Evolution of a phytoplankton bloom

CGSN: Evolution of a

phytoplankton bloom A phytoplankton bloom dominated by the species Phaeocystis pouchetii was detected on the New England Shelf during the first Shelfbreak Productivity Interdisciplinary Research Operation at the Pioneer Array (SPIROPA) cruise in April 2018. Phaeocystis blooms are of interest because they alter water column chemistry (by releasing dimethyl sulfide into the surface layer) and influence food web dynamics (due to interactions with zooplankton grazers). The extent to which Phaeocystis is important to annual primary production on the New England Shelf is not well known, and there are relatively few case studies from which the interplay between physical conditions and the phytoplankton bloom can be determined. The authors used in-situ data from the SPIROPA cruise and from the New England Shelf Long-Term Ecological Research (NES-LTER) project, both conducted in the vicinity of the Pioneer Array. The LTER data were collected as an ancillary activity during the Pioneer-10 mooring service cruise. Insitu measurement products included phytoplankton abundance, net primary productivity, net community production, zooplankton abundance, microzooplankton grazing, and particulate matter concentrations. Long term records of Phaeocystis abundance from the Martha’s Vineyard Coastal Observatory (MVCO) were used to provide historical context. Satellite imagery showed that during February and March of 2018 the surface manifestation of the bloom was concentrated in Vineyard Sound and over Nantucket Shoals (Figure 27b-c). During late April, a dramatic surface filament of enhanced chlorophyll was seen extending to the southwest, intersecting the Pioneer Array and the SPIROPA

The authors used meteorological data from the Pioneer Offshore surface mooring to develop a hypothesis for bloom evolution: During February and March, the bloom was enhanced over Nantucket Shoals while being held there by the persistent tidal mixing front. A period of sustained northwesterly winds during 18-22 April caused the surface layer, and the phytoplankton within it, to be advected to the southwest. Once the surface layer was detached from the vigorous tidal mixing over the shoals, nutrients were depleted. Particles then sank into the bottom boundary layer where they were detected in late April. This fascinating case study draws on data from three longterm observing systems (OOI, LTER and MVCO) to enhance the analysis of, and provide context for, a process study focused on primary productivity over the New England continental shelf. It is gratifying to see the convergence of these efforts and will be exciting to see such work continue.

Smith, W.O., W.G. Zhang, A. Hirzel, R.M. Stanley, M.G. Meyer, H. Sosik, et al, 2021. A regional, early spring bloom of Phaeocystis pouchetii on the New England continental shelf. J. Geophys Res., Oceans, 126, e2020JC016856. https://doi.org/10.1029/20 20JC016856 . .