Daughters of Trichonauts* in the Gulf of Mexico (DOTGOM)

Fate of recently fixed nitrogen in the eastern Gulf of Mexico: Does the regeneration of nitrogen fixed by Trichodesmium support the growth development of G. breve blooms?

Funded by NSF-Biological Oceanography. Cindy Heil (PI, University of South Florida), Deborah A. Bronk (Co-PI, VIMS), and Margie Mulholland

Abstract

Blooms of the toxic red tide dinoflagellate Gymnodinium breve Davis (now Karenia brevis), are an important feature of the Florida Shelf ecosystem. It has been hypothesized that blooms 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), it is thus 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 from riverine, upwelling, and in situ N regeneration are insufficient to support the observed G. breve blooms in this region. Little is known about the fate and significance of new N inputs derived from recently fixed N or of the pathways of trophic transfer whereby this new N is assimilated into oligotrophic marine ecosystems. It is critical that we determine the fate of new N in oligotrophic systems and the effects of N inputs from N fixation on the community structure and function so that we can accurately assess the impact of new N on regenerated and export production. We hypothesized that that blooms of G. breve in west Florida shelf waters are supported by the release and regeneration of DIN and DON from N recently fixed by co-occurring or preceding blooms of Trichodesmium spp.

Results

Kinetic studies

Karenia brevis has a high affinity for ammonium, a lower affinity for nitrate, and an intermediate affinity for organic substrates. These data suggest that when high rates of N regeneration by Trichodesmium occur on the shelf, K. brevis have the physiological capabilities to efficiently exploit this source of regenerated N.

Trichodesmium N fixation

We report the first rates of N fixation for the Gulf of Mexico. Trichodesmium colonies from the west Florida shelf fixed N₂ at high rates, comparable to other studies.

Trichodesmium uptake

Ambient nutrient concentrations were low, as would be expected of an oligotrophic area such as the eastern Gulf of Mexico. Preliminary results from uptake experiments indicate that ammonium uptake was 4-11 times higher (on a per colony basis) than the uptake of other forms of N. NO uptake rates were very low compared to the others analyzed. These results are consistent with studies done in other oligotrophic regions.

Dialysis experiments

We performed a series of experiments to quantify the direct transfer of regenerated N, as DON and/or ammonium, resulting from active N₂ fixation by Trichodesmium. Experiments were conducted on 5 of the 6 cruises for the project. In each experiment, Trichodesmium colonies were placed into dialysis bags that had pore sizes of 1K or 100K Dalton. The bags were filled with filtered seawater that was enriched with ¹⁵N labeled N gas. At the start of the experiment, the dialysis bags were immersed in whole surface water and ¹⁵N gas was added; ¹⁵N labeled gas was added to whole water with no Trichodesmium as a control. In one set of experiments, copepods were added to determine the affect of grazers. Plankton in the whole water surrounding the dialysis bags were significantly enriched in ¹⁵N in all treatments, except the controls, indicating release and subsequent uptake of regenerated N from Trichodesmium.

Results indicate that uptake of recently released N in the <1 K Dalton size range was 26 to 50% of the rate measured in the <100K Dalton treatment.

*What is a Trichnaut?

One who studies Trichodesmium
Can be identified by their participation in extremely fun cruises to exotic tropical locations.
Characterized by the need for reggae and rum.
The mighty leader of the Trichonauts is Dougie Capone (aka the guy in the speedo)