Influence of the human perturbation on carbon, nitrogen, and oxygen biogeochemical cycles in the global coastal ocean

Abstract

The responses to human perturbations of the biogeochemical cycles of carbon (C), nitrogen (N), and oxygen (O) in the global coastal ocean were evaluated using a process-based model. In this model, the global coastal ocean is represented by two distinct zones: the proximal zone which includes large bays, the open water part of estuaries, deltas, inland seas, and salt marshes; and the distal zone which includes the open continental shelves down to a depth of 200 m. The biogeochemical model of the elemental cycles in the coastal ocean describes the dynamics of transfer processes and their interactions (primary production, mineralization, sediment deposition, and burial). The coastal sediment submodel describes element recycling in the sediments and allows the assessment of the evolution of denitrification under various environmental forcings. Initial values for the biogeochemical fluxes of carbon, nitrogen, and oxygen between the various reservoirs of the coastal ocean and the exchange of materials at the boundaries with the rivers and the open ocean are estimated based on current literature data. Before anthropogenic activities, the global coastal ocean was a net autotrophic system with a net export flux to sediments and open ocean of 20 Tmol organic C/yr. The magnitude and direction of this net flux is strongly dependent on the flux of organic carbon from the coastal ocean to adjacent reservoirs. The proximal coastal region was slightly heterotrophic at 8.4 Tmol C/yr, consistent with estimates by other authors; the distal coastal ocean was autotrophic at the rate of 28.4 Tmol C/yr. To simulate the evolution of the coastal ocean under human influence during the past 50 yr, the global coastal ocean model was perturbed by increasing the global riverine fluxes of dissolved inorganic nitrogen (DIN) and total organic matter (TOM) and the depositional flux of atmospheric nitrogen relative to the preanthropogenic condition. Model results show that over the past 50 yr, primary production in the coastal ocean has doubled, and resulted in an accumulation of biomass in all compartments of the system. The coastal ocean became more heterotrophic in response to the dominant perturbation of the increased flux of terrestrial organic matter via the rivers. Contrary to expectations, denitrification rates do not increase and the denitrification efficiency of the coastal ocean system decreases. This suggests that the coastal ocean is not likely to self-regulate the effects of human-induced perturbations on nutrient cycles. Finally, simulation results for a future condition where the rates of riverine organic and inorganic C and N inputs to the coastal ocean continue to increase at their current exponential growth patterns indicate that the proximal coastal ocean could become increasingly heterotrophic, the organic matter content could increase, and primary production could be enhanced. These changes could potentially cause noxious blooms to occur and become a generalized phenomenon of the proximal coastal ocean. There is a strong likelihood that unless human-derived inputs are regulated at the source, substantial biogeochemical modification of the global coastal ocean will occur because cyclic processes within the coastal ocean system are not rapid enough to dissipate the effects of the perturbations.