Anthropogenic forcing leads to an abrupt shift to phytoplankton dominance in a shallow eutrophic lake

The timing and causes of lake eutrophication are often obscured when multiple anthropogenic disturbances coincide in space and time. This issue is particularly problematic for shallow lakes in arid regions that experience strong climatic forcing which alters lake hydrology and water levels, and further conflates causal drivers. We used Utah Lake (Utah, U.S.A.) as a model system to examine how natural hydrological variability and anthropogenic forcing influence ecosystem structure of large shallow lakes in arid climates. Paleolimnological analyses of sedimentary biogeochemistry, pigments, DNA, and morphological fossils were used to identify shifts in primary production and evaluate the relative influence of regional climate‐driven hydrological variability and of humans on ecosystem structure. Sediment cores revealed that the phase prior to non‐indigenous settlement included numerous macrophyte and gastropod remains, sedimentary DNA from plants, low organic matter, and low algal production. An abrupt transition occurred in the late 19th century concomitant with agricultural and urban expansion and the introduction of common carp, which was characterised by a loss of macrophytes and an increase in phytoplankton abundance as indicated by sedimentary DNA and pigment concentrations. A further shift to increased cyanobacteria occurred c. 1950 when exponential population growth increased wastewater influx, as recorded by sedimentary δ15N values. Taken together, our data demonstrate that the current eutrophic state of Utah Lake is a function of anthropogenic forcing rather than natural climate‐driven hydrological fluctuations. Furthermore, large lakes in arid regions can exhibit similar patterns of abrupt ecosystem change between alternate states as those observed in northern temperate/boreal and subtropical ecosystems.