Eutrophication and Trophic State Classification of Seagrass Communities in the Florida Keys

Seagrass communities in the Florida Keys are receiving increased nutrient loadings from a variety of land-based human activities that are accelerating coastal eutrophication. We assessed relationships among total nitrogen (TN) and total phosphorus (TP) concentrations of the water column and the productivity, biomass, and epiphyte levels of the seagrasses Thalassia testudinum and Halodule wrightii along three onshore-offshore transects (Key West, Big Pine Key, and Long Key) stratified a priori into hypereutrophic (HYPER), eutrophic (EUTR), mesotrophic (MESO), and oligotrophic (OLIGO) communities with increasing distance from shore. Macroalgal biomass and alkaline phosphatase activity (APA) of macroalgae and attached seagrass epiphytes were also determined along the eutrophication gradients. H. wrightii was the dominant seagrass within inshore HYPER strata whereas T. testudinum was dominant at the EUTR. MESO and OLIGO strata. Seagrasses at the HYPER and EUTR strata had low shoot densities, low shoot production rates, low areal biomass values, low areal production rates, but high levels of attached epiphytes and mat-forming macroalgae. Sea-grasses at the OLIGO strata had the highest shoot densities, highest areal biomass values, highest areal production rates, and typically the lowest or second lowest epiphyte levels of all strata. APA was lowest for macroalgae at the offshore OLIGO strata, and highest at the nutrient-enriched HYPER and EUTR strata where extensive populations of mat-forming mac-roalgae occurred. Microcosm studies showed that both N and P cnrichment increased epiphyte levels and reduced rhizome growth rates in T. testudinum whereas P enrichment alone increased epiphyte levels and reduced rhizome growth rates of H. wrightii. Higher APA in macroalgae and attached blade epiphytes in HYPER and EUTR strata reflected increased P-limitation in these dystrophic environments resulting from high concentrations of TN relative to TP. Sustained nutrient enrichment from land-bascd activities results in increased biomass of attached epiphytes and macroalgae, which attenuate light, reduce dissolved oxygen, and lead to decline of T. testudinum and a gradient of habitat damage from nearshore to offshore waters.