Altered periodicity in long-term phytoplankton dynamics in response to drought

Background/Question/Methods

Phytoplankton biomass in north temperate lakes is hypothesized to exhibit predictable annual cycles.  This pattern underlies common approaches to modeling phytoplankton dynamics, such as the Plankton Ecology Group (PEG) model, which describes the annually-repeated process of community assembly as a result of competitive phytoplankton interactions, herbivory, and physical forcing.  Under this paradigm, phytoplankton biomass exhibits its yearly maximum in the summer, with smaller peaks in the spring and autumn.  These predicted patterns are supported by meta-analyses indicating that an annual pattern of total phytoplankton biomass dominates for most north temperate lakes.  However, little is known about the periodicity of the individual phytoplankton groups that compose the aggregated signal: do they also exhibit a dominant annual scale?  In addition, does external forcing (e.g., drought) alter the scale of variation for the biomass of individual phytoplankton groups differently than for total phytoplankton biomass?  Here, we investigated the dominant scales of variation for major phytoplankton groups and important species in Lake Mendota, Wisconsin, USA.  We analyzed a 16-year time series of biweekly and monthly phytoplankton data with wavelet coherence and hypothesized that individual phytoplankton groups would exhibit a dominant annual scale similar to the scale for aggregated biomass.

Results/Conclusions

Contrary to our hypotheses and the PEG model, the dominant scale for total phytoplankton biomass and individual phytoplankton groups in Lake Mendota exhibited substantial variation during 1995-2010.  For most years, the annual scale dominated for both total and individual groups of phytoplankton biomass, except for 2002-2006, when sub-annual scales were most important for total phytoplankton biomass and certain phytoplankton groups, especially cyanobacteria and diatoms.  At the beginning of this period, precipitation levels were lower, resulting in reduced phosphorus loads entering the lake.  As a result, the biomass of cyanobacteria and diatoms decreased, resulting in smaller blooms, especially of species with high phosphorus requirements.  Chlorophytes (green algae), dinoflagellates, and other phytoplankton groups did not respond to the drought, showing a dominant annual scale throughout the time series, and a decoupling from total phytoplankton biomass.  In sum, our results indicate that environmental forcing may have substantial effects on the dominant scale of variation for phytoplankton, but some groups may be more sensitive to certain changes.  Consequently, drought events exacerbated by climate change may affect the ability of the PEG model and other similar conceptual frameworks to predict phytoplankton succession in lakes.