Understanding Controls on Margalefidinium polykrikoides Blooms in the Lower Chesapeake Bay

Abstract:

A time-dependent model of Margalefidinium polykrikoides cell growth was implemented to assess controls on blooms in the Lafayette River, a shallow, tidal sub-tributary of Chesapeake Bay. Simulated cell growth included autotrophic and mixotrophic contributions. Autotrophic cell growth with no nutrient limitation resulted in a bloom progression consistent with observations, but produced chlorophyll concentrations that were an order of magnitude less than observed bloom concentrations. Excystment was important for bloom initiation, but did not influence the development of algal biomass or bloom duration. Encystment 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, mixotrophy significantly impacted cell density during blooms and bloom duration. When mixotrophy contributed a constant 30% to cell growth, and dissolved inorganic nitrogen was not limiting to growth, simulated chlorophyll concentrations were within those observed during blooms. However, nitrogen limitation quenched the maximum chlorophyll concentration by a factor of three. Specifying mixotrophy 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 110 mg Chl m^-3 and 180 mg Chl m^-3, respectively. This suggests that blooms of M. polykrikoides in the Lafayette River are fortified and maintained by substantial mixotrophic nutritional inputs. Timing of simulated blooms was controlled by seasonal temperature, with initiation at 23°C to 24°C, maximum growth at 26°C to 28°C, and decay in the fall as water temperatures cooled. Temperature increases of 0.5°C and 1.0°C, consistent with observed warming 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 remained similar. Warming by 5°C suppressed the summer bloom and shifted the bloom into fall. The simulations suggest 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.