Search

Of all the Antarctic coastal polynyas, the Amundsen Sea Polynya is the most productive per unit area. Observations from the 2010–2011 Amundsen Sea Polynya International Research Expedition (ASPIRE) revealed that both light and iron can limit the growth of phytoplankton (Phaeocystis antarctica), but how these controls manifest over the bloom season is poorly understood, especially with respect to their climate sensitivity. Using a 1‐D biogeochemical model, we examine the influence of light and iron limitation on the phytoplankton bloom and vertical carbon flux at 12 stations representing different bloom stages within the polynya. Model parameters are determined by Bayesian optimization and assimilation of ASPIRE observations. The model‐data fit is most sensitive to phytoplankton physiological parameters, which among all model parameters are best constrained by the optimization. We find that the 1‐D model captures the basic elements of the bloom observed during ASPIRE, despite some discrepancies between modeled and observed dissolved iron distributions. With this model, we explore the way iron availability, in combination with light availability, controlled the rise, peak, and decline of the bloom at the 12 stations. Modeled light limitation by self‐shading is very strong, but iron is drawn down as the bloom rises, becoming limiting in combination with light as the bloom declines. These model results mechanistically confirm the importance of climate‐sensitive controls like stratification and meltwater on phytoplankton bloom development and carbon export in this region. A one‐dimensional physical‐biogeochemical model reproduces the basic elements of the summer phytoplankton bloom in the Amundsen Sea PolynyaPhytoplankton physiological parameters have the strongest influence on the model's ability to reproduce observations of dissolved nutrientsLight is limiting over the course of the bloom's rise, but self‐shading combines with iron‐limitation to drive the decline of the bloom

The genus Phaeocystisis distributed globally and has considerable ecological, biogeochemical, and societal impacts. Understanding its distribution, growth and ecological impacts has been limited by lack of extensive observations on appropriate scales. In 2018, we investigated the biological dynamics of the New England continental shelf and encountered a substantial bloom of Phaeocystis pouchetii. Based on satellite imagery during January through April, the bloom extended over broad expanses of the shelf; furthermore, our observations demonstrated that it reached high biomass levels, with maximum chlorophyll concentrations exceeding 16 µg L−1and particulate organic carbon levels > 95 µmol L−1. Initially, the bloom was largely confined to waters with temperatures <6°C, which in turn were mostly restricted to shallow areas near the coast. As the bloom progressed, it appeared to sink into the bottom boundary layer; however, enough light and nutrients were available for growth. The bloom was highly productive (net community production integrated through the mixed layer from stations within the bloom averaged 1.16 g C m−2d−1) and reduced nutrient concentrations considerably. Long‐term coastal observations suggest that Phaeocystisblooms occur sporadically in spring on Nantucket Shoals and presumably expand onto the continental shelf. Based on the distribution of Phaeocystisduring our study, we suggest that it can have a significant impact on the overall productivity and ecology of the New England shelf during the winter/spring transition. The coastal ocean off New England supports substantial fish harvests and large numbers of marine mammals and birds. These are in turn supported by the growth of phytoplankton, photosynthetic microorganisms that are the base of the marine food web. A large accumulation of a particular type of phytoplankton, Phaeocystis, was observed both during a research cruise to the region and from earth‐orbiting satellites. Phaeocystisis unusual, in that it forms gelatinous colonies that are millimeters in diameter. The colonies are considered to be unpalatable to many organisms that feed on phytoplankton, and accumulations of this genus may be harmful to the regional food web. Waters with elevated abundance of Phaeocystiswere largely confined to the Nantucket Shoals area, likely by physical processes, but occasionally high concentrations of Phaeocystisextended over broader areas and sank to depth. After the bloom disappeared from surface waters, it was sustained in the bottom boundary layer landward of the 60‐m isobath where light was sufficient to support growth. Understanding the physical‐biological interactions in the coastal system provides a basis for predicting these blooms of potentially harmful algae, and may lead to a better prediction of their impacts on coastal systems. A large bloom of the haptophyte Phaeocystis pouchetiiwas observed off the coast of New England during early spring, 2018The bloom was centered on Nantucket Shoals, but relaxation of physical constraints allowed it to propagate over larger areasBlooms like the one we assessed likely occur sporadically over the continental shelf and potentially disrupt regional food webs A large bloom of the haptophyte Phaeocystis pouchetiiwas observed off the coast of New England during early spring, 2018 The bloom was centered on Nantucket Shoals, but relaxation of physical constraints allowed it to propagate over larger areas Blooms like the one we assessed likely occur sporadically over the continental shelf and potentially disrupt regional food webs