Data were collected on several single-day cruises on small privateers out of the Virginia Institute of Marine Science, Gloucester Point, VA

Stations on each date were visually selected to cover a wide range of chlorophyll-a concentrations, including sites with no clear presence of a bloom. At each site, a YSI EXO2 datasonde was used to collect vertical profiles of water temperature, salinity, chlorophyll-a, and turbidity. On the first cruise (8/19/21), readings were recorded sub-surface (0.1 m) and then in 1 m increments to the bottom. On the second date (8/26/21), the YSI logged continuous vertical profiles. On the three dates in 2022, we again targeted discrete depths including 0.1, 0.5, 1, and 1.5 m, and then in 1 m increments to the bottom, although actual depths varied. Data from 8/26/21 were bin-averaged into the increments from 2022 for ease of comparison (averaging all profile values within 0.1 m of the target depths). Vertical profiles of irradiance were measured with LI-COR surface (LI-190SA) and underwater (LI-192SA) quantum sensors; the LI-COR datalogger (LI-1400) was programmed to record the ratio of irradiance at depth to that at the surface. Depth increments varied among cruises but generally included a measurement just beneath the surface, and then measurements in 0.1-0.25 m increments down to 1-2 m depth.

Values were used to compute the vertical attenuation coefficient using Beer’s Law. Sub-surface (~ 10 cm) water samples were collected in triplicate at each station and returned to the lab where they were filtered through 0.7 um glass fiber filters which were frozen prior to analysis following Arar and Collins (1997, EPA Method 445.0). Samples were extracted in the dark for 24 h in 8 ml of a 45:45:10 dimethyl sulfoxide : acetone: distilled water solution with 1% diethylamine (Shoaf and Lium 1976, Limnol Oceanogr 21: 926−928), and read on a 10 AU Turner Designs fluorometer before and after acidification to compute uncorrected chlorophyll-a, active chlorophyll-a, and phaeophytin.