A Regional, Early Spring Bloom of Phaeocystis pouchetiion the New England Continental Shelf

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