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imageSo to cut right to the chase, let's ask what happens to the food web and to the partial pressure of CO2 in the lakes with nutrient additions. If the CO2 level goes below the line, the lake is a CO2 sink; above the line, it is a CO2 source. Looking first at the reference lake, Paul Lake, it is, as predicted, supersaturated with CO2, and a persistent source of CO2 to the atmosphere. The planktivore-dominated Lake where we added nitrogen and phosphorus, became a CO2 sink for the first year. These results are the similar to the ELA results and gives us some confidence that we know parts of the lake system. In Peter Lake, there are no large fish; there are small planktivores minnows, but large zooplankton such as Daphnia are very rare. The algae bloom like crazy, and we get a nice CO2 drawdown from the very beginning.

This other lake, which is a piscivore-dominated lake, with nitrogen and phosphorus added, starts out somewhat as a CO2 sink, but becomes a source very fast. In other words, in the piscivore-dominated lake with nutrients added, we were unable to create a CO2 sink in this year.

imageFor those of you who aren't scientists, this looks worse than it is. I've just showed you one year of data. Now I'm going to show you the data for the entire five-year experiment using partial pressure of CO2 and partial pressure of oxygen, which should mirror each other. That is, as CO2 levels decrease, the oxygen levels should increase since the phytoplankton use CO2 and produce oxygen in photosynthesis.

imageBefore we started fertilizing the experiment, the lakes were supersaturated in CO2, and correspondingly undersaturated in oxygen. We add nutrients for five years and then we stop. At the end of the experiment, the lake is supersaturated in CO2 and under-saturated in oxygen. During the experiment, we see the control lake and the piscivore lakes are supersaturated with CO2. Only the planktivores lake is consistently a CO2 sink, and we see the mirror image in the increase in oxygen. It's a very consistent set of data.

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So the control lakes are consistently CO2 sources, as we would expect. As we manipulate the nutrients in the piscivore-dominated lakes , they have very little effect, and the lake continues as a CO2 source to the atmosphere. We are unable to create a CO2 sink by adding nutrients alone over five years. When you tinker with the food web and remove the large fish, you create a CO2 sink in the planktivore-dominated lake.

imageSo the results of these experiments were somewhat surprising. Fish in fact do affect both the magnitude and direction of gas flux in small lakes. Lakes are CO2 sinks only when nutrients are added, and piscivore are reduced. With the nutrient addition under a piscivore food web, these lakes are net sources of CO2 to the atmosphere. So what's going on?

This is a simplified model of carbon cycle in a lake. If you add nutrients, these arrows--which represent gross primary production and respiration become large because the cycling rate goes much, much faster.

imageBut adding nutrients, primary production increases about as much as respiration by bacteria, and we're not actually altering the ratio between primary production and respiration. So under the piscivore-dominated food web, we add nutrients, we increase primary production but we also increase the respiration with no net increase in carbon storage in the deep lake.

So regarding CO2 storage as organic carbon in the deep lake, the food web matters, and time matters. It's not a sure thing that adding inorganic nutrients are going to produce a CO2 sink over time.

imageWhat about the question of increased fish production with increased nutrient additions? Limnologists have studied this question for some time.

 

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