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.
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.
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
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.