Covariates of cyanobacterial biomass over 1.5 decades in a eutrophic lake

Background/Question/Methods

Cyanobacterial and other harmful algal blooms (HAB) are a global problem resulting from increased rates of eutrophication. Phosphorus is typically managed as the limiting factor for HAB growth. Analysis of long-term ecological data may provide clues as to the interannual trends promoting consistent blooms. Here we analyzed fourteen years (1995-2004, 2007-2010) of nutrient and physical limnological data along with cyanobacterial community composition data collected by the North Temperate Lakes Long Term Ecological Research Project (LTER) from Lake Mendota. The data were statistically analyzed using custom R scripts. Additional lake physical characteristics, including Schmidt Stability, Lake Number, Wedderburn Number and thermocline depth were calculated using LakeAnalyzer 3.4 (lakeanalyzer.gleon.org) implemented in Matlab. The annual average (June - August) cell concentration, biomass, maximum biomass and cell concentration were compared to nutrient loading (Spring maximum) and average (June - August) nutrient levels to examine interannual variations in cyanobacterial biomass and nutrients. The same analyses were conducted for the dominant cyanobacterial genera including Microcystis and Aphanizomenon.

Results/Conclusions

Significant correlations were found between maximum and average Kjelldahl nitrogen (1995-2004, 2007-2010) and average and maximum Aphanizomenon and Cyanophyta biovolume, biomass and cells per mL (R = 0.63-0.74, p< 0.05). Additionally, average and maximum concentrations of total dissolved nitrogen (calculated by adding Kjelldahl nitrogen and nitrate plus nitrite measurements) were significantly correlated with measurements of Cyanophyta and Aphanizomenon (R = 0.53-0.60, p< 0.05). Total phosphorus also significantly correlated with Cyanophyta and some measurements of Aphanizomenon (R = 0.53-0.6, p< 0.05; R = 0.52-054, p< 0.05, respectively). No other nutrients (nitrate plus nitrite, dissolved reactive phosphorus) correlated with any cyanobacterial measurements. The data indicate that nitrogen loading is a significant factor contributing to interannual variations in blue-green algal growth. In addition, the data indicate that Aphanizomenon is likely involved in providing new nitrogen to the lake supporting algal growth. Further analyses of this important dataset could be applied to examine changes in blue-green algae due to lake stratification and global warming as additional future years of data are included.