Table of Contents

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

This dataset includes species composition data from plankton net samples and associated CTD information.

Related Datasets:
Acartia tonsa mortality with CTD data: 5 daytrips

This abundance and diversity data was obtained from several ~8 hour small-boat cruises done about every other week from August to October. Vertical tow samples were taken from the mid-bay of the Chesapeake from several stations in a transect, corresponding to the mid-line transect sampled by the two week-long cruises (stations M1-M3, cruises 1301/1302). More stations along the same transect were included in the collection of this data set.

The plankton nets used for sampling were 0.5m in diameter and made of either 200µm or 64µm mesh. These were equipped to close with a mechanical messenger trigger and were deployed from the winch arm of the R/V Parker.

A CTD cast was done at each station prior to sampling with the plankton net. Sampling depths were determined based on the location of the pycnocline; the aim was to capture zooplankton below and above the pycnocline. For sampling below the pycnocline, the net was deployed below the pycnocline, towed up to the pycnocline, triggered to close, and returned to the surface. Sampling above the pycnocline was done by standard vertical tow: deploy net to desired depth and winch to the surface.

Once brought on board, the nets were rinsed down with to seawater to collect plankton in the codend. Codends were filtered onto 64µm sieves, then the samples were transferred to glass jars labeled on the lid with the date, time, station, and depth sampled. Sieves with 64µm mesh were selected to catch all life stages of the copepod Acartia tonsa since Acartia tonsa eggs are about 75µm in diameter and all subsequent life stages are larger. Buffered formalin was added to each jar to preserve the sample in a 4% solution.

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 25µm 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 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.

Zooplankton samples were sorted under a stereo dissecting microscope within one year 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:
- added conventional header with dataset name, PI name, version date
- modified parameter names to conform with BCO-DMO naming conventions
- replaced commas with semicolons
- changed blank cells and NaN's with nd for 'no data'
- converted longitudes to negative degree (west)
- split Net_mesh and Tow into separate columns

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.