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 MOCNESS 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 MOCNESS had a 0.5 meter square opening for each of 5 nets, which were 200um mesh. It was deployed from the aft A-frame of the RV Sharp, along with an array of sensors connected to the MOCNESS frame, including Sea-Bird temperature, salinity, and dissolved oxygen (SBE 43) sensors, a WetLabs FLNTU to measure chlorophyll a fluorescence and turbidity, and a LiCor 4π PAR sensor.

A CTD cast was done at each station prior to sampling with the MOCNESS. Sampling depths were determined based on the location of the pycnocline; the aim was to capture zooplankton below, within, and above the pycnocline. A drogue net (net 0) without a codend was used to deploy the MOCNESS to within 3m of the bottom. On the upcast, nets were triggered to close electronically from the wet lab so that they captured a depth range representing one of the three areas of interest.

Once brought on board, the nets were rinsed down with filtered seawater (provided by the Hugh R. Sharp) to collect plankton in the codend. Codends were filtered onto 64um sieves, then the samples were transferred to glass jars labeled on the inside and outside with the cast number, date, time, net number, and depth sampled. 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. Buffered formalin was added to preserve the sample in a 4% solution, and then the jars were stores in labeled boxes.

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 64um mesh (to avoid loss of organisms), resuspended, and a subsample taken with a stemple pipette 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. All organisms were identified to lowest possible taxonomic level. When species composition analysis was complete, the processed aliquot was photographed for size measurements and stored in 4% buffered formalin solution in a glass vial. The unused portion of the sample was returned to the original glass jar and returned to storage.

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

MOCNESS electronic data was post processed using a series of MATLAB scripts to read the raw and processed data, and to calculate summary statistics for each net. These are usually generated from the MOCNESS software in a “.TAB” file for each cruise, but the MOCNESS program does not use the incoming GPS data for calculation of time and instead used computer time. The MOCNESS scripts calculate time and location using GPS and also include the time from the computer.

Zooplankton samples were sorted under a stereo dissecting microscope within two years of collection. Sub samples were taken with a stempel 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.