Abstract
Missisquoi Bay (MB) is a temperate eutrophic freshwater lake that frequently experiences toxic Microcystis-dominated cyanobacterial blooms. Non-point sources are responsible for the high concentrations of phosphorus and nitrogen in the bay. This study combined data from environmental parameters, E. coli counts, high-throughput sequencing of 16S rRNA gene amplicons, quantitative PCR (16S rRNA and mcyD genes) and toxin analyses to identify the main bloom-promoting factors. In 2009, nutrient concentrations correlated with E. coli counts, abundance of total cyanobacterial cells, Microcystis 16S rRNA and mcyD genes and intracellular microcystin. Total and dissolved phosphorus also correlated significantly with rainfall. The major cyanobacterial taxa were members of the orders Chroococcales and Nostocales. The genus Microcystis was the main mcyD-carrier and main microcystin producer. Our results suggested that increasing nutrient concentrations and total nitrogen:total phosphorus (TN:TP) ratios approaching 11:1, coupled with an increase in temperature, promoted Microcystis-dominated toxic blooms. Although the importance of nutrient ratios and absolute concentrations on cyanobacterial and Microcystis dynamics have been documented in other laboratories, an optimum TN:TP ratio for Microcystis dominance has not been previously observed in situ. This observation provides further support that nutrient ratios are an important determinant of species composition in natural phytoplankton assemblages.
Highlights
Eutrophication [1] is part of the natural evolution of aquatic ecosystems, especially of rivers, ponds, lakes and other shallow water bodies [2]
We evaluated whether E. coli counts could be a good indicator of recent nutrient input from surface runoff or sewage overflows
In November, the genus Microcystis showed a 25% increase in relative abundance in the pelagic area of Philipsburg which was consistent with an increase in nutrient concentrations as follows: total nitrogen (TN) = 0.49 mg/L; total phosphorus (TP) = 56.74 μg/L; TN:TP mass = 9:1
Summary
Eutrophication [1] is part of the natural evolution of aquatic ecosystems, especially of rivers, ponds, lakes and other shallow water bodies [2]. Cyanobacterial blooms can negatively affect human and animal health [5,6,7,8,9,10,11], the health of the environment [3] including the contamination of potable water supplies [12,13,14] and complications in water treatment processes [12,15,16] It can impact the economy, since the resulting deterioration in water quality contributes to declines in fisheries productivity and hampers recreational activities as well as tourism [17]. Cyanobacterial blooms have been observed in freshwater, estuarine, marine environments and can develop in the photic zone [18] or at the interface between the epilimnion and metalimnion [19]
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