PS 60-32
Thermocline-induced nutrient limitation of toxic cyanobacterial bloom in a shallow, hypereutrophic Mediterranean climate reservoir monitored weekly over two years
The role of eutrophication in determining freshwater cyanobacterial dominance and successional patterns has been studied in detail in temperate and tropical climates. Much is known concerning physical and chemical patterns within these systems, however, the bottom-up influence of climate on cyanobacterial succession in a Mediterranean context is not as clear. Here we characterize a highly toxic cyanobacterial bloom (CB) in a shallow hyper-eutrophic freshwater lake on California's central coast. Our primary goals were to quantify the relative importance of thermocline and nutrients in controlling CB density, taxa succession, and toxicity. Previous study of this lake has indicated the dominance of internal loading of nutrients driven by hypolimnetic oxygen depletion induced by seasonal thermocline. We conducted our study of Pinto Lake (Watsonville, CA) on a regular weekly basis from April 2012-January 2014, collecting discrete samples and hydrological measurements (Hydrolab DS5X) across and below the lake surface. We measured CB concentration, identifying taxa to genera (Anabaena, Aphanizomenon, and Microcystis spp.) by inverted-scope microscopy. We measured chlorophyll a, hepatotoxic microcystins (MCs) by competitive antibody assay (ELISA), and quantified dissolved and total nutrients (NH3, NO3/NO2, SRP, TP, and TOC/TN). Statistical analysis was performed in R programming language using non-parametric statistics (Spearman rank correlation).
Results/Conclusions
Peak CB and MC concentrations were two orders of magnitude lower in 2013 than 2012 (106 cells/mL, 8 ppb MC; 109 cells/mL, >200 ppb MC respectively). The 2013 season was characterized by a highly stable thermocline resulting in early and sustained surface phosphate depletion relative to 2012. The CB taxa Microcystis was the strongest predictor for MCs in 2013, with Aphanizomenon and Microcystis being equally predictive of MC levels in 2012. Microcystis and MCs were both strongly negatively associated with N:P ratio in 2013, while Aphanizomenon and MCs were negatively associated with N:P ratio in 2012. Microcystis was not correlated with Aphanizomenon, however, periods of co-dominance were associated with higher total MCs. Aphanizomenon density was more strongly correlated with changes in thermocline strength in 2012 than in 2013. Our results highlight the crucial role of thermocline strength in determining the succession and density of CB and toxicity in this Mediterranean-climate freshwater lake. Results were surprising given lake shallowness (mean depth=3.75m) and previous years' frequent thermocline interruptions. Our findings are particularly relevant in California, where surface water reservoirs are key to water supply, especially in the face of predicted increases in frequency and duration of drought via global climate change.