Nutrient input and the competition between Phaeocystis pouchetii and diatoms in Massachusetts Bay spring bloom

• There is significant inter-annual variability in diatoms and P. pouchetii abundances in Massachusetts Bay spring bloom.
• Winter nitrate and N residual (N-Si) play an important role in the competition between diatoms and P. pouchetii during Massachusetts spring bloom.
• Winter river discharges and deep mixing in the western Gulf of Maine are likely key drivers for winter nutrient variability in Massachusetts Bay.

The phytoplankton community in Massachusetts Bay has displayed significant inter-annual variability and possible trends over the last two decades, with increasing frequency and magnitude of strong Phaeocystis pouchetii blooms and generally opposite fluctuations in diatom abundances. An analysis of historical data suggests that changes in winter nitrate and silicate concentrations (both their absolute and relative values) may play a critical role in the competition between diatoms and P. pouchetii. We developed a new ecosystem model to simulate Phaeocystis dynamics and to test the significance of variable winter nutrient levels. Idealized simulations for the years 1992–2009 generally reproduced the observed inter-annual variability of P. pouchetii and diatoms during the spring blooms, with modeled peaks in biomass of diatoms and P. pouchetii significantly being correlated with their observed mean abundances. Moreover, modeled peak biomass ratio and observed mean abundance ratio between diatoms and P. pouchetii during the spring blooms were similarly depending on both the winter nitrate and residual nitrate (nitrate minus silicate) concentrations. These results are consistent with resource competition theory in which relatively low winter nutrient concentrations would favor species with faster growth rate (diatoms, in this case). With sufficiently high winter nutrient concentrations, however, P. pouchetii was able to grow before nitrate being depleted by diatoms, even though winter Si > N. Our observations further indicate that inter-annual nutrient variability and consequently spring bloom phytoplankton variability in Massachusetts Bay are likely driven by changes in winter nutrient fluxes from Gulf of Maine rivers and winter convective mixing. These fluxes may have been modulated by large-scale processes such as the North Atlantic Oscillations and Arctic melting through the river discharges, winter-storm activities (and hence winter mixing and nutrient supply), and the deep waters inflow into the Gulf of Maine.