Abstract
Piehler, M.F., J. Dyble, P.H. Moisander, A.D. Chapman, J. Hendrickson and H.W. Paerl. 2009. Interactions between nitrogen dynamics and the phytoplankton community in Lake George, Florida, USA. Lake Reserv. Manage. 25:1-14.
Nutrient addition bioassays were conducted to examine the relationship between inorganic nutrient enrichment and phytoplankton community structure and function in Lake George in the St. Johns River System, Florida, USA. Additionally, a nitrogen budget for the lake was developed and included data from the period of this study. We identified the factors that affected cyanobacterial productivity, prevalence, and nitrogen (N
2
) fixation. The importance of N
2
fixation to the nitrogen (N) budget of the system was also assessed. We hypothesized that N
2
fixation significantly contributed to the Lake George N budget and that changing the nutrient conditions in manipulative experiments would affect rates of N
2
fixation and composition of the phytoplankton, particularly the N
2
-fixing community. Phytoplankton primary productivity in Lake George was stimulated by the addition of both N and phosphorus (P). Phytoplankton biomass accumulation was most often enhanced by the combined addition of N and P; however, N alone was also often stimulatory. When detected, N
2
fixation was always stimulated by P additions. Short-term changes in phytoplankton community composition included taxonomic shifts in response to nutrient manipulations. Fixation of N
2
appeared to be a significant contributor to the N load to the lake. Human impacts that change the loading of N, P, or both N and P to Lake George may affect phytoplankton community structure (composition and biomass) and function (primary productivity and N
2
fixation). These changes could have consequences for biogeochemical cycling in Lake George and potentially through the freshwater-marine continuum. Continuing nutrient management efforts in this and other similar systems must account for the activity of N
2
-fixing cyanobacteria, as products (carbon fixation) and drivers (sources of new nitrogen) of eutrophication.