imageSo now let's look at a very simple issue. We have phytoplankton that are photosynthetic organisms and use CO2 and produce oxygen. We're going to add nutrients such as nitrogen and phosphorus. Iron is not a part of the story since it is not a limiting nutrient in lakes. The questions addressed were if nutrients were added, could a phytoplankton bloom be stimulated leading to an export of organic carbon from the surface water into the deep water. If so, can we measure an influx of atmospheric CO2 into the surface water?

imageWe're really good at providing nutrients for phytoplankton in lakes. What you're looking at is one of the lakes at the Experimental Lakes Area or (ELA). A neoprene or rubber curtain divides the hourglass shaped lake. One side is fertilized with nitrogen and phosphorus, and the other is not. So, yes, we can add nutrients and we can turn lakes green. In fact, the rationale for first doing these experiments was to find out what nutrients caused lakes to turn green so we could stop putting them into lakes. The experiment was to add nitrogen and phosphorus at realistic ratios for 27 years during the ice-free season. This is what the lake looked like in 1994.

imageNotice the nearby lake with the normal surface water color, also note that the lakes are small. Keep that fact in mind, because the small size makes it hard to always extrapolate results from lakes to larger oceanographic conditions.

imageThis is what happened in this experiment during one year. If the partial pressure of CO2 in the water is above the dotted line, the lake is a net source of CO2 to the atmosphere. If the partial pressure of CO2 in the water is below that solid line, the lake is a sink for atmospheric CO2.


Now I have to add one other bit of information. Typical lakes, are supersaturated in carbon dioxide, which means that CO2 gas generally effluxes from the lake to the atmosphere. Why? Because bacteria in the lakes metabolize terrestrial carbon that flows into the lake and bacteria respire CO2. So assuming the lakes are supersaturated with CO2, the expectation would be that the partial pressure of CO2 would be around this dotted line here and the lake would be a net source of CO2 to the atmosphere. They dumped nutrients into the lake starting in the late spring, usually in May when the ice is gone, and boom -- the partial pressure of CO2 can't be measured. It is undetectable in this lake. They drew the CO2 concentration down to zero because the phytoplankton are growing so fast and utilize the CO2 in photosynthesis to produce oxygen. So after adding nutrients over a single spring season, the lake is a huge sink of atmospheric CO2.

imageNow let's look at the CO2 data over 27 years of applying nutrients. I've plotted a partial pressure of CO2 on a log scale so it fits on the graph and the zero on the y-axis is the atmospheric equilibrium line. Over 27 years of nutrient additions, the lake is both a source and sink of CO2. Every summer there's a bloom and the lake drives the CO2 down to very, very low levels. In the spring and fall, the phytoplankton blooms are not large, and the CO2 levels in the water are high. Here the lake is a net source of CO2 to the atmosphere. By the fall, bacteria are decomposing the phytoplankton bloom and producing CO2 that escapes to the atmosphere. The lake is not really storing much carbon in the form of dead phytoplankton cells; bacteria rapidly metabolize most of the organic carbon formed in photosynthesis.

imageThe previous slides showed the concentrations of carbon dioxide. Now here is the data recalculated in terms of flux of carbon into and out of the surface water. A net flux of zero means that the lake is neither storing nor exporting CO2. Here is the 27 years of data, and you can see a very interesting pattern.

Over the first few years, the lake was a CO2 source of decreasing magnitude, and then it goes into a chaotic period where the lake is more often a source than it is a sink, with occasional years where it is primarily a sink.

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