Quantifying nutrient cycling and retention in coastal waters at the global scale. Geologica Ultraiectina (312)

Coastal waters extend from the mouths of rivers to the edge of the continental shelves, forming the transition zone between land and ocean. This highly dynamic narrow ribbon of coastal ecosystems is of major ecological and economical interest. It also plays a key role in global ocean biogeochemistry through its removal and transformation of terrestrial and marine nutrient inputs. Over the past century, human activities have led to drastic increases in nitrogen (N) and phosphorus (P) concentrations in rivers, while in some areas silica (Si) concentrations have declined due to damming. As a consequence of these changes in nutrient loads and ratios, many coastal ecosystems are suffering from problems related to eutrophication, such as increased primary production, the occurrence of harmful algal blooms and hypoxia. The research presented in this thesis focuses on the biogeochemical cycles of carbon and nutrients in the global coastal ocean and their responses to perturbations, mostly on time scales of decades to a century. To reach this goal, a specific effort is made to go beyond traditional boundaries between scientific fields. A fully transient two dimensional hydrodynamic model is coupled to a biogeochemical model for N and Si cycling to assess the effect of increased nutrient loads and colonization by invasive species on a small tidal Bay (Bay of Brest). The model results emphasize the role of seasonal retention and redissolution of Si in determining the resistance of the system to eutrophication. At the global scale, the budget for reactive Si along the land-ocean continuum is updated and implemented in a first order kinetic box model. A sensitivity analysis of the global Si cycle to temperature rise and river damming demonstrates the role of the coastal zone as a buffer between the terrestrial and oceanic domains. The comparison between the performance of the box model for the global Si cycle and two Ocean General Circulation Models (HAMOCC2 and HAMOCC5) demonstrates that the average global ocean response to changes in riverine Si input is similar on time scales up to 150 kyrs. A spatially-explicit global typology for estuarine ecosystems is presented. This typology is based on morphological, lithological and hydrological criteria and provides new insights into the spatial distribution and inherent heterogeneity of estuarine filters worldwide. It is also used as the basis for a spatially explicit model for N and P cycling and retention. The model consists of a ribbon of 6200 generic box models distributed along the world coastline, and the global estimates for N and P retention calculated for the estuarine filter are lower than values reported in previous studies. Finally, a simplified continental shelf typology is presented and combined with the estuarine typology to reevaluate the global CO2 exchange at the air-water interface of the coastal ocean. The improved surface areas, and spatially-explicit typological approach allow for a more accurate upscaling of average CO2 exchange fluxes from local studies to the global scale