Productivity and Nutrients

Almost all life on earth is supported by plants because they have the unique ability to turn energy from the sun into organic matter which serves as food for everybody else. In doing this they also produce oxygen which practically everyone else needs too. There are also lots of bacteria capable of doing this. These plants and bacteria are producing energy the rest of the ecosystem can use, and so they are called the primary producers. The energy they produce is called primary production, and the rate at which they produce the energy is called productivity.

In the ocean, most of the primary producers are microscopic single-celled algae. These cells are measured in millionths of a meter or micrometers or microns. The algae are typically 5-50 microns which is about the width of a hair. And they are essentially responsible for life in the ocean!

Plants convert solar energy into chemical energy stored as organic matter through the process of photosynthesis. In this process, sunlight removes oxygen from carbon dioxide. The lone carbon atoms are highly reactive and so "grab" water molecules. Oxygen molecules are released. This process of hydration produces carbohydrates which are quite stable and can be stored for later use. Plants also make lipids or fats, and proteins or amino acids with the carbohydrates. These are what organic matter such as yourself is made of.

This stored energy can be released later by burning or oxidizing the carbon. In this case oxygen is added to the carbohydrates---just the reverse of photosynthesis:

C(H2O) + O2 à C02 + H20 + energy

This process is called respiration. This is what provides you with all the energy you need to live. And it all came from the sun originally, via plants. And you thought you were important!

What do plants need to do this for us? Well, consider your houseplants. What do you need to do to keep them alive. You give them light, water, and if you’re really conscientious, fertilizer. Algae in the ocean needs the same stuff. Water is not a problem, of course. Algae is designed to float so it stays near the surface and can get sunlight. What about fertilizer? Just like your plants, these nutrients are the limiting factor in algae growth. Usually nitrogen and phosphorous are in the greatest demand and tend to be rather scarce in the marine environment. Where do they come from?

Since nutrients are the limiting factor in algae growth, the more nutrients the more productivity. First how do we know how much productivity there is? There are several ways of measuring this. First, in general, the higher the productivity in an environment, the greater the abundance of life. So one way of measuring productivity is to quantify the mass of living organisms---say organisms per cubic meter, for example (square meter if you’re talking about land). Since animals usually move around, this is most effectively done with plants. In the ocean this means counting algae cells.

Since algae produce oxygen, another way of quantifying (i.e. measuring) algae is to put a sample of seawater in a sealed container and measure the amount of oxygen increase in the water after a period of time. This is usually done per 24 hours. This method typically compares oxygen increase in a bottle that allows light in compared to a bottle that is darkened. In the darkened bottle only respiration can occur, and so the oxygen that is used up by that process can be determined and added to the oxygen produced by photosynthesis in the light bottle (the same amount of respiration goes on in the light bottle as the dark). In this way the total oxygen production can be measured. This is called the light-dark bottle method.

These methods work in small areas, but how do we look at the whole ocean? A new and easier way of estimating oceanic productivity is by satellite measurement. Satellites can measure ocean color. The greener the surface ocean water, the more algae, therefore the higher the productivity. Thus global maps of productivity such as the one below can be produced. Here the reds indicate very high productivity and the purples indicate low productivity. Obviously the data is color coded to improve clarity. The oceans are not really purple.

Looking at that map, what patterns do you see?

Where are the red areas of highest productivity? The most apparent place is in the Arctic and Antarctic. You probably envision these as cold, hostile places where not much goes on, and yet look at this productivity. What’s going on? Consider what’s needed for productivity: water, sunlight, and nutrients. Water in the ocean is never a problem. Sunlight? Well, it’s sunny around the north and south poles for 24 hours a day for half the year. That’s a lot of sun. What about nutrients? Surely not many nutrients are eroding off of ice? True, but the wind patterns and currents that move around the Arctic and Antarctic cause a lot of upwelling. Remember that this is when the surface water is moved and deep water comes to the surface to replace it. That deep water is full of nutrients that have settled out. Therefore we have lots of sun and lots of nutrients, therefore, there is lots of productivity.

Where else is productivity high?

Along most of the coastlines. Why is this? Well, of course there’s plenty of water and pretty much sun everywhere. What about nutrients? Well, where do nutrients come from? Upwelling is one source and we have several situations such as off South America in the Pacific where we have upwelling due to currents moving water offshore (remember our discussion of El Nino/Southern Oscillation). There is upwelling for other reasons too. Look at the figure:

In addition, water along the continental shelves is relatively shallow and therefore it is turbulent. This turbulence or mixing keeps settling nutrients stirred up and available. This is one of the main reasons why the Chesapeake Bay is so productive. The bottom is very close to the top. Finally, lots of nutrients erode off the continents themselves and so water close to shore tends to be high in nutrients.

Where isn’t productivity high?

You need sunlight and nutrients for productivity. In deep water there are lots of nutrients but no sunlight. This is why it’s reasonable to measure productivity just by looking at surface water. In surface waters away from coastlines, generally there is plenty of sun but not enough nutrients. So most of the ocean surface is not very productive. The water is too deep for storms to stir up nutrients from the bottom and there isn’t upwelling.

The average ocean productivity is about 50 grams carbon per square meter per year. The productivity of the open ocean (ocean away from coasts) is comparable to desert production. This means that most of the ocean, or about 90%, is essentially desert. And since the ocean makes up over 70% of the earth’s surface, it is not too surprising that land is much more productive than ocean, even though life on land would seem to be tougher land plants aren’t bathed in water. Average land productivity is 160 grams carbon per square meter per year. The problem in the ocean is that the water is so deep that nutrients quickly fall below the reach of the algae and it is very difficult to get them back up. In most cases it simply doesn’t happen. In upwelling areas along coasts where the nutrients are brought back up, productivity is 5-6 times that of the open ocean and greater than the land average. It is not as high as rainforests or estuaries/salt marshes though.

This deep sea nutrient trap is apparent if we look at tropical waters and seasonal patterns. We think of the tropics as being highly productive areas. And they are, on land. In the ocean, the surface water gets very warm and remember that warm water is less dense than cold. This means the warm water in effect gets stuck above the cold water where all the nutrients are. In temperate zones, this only happens in the summer. In the fall as the surface waters cool off, they begin to mix with the waters beneath them, and this mixing, with the help of winter storms, stirs up nutrients. So the ocean is more productive in temperate zones. This also explains why the tropics have that pretty clear blue water. There’s not much life in it! Our water looks green because it’s full of algae. This pattern is illustrated below:

So in temperate zones I just said that in winter the water mixes easily and so nutrients are available and so productivity is high, right? Well, no. In the winter, there isn’t enough sun. Come spring though, the nutrients are still there, the sun increases and the algae goes crazy. This is called the spring bloom. The algae multiplies quickly, the water may be a bit murky for awhile, but eventually those nutrients get used up, and because during this time the surface water has been warming, it’s gradually getting stuck on top again eliminating the nutrient source. This ends the bloom by summer.

Too Much of a Good Thing

We said nutrients come from land, and so coastal water is the most productive. This has a down side. Coastal pollution is a big problem. All the pollutants we spew over the land eventually wash into the water one way or another, or we simply directly dump into the water. This used to work when there weren’t very many of us. Unfortunately, humans like to live along the coast, and more and more of us are doing that. And even if you are on a sewer line so your sewage is treated before it winds up in the water, and you are conscientious about not dumping toxins down the drain, you are part of the problem. Your car emits all sorts of nasty things. You probably get your electricity from a steam plant fired by coal which puts mercury in coastal water. When it rains hard such that our sewage treatment systems can’t handle the water, the excess overflows and winds up in the water anyway. There are lots of toxins we could talk about, but since we’re into nutrients, why are they a problem?

First of all, the nutrients, especially nitrogen and phophorous, are limiting. This means that algae would grow more if there were more of these available. Well, guess what? Nitrogen is the primary nutrient in sewage, be it yours or a pig farms. In addition, farms and golf courses use fertilizers on their grounds and a lot of this tends to run off and end up in the water. Phosphorous is found in many of our cleaning products which all end up down the drain and in the water too. And so coastal algae often has a regular smorgasbord of nutrients available. In response to this, it blooms. So? That’s more food for the fish, right? Well yes, to a point. But very quickly there is more food than the fish can eat. In fact there is so much algae that it actually starts blocking its own light source and so it starts dying. Not too much eats dead algae. Mostly bacteria get to work in the decay process. But bacteria have to respire, and so the decay process uses up oxygen. Not much lives without oxygen and this is how you wind up with fish kills. And that’s not the worst case scenario. The algae itself can be toxic. This would be a red tide, or you’ve probably heard of pfisteria which is toxic and seems to be popping up more and more here in the southeast. Toxic tides in general seem to be showing up more frequently. Coral reefs are under increasing stress. Manatees are having a tough time. Is this the result of increased pollution input? Warmer water due to global warming? Do we have the luxury of waiting around to figure this out before the system is permanently damaged?

To end on an up note, sometimes the solution to this problem of excess nutrient input is not too difficult. In the 70’s it was decided to ban the use of phosphate in detergents sold in the Chesapeake Bay’s watershed (a watershed includes all the sources of water for a body of water). This simple step helped improve water quality. Presently, the states around the bay are enacting programs in which you must leave a buffer or edge of trees or wetlands between farmland and water, because trees, being such large plants, are good at sucking up nutrients. Wetlands in general are also very good at this and they both do it for free. Finally, you have probably noticed that there are always plastic barriers or at least hay bales surrounding construction sites. The purpose of these is to keep crud from the work site from washing into the water. Are these steps working? Well, they’re helping. We certainly haven’t solved the problem and we no doubt will have lots of other steps we’ll have to take, but I think it’s good to remember that it is not hopeless.