Nutrient Dynamics of Floating Seagrass Wracks in Greater Florida Bay

Floating aggregations of seagrass wrack (i.e., leaves shed from seagrass beds) can serve as an ecological “hot spot” contributing to the survival of many species in the form of habitat and food source. Considerable research has been done on assessing trophic transfer of nutrients from seagrass wrack that is washed ashore, but little is known about nutrient dynamics in floating seagrass wrack. Here, drifters were deployed to track the location of floating wracks created in Greater Florida Bay and mesocosm experiments were conducted on two different of floating macrophytes (Syringodium filiforme, Thalassia testudinum, and Sargassum sp.) to estimate the sinking rate and the contribution of particulate and dissolved carbon and nitrogen released from the floating wracks. Floating wracks were tracked and carried through channels from Greater Florida Bay S. filiforme beds to near the Gulf Stream 15 km offshore. A large fraction of seagrass leaves remained buoyant for over 14 days in the mesocosms with daily shedding rates of individual leaves statistically decreasing over time from 10-15% initially to less than 5% after 6 days. When individual leaves became aggregated into whorled structures, as found offshore, the shedding rate became negligible at \u3c2% d-1 and wracks can persist for extended times. As the floating wrack ages it contributes to nutrient pools, especially total dissolved nitrogen (TDN), dissolved organic carbon (DOC), and colored dissolved organic material pools. TDN was composed of approximately 80% dissolved organic nitrogen, with remainder dissolved inorganic nitrogen largely composed of ammonium. Both TDN and DOC showed variable rates of change but ultimately increased concentration from initial to final days of incubation. With the duration of wracks and the amount of nutrients produced, seagrass wrack can serve as an important resource for organisms living in oligotrophic waters