Structure and dynamics of South East Indian seagrass meadows across a sediment gradient

In this study we examine the influence of non-monsoon sediment arrival on the high-diversity SE Indian seagrass meadows of the Palk Bay and the Gulf of Mannar. We used a gradient-based approach to examine the influence of increasing sediment loads on species composition and shoot density. In addition, for the ubiquitous seagrass (Cymodocea serrulata), we tested the influence of sediment on its biomass and productivity. We identified three sites in Palk Bay and four sites in Gulf of Mannar (SE India) along a gradient of sediment input. At each of the seven locations, sediment traps were deployed to measure sedimentation rates. Nine seagrass cores were taken systematically along 50 m transects at a constant sub-tidal depth to measure shoot density and biomass. A few shoots of C. serrulata were marked to estimate the above ground seagrass growth rate. Our results indicate that sedimentation rates that ranged from 8.6 to 62.4 mg DW cm⁻² d⁻¹ could not explain species composition of the meadow or shoot density of the observed species. C. serrulata was, by far, the most abundant species and present in all sediment condi- tions. Sedimentation rates did not alter shoot elongation rates in C. serrulata, ranging from 1.54 ± 0.29 SD to 0.25 ± 0.02 SD cm d⁻¹ , but in contrast, increased vertical rhizome elongation rate. This increase was reflected in an increase in below ground biomass along the sediment gradient (R2 = 0.582, p = 0.01). C. serrulata appears to be able to adapt to the sediment dynamics in this area by allocating resources to rhizomes and roots to counteract burial and stabilizing sediments. Given that siltation is one of the most important threats to seagrass meadows, understanding the species-specific adaptive mechanisms of seagrass species in these high-sediment, high diversity South Asian meadows is an important first step in ensuring their long-term survival and functioning