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Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River |
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| Authors: | Antonietta Quigg Jason B Sylvan Anne B Gustafson Thomas R Fisher Rod L Oliver Sasha Tozzi James W Ammerman |
| Institution: | 1. Department of Marine Biology, Texas A&M University at Galveston, 200 Seawolf Parkway, Galveston, TX, 77553, USA 2. Department of Oceanography, Texas A&M University, 3146 TAMU, College Station, TX, 77843, USA 3. Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ, 08540, USA 4. Department of Biological Sciences, University of Southern California, 3616 Trousdale Parkway, Los Angeles, CA, 90089-0371, USA 5. Horn Point Laboratory, Center for Environmental Science, University of Maryland, 2020 Horn Point Road, Cambridge, MD, 21613, USA 6. Catchment Biogeochemistry and Aquatic Ecology, CSIRO Land and Water, Waite Campus, Urrbrae, SA, 5064, Australia 7. Ocean Sciences, University of California, Santa Cruz, CA, 95064, USA 8. School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY, 11794, USA |
| Abstract: | To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (Pi), silicate (Si)), nutrient ratios (DIN/Pi), alkaline phosphatase activity, chlorophyll and 14C primary productivity on fine spatial scales during cruises in March, May, and July 2004. Additionally, resource limitation assays were undertaken in a range of salinity and nutrient regimes reflecting gradients typical of this region. Of these, seven showed Pi limitation, five revealed nitrogen (N) limitation, three exhibited light (L) limitation, and one bioassay had no growth. We found the phytoplankton community to shift from being predominately N limited in the early spring (March) to P limited in late spring and summer (May and July). Light limitation of phytoplankton production was recorded in several bioassays in July in water samples collected after peak annual flows from both the Mississippi and Atchafalaya Rivers. We also found that organic phosphorus, as glucose-6-phosphate, alleviated P limitation while phosphono-acetic acid had no effect. Whereas DIN/Pi and DIN/Si ratios in the initial water samples were good predictors of the outcome of phytoplankton production in response to inorganic nutrients, alkaline phosphatase activity was the best predictor when examining organic forms of phosphorus. We measured the rates of integrated primary production (0.33?C7.01 g C m?2 d?1), finding the highest rates within the Mississippi River delta and across Atchafalaya Bay at intermediate salinities. The lowest rates were measured along the outer shelf at the highest salinities and lowest nutrient concentrations (<0.1 ??M DIN and Pi). The results of this study indicate that Pi limitation of phytoplankton delays the assimilation of riverine DIN in the summer as the plume spreads across the shelf, pushing primary production over a larger region. Findings from water samples, taken adjacent the Atchafalaya River discharge, highlighted the importance of this riverine system to the overall production along the Louisiana coast. |
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