Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Species composition via MOCNESS and associated CTD information

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.

Niskin electronic data was post processed using a series of MATLAB scripts to read the raw and processed data, and SBE Data Processing software was used to calculate summary statistics for each bottle.

Zooplankton samples were sorted under a stereo dissecting microscope within two years of collection. Sub samples were taken with a pipet such that a minimum of 200 individuals were counted from each sample. Zooplankton were identified to lowest possible taxonomic level, to species where possible for copepods, and copepod adults were sexed.

BCO-DMO Data Processing Notes:

- replaced blank cells with nd
- reformatted column names to comply with BCO-DMO standards
- reformated dates to YYYY/MM/DD

Description from NSF award abstract:
The PIs will develop a mechanistic understanding of how circulation interacts with hypoxia-induced behavioral and physiological changes to affect the population dynamics of coastal zooplankton. They will do this by assessing two potentially contrasting mechanisms influencing the dynamics of the copepod Acartia tonsa in the hypoxic zone of Chesapeake Bay. The first hypothesis is that maintenance of copepod populations in the hypoxic region requires replenishment by advection (immigration) of animals through wind-driven lateral transport processes. The second, counteractive, hypothesis is that bottom water hypoxia alters the vertical distribution of A. tonsa, thereby making them more susceptible to advective losses from the region (emigration) via surface water transport in the estuarine circulation. They will take advantage of a current NSF-funded physical oceanography research program in Chesapeake Bay that will comprehensively measure and model axial and lateral water exchanges in the mid-Bay region.

The present study will use the physical oceanography study site as a Controlled Volume (CV) in which the oceanographic exchanges of water and the driving mechanisms for those exchanges will be well defined. The PIs will conduct high-resolution spatial and temporal sampling of zooplankton and combine the data with measurements of copepod behavior, mortality and egg production in the hypoxic region. They will use an improved Individual-Based Model of the life history of A. tonsa coupled with the circulation to explore the combined effects of advection, behavior, egg production, and mortality on population dynamics. In addition to increasing our knowledge of the impacts of bottom water hypoxia on copepod populations in Chesapeake Bay, the study will improve our general understanding of the regulation of zooplankton populations by physical and biological processes and the impacts of hypoxia on secondary production and food webs in coastal waters.