Beschreibung:
<jats:p>During a 3‐yr study in the Gulf of Mexico, we measured dinitrogen (N<jats:sub>2</jats:sub>) fixation and nitrogen (N) release by <jats:italic>Trichodesmium</jats:italic> and compared these rates with water column N demand and the estimated N necessary to support blooms of <jats:italic>Karenia brevis</jats:italic>, a toxic dinoflagellate that severely affects the West Florida shelf. Net and gross N<jats:sub>2</jats:sub> fixation rates were compared in simultaneous incubations using δ<jats:sup>15</jats:sup>N<jats:sub>2</jats:sub> uptake and acetylene reduction, respectively. The difference between net and gross N<jats:sub>2</jats:sub> fixation is assumed to be an approximation of the rate of N release. Results demonstrate that <jats:italic>Trichodesmium</jats:italic> in the Gulf of Mexico are fixing N<jats:sub>2</jats:sub> at high rates and that an average of 52% of this recently fixed N<jats:sub>2</jats:sub> is rapidly released. Calculations suggest that observed densities of <jats:italic>Trichodesmium</jats:italic> can provide enough N to support moderately sized <jats:italic>K. brevis</jats:italic> blooms. Based on other studies where δ<jats:sup>15</jats:sup>N<jats:sub>2</jats:sub> and acetylene reduction were compared directly, it appears that N release from <jats:italic>Trichodesmium</jats:italic> is common but varies in magnitude among environments. In addition, carbon (C) and N‐based doubling times for <jats:italic>Trichodesmium</jats:italic> vary among studies and environments. Comparing gross N<jats:sub>2</jats:sub> fixation and C fixation directly, C‐based doubling times exceeded N‐based doubling times, which suggests an imbalance in elemental turnover or a failure to fully quantify <jats:italic>Trichodesmium</jats:italic> N use.</jats:p>