Diagenesis in seagrass vegetated sediments: biogeochemical processes on diurnal time scales

Seagrass productivity is largely limited by nutrient and light availability. However, increasing evidence suggests that sedimentary geochemical processes may play an essential role in seagrass productivity/health. Much of this work has been largely phenomenalistic and has not clearly identified the spatio-temporal behavior of the major geochemical parameters involved in diagenesis of seagrass sediments. In this study, a much broader range of both dissolved and solid phase chemical parameters in eelgrass vegetated sediments was investigated. Parallel measurements were made on adjacent unvegetated sediments (<10 m) to more clearly refine the specific influences of seagrass (Zostera marina) on chemical gradients in associated sediments. Previous studies have pointed strongly toward diurnal ??ventilation?? of sediments vegetated with seagrass by the exudation of photosynthetically produced oxygen. However, strong lateral variability of sediment geochemical parameters among and between seagrass vegetated and unvegetated sediments made the observation of diurnal effects sufficiently difficult. Changes resulting from temporal variability were difficult to discern within the spatial variability. A critical question that is often not dealt with in the study of the early diagenesis of sediments is what spatial and temporal sampling intervals are required to account for the dominant source of variability. The auto-covariance function (ACF) was used to determine the optimum scaling length for sample intervals (?x) of ?H2S and Fe2+. Characteristic scale lengths obtained for sediments from seagrass environments are not significantly different from those observed for unvegetated sediments and averaged 13.7?? 2.2 mm. Lateral variations in our scales analyses showed that scale length approximated our sampling interval and that lateral sampling intervals were smaller than the vertical sampling intervals. Our results indicate that macrofauna dwelling in the sediment, the seagrass root/rhizomes, and aggregations of bacteria, microalgae, and meiofauna may be responsible for the vertical and lateral variability. Model calibrations and sensitivity analyses from a sediment-seagrass diagenetic model revealed that changes in physical parameters of the sediments (irrigation, advection, and porosity, for example) had the greatest effect on organic carbon and total dissolved sulfides. This study revealed that sedimentary geochemical parameters that are both vertically and laterally heterogeneous may also affect seagrass productivity.