Earth observation remote sensing of spatial and temporal relationships between rivers and receiving coastal waters

Abstract

River discharge can have a significant and profound influence on coastal ocean water quality and productivity and therefore the global biogeochemical cycle. However, there is little acknowledgement or understanding of the influence of river discharge on coastal ocean productivity or water quality at local and regional scales. To a greater extent, there is a paucity of knowledge on the global scale influence of river discharge on global productivity (as indicated by chlorophyll-a concentrations), though there have been previous attempts to estimate the global change in chl-a of the open oceans at decadal time scales. Here, the influence of river discharge is assessed from local and regional to global scales via analysis of river discharge time series and remotely sensed MODIS satellite imagery products. In the first research chapter (Chapter 2) these products are applied to investigate how a lack of river discharge from Australia’s Murray River (due to a drought period) and a flooding event affected the water quality and productivity of the adjacent coastal ocean waters. Findings indicate that Murray River discharge can stimulate productivity of the Southern Ocean coastal waters, up to 60 km from the river mouth. This effect was reduced to background seasonal levels during the a prolonged drought. The following chapter (Chapter 3) investigates the long-term influence of river discharge on coastal ocean chlorophyll-a to objectively identify where river discharge has influence under a broad range of flow conditions for 11 global rivers. This is done with a non-parametric spatiotemporal correlation analysis. The method is found to be more effective on rivers which do not have strong seasonal patterns, such as the Rhone (of France) and Murray (of Australia) rivers in temperate latitudes. The final research chapter (Chapter 4) uses the method developed in Chapter 3 to extract time series data from areas identified as influenced by river discharge, to then investigate the presence, absence and direction of trend. This is done to investigate whether the magnitude and scale of influence of river discharge may be changing over time. Some trends are detected contrary to that of previous research. A number of recommendations for future research are identified and proposed. This research is important to understand the current and potential future impacts of river management on coastal ocean environments, and to enable more accurate and rigorous estimation of regional and global scale trends in ocean productivity and biogeochemistry. Future research is imperative in understanding the role and influence of river discharge and river and coastal management on coastal ocean productivity and water quality in the future, at local to global spatial scales.