Aquatic Sciences Meeting, Albuquerque 2001

CS15 Harmful Algal Blooms

Date: Tuesday, February 13, 2001, Time: 11:30:00 AM

Location: Cochiti/Taos

Walsh, J, J, University of South Florida, College of Marine Science, St. Petersburg, USA, walsh@seas.marine.usf.edu

Heil, C, A, University of South Florida, College of Marine Science, St. Petersburg, USA, cheil@seas.marine.usf.edu

Vargo, G, A, University of South Florida, College of Marine Science, St. Petersburg, USA, vargo@seas.marine.usf.edu

ISOTOPIC EVIDENCE FOR NITROGEN SOURCES SUPPORTING GYMNODINIUM BREVE BLOOMS IN THE WEST FLORIDA SHELF WATERS

The sources of nitrogen (N) supporting high Gymnodinium breve bloom biomass in oligotrophic Florida shelf waters are enigmatic. Dissolved organic nitrogen (DON) and Del15PON (delN) measurements from a 1998-9 bloom suggest that G. breve utilizes DON from several sources. The delN for offshore surface phytoplankton populations on the shelf from May-June ranged from -1.6 to 2.8 ppt (mean = 0.3 ppt), suggesting that populations of the N fixing cyanobacterium Trichodesmium accounted for most of the phytoplankton biomass. The delN of estuarine and seagrass derived material had a similar but slightly heavier range: +1.1 - 2.9 ppt, while the diatom-dominated near-bottom chlorophyll maximum found during September 1998 after nitrate depletion had a delN signature of Gulf of Mexico sub-thermocline nitrate, +6.7-8.3 ppt. The December 1998 nearshore red tide had a delN of +4.9 ppt, reflecting use of a 15N-enriched DON substrate which has been modified by bacterial or herbivore fractionation or mixing with DON derived from near bottom nitrate enriched diatom stocks. Several potential N sources are therefore available to support G. breve blooms on the west Florida Shelf.

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Aquatic Sciences Meeting, Albuquerque 2001

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