The Story of Nitrogen

One spring a few years ago, after a couple long days of taking soil samples and spreading fertilizer, I got in my car and started heading south from my home in Rochester, Minnesota, driving down the famous Highway 61 that winds along the mighty Mississippi River.� As I stopped to watch the river flow by, flocks of ducks, pelicans, and cranes wheeling overhead, I thought about the work I'd been doing, the nitrogen fertilizer I'd been spreading.

Science tells us that nitrogen from fields, lawns, combustion engines, and power plants is a fickle thing, easily moving across the landscape, through air and water and soil, creating a cascade of effects that are hard to predict and even harder to control.� The river in front of me was part of a vast system carrying hundreds of thousands of tons of nitrogen from the middle of the United States to the Gulf of Mexico every year, where experts tell us it supercharges the system, ultimately leading to large areas where the water gets depleted in oxygen, driving fish and other creatures away.

In my role as an agronomist, I see every day how nitrogen fertilizer boosts the productivity of farm fields, increasing yields of food, fiber, and forage.� But that same nitrogen, if it were in a nearby forest, might cause trees to grow faster or to grow weaker and die; in a freshwater lake it might have little effect at all, but in saltwater systems such as the Gulf of Mexico, it might cause a large boost in biological productivity and algae growth that we generally perceive to be negative. Watching the river quietly flow by, I found myself filled with questions: first and foremost, why would the same atom of nitrogen behave so differently, cause such a wide range of effects in various environments?� And, could I use that information to become a better agronomist?

Lastly, it made me wonder about a�question that drives the curiosity of many of us, as we see the types of plants change while walking through the forest, as we hunt for bass among the lily pads, watch the plants in our yards and gardens grow or die, or try to diagnose some strange condition in our crop fields: Why am I seeing what I'm seeing?

My name is Greg Klinger and I'm an agronomist and educator at the University of Minnesota Extension.� Together with my friend and colleague Shane Bugeja, I've spent the last few months interviewing experts in agronomy, biology, nutrient management, and ecology, trying to understand the story of nitrogen, in the hopes of explaining the phenomena we see out in the field, woods, and water.� Join us as we explore the different facets of this complex issue.

Part One, The Leaky Barrel.

Why have a podcast series about nitrogen? Unless you�re a farmer, it�s probably not a subject you think about too much. But few things have been more responsible than nitrogen in shaping the world around us. While its effects are often overlooked and underappreciated, few things have changed the world more dramatically than the 20th century development of synthetic nitrogen fertilizers. You need look no farther away than your own body to appreciate this. Nitrogen is the building block of proteins, and scientists have calculated that more than half of the proteins in your body originated in nitrogen fertilizers, getting there through the food we eat. Without the use of these fertilizers, roughly half of the global population, particularly in the developing world, would not be able to exist. Nitrogen is life. However, changes in the amount of nitrogen that moves through plants, animals, and people every year have also had major effects on the world around us, creating biological winners and losers in our forests, prairies, lakes, and oceans. The goal of this first podcast episode is to tease out the ways in which nitrogen fertilizers affect the different environments around us.
��.

�Have you ever heard of the concept of we often call it liebigs barrel?�

�yeah, yep. liebigs barrel or liebigs Law of the minimum? Mm hmm. Yeah. So that comes back to this whole idea of nitrogen versus phosphorus limitation. It's the quantity relative to the need, right?�

Liebig�s Barrel- it�s one of the most important concepts we use in agronomy to make crop management decisions. It was popularized by the 19th century German chemist Justus von Liebig. He realized that plants needed the elements nitrogen, phosphorus, and potassium in order to grow, and suggested that manufactured fertilizers made of these elements could improve the growth of crops. Almost two hundred years later, Liebig is still one of most influential people in the world of agronomy.

Liebig visualized plant growth as a wooden barrel (something along the lines of what you�d age wine or whiskey in) full of water (which represents the overall amount of food that plant could produce). And he pictured each of the wooden staves that made up the sides of the barrel as a different thing contributing to the growth of plants. For instance, the nutrients nitrogen, phosphorus, and potassium, which all plants must have to grow, would all be staves on the barrel, as would things like sunlight, rainfall, and heat. If one of the staves was shorter than the rest- let�s say, there wasn�t enough nitrogen in the field to maximize growth- water would spill out the side of the barrel. The plants couldn�t obtain their maximum growth, produce as much grain or fruit, even though they had enough phosphorus, potassium, sunlight, water and everything else to maximize growth. The leaky barrel suggests there�s always SOMETHING limiting growth, and if you can find what it is and improve it, you can get more growth, more grain. And what we�ve found out since Liebig�s day is that, of all the nutrients plants need to grow, the use of nitrogen fertilizers is the single most important thing a farmer can do to boost crop yields.

Of course, some of those factors that hold back a plant�s growth aren�t controlled by us, which is why it never pays to try and fertilize a crop for its theoretical maximum yield. After all, every plant, even plants that are grown under irrigation, get stressed by too much or too little water, or not enough sunlight, for at least brief periods of time. Or here in Minnesota, some years, a growing season that�s too short and too cold can limit plant growth. How many times have you grown tomatoes, only to see the first frost come just as they�re starting to turn from green to red? As an agronomist, managing fertilizers is about trying to strike a balance in how much you�re investing in your crop- enough that you�re not really limiting your yields, but not too much so that the odds are it will be wasted.

As a side note, as influential as Justus von Liebig has been to the field of agriculture, there are plenty of things that he didn�t get right. For instance, for much of his life, he was convinced that plants got lots of nitrogen from rainfall and would not benefit from nitrogen fertilizers. He started one of the world�s first modern agricultural fertilizer companies, but because he didn�t think nitrogen was a necessary component, crop yields really didn�t benefit much, and the operation folded within 3 years. And when other scientists finally showed that nitrogen fertilizers were important to crop development, he dismissed them as �swindlers� and their research as being a �humbug�- an insult I personally find pretty amusing and also didn�t realize existed outside of Charles Dickens stories. In short, he was a complicated fellow.

In order to get some answers to the questions of why nitrogen acts in various and unanticipated ways in different environments, how and when a lack of nitrogen limits the growth of living things, and what happens when the amount of nitrogen coming into an ecosystem increases, I reached out to some experts who�ve dedicated themselves to understanding how nitrogen impacts 3 unique environments: the lakes of Minnesota, the forests of eastern North America, and the Gulf of Mexico. First up was Chris Filstrup, a researcher at the University of Minnesota-Duluth who�s made a career of studying how nutrients like nitrogen affect freshwater lakes and rivers. What I learned was that nitrogen does not exist in a vacuum, that in order to understand its behavior, you have to understand the behavior of other nutrients. Because as Chris explained, in freshwater, that shortest stave on the Leaky Barrel, the nutrient that prevents bodies of water from growing more and more plants and algae, is generally not nitrogen but phosphorus.

�Why study things like nitrogen, phosphorus in lakes to begin with?

�So the reason we study the nutrients that organisms need to grow and live is because as you start changing the levels of nutrients you see an overall response in the health of that ecosystem. So that lake or that river, it's really like agricultural systems. So if you have more nutrients in the systems, you tend to get greater productivity, more plant growth, more algae growing in those systems, the interesting thing is, unlike your garden, we really don't like to see those high levels of algal growth {or macrophyte growth}, because they have unintended consequences for overall health of the ecosystem.�

There�s a process that scientists like to use to describe what happens when bodies of water get increasing levels of nutrients in them: eutrophication, which comes from Greek and basically means �enriched� or �increasingly well-nourished.� While being well-nourished sounds pretty good to me, this process of enrichment can become a not-so-positive over-enrichment, as Chris is alluding to when he talks about ecosystem health. And it is a process that many bodies of water around the world are going through, as they get greater amounts of nitrogen and phosphorus coming into them than they historically did.

Let�s get back to the idea of the Leaky Barrel- in a lake or the ocean, you generally don�t have to worry about a lack of water, and so nutrients like nitrogen and phosphorus are the things that usually limit growth of plants. Just like putting these nutrients on your lawn as fertilizer creates a thicker, lusher turf, a lake getting more nutrients is likely to end up with denser stands of vegetation in the water, or thicker layers of algae on its surface. This is what scientists mean when they talk about increasing biological productivity- they mean there�s more of the plants and algae that get their energy from sunlight. The paradox, though, is that while you and I and every other animal depend on high levels of productivity from these photosynthesizing plants and algae to survive and grow, when more nutrients leads to more eutrophication, there can be some significant downsides.

�Alright, let's say I'm out, I'm out in a boat on a lake, just fishing or whatever, you know, fishing for northerns, walleyes. As I'm just looking around visually, what would- Could I see eutrophication and what would I be able to pick up?�

�Yeah, so when you when you're fishing in an ecosystem, you know, if you're lowering your lure down in the water column or your worm or live bait or whatever you're using, one of the things that you would notice to kind of assess what the level of productivity is in that ecosystem, is how quickly does that vanish as it moves through the water column. So if you drop your lure down there and you lose it right within the surface couple of inches, that's a highly productive system. Whereas when you go on some of these northern lakes, you can drop your lure down there and you can see it from pretty far out as you're reeling in. And you may even see the fish striking it, those would be a low productivity system. And that's really kind of interesting because one of the techniques that we use to gauge how eutrophic a system is or how productive it is, is we have this little black and white disc called a Secci disc that we drop down from the boat and we measure, where does that disappear in the water column. And if it goes down a long ways, and you can still see it before it disappears, that's a really transparent lake, which is low productivity, as opposed to some of the pea soup, green lakes where you lose it within it descending a couple inches into the water column.�

�I think it�s interesting that the nutrients that are the limiting stave of Liebig�s Barrel for lakes- if you add more of them, you get more productivity and you get this process of eutrophication happening that you might not actually want. What nutrient tends to be that limiting factor to growth in the lakes you study?�

�Nitrogen and phosphorus are the two kind of macronutrients that tend to limit productivity in lakes. The interesting thing is with lakes and other freshwater systems, phosphorus tends to be the most limiting nutrient. Because those systems are really embedded close to the watersheds that receive ample levels of nitrogen. So phosphorus is in the scarcest supply relative to the needs of the organism rather than nitrogen. But then as you move into coastal environments, and you start getting further and further away from the land, nitrogen starts becoming the more limited nutrient, because you're just not receiving that influx from the surrounding landscape because it's farther away.�

�That�s interesting to me, because so many lakes in Minnesota are just surrounded by nothing but woods- I wouldn�t think there�d be much nitrogen getting to them. Why would phosphorus be in so much shorter supply than nitrogen?�
56:42
�It�s the relative amount. So although in those systems you have low levels of nitrogen coming in, you have even lower levels of phosphorus coming. So that's why those systems can still be phosphorous limited. And you see it in the lake because you have the really clear transparent water. So overall, the production level is really low. It just tends to be limited by phosphorus as opposed to nitrogen. Because although nitrogen levels are low, those phosphorus levels are even lower.�

The idea of nitrogen coming in to a forest or lake is hard to wrap your head around, but it comes down to this: nitrogen can be a gas, phosphorus really can�t, and gases MOVE through the air. Phosphorus in the environment, wherever it started out, tends to stick really tight to soil, although sometimes it can dissolve in water off of plant surfaces. It only moves as far as soil, or water running over the ground, does. And, if you�re ever out in the woods during a rainstorm, you can see how effective forests are at sponging up water- it�s rare to see much water running over the land surface from the woods into a lake. As a result, not much phosphorus will get into that lake. Nitrogen is just so more complicated than phosphorus, because it moves with soil and water, but it also moves with the air. In farm country, you can smell this nitrogen sometimes, as ammonia. You can smell this most readily downwind of a poultry farm, and sometimes I catch a whiff after a tank of anhydrous ammonia fertilizer is out in a field. The smell is not as distinctive, but in towns and cities there�s also nitrogen moving into the air- in this case, it�s small amounts coming from our car engines as they burn gasoline, and our power plants as they burn coal or natural gas. All of this nitrogen moves through the air until it collides with a piece of dust floating down towards the earth, or gets enveloped by a falling raindrop. And then, it settles out on the earth, perhaps hundreds of miles from where it started, acting as a type of nitrogen fertilizer above-and-beyond the nitrogen already there. Thinking back to the leaky barrel, this is a situation where that nitrogen stave might now be a little taller than the barrel itself- that extra nitrogen goes somewhere, and that somewhere might be the nearby lake.

�When I was reading about the organisms that will respond in some way to any extra nitrogen or phosphorus coming into a lake or river, they mostly break down into a few categories: water plants like cattails, water lilies, wild rice, duckweed; and then things that float in the water and maybe form mats or blooms, like green algae, diatoms, and cyanobacteria (or blue-green algae). What of these organisms do well with more nutrients in the water?�

�Yeah, I mean overall, right, with greater nutrients, you get greater growth��
� and the things that can respond most quickly to those nutrients are the things that are going to grow. So things with really short lifetimes or generation times. Those are things that can respond most quickly. And those are things like cyanobacteria and algae, rather than plants which take a little bit longer to actually grow� �

�Yeah, we actually within algal communities, we do have lots of diatom growth within lakes. Those tend to occur during lower temperature times of the year. So normally you see a spring diatom bloom in different systems. And then as temperatures warm up the problem with diatoms, one of the things they suffer from is they create these glass cases that they live within their silica cell walls, which are really heavy. So as that water becomes less dense, it's harder to hold those up in the water column and they kind of sediment out. And then other types of algae kind of come in with those warmer systems and start out competing them for lights and nutrients. So things like green algae and cyanobacteria, but those are a common part in lakes.�

�Can you just describe what some of these organisms look like? What�s a diatom look like if I'm out on the water versus a blue green algae versus green algae?�

�Yeah, so some of the key things you can pick up are the cyanobacteria are- during bloom conditions- they�re really easy to tell. And lots of times you can actually distinguish what type of cyanobacteria it is. So lots of times on systems that are experiencing an algal bloom, either the algae are going to be floating on the surface and create this scum that looks like a layer of paint on the surface of the lake. those tend to be cyanobacteria because they have gas vesicles and vacuoles within them, so they're really buoyant and can float to the surface. That's one of their competitive advantage against other things is they can float to the surface and shade out other types of algae. But then you get different kinds as well. There's a type of cyanobacteria called a Phanasabenon and on that if you look in the water, it looks like little grass clippings is what most people think it looks like or little chunks of hair even that are floating in the water in these little colonies, and that's a Phanasabenon. And then other times where you see just the pea soup green water, where everything's mixed from top to bottom, and it's really hard to see, that's something called microcystis typically, and that's another one that can float to the surface. But one of the key ways to tell apart these types of algae are all based on their pigmentation because that's largely how they were classified originally. So when you think that cyanobacteria, you know, you think of the common name blue green algae. The reason that they were called blue green algae is they have this pigment called microcyanin, which is kind of a turquoisey type color almost. So lots of times when you see these surface scums, they aren't really dark green or a green but they're more of a turquoise or a teal�
As the name implies, with green algae, so green algae have chlorophyll A but they also have chlorophyll B in them as well, which is really a bright green to the eye pigment that we can see. So in water is where you see kind of more more green to them, lots of times those could be green algae, and then the diatoms have chlorophyll C and a lot of these accessory pigments that almost give them a golden brown color�
So that's a way to start distinguishing them in the water column.�

It�s important to realize that all of these things Chris is describing (cyanobacteria, green algae, diatoms) are photosynthesizers that produce energy from sunlight like plants do. And they�re all tiny, with individual bodies that can only be seen under a microscope. The best way to distinguish them without being an expert with a microscope- something you can do on your home waters- is to look at how they grow, in goopy mats on top of the water or like a paint spill or a haze throughout the water, and what color they are.

�So why is more biological productivity is not considered a positive thing in a body of water?�

�Yeah, it depends on the kind of your overall end measures of, of how you view productivity, right. So in these lakes and natural ecosystem, booms when we tend to get higher productivity, we tend to see a shift in communities towards things that bloom quickly or they grow quickly. They can form surface scums on the surface of the water�
The other thing we tend to see with higher productivity is once all that production dies, it goes down to the sediments. And down there, bacteria eat it. And when they do so they consume oxygen in the bottom water�
So overall, you're shifting the system towards a system that has these really potentially toxic algae growing on the surface. And also there's no oxygen in the bottom water, so things can't really live down there.�

�So does that kill the plants too?�

�So lots of times you won't have plants growing in a system like that, simply because the algae growing in the upper parts of the water column shade any light from reaching the bottom, okay? So they just can't grow it. It's this whole idea of regime shifts. So you go from this relatively clear water community that's dominated by plants and things like that over towards these really turbid waters that are all dominated by algae and cyanobacteria.�

What quickly becomes apparent in our conversation is that, as productivity in a lake increases, some creatures gain more from this than others. It reminds me of the game birds that I knew growing up in Virginia. When my ancestors arrived in the region in the 1700s, the East Coast was pretty heavily forested, a legacy of the loss, mostly from European diseases, of the native people who had previously managed and cultivated the land. As colonists established themselves in the hills and mountains, they cleared land for farms, and quail became abundant in the brushy fencerows. Most of the forests were logged around the time of the Civil War and World War II, and grouse flourished in the young brushy woods that came up after the logging. Since then, the forests have grown up to tall trees with little undergrowth, and the brushy spots at the edges of farms have mostly disappeared. Along with those changes have gone the quail and the grouse. But the squirrels and the turkeys that like the big oak trees and the acorns have done well- our landscape changed, some animals were able to take advantage of that, and some could not.

Chris was describing something similar happening in nutrient-enriched lakes. If you think of all the biomass of photosynthesizing plants and algae in a lake as a pie chart, maybe under normal circumstances, that pie might be � plants like wild rice, cattails, and water lilies, and � things like green algae, cyanobacteria, and diatoms. But if you were to start adding a lot of nitrogen, or even more so, phosphorus, to that lake, that pie chart (the overall amount of photosynthesizers the lake could grow) might double in size. But, and here�s the key point, that pie chart might now only be � plants, and � green algae, cyanobacteria, and diatoms. So, even though the lake is more productive, it has less plants now and way more algae. And that can have some unintended consequences. Here�s Chris:

�in these really high nutrient systems, what we tend to see is a shift towards cyanobacteria or blue green algae�
cyanobacteria can produce things like hepato-toxins or liver toxins that can create liver failure, in animals and wildlife and pets and humans, but they can also produce neuro toxins that are really potent and affect the nervous system really quickly. And so it's a public health problem, but it also impacts the agricultural industry as well. Because I used to work in the state of Iowa, which has really a lot of livestock production. And we would see these family farmers who would have, you know, a quarter of their herd die all at once, because they're drinking from these really high nutrient stock ponds that developed a really nasty bloom of cyanobacteria. And you know, without thinking about it, you know, the cows are drinking from the water source that had toxins in it, and they just died. But that can be so devastating to a family farm operation. You know, where all of a sudden a quarter of your herd is just gone.�

�Why is it so many of these things- some of the diatoms dinoflagellates cyanobacteria- why do these organisms that respond so well to eutrophication, why do so many of them produce toxins? I mean, I just think of you go out swimming in a lake that has a bunch of, like, lotus or cattails or things like that in it- it's not gonna make you sick. But these little organisms so often seem to make people or animals sick.�

�Yeah, I tell you the truth. I think that is the greatest mystery that- � So cyanobacteria have been around for billions of years on earth. And so during some time, when they were evolving, they started producing toxins. And we just don't know why�
The interesting thing is cyanobacteria don't produce toxins to harm us or harm our pets, that's just a kind of negative collateral damage of them producing these things for other reasons that we just don't quite understand.�

While this is primarily a problem for people, their health and their livelihood, there are some aspects that are a direct concern to all the fishermen and women out there. At one point in our conversation, Chris informed me that oftentimes, it�s not actually the lakes with the highest amount of algae and plant growth that grow fish, and especially big fish, most rapidly. And what it seems to come down to is that some of the photosynthesizing algae that form the base of the food web ( which he refers to as a trophic level), and that become more predominant as lakes get more nutrients, just aren�t good fish food.

�Why, at a higher rate of biological productivity, wouldn't you have higher growth rates?
I'm just curious.�

�Lots of it could be a shift in the composition of what you're growing. Right? So there's a really good example of cyanobacteria producing biomass that isn't officially transferred to the next trophic level. So it doesn't make its way up to fish as quickly. So you have this this big pie, right? So this big pie of overall biomass, but only a sliver of that is being used to produce the fish and the rest of it is actually decomposing at the bottom of the lake, and having those unintended consequences that then produces this feedback loop that then impacts the type of fish you're growing and how quickly they grow�
You know, up here if you artificially fertilize these lakes, and you get a shift towards things that can tolerate low oxygen, so you lose all your your Cisco's and your Coldwater fish and your walleye, and you shift the lakes all over to catfish. There's gonna be a lot of upset anglers up here�
So I think of it in terms of when I was living in Texas. I flew up once a year to visit my wife's family and we went to Lake vermilion to go fishing. Why did I do that when I could have just gone out to some big reservoir in Texas and caught the stripers? Part of it was the aesthetics of that system and getting to kind of, you know, the North Woods type landscape and being able to catch walleye and having those clear, cooler waters that people prefer.�

�I want to come back to the idea of the Leaky Barrel- Liebig�s Barrel.
If you were to picture that barrel with the different staves, you know, for different nutrients that limit growth for all the organisms in a freshwater lake, you know, like the diatoms, the flagellates, the cyanobacteria, plants, would those barrels be different at all for the different organisms?�

�Yeah, most definitely. Because remember, it's based on their metabolic needs. So the easiest example just to throw up there would be thinking about silica right? So if you're looking at the barrel for a diatom, it would have this generally high requirement for silica. versus if you were looking at the barrel for a type of green algae, there would be a minor to almost no silica requirement for that organism, so so the barrel for the diatom would be more likely, relatively speaking to be silica limited, in a system versus that for the green alga�

Silica is an important if overlooked nutrient on Liebig�s Barrel, as it�s critical to maintaining any sort of stiff structure. For instance, plants use it as the backbone of their cells to help their stems stay upright, animals need it for good bone and joint development, and algae like diatoms use it to build the hard cases they live in. A lack of silica almost never limits growth on land or in freshwater, because silica gets dissolved into water as it�s running through soil and rocks. But diatoms use a lot of it- enough to impact its concentration in water under the right conditions.

And so during the spring diatom blooms, lots of that dissolved silica is actually incorporated into those silica cell walls that then settle out from the upper water column. And you can actually see a drop down in the level of silica available in that upper water column, which could impact diatom growth-�

�But mostly downstream, as I learned talking to an expert on the Gulf of Mexico.

�Okay, so phosphorus has gone up, nitrogen has gone up. They�re somewhat stable right now. And the silica, which used to be higher than the nitrate- that's gone down over time�
and that's primarily due to freshwater systems becoming eutrophied, generating more diatoms, those sinking out into the sediments and the silica is not moving downstream because of reservoirs and things like that.�

This is Nancy Rabalais, a marine scientist from Louisiana, talking about longterm trends in nutrient levels flowing down the Mississippi River over time. For over 40 years, she has studied how these changing nutrient levels have impacted life in the Gulf of Mexico. Nancy explained that, like in freshwater lakes, the Gulf of Mexico can now produce more of the tiny organisms- the algae, cyanobacteria, and diatoms- than it used to. But unlike in freshwater, where a lack of phosphorus is generally the shortest stave on Liebig�s leaky barrel, in the saltwater Gulf, it�s mostly a lack of nitrogen that had historically constrained the growth of algae.

�Marine waters are considered to be nitrogen limited�
if you're very close to the river, sometimes you can have phytoplankton growth that is phosphorus limited. And it's phosphorus limited because there's so much more nitrogen than there is phosphorus�
The other thing that can limit growth is light, there's not enough light for the phytoplankton to grow. So very near the river, the phytoplankton grow, even though those tons of nutrients is often light limited. But then as you move away from the river, when the turbidity gets better, there's more light penetration, there's still plenty of nutrients. And that's where the phytoplankton bloom��
These phytoplankton, a term which encompasses all the microscopic diatoms, green algae, and cyanobacteria, have more nutrients and can grow to a greater abundance than they used to a hundred and fifty years ago, as cores of sediment, pulled out of the bottom of the Gulf and age-dated by Nancy and other researchers, can attest. How much these algae can grow these days over historical levels is hard to say. Nancy shared with me the results of some studies she�s done where she takes water samples from all over the Gulf, adds various amounts of different nutrients to them, and sees how much more growth of algae they get. Added nutrients tend to double to quadruple the growth rates of the algae, and the experiments also show how complicated Liebig�s Barrel can be for the Gulf of Mexico.
�if you look at all these vials over time and which ones responded the best, the ones that responded the best were given both nitrogen and phosphorus. So they're co-limiting basically and they co-limit in different places at different times of the year. If you add only nitrogen versus only phosphorus, the nitrogen-only additions grew much more than the only-phosphorus addition. So if there were one dominant nutrient in the Gulf, it would be the nitrogen. And that's the one we've seen change the most over the years.�

I asked Nancy about how nutrients like nitrogen, phosphorus, and silica coming into the Gulf from the Mississippi River have changed over the years:

�So over the years, the silicate has gone down, nitrogen has gone up, phosphorus has gone up. And it's, it's out of ratio now as to the best combination of the three together...
We know that the silicate to nitrate ratio has changed from four to one from four to basically one or lower, okay. And the nitrogen to phosphorus has changed. From maybe 16 to one to 30 to 50 to one so much more nitrogen now per phosphorus than historically.�
Scientists look at these ratios of different nutrients because the amount of one nutrient RELATIVE to another impacts what�s going to be the limiting stave on Liebig�s Barrel. For instance, think about algae that needs 16 times as much nitrogen as it does phosphorus to build its cells. If the water it lived in had 30 times as much nitrogen as phosphorus, you could sit there and dump bags of nitrogen fertilizer in the water (I don�t recommend actually doing this) and it probably wouldn�t grow any more algae, because what they need to grow more is phosphorus. And these nutrient ratios change as the Mississippi River flows its way down towards Louisiana.

If, like some sort of Northwoods Huckleberry Finn, you were to drift a raft down the Mississippi, it would take you about 3 months to travel from the headwaters at Lake Itasca to the Gulf of Mexico. Along the way, hundreds of rivers would join the Mississippi. Each one of these rivers would carry more nutrients like nitrogen, phosphorus, and silica with them, but at the same time, nutrients would be lost along the way. For one thing, those nutrients would be used by plants and algae in the rivers, and when those living things died, some of those nutrients would settle to the bottom of the river and get buried in sediment. For another thing, any time the river slowed down, like at a dam or lock, some soil would settle to the bottom, and nutrients attached to that soil would go with it. But- and this is a key point- some nutrients would disappear to a greater extent than others. And so you end up with a situation where the proportions of different nutrients COMING IN to the river at Lake Itasca are very different from their proportions GOING OUT at New Orleans. I want to go back to a conversation point with Chris Filstrup:

�Yeah. I think it's kind of thinking about how far do those molecules travel, right. So anything that's going to be incorporated into particularly forms such as an algal cell or bound to a thing like a clay will sell settle out relatively quickly in the upper part of that watershed, and then not be transported down right? So earlier we were talking about nitrogen versus phosphorus limitation. And in the inland waters, [Here he means lakes and rivers] these tend to be phosphorus limited, right? So you have a surplus of nitrogen relative to the phosphorus. So all the phosphorus can be consumed by organisms to meet their demand and sediment out. But you're still going to have a lot of nitrogen in that system that can't be used by organisms because they just don't have the need for it. Because they're actually limited by something else. So a thing like nitrate, or something like that, could travel further along that conveyor belt and make it further into the oceans and the coastal systems-�

Think back to the idea of the leaky barrel. We add nitrogen and phosphorus fertilizers to farms and lawns, and some of that fertilizer gets into our waters. And the algae and cyanobacteria in those waters can use a lot of the phosphorus fertilizer, so they remove a big portion of that, and they can use some of the nitrogen, so there goes a little of that. But there are all these other things we�re NOT adding to our farms and our lawns- I�ll use the nutrient silica as an example- because there is more than enough of those elements available in most soils for plants to use- they almost never limit plant growth. And silica, at least in small quantities, makes its way into lakes and rivers from the soil and rock it came from. Plants and algae will use some of this silica for their own growth, removing it from the water. Now, if you double the growth of plants and algae in the water because you increase their phosphorus and nitrogen supply, you double the amount of silica they remove from the water. The net result of all this is that, at the same time that we have INCREASED the supply of phosphorus and nitrogen to the Mississippi and Gulf of Mexico, we�ve DECREASED the supply of silica. And that has major implications for the Gulf, because silica is really important for diatoms, and diatoms represent a huge portion of life in the Gulf.

�I was talking with Chris Filstrup, a researcher who studies eutrophication in freshwater lakes, about what organisms benefit from increased nutrients in a lake, and it seemed to be mostly cyanobacteria and green algae, and diatoms to a lesser extent. What organisms benefit from increased nutrients flowing into the Gulf of Mexico?�
�Well in the coastal waters, I would have to say it's all diatoms if the nutrient ratios are favorable for their growth. �

�Okay. �

�diatoms are primarily the base of the foodweb in many coastal systems and especially the Gulf of Mexico, although the makeup of the phytoplankton changes over the year...�
dissolved silica is necessary to make up what's called the crustule of the diatom. They come in, you know, pieces together or and they can form chains or they can sit on the bottom or they have these shell like characteristics made of silica�

�The diatoms can be classified as heavily silicified- in other words, they have lots of silica in their crustules- or moderately or lightly solicified. And what's happened over the years in this 50 year, 60 year, going on 70 year process now is that there's less silica available, so there's different types of phytoplankton now than in the past. The heavily silicified ones that used to be common in the 50s are not very common anymore except very close to the river delta. And they're made up of these more lightly silicified, including the harmful algal pseudonitzschia, which causes� short term memory loss. Old age!�

�Are there toxin-producing diatoms that have a lot of silica or have we selected for more toxic diatoms over time?�

�The toxin producing diatoms are those that have less silica in their cell walls and over time, the number of the toxic diatoms has increased��
�Why is that?�
�That's because the diatoms that have adapted to that change in the ratio are more efficient at both using nitrogen and silica. They outcompete those that aren't as good as the ones that were heavily silicified.�

The Gulf of Mexico gets more nitrogen than it used to, but less silica, because that silica is being used up by algae blooms farther upstream. Because the Gulf has more nitrogen, more diatoms can grow and form blooms. At the same time, with more �mouths to feed�, there�s more competition for silica, and so the diatoms that need more silica overall lose out to those that don�t need as much- which also happen to be ones that can produce harmful toxins. It�s another reminder that, in the natural world, more growth of plants and algae does not always translate to outcomes we really want. It�s as if you were to go into the woods behind your house and spread nitrogen fertilizer- and that didn�t do anything to help your oak trees grow, it just caused the poison ivy and stinging nettles to grow like crazy.

------

Justus von Liebig, in the book that made him famous in the agricultural world, envisioned a future where �fields will be manured�with the salts of phosphoric acid prepared in chemical manufactories, exactly as at present medicines are given for fever and goiter�, a startlingly similar take on agriculture to what the industry is today. Where he went wrong was in discounting the importance of nitrogen to our crops. Today, we grow 9 times as much corn on an acre of land as we did in his day, and while this isn�t entirely because of nitrogen fertilizers, they are a major reason for it. At the same time, I imagine he�d be surprised to see how widely his ideas are used in the field of ecology- how major changes in the availability of elements like nitrogen, phosphorus, and silica have altered the biological winners and losers in the natural world.

But the concept of the Leaky Barrel that Liebig popularized also reveals a mystery that needs explaining. Life in freshwater lakes and rivers isn�t often limited by a lack of nitrogen, but in the Gulf of Mexico it is. And if you were to go around the world and run experiments like Nancy�s vials- applying different nutrients as fertilizers in every different setting you could think of- forests, cornfields, a prairie with grasses bending in the wind, a desert of cactus and sagebrush, a mangrove swamp teeming with fish and alligators- you�d find that nitrogen was most often the factor limiting growth in these different settings. As far as nutrients go, it�s most often the shortest stave on the barrel. The mystery is why this would ever be the case.

Consider this: A grown man walks around blissfully unaware of the fact, but in a sense carries on his shoulders the crushing weight of around 440,000 lbs- the weight of the air that extends from the ground to about 300 miles above the earth. Most of this huge weight of air is composed of nitrogen. Although this nitrogen gas in the air is in a form that plants and animals can�t use directly to build proteins, there are living things in every environment on earth, called nitrogen fixers, that can take this gas and convert it into the forms livings things can use. You have an unlimited SUPPLY of nitrogen, and a lot of DEMAND: the mystery is why, in most environments, that demand is not fully being met. In the next part of this series, we�ll dive into that mystery a little more, as we talk more about the Gulf of Mexico and an experimental forest in the Allegheny Highlands of West Virginia.