Optical and Environmental Signatures of Cochlodinium polykrikoides in Narragansett Bay, Rhode Island

The dinoflagellate Cochlodinium polykrikoides produces toxins lethal to fish and shellfish and can negatively impact local fisheries when it reaches high densities. When it blooms, this species forms distinct rust colored patches that were noticeable throughout the Bay. Here we focus on the environmental drivers and optical signatures of C. polykrikoides occurrence in Narragansett Bay, Rhode Island. C. polykrikoides was first detected in Narragansett Bay in the late 1970s, but the size and extent of the bloom has expanded in recent years (2016-2018), reaching quantities as high as 344,000 cells L^-1 in August 2018. We combine weekly sampling at the Narragansett Bay long term phytoplankton survey station with an Imaging FlowCytobot (IFCB) that was deployed on a dock at the University of Rhode Island’s, Graduate School of Oceanography’s Bay Campus to investigate the bloom characteristics at variable time intervals. The 2018 bloom was the largest observed and was captured in the IFCB which allowed cell counts and images of the plankton to be taken continuously every 20 minutes. Coincident with the IFCB, a YSI sonde recorded temperature, salinity, dissolved oxygen concentration, and pH every 15 minutes. A partial least squares regression analysis is being applied to determine which variables have the greatest impact on the abundance of C. polykrikoides at our sample site. Understanding the influence of different environmental parameters on this species occurrence and bloom severity is key for interpreting why the 2018 bloom was the largest in recent history. Further, given the dark coloration of the bloom, it may be able to be distinguished remotely from patches of water that do not contain the rust-tide bloom. Cultures of this species are being used to determine the absorption and scattering coefficients of C. polykrikoides for the use in phytoplankton-detecting satellite algorithms.