The Role of Nitrogen Availability on the Dominance of Planktothrix Agardhii in Sandusky Bay, Lake Erie

Sandusky Bay and Lake Erie are plagued with harmful algal blooms every summer. Sandusky Bay is a drowned river mouth that is very shallow and turbid and is dominated by Planktothrix agardhii, while Lake Erie is dominated by Microcystis aeruginosa. Both species of cyanobacterium are non-diazotrophic and produce microcystin, a hepatotoxin. A competition experiment was conducted culturing both species alone and in coculture at nitrogen (nitrate) replete, nitrate restricted, and nitrogen-free environments. Planktothrix grew better alone at nitrogen restricted medium than in co-culture with Microcystis. In coculture, Microcystis was dominant over Planktothrix however, that dominance decreased as nitrogen was reduced in each treatment. In the nitrogen replete environment, the coculture produced significantly more toxin than the monocultures and in the no nitrogen environment the Planktothrix monoculture produced more toxin than the Microcystis monoculture or the coculture. The community composition in Sandusky Bay was monitored over the winter and spring months (January-April) to see how it changed as time progressed. Nutrient amendment experiments were also conducted adding nitrate, phosphate, and a combination of nitrate and phosphate to stimulate growth and identify any possible nutrient limitations. The initial community yielded low cell densities until the temperature increased and cell abundances followed shortly thereafter. Planktothrix dominated over the winter followed by a transitional period of cryptomonad and diatom dominations before transitioning back to Planktothrix. Both nitrate and phosphate were limiting Planktothrix growth in the spring, while nitrate alone was limiting the overall community.