Sub-tropical seagrass ecosystem metabolism measured by eddy covariance

Abstract

The metabolism of seagrass ecosystems was examined at 4 sites in south Florida, USA, using the eddy covariance technique under in situ conditions. Three sites were located across a phosphorus-driven productivity gradient to examine the combined effects of dynamic variables (irradiance, flow velocity) and state variables (sediment phosphorus and organic content, seagrass biomass) on ecosystem metabolism and trophic status. Gross primary production and respiration rates varied significantly across Florida Bay in the summer of 2012 with the lowest rates (64 and -53 mmol O2 m-2 d-1, respectively) in low-phosphorus sediments in the northeast and the highest (287 and -212 mmol O2 m-2 d-1, respectively) in the southwest where sediment phosphorus, organic matter, and seagrass biomass are higher. Seagrass eco systems offshore of the Florida Keys had similar large daily production and respiration rates (397 and -217 mmol O2 m-2 d-1, respectively) and were influenced by flow through the permeable offshore sediments. Across all sites, net ecosystem metabolism rates indicated that the seagrass ecosystems were autotrophic in the summertime. Substantial day-to-day variability in metabolic rates was found due to variations in irradiance and flow velocity. At all sites the relationship between photosynthesis and irradiance was linear and did not show any sign of saturation over the entire irradiance range (up to 1400 μmol photons m-2 s-1). This was likely due to the efficient use of light by the large photosynthetic surface area of the seagrass canopy, an effect which can only be examined by in situ measurements that integrate across all autotrophs in the seagrass ecosystem.