Effect of upwelling on phytoplankton productivity of the outer southeastern United States continental shelf |
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| Authors: | James A Yoder Larry P Atkinson S Stephen Bishop Eileen E Hofmann Thomas N Lee |
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| Institution: | 1. Skidaway Institute of Oceanography, P.O. Box 13687, Savannah, GA 31406, U.S.A.;2. North Carolina State University, Department of Marine Science and Engineering, P.O. Box 5923, Raleigh, NC 27650, U.S.A.;3. University of Miami, Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami, FL 33149, U.S.A. |
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| Abstract: | Gulf Stream frontal disturbances cause nutrient-rich waters to frequently upwell and intrude onto the southeastern United States continental shelf between Cape Canaveral, Florida and Cape Hatteras, North Carolina. Phytoplankton response in upwelled waters was determined with three interdisciplinary studies conducted during April 1979 and 1980, and in summer 1978. The results show that when shelf waters are not stratified, upwelling causes productive phytoplankton (diatom) blooms on the outer shelf. Phytoplankton production averages about 2 g C m−2 d−1 during upwelling events, and ‘new’ production is 50% or more of the total. When shelf waters are stratified, upwelled waters penetrate well onto the shelf as a subsurface intrusion in which phytoplankton production averages about fives times higher than the nutrient-depleted overlying mixed layer. Phytoplankton within the intrusion deplete upwelled NO3 in about 7 to 10 days, at which point no further net increase in phytoplankton biomass occurs.Current meter records show that upwelling occurs roughly 50% of the time on the outer shelf during November to April (shelf not stratified), and we estimate that seasonal primary production in upwelled waters is 175 g C m−2 6 months−1 of which at least 50% is ‘new’ production. More than 90% of outer shelf primary and ‘new’ production occurs during upwelling and thus upwelling is the dominant process affecting primary productivity of the outer shelf. Our seasonal estimates of outer shelf primary and ‘new’ production are, respectively, three and ten times higher than previous estimates that did not account for upwelling. |
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