This abundance data was obtained from two week-long cruises (1301 in August and 1302 in September) during which niskin samples were taken from the mid-bay of the Chesapeake from 9 stations in a box formation; 3 stations in a northern transect across the bay (N1-N3), 3 in a midline transect (M1-M3), and 3 in a southern transect (S1-S3).

The niskin bottles used were General Oceanics 1010x External Spring Water Sampler with a 10L capacity (part number: 101010X). 12 of these were deployed rosette style on the SBE 32 Carousel Water Sampler from the starboard winch of the RV Sharp, along with the SBE 9plus unit which was attached to the rosette. On each downcast, instrument readings were sent from the SBE 9plus unit on the rosette to the SBE 11plus V2 Deck Unit. Based on these data, sampling depths were selected which fell into three areas of interest: above, within, and below the pycnocline. If a pycnocline was not evident, only two samples were taken. On the upcast, niskins were triggered to close electronically from the dry lab.

For zooplankton abundance assessment, one niskin bottle was triggered at each depth. Once on board, niskin bottles were drained through sieves with 64µm mesh. Sieves with 64um mesh were selected to catch all life stages of the copepod Acartia tonsa since Acartia tonsa eggs are about 75um in diameter and all subsequent life stages are larger. The captured zooplankton were transferred to glass jars and preserved with buffered formalin (about a 4% formalin solution), labeled with the cast number, date, local time, and sampling depth, and then stored in labeled boxes for later analysis.

After returning from the cruises, samples were stored indoors in climate-controlled laboratory space. To process the samples, the contents of the jars were filtered onto 25um mesh (to avoid any loss of organisms), resuspended, and a subsample was transferred to a counting wheel where it was checked for density and diluted if necessary, the goal being at least 200 individuals of Acartia tonsa present but less than 300.

The sample was then examined for species composition under dissecting microscope with darkfield illumination. Length and width measurements were taken for the first 50 individuals in the sample, and all were identified to lowest taxonomic level. After processing, samples were returned to their original jars for storage.

Abundance and size data were entered into Excel spreadsheets and checked for transcription errors, then imported into MatLab for data analysis.