Plankton Community Changes and Nutrient Dynamics Associated with Blooms of the Pelagic Cyanobacterium <i>Trichodesmium</i> in the Gulf of Mexico and the Great Barrier Reef
Judith M. O’Neil | Cynthia A. Heil | Patricia M. Glibert | Caroline M. Solomon | Joan Greenwood | Jack G. Greenwood
Blooms of the harmful dinoflagellate <i>Karenia brevis</i> on the West Florida Shelf (WFS), Gulf of Mexico, are hypothesized to initiate in association with the colonial cyanobacterium <i>Trichodesmium</i> spp. and benefit from dissolved organic nitrogen (DON) release derived from N<sub>2</sub>-fixation by the cyanobacteria. Previous studies have detected DON release using direct experimental measurements, but there have been few studies that have followed nutrient release by in situ blooms of <i>Trichodesmium</i> and the associated plankton community. It was determined that long-term <i>Trichodesmium</i> spp. and <i>Karenia brevis</i> abundances on the WFS were related, following a 2-month lag. A separate Eulerian study of a <i>Trichodesmium erythraeum</i> bloom event was conducted over 9 days on the Great Barrier Reef. Concentrations of <i>T. erythraeum</i> increased over the course of the study, with coincident increases in dinoflagellate abundance and decreases in diatom abundance. Inside the bloom, concentrations of NH<sub>4</sub><sup>+</sup>, PO<sub>4</sub><sup>3−</sup>, and DON increased significantly. The copepod grazer <i>Macrosetella gracilis</i> also increased in abundance as <i>T. erythraeum</i> numbers increased, contributing to nutrient release. Copepod grazing rates were measured, and N release rates estimated. Together, these studies show that <i>Trichodesmium</i> blooms have consequences for dinoflagellate abundance at both seasonal and ephemeral scales via direct and indirect N release.
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