Table of Contents

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

Methods & Sampling

Bottom water samples were obtained by lowering a submersible pump to near the seafloor and filling two 26-liter carboys. Subsamples for salinity were usually taken from each sample and measured with a YSI conductivity meter to confirm the CTD measurement. Radium samples were filtered through a column of Mn-fiber to quantitatively remove Ra [Moore, 1976]. The Mn-fiber was returned to the lab, where short-lived radium isotopes (223Ra, half-life = 11.4 days, and 224Ra, half-life = 3.66 days) were measured using a delayed-coincidence counting system [Moore and Arnold, 1996]. After the short-lived measurements were complete, Ra and Mn were stripped from the Mn-fiber and radium was coprecipitated with BaSO4 and transferred to a small tube. 226Ra and 228Ra were measured by gamma spectrometry after 222Rn had equilibrated with 226Ra [Moore et al., 1985]. The RaDeCC systems were calibrated for 224Ra with a NIST standard solution of 232Th adsorbed to Mn-fiber and for 223Ra by the technique of Moore and Cai [2013]. The gamma detector was calibrated with NIST standard solutions for 226Ra and 228Ra in a BaSO4 matrix [Moore, 1984].

Samples were taken from R/V Trinity, owned by a private charter company called Charleston SCUBA. This company is no longer in operation. 

Processing notes from researcher:

The 224Ra and 223Ra counting data were analyzed using the procedure detailed in Moore and Arnold [1996] to correct for chance coincidence events. The gamma spectra for 228Ra and 226Ra were analyzed using the program HYPERMET, which employs multiple iterations to fit each peak in the spectra to an energy range [Phillips and Marlow, 1996]. The energy ranges are assigned to specific radionuclides based on the measurements of NIST standards.

BCO-DMO processing notes:

• Added a date field (a concatenation of the year, day and month fields)
• Replaced spaces in column names with underscores "_"
• Removed units and special characters from column names
• Renamed column names that started with numerical characters (e.g., 226Ra is now Measured_226_Ra)

NSF Award Abstract:
Continental shelves are highly productive, with both ecological and economic importance. Benthic microalgae (BMA) are key primary producers in these location. As much as 6x the water column biomass of primary producers is compressed into a layer only a few mm thick on the sediment surface. The source(s) of fixed nitrogen (N) supporting such highly concentrated BMA biomass is currently unknown. Recent studies of sub-seafloor groundwater flow at the University of South Carolina have demonstrated that upwelling saline groundwater likely supplies high concentrations of nutrients in the ridge-swale habitats in the South Atlantic Bight (SAB). The investigators suggest that groundwater input of fixed N into surficial sediments is the primary source of N supporting BMA biomass and production in the mid-shelf region of the SAB. The purpose of this project is to determine the primary source of fixed N supporting BMA biomass in the surface sediments of the shallow shelf waters (<30 m), using the SAB as a field area. A secondary objective is to apply novel and innovative methods to directly quantify groundwater inputs of N into surficial sediments. Research results will fully document the spatio-temporal distributions of BMA and phytoplankton biomass and community structure in the mid-shelf region of the SAB and relate the observed patterns to groundwater inputs of fixed N sources as well as hydrographic and climatic conditions. This research will provide full support and tuition for 2 graduate students, summer support for undergraduate assistants, and involve upper level undergraduates as lab interns. The study team will also work with the Baruch Institute and other partners to develop an "Ocean Schoolyard" program to meet the needs of teachers, students, and community audiences. The project will also provide partial support for Girls Go for I.T., a coding summer camp designed to attract middle-school-aged girls to careers in I.T. and STEM fields.

The specific objectives of the study are to (1) quantify spatial and temporal variations in N fluxes associated with hydrodynamic exchange and upward groundwater flow (2) document spatial and temporal variations in BMA biomass and (3) measure the delta15N of fixed nitrogen sources (well water, porewater and water column ammonium and nitrate; sediments), the BMA, and phytoplankton. The sampling area will be restricted to the 10 - 30 m isobath region of the SAB off the coast of Charleston, SC. Samples will be collected at both the existing groundwater well field and other regions of the shelf. At each of the groundwater wells in the well field, SCUBA divers will collect fluids from the wells to determine well water inorganic nutrient concentrations (nitrate + nitrite, ammonium, orthophosphate, silicon) and the delta 15N of well water ammonium and nitrate (when present). In nearby sediments, samples will be collected for BMA biomass and community composition, surface porewater inorganic nutrients (nitrate + nitrite, ammonium, orthophosphate, silicon), C and N of sediments, sediment grain size analysis, and delta 15N of BMA, ammonium, nitrate (when present), and sediments. Line transects, consisting of 5 sampling locations along a 50 m transect, will be conducted in each of the 4 depth strata. At 10 m intervals along each transect, divers will collect samples the same as above for the well field. Water column samples will be collected for HPLC measurements of phytoplankton biomass and community composition, inorganic nutrient concentrations (nitrate + nitrite, ammonium, orthophosphate, silicon), seston CHN, delta 15N of phytoplankton, and the delta 15N of ammonium and nitrate. The researchers will use heat as a tracer to map the depth of hydrodynamic exchange and monitor the rate of vertical groundwater flow. Results from that analysis will also allow them to then simulate transport of a conservative tracer that can be compared to observed nutrient concentrations to BMA abundance and community composition.