US Long-Term Ecological Research Network

Lake Mendota is an eutrophic lake that harbors an abundant and diverse array of bloom-forming cyanobacteria (also called blue-green algae). The cyanobacterial community is highly variable, contains numerous nitrogen (N2) fixing and non-N2 fixing genera, and has multiple genotypes capable of forming ephemeral, and possibly toxic, blooms. Nutrients play an integral role in structuring the cyanobacterial community, but it is unclear how phosphorus (P), nitrogen (N), and trace metal limitation might influence seasonal to decadal community dynamics. My research focuses on linking nutrient cycling to cyanobacterial community structure and succession in lakes. I am particularly interested in N2 fixation as a potential driver of cyanobacterial community structure and cyanotoxin production, especially on temporal scales relevant to protecting public health (e.g. days) and long-term ecosystem function (e.g. decades). On a seasonal scale, N2 fixing and non-N2 fixing cyanobacteria appear to dominate phytoplankton biomass on alternating time scales. Evidence suggests that this may be related to nitrogen stress and the subsequent “new” nitrogen production from N2 fixation. However, it is unclear how this may affect/regulate toxin production. On decadal time scales, it is unclear how climate change, land use change, and the management of nutrient loading may alter the current cyanobacterial community. Thus, it will be important to understand what shapes cyanobacterial populations on a more mechanistic level. I hope to combine my current research with Long Term Ecological Research to develop a model of Lake Mendota that will allow us to explore those future scenarios.

The figure on the right show the relationship between nitrate + nitrite concentrations (N+N), N2 fixation rates, and cyanobacterial relative abundance within the photic zone of the Lake Mendota University Bay location from May 18th to August 31st (days138-243), 2010. Top panel: As N+N concentrations (circles) begin to drop, N2 fixation rates (bars) increase. Following a N2 fixation bloom on day 181, N+N concentrations increase slightly and N2 fixation rates remain low until day 220. On day 220, N+N concentrations are near detection and N2 fixation rates increase again. Bottom panel: Relative abundance of 5 major genera in Lake Mendota. Non-N2 fixers: Chr = Chroococcus and Mic = Microcystis; N2 fixers: Aph = Aphanizomenon, Ana = Anabaena, and Glo = Gloeotrichia. Unk = Unknown. A large Aphanizomenon and Gloeotrichia bloom was observed on day 181. Between days 190 and 220, Chroococcus and Microcystis dominated the photic zone. Again, Aphanizomenon bloomed on day 220. The vertical dashed lines separate these shifts where N2 fixers and non-N2 fixers switch community dominance.

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