Marine & Environmental Sciences Faculty Reports

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This report discusses a sequence of six cruises in the vicinity of the Boynton-Delray (South Central) treated-wastewater plant outfall plume (26°27'43"N, 80°2'32"W), the Boynton Inlet (26°32'43"N, 80°2'30"W), and the Lake Worth Lagoon, Palm Beach County, Florida. The sampling cruises took place on June 5-6, 2007; August 28-29, 2007; October 18-19, 2007; February 14 and 18, 2008; May 19-20, 2008; and July 11-13, 2008. Water was sampled at 18 locations at the surface, middle, and near the seafloor (where there was sufficient depth) for a total of 45 samples; these samples were analyzed for a variety of nutrients and related parameters. The water sampling unit contained a conductivity-temperature-depth (CTD) instrument from which data were obtained at each sampling site. Synchronal ocean current data were measured by a nearby acoustic Doppler current profiler (ADCP) instrument.

The inlet measurements were consistently lower in salinity and more acidic (lower in pH) than the coastal ocean and were warmer during the May and, especially, during the February cruises. For most analytes, viz., nitrite+nitrate (N+N), total suspended solids (TSS), chlorophyll-a, silica (Si), and total dissolved nitrogen (TDN), the lagoon concentrations were significantly higher than the coastal ocean; the inlet concentrations appeared to be consistent with lagoon water with partial mixing with the coastal ocean, as expected. Estimates of the nutrient flux to the coastal ocean were computed: approximately 1,500 kg of dissolved nitrogen (N), 2,350 kg of silicate (Si), 33 kg of orthophosphate (P), and 59 kg of ammonium (NH4) per day were delivered to the coastal ocean through the inlet.

The outfall boil at South Central outfall (the smallest in volume of the six outfalls in southeast Florida) is only visible under ideal conditions. In the six cruises described in this document, the outfall boil could be found in only one cruise (August 28-29, 2007). Elevated concentrations of nutrients (N+N, P, Si, and P) at the outfall vicinity were measured, and these concentrations decreased rapidly away from the outfall for most analytes, to become undistinguished from the background within 3 km or less. Not finding the boil, however, in five of six cruises meant that the waters with the highest concentrations were probably missed. When the boil was sampled in August 2007, N+N, P, and total dissolved phosphorus (TDP) concentrations at the boil were roughly the same as from the inlet. For other analytes (chlorophyll-a, TSS, Si, and dissolved organic carbon [DOC]), the concentrations at or near the outfall were significantly less than those from the lagoon and inlet on most of the cruises.

The coastal ocean appeared to be significantly impacted by the Boynton Inlet and less so from the inlet. A suggestion of a source to the south was seen in some analytes. Measurements from the Gulf Stream Reef area were the lowest in the study, and may provide “background” concentrations for this region. As expected, the coastal ocean was warmer and more stratified in the summer compared to the winter, e.g., whereas no thermocline was noted in the CTD data from February 2007, a strong thermocline was observed in most casts during July 2008. In certain cases (e.g., N+N in June 2007, pH in July 2008), an increase in the concentration (decrease for pH) from north to south implied a source from the south, e.g., the Boca Raton Inlet or Boca Raton outfall.

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NOAA Technical Report, OAR AOML-39