Spatial heterogeneity in sediment and carbon accretion rates within a seagrass meadow correlated with the hydrodynamic intensity

Abstract

The majority of the carbon stored in seagrass sediments originates outside the meadow, such that the carbon storage capacity within a meadow is strongly dependent on hydrodynamic conditions that favor deposition and retention of fine organic matter within the meadow. By extension, if hydrodynamic conditions vary across a meadow, they may give rise to spatial gradients in carbon. This study considered whether the spatial gradients in sediment and carbon accretion rates correlated with the spatial variation in hydrodynamic intensity within a single meadow. Field measurements were conducted in three depth zones across a Zostera marina L. (eelgrass) meadow in Nahant Harbor, Massachusetts. Four sediment cores were collected in each zone, including one outside the meadow (control) and three within the meadow at increasing distances from the nearest meadow edge. Sedimentation and carbon accretion rates were estimated by combining the measurements of dry bulk density, organic carbon fraction (%OC), 210Pb, and 226Ra. Tilt current meters measured wave velocities within each zone, which were used to estimate turbulent kinetic energy (TKE). Both sediment and carbon accretion rates exhibited spatial heterogeneity across the meadow, which were correlated with the spatial variation in near-bed TKE. Specifically, both accretion rates increased with decreasing TKE, which was consistent with diminished resuspension associated with lower TKE. A method is proposed for using spatial gradients in hydrodynamic intensity to improve the estimation of total meadow accretion rates.