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To investigate nutrient release from decaying fish carcasses as a potential source fueling blooms of the toxic dinoflagellate, Karenia brevis (C.C. Davis) G. Hansen & Ø. Moestrup this study evaluated the type and quantity of nutrients (nitrogen (N), phosphorus (P), and carbon (C)) released from decaying fish using both wet chemistry and compound specific (Fourier transform ion cyclotron resonance mass spectrometry, FT-ICR MS) techniques. A 3 day bioassay experiment was then conducted to examine how eutrophic (Charlotte Harbor, CH) and oligotrophic (Blue Water, BW) microbial communities in the eastern Gulf of Mexico (GoM) respond to fish-derived nutrients (FDN) relative to other in situ N sources. Decaying fish released a suite of compounds, primarily in the form of ammonium (NH 4 + ); release rates ranged from 37.7 ± 12.6 to 102.3 ± 4.7 μmol N L −1 g −1 wet wt. d −1 . Phosphate release was 13.9 ± 2.8 to 21.4 ± 2.9 μmol P L −1 g −1 wet wt. d −1 . A total of 3515 organic compounds were produced, which were dominated by lipid and protein-like compounds. In the bioassay, FDN were utilized by microbial communities collected from both CH and BW, with complete drawdown of fish-derived NH 4 + in the CH treatment in conjunction with a 3-fold increase in Chl a . FDN provided the necessary nutrients to alleviate P limitation in the BW treatment, resulting in a 13-fold increase in Chl a . Results from this experiment indicate FDN are important N and P sources to the microbial community along the eastern GoM and have important implications for maintenance of associated ichthyotoxic K. brevis blooms. [ABSTRACT FROM AUTHOR]