Seagrass meadows are important marine carbon sinks, yet they are threatened and declining worldwide. Seagrass management and conservation requires adequate understanding of the physical and biological factors determining carbon content in seagrass sediments. Here, we identified key factors that influence carbon content in seagrass meadows across several environmental gradients in Moreton Bay, SE Queensland. Sampling was conducted in two regions: (1) Canopy Complexity, 98 sites on the Eastern Banks, where seagrass canopy structure and species composition varied while turbidity was consistently low; and (2) Turbidity Gradient, 11 locations across the entire bay, where turbidity varied among sampling locations. Sediment organic carbon content and seagrass structural complexity (shoot density, leaf area, and species specific characteristics) were measured from shallow sediment and seagrass biomass cores at each location, respectively. Environmental data were obtained from empirical measurements (water quality) and models (wave height). The key factors influencing carbon content in seagrass sediments were seagrass structural complexity, turbidity, water depth, and wave height. In the Canopy Complexity region, carbon content was higher for shallower sites and those with higher seagrass structural complexity. When turbidity varied along the Turbidity Gradient, carbon content was higher at sites with high turbidity. In both regions carbon content was consistently higher in sheltered areas with lower wave height. Seagrass canopy structure, water depth, turbidity, and hydrodynamic setting of seagrass meadows should therefore be considered in conservation and management strategies that aim to maximize sediment carbon content.

The study was conducted at Moreton Bay, SE Queensland, Australia (27°S, 153°E). Sampling was conducted in two regions: (1) Canopy Complexity, on the Eastern Banks, where seagrass canopy structure and species composition varied while turbidity was consistently low; and (2) Turbidity Gradient, where turbidity varied among sampling locations. In the Canopy Complexity region, 98 sites were sampled in June 2012, and 32 sites in February 2013. In the Turbidity Gradient region, six sites were sampled at each of four locations on June 2012, February 2013 and June 2013.One seagrass core sample was collected at each of the Canopy Complexity sites. For the Turbidity Gradient, seagrass biomass cores were collected at each of six random sites within a 50 × 50 m plot which was haphazardly defined at each location.Seagrass structural complexity (above and below ground biomass, shoot density, leaf length, width and area) were measured from the seagrass biomass cores at each location. Briefly:Each biomass core was gently rinsed free of sediment using a 1 mm mesh bag to retain seagrass material. Biomass samples were kept on ice in the field, then stored frozen (-20°C) until further processing. The number of shoots per species in each core was quantified. Biomass material from each core was separated per species into above ground (leaves and leaf stems) and below ground (roots, rhizomes, and leaf sheaths) material. Prior to drying, leaves were submerged in 10% hydrochloric acid (HCl) and rinsed with fresh water to remove calcareous epiphytes. Foliar epiphytes were gently scraped off using laboratory forceps. Each component was then dried at 60°C and final biomass (g dry weight (DW)/m2) calculated. Images were taken of three representative shoots per each species from each core for estimation of leaf area using Image J. Leaf area index (LAI; m2 leaf material/m**2 surface) was calculated for each species per sample. LAI of each sample was defined as the sum of LAI of all species present. Metrics for representing seagrass structural complexity were LAI and seagrass biomass.

Supplement to: Samper-Villarreal, Jimena; Lovelock, Catherine E; Saunders, Megan I; Roelfsema, Christiaan M; Mumby, Peter John (2016): Organic carbon in seagrass sediments is influenced by seagrass canopy complexity, turbidity, wave height, and water depth. Limnology and Oceanography, 61(3), 938-952