Vertical profiles and attenuation coefficients during intense blooms in the Lower York River Estuary between August 2021 and September 2022.

The dinoflagellates Margalefidinium polykrikoides and Alexandrium monilatum form intense summer blooms in the lower York River Estuary (LYRE), typically from late July through mid-September. While surface concentrations of chlorophyll-a during these blooms are well-documented, we lack information on the vertical distribution of biomass in bloom patches. Both species are known to vertically migrate and accumulate near the surface at midday. We also lack direct measurements of light attenuation through intense bloom patches for comparison to attenuation through the background phytoplankton assemblage. We conducted vertical profiles of water column structure, phytoplankton chlorophyll-a, and irradiance on five dates during these intense summer blooms in the LYRE. Ten to twelve stations were occupied on each date spanning a wide range of bloom densities, including background stations outside of bloom patches. Profiles were used to determine the vertical distribution of bloom biomass, while irradiance measurements were used to compute the vertical attenuation coefficient as a function of bloom chlorophyll-a biomass, determined from laboratory extraction of surface samples.