Since its inception the theory of alternative equilibria in shallow lakes has evolved and been applied to an ever wider of ecological and socioecological systems. The theory posits the existence of two alternative stable states each displaying resistance to changing environmental conditions with shifts between states being unpredictable. Here, use data covering a range of timescales to assess the generality of the theory in space and time. We show that a simple, deterministic link between shallow lake phytoplankton biomass (chlorophyll-a) and nutrient concentration becomes evident once a long-term (>3 years) perspective is taken. Specifically, we find a linear relationship between multi-year means of nutrient concentration and chlorophyll-a, for two independent shallow lake datasets in Denmark and North America (902 lakes in total). In addition, palaeolimnological data generated for three shallow lakes show that loss of submerged plants was preceded by decades of progressive, predictable biological change, with no clear evidence for sudden changes as indicative of alternative stable states shifts. This demonstrates that with a longer-term perspective, the notion of alternative equilibria is unnecessary for explaining the response of chlorophyll-a to nutrient enrichment. This in turn, questions the utility of the theory for explaining shallow lake response to and recovery from eutrophication.

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

Authors:

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

Carl Sayer, University College London  (c.sayer@ucl.au.uk)

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

Martin Lauridsen, Aarhus University  (tll@ecos.au.dk)

Ambroise Baker, Teeside University  (A.Baker@tees.ac.uk)

Daniel Graeber, UFZ  (daniel.graeber@ufz.de)