Spatial and temporal variability of algal fluorescence in eutrophic Lake Mendota

Background/Question/Methods

Harmful algal blooms are a frequent occurrence in Lake Mendota, a large eutrophic lake in Madison Wisconsin.  Lake Mendota has been studied for over a century, with frequent sampling efforts being focused at the deepest location in the pelagic zone.  However, anectodal observations suggest that algal growth can vary spatially throughout the summer, making it difficult to predict the timing and location of harmful algal blooms.  Reducing predictive uncertainty remains a top priority for scientists and lake managers.

The purpose of this study was to examine the spatial and temporal variability of algal growth in Lake Mendota over the course of the summer to quantify spatio-temporal heterogeneity and determine any areas experience increased frequency, size, or duration of algal blooms.  Field sampling was completed weekly for 21 weeks from early June to mid October in 2010.  Algal pigment fluorescence (both chlorophyll and phycocyanin), temperature, and dissolved oxygen were measured using a YSI Multiparameter Water Quality sonde at 10 locations along a transect and at 1 to 2-meter increment depths.

Results/Conclusions

Results revealed both spatial and temporal variability of algal fluorescence across the sampled transect.  Across the transect, two different bays were sampled in addition to part of the pelagic zone near the deepest location.  The bays, in general, showed higher variability and magnitude of algal fluorescence than the pelagic.  The two bays  varied substantially over the summer, reflecting their unique bathymetries and direction of exposure to pelagic areas.  Overall, lake patterns changed seasonally and with meteorological conditions.  Additionally, potential algal growth driver data, including nutrients, dissolved oxygen, and temperature, varied over time and space.  These results indicate that algal growth in Lake Mendota is both spatially and temporally dynamic and that measuremements made at traditional sampling location, the deepest part of the lake, do not capture important system characteristics.  Horizontal heterogeneity tended to be greater than vertical heterogeneity, even when no surface scums were present.  Furthermore, results suggest local growth conditions, in addition to lake hydrodynamics, may be important to explaining spatial heterogeneity.