Abstract
Coastal eutrophication is one of the main factors causing the loss of seagrass meadows worldwide. The loss of seagrass canopy following a disturbance may increase the loss of sedimentary Organic Carbon (Corg) and Nitrogen (N) stocks through resuspension and remineralisation, contributing to increased greenhouse gas emissions. This study provides comprehensive estimates of changes in Corg and N stocks and accumulation rates in seagrass meadows due to anthropogenic activities (e.g. eutrophication) and determines how habitat characteristics (e.g. bottom shear stress, water depth) influence the extent of nutrient fluxes at Cockburn Sound, WA. A total of 17 soil cores were sampled: 7 cores from living and persistent seagrass meadows, and 10 cores from bare sediments previously vegetated with seagrass. Sediment grain size was analysed to characterise sedimentary conditions, and δ13C of sedimentary organic matter was used to determine the sources of Corg stocks at vegetated and unvegetated sites. Accumulation rates of Corg and N were determined using 210Pb. On average, seagrass soils contained up to 2.5 times higher Corg and N stores (3.32±0.5 and 0.16±0.15 kg m-2, respectively) than bare but previously vegetated sediments (1.45 and 0.11 kg m-2) in 50 cm-thick deposits. Industrialisation and land-use change in Cockburn Sound from the 1960s onwards have led to the loss of 3,500 ha of seagrass ecosystems, linked to a loss of seagrass Corg and N sequestration capacity and the loss of sedimentary Corg (1.9 kg m-2) and N (0.05 kg m-2) stores accumulated over the last ~200 years. However, the loss of Corg and N stocks at Cockburn Sound was not uniform. Areas with higher hydrodynamic energy experienced greater erosion and loss of stocks. This information is critical for the implementation of Corg storage credit offset policies focused on avoiding the conversion of seagrass ecosystems and contributing to their preservation, as well as understanding the role of seagrasses as filters and sinks of nutrients in a changing environment.
