Nutrients enhance phytoplankton growth in shallow lakes, but lake-specific responses to nutrient changes are highly variable. Here, we hypothesize that this large variability results from stochastic changes in short-term drivers, which obscure a ubiquitous and predictable, long-term stoichiometric relationship between phytoplankton and nutrients. To test this hypothesis, we extracted long-term variation in multiannual time series by calculating 5-year simple moving averages (SMAs) based on growing season averages of total nitrogen (TN), total phosphorus (TP), and chlorophyll a (Chla, as a phytoplankton biomass surrogate) concentrations for 159 shallow lakes from around the globe. The residuals of these 5-year SMAs represent short-term variability. For both datasets, we calculated regressions between TN, TP, and Chla along a gradient of molar TN : TP. Using a rigorous bootstrap approach, we show that regressions between nutrients and Chla are highly robust for 5-year SMAs (median R² = 0.87), where we find predictable, ubiquitous relationships between nutrients and Chla along the TN : TP gradient. The remaining short-term variation in Chla was not related to nutrients. This ubiquitous, stoichiometric, long-term relationship between nutrients and phytoplankton provides a new approach for developing effective management strategies for nutrients in shallow lakes and catchments.

Primary Presenter: Daniel Graeber, Helmholtz-Centre for Environmental Research - UFZ (daniel.graeber@ufz.de)

Authors:

Mark McCarthy, Estonian University of Life Sciences  (mark.mccarthy@emu.ee)

Tom Shatwell, Helmholtz-Centre for Environmental Research - UFZ  (tom.shatwell@ufz.de)

Dietrich Borchardt, Helmholtz-Centre for Environmental Research - UFZ  (dietrich.borchardt@ufz.de)

Erik Jeppesen, Aarhus University  (ej@bios.au.dk)

Martin Søndergaard, Aarhus University  (ms@bios.au.dk)

Torben Lauridsen, tll@bios.au.dk  (tll@bios.au.dk)

Thomas Davidson, Aarhus University  (thd@ecos.au.dk)