Blooms of Karenia brevis occur almost annually in the eastern Gulf of Mexico (Gulf of America). Submarine ground water discharges (SGD) have been previously suggested as a potential nutrient source in sustaining blooms. The West Florida Shelf is dotted with blue holes, or underwater sinkholes, which may be a route by which SGD seeps into offshore waters. Previously reported measurements within the holes demonstrated potentially high rates of denitrification, and dissimilatory nitrate reduction to ammonia (DNRA).  Here, multiple stations were monitored monthly for several years at sites near to–and away from–these sinkholes to assess the nitrogen (N) isotopic signatures of the particulate material. One offshore site, but not others, had mean d¹⁵N values of 13.28 in surface samples and 12.58 in bottom samples, with the highest value reaching 38.86, reflecting highly processed N. Other sites had much lower d¹⁵N values. Higher d¹⁵N vales were associated with lower river flows, wherein N would have a longer time to undergo processing. The microbiome of the free-living (0.22 μm) planktonic communities of this site, examined via 16S rRNA amplicon sequencing, confirmed the presence of putative denitrifying bacteria. Following Hurricane Ian, a Karenia brevis bloom formed with low d¹⁵N (<5), but as the bloom was sustained, the d¹⁵N of the particulate material increased significantly, reflecting greater reliance on recycled N. Historical reports of Karenia within 10 km of known blue holes show that K. brevis can concentrate in these locations, which may be hot spots for a diversity of Karenia species.