microscopic plants in the ocean, collectively referred to as phytoplankton and I'm not going to ask you to know them all, but I would like you to know some features of some of the

14:36

Speaker A

plants in the ocean. So one of the major groups of plants in the ocean is the diatoms.

14:43

Speaker A

Diatoms are eukaryotic algae. In fact, these first three groups that I'm going to talk about are all eukaryotic organisms.

14:50

Speaker A

And diatoms have a glass shell, silica dioxide shells. SiO2 is the molecular formula for glass.

15:00

Speaker A

They also like high levels of nutrients and so diatoms tend to be found in areas where there's upwelling or nutrients that are delivered from coastal runoff.

15:10

Speaker A

So glass shells like high levels of nutrients abundant near shores. Another type of eukaryotic algae that has a different type of shell is the group called coccolithophores and coccolithophores have a calcium carbonate type of a shell. Uh and their

15:27

Speaker A

shell is sort of comprised of all these uh intricate calcium carbonate plates that cover the organism.

15:35

Speaker A

Um and the coccolithophores tend to live a little further offshore in environments that aren't quite as rich in nutrients, but they can still be really abundant and form an important base of the food chain for the oceans.

15:49

Speaker A

Uh the next group of eukaryotic phytoplankton that I want you to know about are the dinoflagellates. They have the word flagella in the name, and as the name of implies, they have not one but two flagella per cell. So, the

16:04

Speaker A

flagella is like the tail of a sperm cell, and so the dinoflagellates have one that they use for movement, and one that they wrap around their waist, and they can use to sort of spiral as they move through the water.

16:17

Speaker A

I mean, this They're single-celled. They don't really have a waist, but they're sort of this groove on the center of the cell, and that's where this uh flagella sort of fits into this groove, and um that helps them sort of spin as they

16:30

Speaker A

move through the water. So, you don't usually think of plants as moving, but though when you're talking about single-celled plant-like organisms, the lines are a little bit blurry, and many of these photosynthetic organisms actually are mobile.

16:44

Speaker A

Uh the dinoflagellates uh they don't have any type of special shell or skeleton, but they have some other special abilities. Some are bioluminescent, so you can see the one that's glowing in this picture. Others are able to produce toxic compounds for

16:57

Speaker A

defense and and other purposes. Um and that can make them harmful when they're in large numbers because they can form what we call toxic algae blooms or harmful algae blooms. Uh like red tide is a type of harmful algae bloom caused

17:12

Speaker A

by a type of dinoflagellate. Um the last group of marine phytoplankton that I want you to know about are the cyanobacteria.

17:22

Speaker A

So, the cyanobacteria include a really high diversity of organisms from simple simple little round balls like the Synechococcus algae to more complex clumps of cells that are adhered together in filaments like this Trichodesmium. Some of them are like the

17:43

Speaker A

Synechococcus are adapted to living in low light levels where there's just barely enough light to grow. Others like the Trichodesmium, they live in the bright waters right at the surface of the ocean.

17:55

Speaker A

And one of the sort of special abilities of the dinoflagellates is they can turn nitrogen gas N2 into more usable forms of nitrogen like ammonia NH3.

18:09

Speaker A

And this means that they can take they can find a nitrogen source where there would not otherwise be enough nitrogen for primary production to happen. And so they can sort of make primary production possible in nutrient-limited areas.

18:27

Speaker A

To measure primary production, we need to be careful about the terminology that we use. So, primary production overall is the production of organic matter, but there's sort of different terms that describe the gross production versus the net production. And they're related by how

18:49

Speaker A

they deal with this other thing, respiration. So, I'll explain those three things, gross production, respiration, and net primary production on this slide here. So, gross primary production is the overall rate that producers create organic matter. So, basically how fast are they taking

19:05

Speaker A

carbon dioxide and water and smooshing them together into complex organic molecules. So, the units that gross primary production is typically measured in are grams of carbon per meter squared per year. So, to sort of picture that, you can picture a meter squared

19:24

Speaker A

size patch of ocean or or maybe for convenience we can just picture a meter squared size patch of lawn.

19:31

Speaker A

Most of us have seen, you know, grass grow in a lawn and um so in that 1 m squared area, the amount of growth in a certain amount amount of time is the primary production, the gross primary production.

19:47

Speaker A

Uh so, if you measure that over a year, how much grass was made by this patch of ground, you could express that gross primary production in grams of carbon per meter squared per year.

20:00

Speaker A

And the one of the things that complicates the measurement of primary production though is that just as fast sometimes as organic matter is being produced, it is being destroyed. And what destroys organic matter is the process called respiration.

20:18

Speaker A

Um and all organisms, even the ones that are primary producers and make organic matter, they also have to burn organic matter. So, they they can sort of store energy as organic matter, but they need to get it back to do the things that

20:33

Speaker A

keep them them alive and growing and reproducing. So, they need to burn that organic matter um through this process called respiration. So, uh there's some rate at which they're creating organic matter, but there's also some simultaneous rate of respiration

20:49

Speaker A

um where they're destroying organic matter. And so, it's really the balance between the gross primary production and the respiration that terms the determines the overall amount of uh organic matter that's produced in a given time, which is the net primary

21:03

Speaker A

production. Uh an The that I sometimes use is uh imagine it's a a restaurant and the sales of the day, the the total amount of money in the cash register at the end of the day is the gross primary

21:20

Speaker A

production, but then to for the restaurant to maintain itself it has to buy the food and pay the staff and so you have to sort of expand that to keep the system going and that's the respiration and then sort of what's left

21:34

Speaker A

over as the overall profit is is the net gain of the net primary production. So you've got your gains and your losses and the balance between them which is the net primary production. So net primary production it can be positive if you're making

21:50

Speaker A

gains but net primary production could actually be negative if you're sort of running a deficit and sort of like a restaurant that lost money that day.

21:59

Speaker A

You know, it wouldn't be able to sustain that forever cuz it would eventually go out of business or if this is a plant it'll eventually run out of energy reserves and die out. But net primary production can be positive or negative

22:10

Speaker A

or zero and that's that's why it's really important to know what net primary production is because it sort of tells you is this production sustaining itself or depleting itself and these are things that scientists like to know.

22:27

Speaker A

So the thing about gross primary production is that it's hard to figure it out.

22:33

Speaker A

Um you can't really measure it directly because you can't get a handle on um something that's being created and destroyed at the same time. So what you have to do is measure the total amount of primary production that you end up with

22:54

Speaker A

net primary production and measure the amount of respiration and from the difference between those two you can calculate gross primary production.

23:05

Speaker A

So, here's here's how that works. We use as our indicator of primary production uh oxygen.

23:13

Speaker A

The reason we use oxygen as our indicator of primary production instead of organic matter, which would be a little more intuitive, is that uh it's easier to measure oxygen than it is to measure the amount of organic matter.

23:26

Speaker A

So, primary production is the amount of organic matter that's produced over a given time, but uh since it's hard to measure that, uh we measure the amount of oxygen that's produced over a given time, because that is proportional to the

23:39

Speaker A

amount of organic matter that's produced that's produced over that same time um due to the balance of the equation here.

23:46

Speaker A

So um we we put everything in terms of oxygen, and then later on, if we want to, we can convert that into whatever the amount of organic matter uh was based on the uh equation.

23:57

Speaker A

Uh so, net primary productivity, NPP, we measure that as the amount of oxygen produced in the light.

24:03

Speaker A

And respiration we measure as the amount of oxygen used up in the dark. Um respiration happens in the light also, uh but you measure it in the dark so that you can make sure that you're measuring just respiration and um not

24:19

Speaker A

that mix of respiration and gross primary production, which is net primary productivity. So, uh in practice in the field, if you're a marine biologist and you want to um figure out the primary productivity of the ocean, you go out in

24:34

Speaker A

your boat, um and you take a sample of water, which includes all of the plankton in the water. Uh you probably collect it in a big bucket like this, and then you can divide that bucket up into smaller jars, clear jars that let

24:51

Speaker A

light in and opaque jars that are dark brown or black and don't let any light in, and you put oxygen probes in the jars. Uh you measure the oxygen level when you start, which should be the same in each jar at the start because they

25:03

Speaker A

just came straight from the ocean. Uh but then over time, the jar that's in the light will probably have an increasing amount of oxygen because there's photosynthesis happening there and if it's in the light, the photosynthesis is probably exceeding the

25:18

Speaker A

rate of respiration, so oxygen will build up and you'll get this change in oxygen. Triangle means change in, so change in oxygen levels in the light, uh which are proportional to net primary production, and the change in oxygen

25:30

Speaker A

levels in the dark, which are proportional to respiration. Um and then you combine the two.

25:38

Speaker A

So, to combine the two to figure out gross primary production, uh you take the change in oxygen levels in the light, and then you account for the change in oxygen levels in the dark, uh and add that in, and um

25:53

Speaker A

you figure out the the gross primary production from that. So this uh works if you have a pure sample of just primary producers like just phytoplankton.

26:03

Speaker A

But what's actually more likely is that in your jar of ocean, you've got not only the microscopic plants, but you've also got microscopic animals, uh the zooplankton in there as well. So, you've got some organisms in there that are not

26:16

Speaker A

primary producers at all, that are just consumers. And so, they add to the respiration uh without contributing any to the primary production. And so, the overall change is a result not just of how much of the uh photosynthesis respiration balance of

26:33

Speaker A

the primary producers, but also of the heterotrophic organisms that are in the system, and that we refer to as uh community production.

26:41

Speaker A

So, um it's possible even if the primary producers are photosynthesizing, your net community production might be zero if the organic matter that's being produced is being eaten at the same rate that it's being produced.

26:56

Speaker A

So, in the remainder of this series of lectures, we're going to talk about these three major factors that influence the amount of primary production in the ocean.

27:05

Speaker A

Uh we mentioned that the amount of primary production in the ocean is really important to know because of, you know, the role of primary production in creating oxygen and the role of primary production creating organic matter that supports all the food chains of the

27:17

Speaker A

world, including the food chains that lead to humans. So, um these are three major factors that affect primary production. In this uh video, we're only going to talk about light. Uh it turns out that understanding the effects of

27:29

Speaker A

light on primary production is pretty complicated, so that should be plenty to fill I want to introduce this term here called attenuation.

27:40

Speaker A

Attenuation means the diminishment of something. So, uh light attenuation means the uh suppression of light or the muffling of light. Um another type of attenuation that scientists sometimes talk about is the attenuation of sound, how sort of sound fades out as uh you go a further

28:01

Speaker A

distance away from the sound source. So, um water is not perfectly transparent to light. Even if it's uh pure water, light cannot penetrate an infinite distance between water. It will It will gradually diminish uh the more water it passes

28:18

Speaker A

through. Um and understanding that rate of attenuation is important for determining how much light is still left at a given depth uh because is there You want to know if there's going to be enough light left at that

28:33

Speaker A

depth for plants to grow at that depth. Uh and so, the obvious thing is that the deeper you go in the ocean, the less light there is. And at some point, there's no longer enough light for plants to grow. Um but exactly what

28:47

Speaker A

point is that? That's where we need to use a little bit of math. And the math that we use is called the Lambert-Beer equation.

28:57

Speaker A

And it looks kind of scary, but it's really not that bad as equations go, and I think you'll be able to understand each of the parts of this equation as I explain them. So, in this equation, capital E is the

29:12

Speaker A

letter that stands for light. And we use the word irradiance sometime, and irradiance just means light.

29:19

Speaker A

Sometimes in this equation they'll use a capital I instead of a capital E. This is exactly the same equation, it's just some people prefer to use an I and some people prefer to use an E.

29:29

Speaker A

So, irradiance capital E or capital I is is what this means. And it's not just the capital E, it's got a little subscript with it. So, here it's a subscript Z, here also you see the subscript Z, or

29:44

Speaker A

here it's a subscript zero. So, the Z is the depth. So, if it's zero, it means that's the light level at the surface. If it's Z, it means it's a light level at depth Z.

29:55

Speaker A

So, so Z is usually some number in meters, so you could be 1 m deep or 100 m deep.

30:02

Speaker A

And so, you're basically figuring out like in relation to how deep you are, how much light there is. That's what this whole equation is about.

30:10

Speaker A

Why do they use the letter Z? Well, it relates to the coordinate plane. So, you've probably seen graphs where you've got an x-axis and a y-axis.

30:20

Speaker A

And if it's a three-dimensional graph, it also has a z-axis. And so, if this is the ocean, the X and the Y can be sort of like latitude and longitude. And then Z is sort of like your depth under the water.

30:37

Speaker A

There's a little fish. Um, and so, so basically Z Z equals zero would be at the surface of the ocean and um the bigger numbers of z would be deeper depths.

30:50

Speaker A

Uh, the other variable in this equation is a lowercase e. Uh, it's Euler's number 2.718. Um, Euler's number is related to natural logarithms and it's a number that you see in a lot of exponential growth or decay equations.

31:04

Speaker A

Another variable that's in this uh equation and and kind of the key variable for describing the light attenuation is lowercase k, the light attenuation coefficient. So, k varies depending on how clear or murky the water is. So, if the water is clear, the

31:24

Speaker A

k is small and light attenuates slowly, meaning that um the rate that light fades out as it passes through clear water is not as fast as the rate that light fades out when it passes through murky water where the k is a large

31:37

Speaker A

number. So, uh I always like to test out equations to sort of prove to myself that they really work the way I think they do. So, I tested out this equation by entering it into an Excel spreadsheet and plotting a graph.

31:55

Speaker A

This is the graph that I plotted when I put the Lambert-Beer equation into Excel and I used two different values of k.

32:05

Speaker A

Um, k equals .04 is a typical value for really clear water like you would find in the middle of the open ocean gyre gyres and k equals .2 is a value that you might find in murky water near the

32:19

Speaker A

coast. So, you can see that regardless of what k value the k value is, that uh light decreases as we go towards deeper depths.

32:32

Speaker A

But, how fast light decreases is dependent on the k value. So, with this uh low k value, you still got a decent amount of light a little bit left even at 100 m deep.

32:43

Speaker A

Whereas with the larger K value, you've pretty much lost all of your light by the time you're you're just 20 m deep, which is only about 60 ft deep.

32:53

Speaker A

If we turn this graph on its side and now put depth on the vertical axis here, this might be a little more intuitive. I also put this color gradient here so you can kind of get the idea that we're

33:04

Speaker A

talking about sunlit surface waters near Z equals zero and sort of these black lightless waters by the time we're over 100 m deep.

33:15

Speaker A

And if you're in these clear open ocean conditions with a low value of K, you still have some light left even when you're pretty deep down. 40 m deep is probably just a little bit deeper than you could safely go with scuba gear.

33:32

Speaker A

Um and in the open ocean, there'd still be some light at that depth. In murky waters, it would pretty much be black at that depth.

33:42

Speaker A

So, let's put plants back into this picture here. The reason we care about light levels and how they diminish underwater is because we care about whether there's enough light for plants to grow. How much light do plants need

33:55

Speaker A

to grow? It It depends on the plant, but the general rule is that you need about 1% of full strength sunlight for ocean plants to grow. So, by the time you get deep enough that there's less than 1% of the surface light left,

34:12

Speaker A

there's there's little or no plant growth occurring anymore. And understanding this sort of breakpoint where there's no longer enough light to grow requires considering the balance between photosynthesis and respiration, these two terms that we talked about earlier. So, photosynthesis being the

34:31

Speaker A

production of organic matter, respiration being the breakdown of organic matter. So, um all organisms, including plants, constantly need to be respiring to keep themselves alive.

34:44

Speaker A

And so, they constantly need to be burning organic matter. Um and it doesn't matter what depth you're at, you need to be burning organic matter. That's why this line indicating respiration is a uh sort of a constant vertical line as you

34:59

Speaker A

go through here. And but the amount of photosynthesis is dependent on the amount of light, so it's um high near the surface and then it fades out as you go down deeper.

35:10

Speaker A

Um there are some complicated reasons why it doesn't just keep increasing as the as you get into brighter and brighter, and we'll talk about that on the next slide, but don't worry about that right now. Uh anyways, um

35:21

Speaker A

uh this as you get deeper and the light gets dimmer, uh you hit some point where photosynthesis is decreasing to the point where it's exactly equal to respiration, and they call that the compensation depth. Um so, your net

35:38

Speaker A

primary production is zero at the compensation depth, meaning that uh your photosynthesis is exactly matching the respiration, and that means that you're not making any more organic matter than you're consuming if you're a plant. And um so, a plant that was at its compensation

36:00

Speaker A

point would not be able to grow or reproduce. It would just be able to maintain itself alive. If the plant was a little bit higher up, um then it could make gains and it could grow and reproduce. If it was a little bit lower

36:16

Speaker A

down, it would be running a deficit. So, um being below the compensation depth doesn't mean you instantly die, but it means that you're not quite meeting your needs on a day-to-day basis and you're gradually burning up your reserves and

36:30

Speaker A

eventually you'll die. Okay. So, uh I hope that explains compensation depth where sort of the photosynthesis line and the respiration line cross. Um but there's this other threshold indicated on this graph here, the critical depth. The critical depth

36:46

Speaker A

is a little bit more complicated and I'm going to uh do my best to explain that on the next slides. If you are having trouble understanding the critical depth from my explanation, I encourage you to uh read in the lecture notes about it

37:00

Speaker A

and also look up other resources on it because the the concept of critical depth is a is a not an easy one.

37:08

Speaker A

Oh, uh just breaking up the zones here. Um where photosynthesis exceeds respiration, the plants are able to make gains. Where respiration exceeds photosynthesis, the plants are running a deficit. Uh and at the compensation point, All right. So, um

37:24

Speaker A

I'm introducing this these concepts on this slide here to get at the idea of what the critical depth is here. Um but before we can understand the critical depth, we need to know about uh different zones in the water

37:37

Speaker A

um and how they relate to that depth. So, you can see in the pictures here that there's sort of this uh light blue water and then uh dark blue water below the light blue water. So, light blue water

37:51

Speaker A

is they're calling it the well-lit zone here. Uh a more technical term for it is the euphotic zone. It's a zone where there's enough light for plants to grow.

38:00

Speaker A

So, if plants are in the um euphotic zone, that's they're happy. I mean, plants don't have feelings, but they're they're able to grow if they're in the sunlit zone. If they are below that, then um they don't have enough light to grow and

38:16

Speaker A

they're dying or in the process of dying. Um so uh those are zones based on light.

38:27

Speaker A

But, there's this other type of zones that we can divide the water up into that are based not on light, but on density of the water. So, this zone here above the dotted line is called the mixed layer.

38:42

Speaker A

And then this zone here where the density begins changing rapidly is called the pycnocline.

38:48

Speaker A

Um so if water is all the same density, which it is within the so-called mixed layer zone, um that means that the water can mix easily within the mixed layer zone. And so, uh a single-celled organism that was

39:03

Speaker A

in the mixed layer zone is is sort of not just fixed at one elevation, but it's kind of like moving and swirling around at all different depths within the mixed layer zone, and that's sort of what's indicated by the um

39:15

Speaker A

little circular motion here. And uh in this case, in this diagram here on the left-hand side, the mixed layer zone is pretty much entirely within the well-lit zone. So, even as the cell is moving up and down, it's

39:31

Speaker A

staying within uh the sunlight and and not spending very much time in the darker waters here. So, it's uh able to make net gains.

39:42

Speaker A

Um if the mixed layer was thicker, though, like in this diagram on the right-hand side, then a cell that was in the water would be spending a lot of its time uh down in this deep dark water where it's not uh

39:58

Speaker A

able to photosynthesize. And so, even though when it did happen to be mixing near the surface, it would be photosynthesizing, it would be spending so much of its time running a deficit uh in this um deep dark water that it would not be

40:11

Speaker A

able to um ultimately grow and and survive. So, um whether or not the mixed layer, um, basically basically the depth of the mixed layer, whether um, it's mostly confined within the sunlit zone or extending much deeper into the dark

40:31

Speaker A

zone, um, the depth of the mixed layer is very very important for determining whether a phytoplankton cell is mostly staying in the light or mixing down into the dark a lot.

40:42

Speaker A

Um, the the thing that uh determines whether the um waters can sort of mix or if they're sort of layered off from mixing is this density transition called the pycnocline, which is related to uh temperature changes or salinity changes

40:59

Speaker A

in the water. And, um, so it tends to change with seasons as uh like when the surface waters warm up that, um, makes them less dense and sort of helps them be separate from the deeper waters and it prevents the

41:15

Speaker A

mixing. Whereas in the wintertime, um, there tends to be deeper mixing. So, this uh picture uh B here is something that you'd be more likely to see in the wintertime when there's uh not much temperature difference between the

41:30

Speaker A

surface and deep waters to prevent, um, mixing and there's also a lot of strong wind that would uh tend to mix the water as well. So, um, those kind of weather factors can determine how much mixing there is and how deep the mixed layer is

41:45

Speaker A

and it turns out that that's very important for whether the phytoplankton live or die.

41:50

Speaker A

So, I'm going to show some, uh, pictures on the next page that show, uh, the difference in mixed layer depth in different parts of the world over time.

42:02

Speaker A

This picture here is color coded by the depth of mixed layer. So, the, um, red and orange and yellow colors indicate a deep mixed layer depth, hundreds of meters deep. So, this means that uh, the waters from the surface

42:16

Speaker A

down to hundreds of meters deep are all swirling together, and any phytoplankton cell that's caught up in that layer is sort of spending a lot of time in deeper, dark waters. Whereas, if the mixed layer depth is shallow,

42:29

Speaker A

it means that things that are in that mixed layer are mostly within the sunlit zone and have the potential to photosynthesize productively. So, this is in January, winter in the northern hemisphere and and summer in the southern hemisphere.

42:44

Speaker A

So, you can see the mixed layer depth is deep in the sort of stormy, mixed waters of the northern hemisphere, and it's shallow in the southern hemisphere in January.

42:55

Speaker A

In April, the mixed layer depth begins to get shallower in the northern hemisphere as the winter winds calm down and the surface waters warm up and become more thermally stratified. But, the mixed layer depth is deepening in the southern

43:11

Speaker A

hemisphere as winter comes to those Antarctic waters. And then, in June, the northern hemisphere summer, you can see that there's the shallow mixed layer depth in the northern hemisphere and a deep mixed layer depth in the southern hemisphere.

43:25

Speaker A

And then, it begins to change again in November. And of course, this is a cycle that repeats every year.

43:32

Speaker A

So the the idea of the critical depth, which is sometimes also called the critical depth of mixing, is basically the critical depth is the depth that the mixed layer has to be shallower than for a phytoplankton cell in the mixed layer

43:50

Speaker A

to have net primary production. So, if the depth of mixing is like this deep, like it is in this uh picture, then the phytoplankton cell, which is sort of mixing in the water around these depths, it's mostly in this zone of gains and

44:11

Speaker A

it's uh it's overall net gains are going to be positive. Whereas if that cell is um mixing to a deeper depth, it's exceeding the mixed layer, that that exceeding the critical depth, then um it's spending a lot of time below the

44:28

Speaker A

compensation depth and where it's running a deficit and it's ultimately not going to have positive net primary production. It's going to die.

44:37

Speaker A

So um that's why understanding the depth of mixing in relation to the uh critical depth is is really important for oceanographers trying to predict how productive the um primary producer community of the ocean is going to be in

44:53

Speaker A

that point of the ocean at that time of year. So um it's something worth mentioning that the relationship between photosynthesis and light is not always a simple one. So, um there there's some general things that we can predict like the more light there

45:15

Speaker A

is, the more photosynthesis there will be and that's sort of like true up to a point, but because there's a lot of complexities to it, um we need to for every individual type of uh primary producer uh understand in more detail uh what we

45:31

Speaker A

call the PE curve, which is the photosynthesis versus irradiance curve. Um so, this is basically like imagine a graph with photosynthesis on the Y axis and irradiance on the X axis. So, irradiance is the amount of sunlight. So, if if it's dark, that

45:53

Speaker A

would be zero light and um if it's uh really, really bright, that would be, you know, a lot of light, full strength sunlight. Um and so, obviously if there's no light at all, there's going to be no photosynthesis at all. So,

46:08

Speaker A

there's one point on this graph that we know for sure is going to be zero.

46:13

Speaker A

Um and then we would expect that there would this would go up somewhat as the light increased, but exactly what pattern it would have, uh we don't know for sure. It might depend on the type of algae. And so, I'm

46:26

Speaker A

going to show you some examples of um more realistic photosynthesis versus irradiance curve so you can get an idea of how the pattern of primary production might change for a plant in relation to how much So, here's a more realistic

46:42

Speaker A

photosynthesis versus irradiance curve, PE curve. Um so, as we said, when there's no light at all, there's no photosynthesis. Um and when light is very low, there's some photosynthesis, but it's not enough photosynthesis to exceed the um amount of respiration. So,

47:03

Speaker A

uh you have to get to some certain minimum amount of light before you reach that compensation point where now photosynthesis um it now photosynthesis is um matching the production of organic matter to the consumption of organic matter by

47:20

Speaker A

respiration. Uh and then as you get into still more light, uh you start making gains where now uh you're photosynthesizing creating more organic matter than is being used up by respiration, and there's a roughly linear increase in photosynthesis for a

47:38

Speaker A

while. So, just kind of as you probably predicted, more light equals more photosynthesis. Um what you see though is that after a while it kind of levels off. The light is still getting brighter, but the amount of photosynthesis of this um particular

47:53

Speaker A

algal species, whatever plant that's being represented in this diagram, it's it's maxed out. It's not getting any higher. Uh and that's because you we sort of reach the physiological capacity of that uh algal cell or plant. It's It

48:08

Speaker A

just can't photosynthesize any faster. It's It's machinery is getting backed up. Um and so even though there's more light available, it just can't um photosynthesize any any faster. And if the amount of light keeps going up and up and up, eventually uh it starts to

48:24

Speaker A

backfire. The photosynthetic system of the plant gets so overloaded that the rate of photosynthesis uh actually goes down. Um the uh enzymes start running in reverse and things like that. And um photosynthesis actually uh declines. Um this is something that you might notice

48:44

Speaker A

if you've ever sort of bought a plant for your house. Usually the plants that you buy for your house are adapted to do well in in low light levels cuz there's usually not a lot of light in your house

48:55

Speaker A

unless you put the plant right in the window. Um so so we pick house plants to be plants that can survive on low amounts of light. Uh and if you put them in brighter light, it doesn't really help them. In fact, if you put them in

49:07

Speaker A

direct sunlight like outside, um it might hurt them. They might have so much uh light that they actually um start dying.

49:16

Speaker A

Um so beware of that if you're ever trying to take care of a house plant and and because more light is not necessarily always a good thing for some plants.

49:26

Speaker A

So uh there's a lot of variation between different species of plants and algae in terms of uh how they respond to light. So uh no plant can grow without any light.

49:41

Speaker A

Um and no photosynthetic plant can grow without any light, but um the the overall shape of their light response curve does vary between species. So there's some species that are able to uh pretty efficiently make use of low light

49:57

Speaker A

levels and those do well in the shade. But, those ones that do well in the shade, they usually sort of top out their photosynthetic rate um at a modest level. Whereas, some of the species that might not do so well in the

50:08

Speaker A

shade, uh they can do really well when they get a little bit brighter because they have a high maximum rate of photosynthesis. So, as long as there's enough light, they can just keep photosynthesizing faster and faster. And so, if you sort of take these um

50:23

Speaker A

species-specific response curves um and sort of overlay them on uh uh a gradient of where light be in the ocean, you can see that the species that uh performs better in brighter lights might uh dominate waters near the surface,

50:40

Speaker A

whereas the species that performs better in darker conditions might dominate deeper waters. And we actually do see some of these patterns of uh changing algal species composition with depth and light um as we would predict from our understanding of their physiology.

50:57

Speaker A

This may remind you of uh niche differentiation graph I showed you in a previous lecture where I was showing two tree species that were adapted to different levels of temperature and how one would sort of perform better in hot temperatures and another in cool

51:12

Speaker A

temperatures uh and they sort of uh partition the habitat that way through niche differentiation. Well, it's the same idea here, it's just that the environmental gradient is light now, not temperature.

51:27

Speaker A

So, here we're looking at a couple of uh broad groups of uh primary producers in the ocean, blue-green algae, uh dinoflagellates, green algae, diatoms, and uh these are really stereotyped um light responses for the different groups, but you can get the general idea

51:46

Speaker A

that some groups like the blue-green bacteria and the dinoflagellates would have an optimum light level at a pretty low light intensity. Other groups like diatoms, their optimum light level would be moderate light intensity intensity.

51:59

Speaker A

And then some groups like green algae, they're basically the brighter the better for those species and they would grow well in in very shallow waters or even intertidal zones of the ocean. So, um uh there's a lot of variation on a uh

52:13

Speaker A

finer um taxonomic resolution. So, like there might be some species of blue-green bacteria that actually would like bright light. Um but this is just a uh an overall trend of different groups and how they respond to the light.

52:29

Speaker A

So, I've been talking about light as if light is all one thing and there's no difference between uh one one type of light or another, but that's that's actually not true. So, there's all kinds of different light. Uh light in general

52:43

Speaker A

is really just a subset of electromagnetic uh and it's electromagnetic radiation that has a particular range of wavelengths between 380 and 740 nm. So, um it's kind of hard to visualize, but light uh functions sort of as a wave and

53:02

Speaker A

wavelength is the distance between uh two humps of the wave. So, when we say that visible light is electromagnetic radiation with a wavelength of 380 to 740 nm, that means that that distance between um you know, like if we're talking about

53:18

Speaker A

green light, the distance between the wavelengths of that electromagnetic radiation that we call green light is 500 uh nm. Um so, wavelength is one of the things that we can describe uh light waves with and another thing that we can describe light waves

53:35

Speaker A

with besides the wavelength is uh the frequency. So, um frequency means like if you're uh monitoring at a certain point, how many waves pass that point and sort of your timing it like with a little watch here.

53:54

Speaker A

Um the number of waves that pass that set point there in a second is called hertz.

54:04

Speaker A

So hertz means waves per second. Um so if you're at 10 to the zeroth hertz, 10 to the zeroth equals one. So that means one wave would pass by every second like wave wave wave wave. That would be one hertz. Um

54:23

Speaker A

if it's 10 to the eighth hertz, that's um like 10 to the eighth waves per second which is a lot of waves per second. Um and if you know these two things about the light, the distance between two wave

54:37

Speaker A

peaks and the number of wave peaks that are passing by a point every second, you can actually figure out the speed of light. So the speed of light is equal to the uh the size of the waves times how many waves are passing by per

54:53

Speaker A

second. So you just take the wavelength times the hertz and you can figure out the waves per second. So I matched up the particular frequency that goes with waves of one meter wavelength.

55:05

Speaker A

Um and so if these waves are one meter in size, which is you know, spread your hands a moderate distance apart and that's one meter. Um and then 10 to the 8.477 of those one meter waves passes by

55:20

Speaker A

uh in a second, that means that the speed of light is um 10 to the 8.477 meters per second which is equal to 300 approximately equal to 300 million m/s.

55:41

Speaker A

Um so, all different types of uh electromagnetic radiation actually have the same speed of light. So, whether it's radio waves or light waves, they're they're all going 300 million m/s.

55:52

Speaker A

Um which is really fast. Uh you know, light um takes about uh uh 8 minutes to get from the sun to the Earth um cuz it's really far, but um light waves uh or any kind of electromagnetic radiation could could

56:07

Speaker A

circle the Earth uh more than eight times per second. So, it's a long ways around the Earth, and something that's going around the Earth eight times per second is obviously very fast. 300 million m/s, very fast. And so, that's

56:18

Speaker A

just a little bit of uh physics here. Um so, the reason I'm talking going on this sort of aside about physics and electromagnetic radiation is is so you can understand the differences in these different sort of color wavelengths

56:32

Speaker A

within the light spectrum because it turns out that the different colors of light uh have different effects when it comes to photosynthesis um due to the way plants uh harness light.

56:46

Speaker A

Oh, yeah. And you can calculate the speed of light from this chart as 300 million m/s.

56:50

Speaker A

Uh okay. So, the main uh chemical that plants use to capture light energy is called chlorophyll. Uh so, chlorophyll, this light-capturing chemical, it comes in different forms.

57:02

Speaker A

There's chlorophyll A, B, C, and D. Um and they cap Each form of chlorophyll captures slightly different wavelengths of light, but in general, the chlorophyll um molecules uh are good at capturing bluish light and reddish light.

57:18

Speaker A

Uh and not so good at capturing, you know, like green light. So, um bluish light and reddish light, those are the the main wavelengths of light that the chlorophyll captures.

57:30

Speaker A

So um all plants have uh chlorophyll A. That's a basic type of chlorophyll, but um some some plants have some other types of chlorophyll as well. And the different types of chlorophyll capture slightly different wavelengths of light, which

57:44

Speaker A

may give those plants an advantage in environments that have a little bit different um light spectrum in them.

57:51

Speaker A

Um and some plants and algae have extra light-catching chemicals besides just chlorophyll, uh like xanthophyll, phycocyanin, phycoerythrin, and that helps them capture wavelengths of light that are different than um the ones that that chlorophyll captures. Um and that may give them a further

58:11

Speaker A

advantage in in some environments like where light is really dim or or certain wavelengths are are filtered out of the water and you need to catch other wavelengths of light.

58:22

Speaker A

So, um you know, most plants that we see on land anyways are are green. Uh at least they look green to us. Um and it's funny because chlorophyll actually does not absorb green light. You could shine just pure green light on a plant and it

58:38

Speaker A

would die because it's not able to absorb and use that light. And so, why is it that a plant um and here's like a leaf of a plant. Um why is it that it looks uh green um when

58:53

Speaker A

to to our eyes. This is a person's eye. Um when actually um it's not absorbing green wavelengths of light. It's because the light that uh we see um coming off of the plant is is what's reflected at us. And so, the

59:14

Speaker A

light that we're seeing bounce off the plant and into our eyeballs is the light that the plant is not absorbing. So, the color that something appears to our eyes is actually the color that it's reflecting, not the color that it's

59:25

Speaker A

absorbing. Okay, so all this wavelengths of light stuff comes into play when we're talking about the light environment underwater. So, we know that water attenuates light.

59:41

Speaker A

Light diminishes as we go deeper in water. But, it's not an equal attenuation for all wavelengths of light. So, some wavelengths of light are attenuated really rapidly by water. So, red light it is rapidly attenuated by water. So,

59:58

Speaker A

um only in very shallow water would you be able to see red things. And by the time you get just a little bit deep underwater, all the red light has been blocked and you can't see red stuff anymore. So, if

60:10

Speaker A

you had like a beautiful red pair of swim trunks and you were snorkeling underwater, you'd see the red color when you were shallow. But, if you went a little bit deeper down to where the red wavelengths have all been attenuated away by the

60:24

Speaker A

water, um you your swim trunks would just look black or gray. You wouldn't be able to see the red at all. Um that's one of the reasons why underwater pictures that you take when you're scuba diving aren't that colorful. Everything looks

60:37

Speaker A

kind of green and blue unless you adjust the colors or bring a flashlight with you underwater to to bring out the colors again. Uh anyways, the wavelengths of light that penetrate furthest into the water are these longer wavelengths of light like green, blue,

60:52

Speaker A

and violet. So, it's nice that plants can absorb wavelengths because blue wavelengths of light are the ones that make it furthest underwater.

61:04

Speaker A

So, um it's not just water itself that blocks light um from from penetrating. There are lots of other chemicals that can be dissolved in the water that can change its light blocking properties.

61:21

Speaker A

And they can be dissolved in the water or they can be particles that are floating in the water Uh that make the water different colors or murkier.

61:29

Speaker A

And I'm sure you've noticed water that was more clear or more murky or discolored one way or the other in your observations of the world in your life so far. One of the things that you might have seen, especially if you live in

61:41

Speaker A

Florida, is that there's some water bodies like in swamps or some estuaries around the mangroves where the water has this really dark and kind of reddish color. It's not necessarily murky, but it has a tint to it, a reddish or amber

61:56

Speaker A

tint. And that comes from this class of organic molecules called tannins or colored dissolved organic matter. They're plant chemicals that leach off of decaying sticks and leaves and um discolor the water.

62:13

Speaker A

They look red to us because they reflect red wavelengths. And actually, they absorb blue light really well.

62:22

Speaker A

Because these colored dissolved organic matter compounds absorb blue light, they prevent blue light from penetrating very far into the water. And so, water that has a lot of this type of dissolved organic material in it may not have a lot of photosynthetic

62:41

Speaker A

activity because the valuable blue wavelengths for photosynthesis don't make it very far through this kind of water. So, if you're a plant and you're trying to grow in these tannic swamp waters, you need to make sure you're really near the surface

62:55

Speaker A

to be able to get enough light to grow.

Topics: primary production marine ecology photosynthesis chemosynthesis phytoplankton organic matter marine food chain bottom-up control oxygen production Florida Gulf Coast University


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