In hypertrophic lakes, where nutrients are in ample supply, phytoplankton
growth is often limited by light availability. In a large-scale
experiment, we manipulated the turbulence structure of an entire lake
using artificial mixing. Changes in turbulent mixing of the lake caused a
dramatic shift in phytoplankton species composition. Consistent with the
model predictions, sinking diatoms and green algae dominated during
intense mixing, while buoyant and potentially toxic cyanobacteria became
dominant when mixing was reduced.
In the oligotrophic ocean, phytoplankton species face two opposing
resource gradients: light supplied from above and nutrients supplied from
below. Here, our model predicts that reduced mixing will bring less
nutrients into the euphotic zone, which generates oscillations and chaos
in the phytoplankton populations. These intriguing model predictions are
compared with the complex species dynamics observed in the long-term time
series of the Hawaii Ocean Timeseries program.
According to climate models, global warming will yield a stronger vertical
stratification of lakes and oceans, which reduces vertical mixing in the
water column. Our findings warn that reduced mixing, driven by climate
change, can induce major shifts in the population dynamics and species
composition of plankton communities.
References:
Huisman, J., J. Sharples, J.M. Stroom, P.M. Visser, W.E.A. Kardinaal,
J.M.H. Verspagen & B. Sommeijer. 2004. Changes in turbulent mixing shift
competition for light between phytoplankton species. Ecology 85: 2960-2970.
Huisman, J., N.N. Pham Thi, D.M. Karl & B. Sommeijer. 2006. Reduced
mixing generates oscillations and chaos in the oceanic deep chlorophyll
maximum. Nature (in press).