Chesapeake Bay Benthic Monitoring Program

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Data Collection and Processing


Samples are collected using four kinds of gear depending on the program element and habitat type.  At fixed stations, a hand-operated box corer ("post-hole digger"), which samples a 250 cm2 area to a depth of 25 cm, is used in the nearshore shallow habitats of the mainstem bay and tributaries.  A Wildco box corer, which samples an area of 225 cm2 to a depth of 23 cm, is used in deep-water (more than 4 m) habitats in the mainstem bay and tributaries.  A Petite Ponar grab, which samples 250 cm2 to a depth of 7 cm, is used at the fixed site in the Nanticoke River to be consistent with previous sampling in the 1980s.  These different gears are used to be consistent with historical methods and for data collected at historical sites to be comparable.  At two fixed stations first sampled in 1995 and at all probability sites, a Young grab, which samples an area of 440 cm2 to a depth of 10 cm, is used (the upper picture on this page illustrates a Versar crew deploying the Young grab).

At each sampling location, sample volume and penetration depth are measured for all samples.  If the Wildco and hand-operated box cores penetrate less than 15 cm, or the Young and Petite Ponar grabs penetrate less than 7 cm into the sediment, the sample is rejected and the site is re-sampled. 

Three samples are collected for benthic community analysis at each fixed station.  One sample is collected at each probability site.  Samples are sieved in situ through a 0.5-mm screen using an elutriative process.  Organisms and detritus retained on the screen are transferred into labeled jars and preserved in a 10% formaldehyde solution stained with Rose Bengal (a vital stain that aids in separating organisms from sediments and detritus). 

Two surface-sediment sub-samples of approximately 120 ml each are collected for silt-clay, organic carbon, and nitrogen analysis from an additional grab sample at each sampling location.  They are frozen until processed in the laboratory.  In addition, the basic water quality parameters of dissolved oxygen, salinity, conductivity, temperature, and turbidity are measured at each location near the surface of the water column and at approximately 1 meter from the bottom using a Hydrolab multiprobe CTD.  At fixed stations, water column profiles are conducted at 1-3 m intervals, depending on the depth of the station.




Benthic samples are processed to identify and enumerate each species present, and to measure species-specific ash-free dry weight biomass.  Organisms are sorted from detritus under dissecting microscopes, identified to the lowest practical taxonomic level, and counted.  Oligochaetes and chironomids are mounted on slides and examined under a compound microscope for genus and species identification.

Samples sorted by each technician are resorted on a regular basis to ensure that all organisms are removed from extraneous material.  A sorting efficiency of 95% is typically considered acceptable.  Approximately 10% of all samples processed are resorted for quality assurance.  The level of effort expended on resorts depends on the specific needs of each project but typically ranges from 5% to 20%, depending on whether the intent of the resorts is to confirm efficiency levels and support training efforts, or to guarantee a specified efficiency level within defined statistical confidence limits.

Species identifications are verified when organisms are transferred for biomass measurements.  A reference collection containing representative specimens of each taxon identified is maintained in the laboratory.  Questionable or unusual species identifications are confirmed by recognized experts in the appropriate taxonomic specialties.  Contacts for taxonomic consultation include the Smithsonian Institution, the National Museum of Canada, and the Institute of Ocean Sciences.  An extensive and current library of taxonomic and biological literature is available in-house for reference by technical specialists processing samples.

Ash-free dry weight biomass is measured directly for each species by drying the organisms to a constant weight at 60C and ashing in a muffle furnace at 500C for four hours.

Sediment samples are analyzed for percent silt-clay content, and carbon and nitrogen content.  Sand is separated from mud by wet-sieving through a 63-m sieve, and the silt-clay fraction of the sediment is weighed.  The carbon and nitrogen contents of dried sediments are measured with a Model CE440 Carbon Analyzer (Exeter Analytical, Inc.) by nondispersive infrared detection. This analysis is conducted at the Chesapeake Biological Laboratory.


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Revised: April 22, 2005.