Connecting Sediment Load Targets to Ecological Outcomes for Seagrass: Report to the National Environmental Science Program

Catchment activities, such as logging, grazing, agriculture and urbanization, generate elevated sediment loads which impact downstream water quality and coastal ecosystems. Quantifying the complex link between catchment sediment sources and downstream ecosystems is challenging but important for the development of reliable land-based ecologically relevant load targets. With this goal in mind, we compared condition of seagrass (area and biomass) in Cleveland Bay, northeast Australia, to river discharge and associated sediment loads, fitting linear models to 12 years of routine monitoring data. The data demonstrate that the Burdekin catchment dominates sediment delivery to Cleveland Bay. Annual changes in the area and biomass of shallow subtidal seagrass were significantly correlated with annual total suspended solid (TSS) loads from the Burdekin River (and to flow, in the case of area). However annual TSS loads were not good predictors of change in area and biomass across all seagrass communities. Neither area nor biomass was significantly correlated to annual sediment (i.e. TSS and fine sediment) loads for both the shallow subtidal community, and all communities combined, but area was significantly correlated to 4-year antecedent TSS and fine sediment loads. The results demonstrate that the trajectory of decline and recovery differed between biomass and area, and suggest that processes occurring on annual timescales drive year-to-year variation, but that seagrass state is affected by conditions accumulating over longer time periods. The findings also highlight different responses of subtidal and intertidal seagrasses to TSS loads. Fine sediment (particle size 0.55, p