Using Sediments to Identify Drivers of Cyanobacteria, Cyanotoxins, and Eutrophication in the Shallow, Subtropical Lakes of Florida, USA

Abstract

Nutrient eutrophication and cyanobacteria dominance has increased in lake systems throughout the world over the last two decades, with growing concerns over decreases in ecosystem services and the toxins that some cyanobacteria produce. Monitoring of cyanotoxins and regulation of external nutrient inputs into water bodies have been enacted to mitigate increases in harmful algal blooms, but infrequency of sampling and the need to add additional measurements not normally afforded to monitoring programs, suggests the need for other analyses capable of longer timescales and other measurements. This thesis utilized the analysis of microcystin concentrations and paleolimnological measurements to accomplish two primary objectives: 1) determine the relationships of microcystin with nutrient, biological, and morphological variables to define the triggers that favor microcystin production, and 2) characterize changes in the past ~150 years in nutrient concentration, nutrient stoichiometry, and primary productivity through sediment samples of Florida lakes. Surface sediment samples (1-5 years) and deeper core samples (~150 years) were collected from a large sample size of lakes (n=47) throughout the state of Florida, USA. Four measured parameters (Fe, P, aphanizophyll, and secchi depth) demonstrate statistical relationships with total microcystin concentration and could serve as potential triggers of microcystin production. Most notably, stoichiometric ratios of Fe:P had the clearest direct relationship with microcystin concentrations, but Fe is rarely included in monitoring programs where eutrophication and/or cyanotoxin occurrence is of concern. This study demonstrates the potential to temporally reconstruct microcystin production from surface sediment samples and subsequently the drivers of microcystin production. In the temporal comparison of sedimentary and water nutrient concentrations, the differences in nutrient trends suggest that even though external nutrient inputs have decreased, nutrient concentrations in the sediment could be internally loaded and sustaining phytoplankton populations through resuspension events. The analyzed subset of Florida lakes appears to have gone through much less limnological change in the past 40 years when compared to the 100 years prior, suggesting that substantial ecological shifts might have preceded monitoring data.