Winter Nutrient Pulse and Seagrass Epiphyte Bloom: Evidence of Anthropogenic Enrichment or Natural Fluctuations in the Lower Florida Keys?

Nutrient enrichment continues to disrupt marine ecosystem function worldwide. Assessing eutrophication in seagrass ecosystems such as the Great White Heron National Wildlife Refuge (GWHNWR), Florida Keys is critical for protecting the diverse community that depends on the intertidal and subtidal seagrass beds. We quantified water column nutrients, chlorophyll a, tissue nutrients in macroalgae and Thalassia testudinum, and epiphyte percent cover on seagrasses on tidal flats seasonally over 1 year at three sites: Howe Key, Water Keys, and Upper Harbor Key. Water column nutrients (total nitrogen, total phosphorus, nitrate–nitrite, ammonium, dissolved inorganic nitrogen, dissolved organic nitrogen, soluble reactive phosphorus, dissolved organic phosphorus) increased from fall to winter at Water Keys. These increased nutrients coincided with a bloom of the epiphyte Spyridia filamentosa on seagrasses that exceeded 40 % cover at Water Keys and Upper Harbor Key in two consecutive seasons. Seagrasses at all sites had chronic percent cover of small epiphytes exceeding 60 %. Additionally, low δ13C in T. testudinum tissues at Upper Harbor Key compared to the other sites suggested variations in carbon sources across the study area. Spatial patterns in macrophyte nitrogen-tophosphorus (N:P) ratios point to sources of nutrients from the inhabited islands of the Keys and from the Gulf of Mexico. Water column total nitrogen and total phosphorus concentrations exceeded the numeric nutrient criteria for the study area suggesting that the area should be monitored closely.