Collaborative Research: Fate of Recently Fixed N2 in the Eastern Gulf of Mexico: Does the Regeneration of N by Trichodesmium Support the Development of Gymnodinium Breve Blooms?

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Blooms of the toxic red tide dinoflagellate, Gymnodinium breve, are an important feature of the Florida Shelf ecosystem. Blooms appear to initiate and develop in an oligotrophic, mid-shelf region 18-75 km offshore of west Florida. However, these waters are characterized by low concentrations of both dissolved inorganic and organic nitrogen (DIN and DON), and it is unclear how G. breve blooms meet their N demand for growth. Ambient N concentrations, model predictions, and stoichiometric calculations of nutrient requirements for growth based on observed biomass suggest that N inputs fromriverine, upwelling and in situ N regeneration are insufficient to support the observed G. breve blooms in this region. Furthermore, unpublished research, anecdotal information and historical red tide monitoring data suggest a correlation between the timing and magnitude of G. breve blooms and the occurrence of the filamentous, N2 fixing cyanobacteria, Trichodesmium spp. in both the Gulf of Mexico and Atlantic coastal waters. Little is known about the fate and significance of new N inputs derived from recently fixed N2 or of the pathways of trophic transfer whereby this new N is assimilated into oligotrophic marine ecosystems. It is critical to determine the fate of new N in oligotrophic systems and the effects of N inputs from N2 fixation on the community structure and function so that we can accurately assess the impact of new N on regenerated and export production.We propose that the oligotrophic west Florida Shelf ecosystem, which is characterized by large blooms of these species, is an ideal model system in which to examine the fate of new N inputs from N2 fixation by Trichodesmium spp. and the pathways facilitating the trophic transfer of this N. We also suggest that that blooms of G. breve in west Florida shelf waters are supported by the release and regeneration of DIN and DON from N2 recently fixed by co-occurring or preceding blooms of Trichodesmium spp. The present project will examine these hypotheses with a three-year program consisting of laboratory and field studies to: 1) determine the capacity for and kinetics of N uptake and regeneration by G. Breve; 2) determine whether direct or indirect regeneration of NH4+and DON from recently fixed N2 by Trichodesmium provides sufficient N substrates to support the observed growth of G. Breve; and 3 identify associated communities and examine trophic pathways whereby newly fixed N2 derived from Trichodesmium spp. stimulates production in otherwise oligotrophic waters. There will be active collaboration with modeling projects focused on both Trichodesmium and G. breve blooms in the west Florida shelf region.