Microbial Observatory at North Temperate Lakes LTER Spatial and temporal cyanobacterial population dynamics in Lake Mendota 2009 - 2011

Toxic cyanobacterial blooms threaten freshwaters worldwide but have proven difficult to
predict because the mechanisms of bloom formation and toxin production are unknown,
especially on weekly time scales. Water quality management continues to focus on aggregated
metrics, such as chlorophyll and total nutrients, which may not be sufficient to explain
complex community changes and functions such as toxin production. For example, nitrogen (N)
speciation and cycling play an important role, on daily time scales, in shaping
cyanobacterial communities because declining N has been shown to select for N fixers. In
addition, subsequent N pulses from N2 fixation may stimulate and sustain toxic
cyanobacterial growth. Herein, we describe how rapid early summer declines in N followed by
bursts of N fixation have shaped cyanobacterial communities in a eutrophic lake (Lake
Mendota, Wisconsin, USA), possibly driving toxic Microcystis blooms throughout the growing
season. On weekly time scales in 2010 and *2011, we monitored the cyanobacterial community
in a eutrophic lake using the phycocyanin intergenic spacer (PC-IGS) region to determine
population dynamics. In parallel, we measured microcystin concentrations, N2 fixation rates,
and potential environmental drivers that contribute to structuring the community.