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 O 2 m −2 d −1 , respectively) in low-phosphorus sediments in the northeast and the highest (287 and −212 mmol O 2 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 O 2 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-today 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.

In situ oxygen dynamics and carbon turnover in an intertidal sediment (Skallingen, Denmark)

“…Berg & Huettel 2008 has proven itself as a very powerful, non-invasive technique for resolving benthic O 2 ex change and metabolism in challenging coastal environments (e.g. Glud et al 2010, Attard et al 2014, Long et al 2015. The approach, however, will only work during inundation, but combined application of AEC and continuous auto nomous in situ microprofiling measurements, as realized here, might represent an ideal procedure for resolving NEM in dynamic intertidal settings and to assess the importance of such environments for marine carbon cycling in general.…”

Technical note: Time lag correction of aquatic eddy covariance data measured in the presence of waves

“…Over the last 10 years, the aquatic eddy covariance technique has become a widely accepted approach for measuring oxygen fluxes between benthic ecosystems and the overlying water . In that time, the number of users has grown rapidly, and the technique has been applied under very different field settings such as muddy and sandy sediments Kuwae et al, 2006;Glud et al, 2010), deep ocean sediments (Berg et al, 2009), coral reefs Cathalot et al, 2015;Rovelli et al, 2015), and seagrass meadows (Hume et al, 2011;Rheuban et al, 2014;Long et al, 2015). With a few exceptions, all of the recently published aquatic eddy covariance studies have focused on oxygen fluxes.…”

The response of seagrass (<i>Posidonia oceanica</i>) meadow metabolism to CO<sub>2</sub> levels and hydrodynamic exchange determined with aquatic eddy covariance

“…Unlike in incubations, saturation occurred for a complete meadow under low in situ light conditions. It contrasts with light-limitation of mature Z. marina and T. testudinum meadows under in situ irradiance (Rheuban et al 2014;Long et al, 2015), and suggests that a factor other than light may limit primary production at peak irradiance. Nutrient limitation may contribute.…”

“…Therefore, the decomposition of leaf litter within the ecosystem was substantially smaller than production. Prior studies utilizing the eddy covariance technique in summer identified greatrer similarity between respiration and 230 primary production in Zostera marina meadows in the mid-Atlantic Bight (Rheuban et al, 2014a) and in Thalassia testudinum meadows in Florida Bay, USA (Long et al, 2015). In those studies NEM was one-fifth of GPP.…”

“…Recent studies include eddy covariance flux measurements over meadows of Zostera marina (Hume et al, 2011;Rheuban et al, 2014a;Rheuban 65 et al, 2014b), Thalassia testudinum (Long et al, 2015), and Zostera noltii (Lee et al, 2017). Open water measurements of P. oceanica productivity confirm the net autotrophy of the ecosystem and reveal a surprising, seven-fold increase in net production due to removal of sloughed seagrass leaves by storms (Champenois and Borges, 2012).…”