Understanding controls on Margalefidinium polykrikoides blooms in the lower Chesapeake Bay.

A time-dependent model of Margalefidinium polykrikoides, a mixotrophic dinoflagellate, cell growth was implemented to assess controls on blooms in the Lafayette River, a shallow, tidal sub-tributary of the lower Chesapeake Bay. Simulated cell growth included autotrophic and heterotrophic contributions. Autotrophic cell growth with no nutrient limitation resulted in a bloom but produced chlorophyll concentrations that were 45% less than observed bloom concentrations (~80 mg Chl m-3 vs. 145 mg Chl m-3) and a bloom progression that did not match observations. Excystment (cyst germination) was important for bloom initiation, but did not influence the development of algal biomass or bloom duration. Encystment (cyst formation) resulted in small losses of biomass throughout the bloom but similarly, did not influence M. polykrikoides cell density or the duration of blooms. In contrast, the degree of heterotrophy significantly impacted cell densities achieved and bloom duration. When heterotrophy contributed a constant 30% to cell growth, and dissolved inorganic nitrogen was not limiting, simulated chlorophyll concentrations were within those observed during blooms (maximum ~140 mg Chl m-3). However, nitrogen limitation quenched the maximum chlorophyll concentration by a factor of three. Specifying heterotrophy as an increasing function of nutrient limitation, allowing it to contribute up to 50% and 70% of total growth, resulted in simulated maximum chlorophyll concentrations of 90 mg Chl m-3 and 180 mg Chl m-3, respectively. This suggested that blooms of M. polykrikoides in the Lafayette River are fortified and maintained by substantial heterotrophic nutritional inputs. The timing and progression of the simulated bloom was controlled by the temperature range, 23 °C to 28 °C, that supports M. polykrikoides growth. Temperature increases of 0.5 °C and 1.0 °C, consistent with current warming trends in the lower Chesapeake Bay due to climate change, shifted the timing of bloom initiation to be earlier and extended the duration of blooms; maximum bloom magnitude was reduced by 50% and 65%, respectively. Warming by 5 °C suppressed the summer bloom. The simulations suggested that the timing of M. polykrikoides blooms in the Lafayette River is controlled by temperature and the bloom magnitude is determined by trade-offs between the severity of nutrient limitation and the relative contribution of mixotrophy to cell growth.

Effluent organic nitrogen (EON): bioavailability and photochemical and salinity-mediated release.

The goal of this study was to investigate three potential ways that the soluble organic nitrogen (N) fraction of wastewater treatment plant (WWTP) effluents, termed effluent organic N (EON), could contribute to coastal eutrophication--direct biological removal, photochemical release of labile compounds, and salinity-mediated release of ammonium (NH4+). Effluents from two WWTPs were used in the experiments. For the bioassays, EON was added to water from four salinities (approximately 0 to 30) collected from the James River (VA) in August 2008, and then concentrations of N and phosphorus compounds were measured periodically over 48 h. Bioassay results, based on changes in DON concentrations, indicate that some fraction of the EON was removed and that the degree of EON removal varied between effluents and with salinity. Further, we caution that bioassay results should be interpreted within a broad context of detailed information on chemical characterization. EON from both WWTPs was also photoreactive, with labile NH4+ and dissolved primary amines released during exposure to sunlight. We also present the first data that demonstrate that when EON is exposed to higher salinities, increasing amounts of NH4+ are released, further facilitating EON use as effluent transits from freshwater through estuaries to the coast.