Session 13: Primary Production (1)

Key Takeaways

The following content is provided under a Creative Common License. Your support will help MIT Open Courseware continue to offer high-quality educational resources for free. To make a donation or view additional materials from hundreds of MIT courses, visit MIT Open Courseware at ocw.mmit.edu. We're going to transition. We're going to have now a set of six lectures on the water column. So, we've been, if you like, We've been looking at all of the ways that material gets into and out of the ocean. So, hydrothermal vents, rivers, air sea gas exchange, um um particles. God, is there a thermome is there a thermostat in here? We could >> I turned the heat off when he got here. >> Oh, he did? Okay. Um this [laughter] >> what we're going to be focusing on next is actually the dynamics in the water column itself. And so we're going to start by having a set of lectures on photosynthesis. Um that will be the main focus for today and a little bit into uh next Tuesday. Um we will then and so the photosynthesis is in the upper illuminated part of the water column. We'll then have some lectures where we look at what happens to that material once it's formed from photosynthesis. So we'll look at the respiration terms and then what's left over which is the the export. Right? So organic matter is formed by photosynthesis. the bulk of it is actually respired in the surface layer. Um the part that isn't respired is either advected away or sinks as particles. We usually call the the stuff left over as export because you can think of it as local production and u the material that that is in excess gets exported out of those local conditions. That exported material is then going to uh respire at depth and then some small part will end up in the sediments. Um we'll be starting out looking at the focus will be on carbon. Um but we will also fold in nitrogen, phosphorus, oxygen, uh some of the key limiting nutrients and also oxygen since it's often used to understand photosynthesis and respiration. We'll have some talks uh that will look at the internal dynamics of calcium carbonate in the water column and then also the silicate cycle uh which is associated with biological formation of particles and sinking. So the over the next over the next six lectures we will will cover all of these topics. the class notes um are are you'll get both um PDFs of my lecture notes that I use to give the lecture but there are also some older typed lecture notes that have been posted on the web. Um, we don't have the electronic figures that go with those uh cuz those are older notes from back in the prehistoric days when this course was taught on overheads um and and lots of xeroxes. Um, so the notes are the notes are really useful, but just be aware that they don't have they don't have figures. I'm trying to dig up and see if maybe we can get those figures scanned in to go with those lecture notes, but um be bear with us on that. So, in terms of, you know, things for the exam and stuff, all you will need to know is what's being taught in the course, but you can use these older lo notes as background as background material. Okay. So today we're going to cover start in on primary production or photosynthesis. And if you're a biologist, bear with me because we're going to skim through a lot of biology in a fairly short period of time. Um, everything I say is hopefully correct, but it's not as detailed as you might learn in a in a full biological a full course on on marine ecology or biological oceanography. The basic reaction we're looking at here is the conversion of CO2 plus water in the presence of light. So, H new to some form of organic matter plus oxygen. This is typically called or or can be called photo litho trophy or photo litho autorophy. Um it's photo because it's it the the main energy is coming from light. So this is the energy source. It's litho because you're using um the the primary the primary carbon source is inorganic. Um and it's it's autorophy because you're you're you're you're creating that organic matter. If instead we had used, you know, for example, chemo trophy would be a case where you were using um instead of using light for the main energy source, you might be using uh some form of organic matter. So you could have chemo chemoliththotrophy where your carbon was coming from inorganic forms or you could have chemo trophy uh where you were actually taking your carbon source from organic matter as well um as your energy source. So there's a whole series of these in most of the upper ocean. We're going to be looking at photoiththoy uh often just called photorophy or or even abbreviated as autorophy. the reverse of this reaction. If we were to to write that reaction backwards, the Corganic plus O2 into CO2 plus water is respiration. The main way that photo auditor trophy is is carried out in the surface ocean. Um almost all of the organisms use chlorophyll as their main pigment for absorbing the light. So remember the equa the equation is you you're taking light energy and you're using that light energy to form organic matter out of inorganic carbon. The chlorophyll molecule, put this over here so I can see it. Is a tetrop tetroparole. And I've got a see if I can do this without. So, it's a it's a magnesium ion surrounded by nitrogens and then those nitrogens are in a are in a in paroles. And one of the things I guess the main thing you need to know about this is that the reason why and this is a long this is a long carbon chain down here. Um, what makes this useful as an antenna for absorbing light is that there is a that the electrons in this in this organic framework surrounding the magnesium are distributed. these double bonds. you know, I could I could write this with a with a with a slightly different configuration where the double bonds were switched to um you know, moved over one and it's actually a series of pi orbitals where the electrons are actually distributed over the entire molecule. So the electrons are not isolated to a single bond. They're actually they're actually shared across the entire parole. Because of that, it allows for uh it it allows for an easy electron transitions. So you can get a photon that comes in and then bo bumps one of these electrons in the pi orbitals up to a higher electronic state. And so that's how you're absorbing you're absorbing a photon in the visible uh into uh into the the chlorophyll pigment. So you light comes in and you're converting light energy into an excited electron and then that excited electron be is transferred out of the chlorophyll through a series of enzyatic reactions where it can then be that energy of the excited electron can be used to break and form chemical bonds. In addition to chlorophyll, this is actually this chlorophyll A. There's a whole series of what are called accessory pigments. Oh, I should just say the pyro. You'll also sometimes hear this as conjugated double bonds. Double bond. Bubble bonds. Double bonds. Um there's a whole series of accessory pigments. So for example, chlorophyll B, chlorophyll C, um, carotenoids, biloprotein or yeah, biloproteins. Um, these accessory pigments do a variety of different things in the cells. The main antenna is typically chlorophyll, but sometimes there are accessory pigments that also absorb light at different wavelengths. And that light can then channel down and be used by the chlor the chlorophyll process because it it's basically transferring the the excited electron down into the reaction center of the antenna. Uh they also have use for protection for photo protection. uh a a lot of times these pigments are are are there in the cell to protect the cell from either UV radiation or from other radiation from damaging the the the chlorophyll reaction center. So the main thing about accessory pigments is they absorb light at different wavelengths. Then chlorophyll. So if I were to draw a absorption spectra ex that absorption spectrum for chlorophyll a function of in nanometers. So this would run that's in nanometers. This would run from in the visible from 400 to 700. You're going to see a peak in the blue and then another peak out in the red. So, what's going to happen if you if you have a pigment that's absorbing in blue and red? What what light is left over? What light are you actually going to see? >> Green. >> Green. Right. So this is this is blue, this is red, and then you have then you have green. Um, and the accessory pigments can fill in in this middle in this middle region and so they can actually channel photons that aren't absorbed by chlorophyll and provide energy into the cell. In some cases, this band of say 400 to 700, sometimes you'll see it written as 350 to 700 is called PAR, which is the uh PAR is the photosynthetic available radiation and it's typically like 45% of total solar somewhere around in that range. Um it does depend upon clouds and time of day and solar zenith angle but that's a pretty pretty decent approximation. So about half the solar light that's coming in can be used by the the photoroes. The other half basically goes to warming the warming the water. >> Wait, so about half can be used or about half does get used? >> A half can be used. Um yeah only a small fraction actually gets used in photosynthesis but but only about half is available. The other half is is just it's gone >> because the it's the photons don't have enough energy >> uh to be used in photosynthesis. Okay. the it's not just uh you you don't think of the cell as as um having dissolved chlorophyll. The chlorophyll is actually highly organized within the cells within membrane structures. And in fact, the chlorophylls are in antenna where you have something on the order of about 500 chlorophyll molecules for a single antenna. And within that antenna, most of the chlorophyll are just there to absorb absorb light. And then they transfer these excited electrons down to what's called the reaction center. And it's at the reaction center that that that electronically that excited electron uh is then transferred into enzyme systems that can do actual work for the uh work for the organism. There's two parts to the photo system called photo system one and photo system two. And I don't expect you to know the full details of these because these get, you know, rather uh rather complicated. But you should know that there are these two different photo systems and so and and they have different functions. Photosystem one is involved in the reduction of a molecule called NADP. And NADP stands for nicotenomide adanine di nucleotide phosphate, which is a bit of a mouthful. which is why we typically call it typically call it NADP. So the when you reduce NAD NADP it goes to NADPH where you've actually added a a proton to it. Photosystem 2 is involved in the liberation of oxygen from water. And the two are actually coupled so that you can think of this as a as a chain where you need both photosystem one and photos system 2 to be functioning. If you combine them and look at the um the net effect of both of them, the reaction looks something like this under light conditions. You're taking two waters plus four NA DP+ with approximately eight photons going to oxygen plus two. Oh, why did I write four over there? Sorry, I had a brain fart. This is a two. Can't even read my own notes. NAD pH plus two protons. In addition to reducing two NADPs, you also generate four ATPs. So you take four ADPs plus four inorganic phosphate and you form four ATPs. Now ATP is the basic energy uh currency within the cell. Um, ATP is adenazine triphosphate. Um, and so when you B ADP has two phos phosphate units on it, ATP has three, you're you're storing energy in that bond between that third phosphate unit, the between the second and the third phosphate unit. And so in the process of forming ATP, you're storing energy in that ATP. That ATP can be used later by enzymes to drive reactions that are that require energy. So this is how the cell generates energy. So far we've liberated oxygen and we've generated reducing power and en and and and stored chemical energy. Right? So this is your reducing power. This is your your energy. But we haven't made any organic matter yet. To make organic matter, there's a series of of what are called the dark reactions or the Calvin Benson cycle and these are involved in the in the formation the actual formation of the of organic matter. So the reaction is two protons plus CO2 plus two NA DPH goes to some generic carbohydrate. We'll get to that in a moment. Um, so this is a a generic carbohydrate plus water plus 2 N A DP++. So essentially you use your reducing power um to reduce the oxidized form of carbon to a more reduced form in the form of the organic matter. And this uses up three ATPs. A DP plus 3pi. Okay, notice that there is a difference. This generates 4 ATP. This generates 3 ATP or uses up 3 ATP. There is an extra ATP that can be used within the cell. Um, and it depends on what you're synthesizing and and all sorts of things along a lot like that. The net reaction if you combine the light reactions and the dark reactions are CO2 plus 2 H2O goes to with with approximately eight photons. Now I say approximately eight photons because not all the photons absorbed end up being used biochemically right um you know for example you can lose photons by fluoresence chlorophyll fluoreses in fact that's one of the ways you you make measurements of chlorophyll is you shine a beam of light into a seawater sample um you shine a beam of light at one wavelength at a high at a high energy of photon. Some of that those photons are absorbed by the chlorophyll and then they fluores they remit a photon at a different way at a different lower wavelength and that can be a very accurate or a very precise measurement because you're measuring photons at a different wavelength than you're putting them into the system. So it's not you're not measuring the absorbance where you have to worry about you know scattering of photons and and all sorts of things like that. You're actually looking at a distinct signature that's specific to chlorophyll which is that refllororesence at a different wavelength. But some of the photons absorbed by the chlorophyll depending upon the conditions might just get refllores or they might be lost into heat. Um, so not all the photons that get absorbed by the by the chlorophyll antenna end up driving this reaction forward. Another thing is I've written this generic carbohydrate, but the cell needs a whole bunch of things that are carbon based, right? It needs it needs to make sugars which go into carbohydrates. It needs to make amino acids which go into proteins. It needs to make fatty acids that go into lipids and it needs to make um nucleotides that go into DNA. Some of you know the car the the amino acids and the nucleotides have a lot of nitrogen in them. So in addition to carbon, you also need nitrogen to form amino acids and the nucleotides. And if you actually look at the cell's bio biochemistry, um they're going to have this equation is going to be slightly different depending upon what it's making. So, you know, for example, if I were making a fatty acid, oh, well, let's go back for a second. If I look at this reaction, I can define something called the photosynthetic quotient. And the photosynthetic quotient is just the ratio of O2 to CO2. So it's the ratio of the O2 that you produce per CO2 molecule that you consume. And so for this reaction here the PQ would be one. Um but in fact the PQ for most for most uh algae and photoroes is greater than one and it's because you're forming uh you know for example fatty acids or other compounds. So like a fatty acid might look like um and the photosynthetic quotient for that would actually be more like around 1.42 rather than one. And so there's been a lot of work within the within the community to try to come up with an estimate of the photosynthetic quotient um for phytolanton. And there's sort of two ways to do that. One is just to take your your your algae and essentially burn them. Um add oxygen until you've taken all the organic matter and converted it into CO2. Um the other way is to look at the the other way is to look at the water column. and to see what the the when you change car inorganic carbon what's the relative change in oxygen and we'll talk about that more when we get to remmineralization about different techniques that have been used for that but the uh sort of canonical numbers are that the oxygen to CO2 ratio for for algae is something in the range of um 150 to 170 over 106 where you have a a ratio that looks something like this where you have C106 some number of of um hydrogens, oxygens, nitrogens and one phosphate. So this would be a a canonical uh algae organic molecule. There is of course no organic molecule that looks like that, but that's sort of what the comp composite might look like. Um, a lot of this work dates back to um, Alfred Redfield who was a u a professor of physiology at Harvard and was also uh, one of the first marine chemists here at Hoie and in fact the Redfield building is named after Alfred Redfield. uh these are sometimes called red field ratios and we'll come back when we do nitrogen and phosphorus we'll come back to this in more detail. Most of the focus has been on on the nitrogen to carbon the phosphorus to carbon but the oxygen to carbon is is equally important and it's um it's still not actually fully resolved. It probably varies in time and space depending upon what the community who's there and what um what kind of uh environmental stresses they're under. Okay, so that's photosynthesis. The reaction CO2 plus water under light conditions to organic matter. The the forward version of this reaction is often called gross primary production. Um you'll sometimes hear it called GPP and it's just it's just jargon. Um the problem is it's it typically we don't measure gross primary production right because the cells are also need the the autootroes respire some of the carbon that they make for energy. So, for example, at night, you know, during the day they photosynthesize. At night, there's a a basal metabolism or basal respiration. Um, so if you were to watch a cell at night, the cell would actually be consuming oxygen and using organic matter. That's also going on during the day. It's just very hard to see with normal techniques. Um, and we will talk, I think, in on Tuesday, more about how these measurements are actually made. Um, so what we typically measure is something more like what's called net primary production which is GPP minus the autoro respiration. Um, for most systems, the autoro respiration is pretty small. Uh, they're fairly efficient. So, it's often less than or equal to 0.1 the respiration is equal to less less than or equal to 0.1 GP. But in for example the there there there are systems for example in the tropics in igotrophic regions where it can get up to 0.4 GP and some of this is what's called photorespiration which is simply um the cells have they have the antenna there for absorbing light. uh they're they're driving the reaction this way, but they don't have enough nutrients to to form or the all the other parts of their cells that they need. They don't have enough nitrogen and phosphorus to build, you know, amino acids and DNA. So, they can't they can't replicate themselves. They can't grow new cells. And so, they just churn out they just recycle that organic organic matter. So sometimes in some environments respiration can get rather large. Any questions so far? Nope. Okay. So who are these autoroes? Um, there's something like about 20,000 phytolanton species. And I'm going to put species in quotes because it unlike, you know, unlike mammals or or reptiles, it's it's often hard to define what a species is. And in fact, one of the one of the um rather well-known and wellstudied phytolanton species uh prochloric caucus. When they actually went in and started to look at the genome of prochloric caucus, they found a variety of different strains and the genomic differences between them were rather stunning. you know, huge amounts of d of of of differences in the DNA code. And then that sort of raises the question of really are these actually are these truly ecotypes or variants of the same species or are they very different species that just happen to look approximately the same to us because we're looking at very small cells. um and and and the techniques we're using are rather crude to characterize what is a species. So, you know, the the the sort of paradigm number is 20,000, but this is a really loose definition of what a species is, particularly because a lot of the um uh bacterial species in particular are able to exchange genomic information across species boundaries uh quite readily. And so you'll see lateral gene transfer um which sort of begs the question of well what's a species then if they're able to change exchange genetic information. There are however some um significant differences that allow you to at least group these species into some larger groups. Um, one of the big ones is proariots or proaryotic cells versus ukareotic cells. Uh, the procariats don't have a defined nucleus within their cell. Um, ukareots do have a defined nucleus and in fact the the best understanding for for most of the phytolanin is the way cells became ukareotic was there was some form of endoindo symbios where uh one proaryotic cell basically swallowed or engulfed a second proaryotic cell and there's been a series of these um uh endo symbiotic events through evolutionary time. And that's how not only did the ukareotic cells get nucleuses, but they also got things like mitochondria and other internal organels, chloroplast, mitochondria. Um the big difference is the ukare in the ukarotic cells, the nucleus is where most of the genetic material is. The mitochondria actually have can have their own genetic material. Um, but you need some biochemical mechanism for transferring the uh proteins that are made in the nucleus or excuse me the genetic information that's made into the nucleus back out into the cell and it's a little bit more complicated uh in terms of the internal biogeeochemistry. Proariats tend to be smaller than ukareats um but that's not always true. There is a um there's one really bizarre procariat. I can't remember its its name, but it was found off of uh the sands of Nambibia, and it's actually visible with the naked eye. It's a it's it's like one of those weird things out of science fiction. um within what you might then call the ukariats there um there are a whole series of different groups um that all are uh considered phytolankton they're not very closely related if you look at them genomically so for example oh within the proaryotic let me just say one of the big ones are cyanobacteria. You'll hear about cyanobacteria a lot. So for example uh prochloric caucus is a cyanobacteria. Trichadesmium is a cyanobacteria. Um they're quite prevalent as photoroes uh particularly in um igotrophic regimes in ukareots you have both green and red. green. That was greed. Green and red algae. Uh the datoms and dinoflagulates are are part of the red algae. You also have haptytes which are where the cockalithophores fall. Cockalithophores are the are the ones that form little calccarious shells. And I have some I have some pictures of some of these things. So um this top image is a syncus cell which is a type of cyanobacteria. Uh it's quite small. This is half a micron. Uh at the other end of the spectrum is tracheadesmium which is a which is a colonial nitrogen fixing cyanobacteria. Uh these are actually visible by the naked eye not individual cells but the whole colony and you can actually pick them out by hand. Um this is a um this is the silica shell from a datom. Datoms form uh little uh almost like uh pill boxes or or or hat boxes. There's two shells uh silica shells that nest in each other. This is has a thickness. It's it's if you turned it on its side, it would look like a a wafer. Um and then finally, uh and then I should say datoms come in all sorts of shapes. Some of them look like these little pill boxes. Other ones look more like elongated capsules. Uh, and they can be either live alone or they can um or they can live in colonies. In colonial form they form long chains often. Uh this is a electron microraph of a cockalithaphor. So you see these little ovals. Those are are actually little they're called caucalus. They're little cal calcium carbonate plates um that surround the shell or surround the cell. Uh and in fact when when uh you have a bloom of these when the bloom starts to die off uh often they will lose these liths and it's like um the water looks like uh water off of a off of a a glacier that has really fine glacial flower. it gets this really pretty looking green uh greenish blue milky color and that's because these are scattering these are very good at scattering light. They're small very fine bits of chalk ground up uh think of as groundup chalk in the water. Uh and you can actually see these from space because of the the amount of back scatter you get in the visible Uh, and then there's a whole series of of other um smaller groups um that have all they they're all [snorts] phytolanton, but they haven't um they haven't evolved from a single group. They've evolved multiple times from multiple different directions within the ukarots. So, it's a very phoggenetically uh rich group of species and so you need to be very careful about talking about the typical phytolankton and I will for the rest of the course talk about phytolankton as if it were a single group of organisms but it's it's not uh it's quite diverse. Another way that you can look look at phytolanton is size. Um and size is quite important for a variety of of reasons. One is grazing. Typically, but not always, um grazers eat things that are smaller than them. Um not always true, but for the most part. And so if you're big, one way to avoid grazing is to be bigger than the than the z plankton or the heterroes that are there. A second is export. If you're really really small and you die, you won't sink very fast and so you don't contribute that much to export flux. Or even if you're if you're graz, somebody eats you and turns you into a fecal pellet. Well, if you're small, your grazer is going to be small and your grazer isn't going to produce a very big fecal pellet. Well, small fecal pellets aren't going to sink very fast either. And so, size has a has a a strong correlation with export production. Um there's sort of a size classification that's based on diameter and they go up by about a factor of 10 in diameter. But a factor of 10 in diameter is going to be a factor of what in volume? Roughly 100. How about a thousand? because it's going to be a you know the the simplest assumption would be it's a sphere. Of course they're not always they're not al always spherical. So um even within these ranges there's actually a huge range in cells but picoplankton are about you know you know maybe 0.2 that's pretty small. Um, most uh bacteria, you know, only the you you only get below a micron when you're a heterotrophic bacteria or a prochloric caucus or or maybe a cynic caucus. Pretty small things up to about 20 microns. Nano is 2 to 20 microns. Uh micro is 20 to 200. And then you have meo and macro etc. Um these are pretty loose. Um people will talk about the pico nanoplankton and they might not have gone out and actually measured the exact size but it sort of gives you a rough um a rough ballpark figure. The last way we distinguish so that was size. The last might be by what you might call guild or and this is um some of this is ecological but some of this is geochemical. Um for a geochemist I might want to know everybody who can fix nitrogen uh can fix nitrogen right because that's a very important geochemical process ge nitrogen fixation is converting uh into gas which is unavailable to my most organisms into a form that can be used by most organisms nitrate ammonia um I might want to know about calcifiers. You know, who forms a calccarious shell? Well, there are different different totally different groups might be calcifiers, but you know, I'm just interested in whether they're forming calcium carbonate. Similar uh solicious shell users, etc. So, you can also group things by these these geochemical functional groups. Okay, so that's who's there. The next question is what controls photosynthesis? Well, the first thing is just phytolanton biomass. If there are more if there are more of these autootroes there. Oh, and I should note I'm trying to be very careful and not call these plants because remember we were talking about how uh many of them are procariats. Well, the plants are only the ukareot you can have ukareotic plants. The procariats are not plants. Um people get a little sloppy and they call all of them you know microscopic plants. Well, in in much of the ocean, plants are actually a minor contributor. So, just, you know, I don't know. It's it's it's one of those little biological quibbles, but I think it actually is important to distinguish um that that that most of the photosynthesis, for example, in the igotrophic gy is being done by procariats, not by ukareots. So, they're not technically plants. Um so the first thing is you need to know how much phytolanton biomass is there and that's it's somewhat circular because you know the the amount of biomass is going to depend upon photosynthesis in the past right if you've had more photosynthesis you can build up more biomass um but it's also going to depend upon grazing um sinking Some of the larger phytolanton actually have a sinking rate or they will get to an end of the bloom and they will sink out. And mortality and within mortality is natural mortality but it also might be for example virus lis um and there's actually a fairly dynamic interaction between viruses and phytolankton. Second thing you need to know is how much light there is. Obviously more light probably more photosynthesis, nutrients, the amount of inorganic carbon or CO2, we'll talk about that. um as well. And then finally temperature. So we're mostly today going to we're going to start into light. I don't know if we'll get all the way to temperature. Uh depends on how things go. But we'll go through these individual controls. We'll talk more about phytolankton biomass um on Tuesday when we start looking more at the whole community. So phytolankton can only grow in the upper ocean. And if you imagine that it if we were to plot versus depth, you have some surface radiance for as an approximation that ardiance is going to drop off approximately exponentially with depth. And at some point in the water column, the radiance is going to equal 1% of the surface irradiance. That was is is typically called that depth where you reach 1% of the surface radiance is called the euphotic zone. And to first order, most of the photosynthesis or the vast majority of the photosynthesis is going to occur within the euphotic zone. It's going to occur above this euphotic depth, the 1% light level. This euphotic zone in very clear waters might be 125 m. And in very turbid waters, in very uh turbid coastal waters with either a lot of phytolanton or a lot of other you know sediments or detridal material might be a few meters. So there's a a a large range. I mentioned that you can approximate this as an exponential if you assume that the that whatever for a single wavelength. The radiance as a function of depth can be approximated as the surface radiance times e to the minus k. Oh, sorry. Let me start that over. E at a depth Z for a particular wavelength lambda is going to equal E0 for that particular lambda e to the minus k lambda z. Right? It's just going to be an exponential falloff. Um it's going to be different for different wavelengths, right? Um so K as a function of of lambda um typically the the the things that are going to absorb light in seawater are water itself chlorophyll uh and what's called sedom which is chromophoric dissolved organic matter. Um, sometimes you'll hear of CDOM as Gelb stuff. GB stuff, which is basically yellow stuff. Uh, otherwise known as a bunch of organic crap that they didn't know exactly what it was, but it was absorbing a lot of light and making a mess out of their experiments. um in in most waters the reds get absorbed early. So red light disappears rather quickly. So, if you go out to the very very clearest open ocean, um like if you go diving in the Saraso Sea, uh the water looks very very blue and that's because you've absorbed most of the red most of the red light. Um if you have a lot of chlorophyll, you'll also be absorbing in the blue, right? Because the chlorophyll absorbs in both the red and the blue. So if you have a lot of biomass, you'll also start soaking up the blue. So there is there are spectral shifts with depth. Um and so this equation while it might hold well for a particular wavelength is only an approximation. And if you were to do it this you you'll you'll sometimes see it written as this approximately I0 e to the minus kar over z where this is now integrated over over par. And that's only an approximation because some of the wavelengths are being absorbed differentially. The last thing on this before we we look at actual photosynthetic ardiance curves is that this K, you know, for example, if I were to write out K would have a term for K water um plus a term for K chlorophyll plus a term for K CDOM um and and and often it's sort of linearly separated like this into different absorption absorption terms. If you actually get into this specific field, you need to worry about not only absorption but also scattering. a lot of the light gets scattered and so you can have um when light's scattered it comes at a different angle and it can get absorbed. I mean the path you increase the path length over which absorption can occur and so actually doing this is a uh is a can be a fairly complicated uh process if you want to do it from first principles. A lot of these are actually what are called effective absorption coefficients because they're not the real absorption if you were to put it in a cell and measure it over a very small cell length, but they're the effective absorption including all the effects of scattering light within the water column. Okay. So, how do how do phytolanton respond to light in terms of their photosyn photosynthesis rates? Um what people often do is they take a phytolankton culture or a phytolankton sample and they expose it to different levels of a radiance and then they measure the photosynthesis rate and they make what's called a PI curve for photosynthesis uh photosynthesis radiance curve. um you will go back and forth. Sometimes the literature uses I for radiance, sometimes it uses E. Um it it and there there are very strong opinions about which you should use and I'll leave it at that. Um and and people have their good reasons for using different things, but in this class I'll just talk about P use it as as as PI. Typically the curves look something like this where you have a region right near in very low light right near the intercept where it's approximately linear. So photosynthesis scales linearly with a radiance. You have some regions where it or you have some region at high light where it saturates and then you have potentially a region out at very high light where you have photo inhibition. Essentially, you're shining so much light on the phytolanton the phytolanton is not happy and it shuts down its photosynthetic apparatus. Um there's a whole series of theoretical slashimpirical curves for representing PI curves. Um, one simple one that that that I like because it's it's it's fairly simple to con conceptualize is that you have photosynthesis is going to be some function of pmax. There's some at high saturation there's some pax value that the cell will will basically it's has sufficient light and that's as much photosynthesis as it can do. And then you have basically a correction term for light. um where if you look at this term um at very low light if I were to go to if if I goes to zero e to the 0 goes to what right one minus one so this goes to zero so it has the correct behavior at low light and if I goes to infinity e to the minus infinity goes to zero so this drops out and you're left at Pmax. So, this does this saturating curve. It doesn't do the photo inhibition part. You would have to add an additional term on for photo inhibition. Um, one little clever trick that I like is if you have an equation e to the b and if you look at the limit of that as b goes to zero, e to the b goes to 1 + b. That's an approximation for for the exponential. The nice thing about that is if you look up at this term that would say that the term e to the minus alpha i pax would go to 1 minus alpha i pax, right? And then the ones drop the um the ones drop out alpha I because the paxes also drop out. And so that's you know a way of thinking about what this alpha term is is it's actually the slope. Alpha is the slope of the pi curve at low light. And sometimes you'll hear alpha described as the initial slope. Okay. So light is going to vary considerably over time and it's going to vary for both for for two reasons. Let's say we have our surface of the ocean. We have depth and we're going to plot time in this direction. So, if I were to look at at the surface of radiance, you're going to get a seasonal cycle in in in most places outside the tropics. Ex in the extratropics, you'll get a seasonal cycle where you'll have low light in the winter and highlight in the summer and then low light again in the winter. At the same time, mixed layer depths tend to be deep in the winter and shallow in the summer and deep in the winter. So this would be that's I zero. This would be mix layer depth. What the phytolanton sees when it's in the mix layer depth or in the mix layer itself is kind of an average light over the mix layer. The overturning time for mix layers are on the on the time scales of you know tens of minutes to maybe an hour much shorter than the generation time for an individual phytolankton. So these phytolanton are getting caught up in turbulent eddies and they're going up and down and up and down and up and down within the mix layer. And as an approximation without going into into uh a a lot of details, you can you can say well they're sort of seeing the average light of the mix layer depth. So when the mix layer depth is deep, remember mix the light was dropping off exponentially. So when the mix layer depth is deep even with the same surface radiance the average light experienced by the phytolanton goes down. So during winter you have lower effective light both because you have lower light at the surface and also you have lower light um because of the deeper mix layers. Often what you'll see in temperate latitudes is during the winter if I were to plot on the same time axis and then I were to plot um phytolankton what I would see is low phytolankton in the summer and then I might see some bloom event and then it might collapse down and then I might see another bloom event in the fall. >> Yeah. Um this is called the spring bloom. It's really original. Uh and this is the fall bloom. This is not true everywhere, but it's sort of canonical for say the the the large parts of the temperate North Atlantic. Um the spring bloom is triggered by the fact that you have both rising surface light and sholing mix layer depths and you reach a point where you suddenly have enough light for the phytolankton community to grow. And there's a a nice uh theoretical discussion of this that was originally put forward by Spherdrip. Still lecture 13. uh and it's called spherrips critical light theory or critical depth theory critical depth theory and it goes something like this. At the surface, photosynthesis rates are high and they tend to drop off with depth because of light. Let's assume for a moment that the phytolanton is well mixed and we're just worrying about uh we're just worried about um the effect of light on photosynthesis. So this would be depth at some depth. You reach a depth and we'll call this depth the compensation depth. The photosynthesis of the community is going to balance by the respiration. So this would be your sort of community respiration. So above this depth, the bugs are growing and they're producing a lot more organic matter that's being than than is being consumed by the community. Below this depth, the phytolanton just are not growing as fast as the respiration of the community. And so up here you have net organic matter generation. Down here you have net organic matter loss and this C is called the compensation depth compensation depth and it's essentially where respiration compensates for photosynthesis. Now let's say this is all in a mix layer, right? So you have these are instantaneous photosynthesis rates. So this is the photosynthesis rate for that bug sitting there at that particular instance at that depth. But if this is all in a mix layer um the whole phytolanton community is seeing this this sort of light the the average light integrated over that depth. We might want to know at what depth if we integrate the respiration DZ would that equal the integrated photosynthesis? And the argument there is that if the mix layer is really deep and you you have a lot of the community that's down in this region where you have net organic loss, you can't have a phytolanton bloom, right? You'll have guys are growing up here, but then they get mixed down and then they're down here starving or they're being getting eaten and they're just all of that organic matter can't go into new biomass and the phytolanton can't bloom. That that depth is called a critical depth where the total respiration integrated over the mix over this critical depth is equal to the total amount of photosynthesis. And you can solve for that if we assume that the um if we assume for a moment that The radiance will approximate as a surface radiance and a single par. Now, as I said, that's not a perfect assumption, but we'll go with it for right now because it makes the math very straightforward. We'll also assume that P scales linearly with I. So we're down in that linear part of the curve. So we have no saturation yet. If you do that and you go back to these integrals, dr dz is just going to equal whatever your sort of background canonical respiration. We're going to assume that that's constant with depth times whatever this critical depth is. Right? That's if it if it's if it's if it's constant and you integrate it, it's just that value times um times the depth. The integral of pdz is then going to be the integral from 0 to z cr of alpha time i. But I is then equal to I E minus K Z DZ. And that's going to equal I do this right. Um 1 / k 1 - e to the minus k zcr. And so if you then set that equal, you can find a depth. You can solve for ZCR where the respiration and the photosynthesis are equal. And the argument is that if Z mix layer is greater than ZCR then you get no bloom. But when mix layer depth is less than or equal to the critical depth that's when you trigger a spring bloom. And as long as you have some estimate of the respiration rate, the community respiration rate of the phytolanton, um, you need to know the surface. If we go back up here, you need to know the surface P 0 or some idea of of of alpha. Uh, either will do. If you know alpha and you know your radiance and you know your background respiration, you can then compute the critical uh you can then compute the critical depth as a function of those parameters and then you can compare it with your mix layer depth. And so when the mixed layer depth schos or we had that little the mix layer depth would go up like this you'll reach a spot where let's say this was your Z critical you'll reach a spot where the mix layer will show and then you'll start to develop the bloom. And so then this this all fills in with and you'll get this big spring bloom event. Um, we've run out of time, so we got through light. We'll start on Tuesday with nutrients and we'll do nutrients, temperature, and DIC.