Carbon cycle in the coastal zone: Effects of global perturbations and change in the past three centuries

The coastal zone, consisting of the continental shelves to -200 m, including bays, lagoons, estuaries, and near-shore banks, is an environment that is strongly affected by two much bigger environmental reservoirs adjacent to it: the land and open ocean. In the coastal zone, as elsewhere in the Earth system, the biogeochemical cycle of carbon is coupled to, and driven by, the cycles of nitrogen and phosphorus through biological transfer processes. Human activities in the past 300 years have become an increasingly important geological factor with respect to the coastal zone through four major environmental perturbations: (1) C, N, and S emissions from fossil fuel burning; (2) changes in land-use activities resulting in gaseous C emissions, increased dissolved and particulate loads, organic matter transport, and feedbacks to biological production; (3) application of inorganic nitrogen- and phosphorus-containing fertilizers; and (4) discharges of sewage containing reactive organic C, N, and P. In addition, the mean global surface temperature of the planet has increased over this period of time by approximately 1°C, perhaps also because of human activities. Starting with the year 1700 as a base for the industrial-age perturbations on land, we analyzed the consequences of these five perturbations to the carbon cycle in the coastal zone using the thirteen-reservoir, process-driven model TOTEM for the coupled C-N-P-S biogeochemical cycles. An indicator of the reliability of the model is the good agreement of its results showing the time course of increasing atmospheric CO₂ concentrations since the year 1700 with the observational results reported in the literature. During the past three centuries, there has been a significant increase in the amount of organic carbon transported from land and stored in coastal zone sediments. Of the total transported, about 65% was stored in sediments and the remaining 35% primarily recycled through exchange with the atmosphere and open ocean. The imbalance between the amounts of organic carbon produced by gross photosynthesis and remineralized has apparently increased slightly in favor of remineralization, corresponding to an increase in the degree of heterotrophy of the global coastal zone. This process, along with the release of CO₂ from the formation of CaCO₃, counteracts the invasion of CO₂ from the atmosphere to coastal waters that is driven by the rise in atmospheric CO₂ concentrations. An analysis of a possible reduction or full collapse of the oceanic thermohaline circulation, as believed to have occurred in the past and a possibility for future centuries, indicates that the CO₂ transfer from the atmosphere to coastal waters would increase while that from the atmosphere to open ocean surface waters would decrease, if such an event took place. This is attributable to a reduced supply to the coastal zone of dissolved inorganic carbon by coastal upwelling from the deeper ocean, a process linked to the global conveyor belt of the thermohaline circulation. To date, fossil fuel CO₂ emissions to the atmosphere, changes in land-use practices, and sewage discharges have been the three main factors affecting the carbon cycle in the global coastal zone. The latter inputs from land have apparently produced a slight increase in the heterotrophy of the global coastal zone. However, increases in the inputs of nutrient nitrogen and phosphorus from land to the coastal zone in the future may drive its trophic state toward net production and storage (autotrophy), thereby also increasing its potential role as a sink for atmospheric CO₂. The direction of future change in net ecosystem production in the coastal zone strongly depends on changes in the relative magnitudes of organic carbon and nutrient N and P fluxes to the coastal zone via rivers, provided the upwelling fluxes remain constant.