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So what's going on? Initially, there is a lot of organic material from the phytoplankton blooms accumulating in the first few years of the study that is piled in the deep water and awaiting decomposition. Later, bacteria are decomposing the pile of organic material and the lake is a net source of CO2. So, why does the lake occasionally become a CO2 sink in some years? I don't know and the fact is the situation is unpredictable. The bottom line is that it is difficult to predict whether the lake is a net source or sink of CO2 when adding nutrients. And it's not a sure thing that the lake continues as a CO2 sink, with continuous fertilization over time.

imageTo summarize the results from this ELA experiment, we're dead sure that limited nutrient inputs into lakes will cause seasonable blooms of phytoplankton and substantial CO2 drawdown from the water. This is a consistent, every-year pattern. However, over short time scales-- months to years--much of that CO2 is returned to the atmosphere, and in fact over the 27 years for which data are available, this lake, under a huge regime of fertilization, is not the net sink of CO2. It's a variable net source of CO2.

imageThat's the ELA experience in a very simplified food web. Now, I'd like to expand our view of what might control CO2 levels in the lake water by looking at the food web. This is a question that marine scientists are going to have to deal with if iron fertilization is going to go forward. Phytoplankton don't exist in a vacuum. To oversimplify the lake food web, phytoplankton are consumed by zooplankton, and zooplankton consumed by small fish, and small fish by large fish.

I'm going to talk now about an experiment that asked the question: do large fish control CO2 release? At first, that sounds kind of silly, but think about it this way: big fish consume little fish, little fish consume zooplankton. If we eliminated large fish, do the small fish do a lot better? They can "chow down" on the large zooplankton. Fewer zooplankton means less grazer control over phytoplankton. So if we remove large fish and add nutrients, we might expect to get a lot more phytoplankton than in the presence of large fish which will affect CO2 levels in the lake.

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imageThese experiments occurred at a place called UNDER, the University of Notre Dame Environmental Research Center, where a number of limnologists have worked. So the question is: do fish affect CO2 dynamics in small lakes?

So it's very much like what you saw in the ELA. In fact, right now this is the United States' closest thing to an Experimental Lakes Area.

imageHere's a lake called Long Lake where we divided it into three basins with a west basin and an east basin, and the area between the curtains we maintained as a control with small bass (or piscivores) in abundance. Piscivores are fish that eat other fish. So East Long Lake was a piscivore lake and had few smaller fish.

imageThese other two lakes are more romantically named--Peter and Paul. It's another hourglass-shaped lake divided by a dike in the 1940s. The reference lake is Paul Lake and is the only one in the series that is not manipulated. We left the food web alone, and we didn't add nutrients. This lake, called Peter, we maintained as a minnow dominated lake. We got rid of the large piscivore and added nutrients.

imageSo here are the four basic experiments. Paul is the one we don't disturb. Peter we maintain as a minnow dominated lake and did not add nutrients. Minnows eat phytoplankton and are called planktiivores. East and West Long Lake, we maintained as a piscivore-dominated lake, and we add nutrients. In a given year the nitrogen and phosphorus added, would be the same in each of the manipulated lakes.

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