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, includin g 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. Zn the coastal zone, as elsewhere in the Earth system, the biogeochemical cycle of carbon is coupled to, and driven by, t he cycles of nitrogen and phosphorus through biological transfer processes. Human activities in the past 300 years have become an increasingly importa nt geological factor with respect to the coastal zone through four major en vironmental perturbations: (1) C, N, and S emissions from fossil fuel burni ng; (2) changes in land-use activities resulting in gaseous C emissions, in creased dissolved and particulate loads, organic matter transport, and feed backs 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 temperat ure of the planet has increased over this period of time by approximately 1 degrees C, perhaps also because of human activities. Starting with the yea r 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 coa stal zone using the thirteen-reservoir, process-driven model TOTEM for the coupled C-N-P-S biogeochemical cycles. An indicator of the reliability of t he model is the good agreement of its results showing the time course of in creasing atmospheric CO2 concentrations since the year 1700 with the observ ational results reported in the literature. During the past three centuries , there has been a significant increase in the amount of organic carbon tra nsported from land and stored in coastal zone sediments. Of the total trans ported. about 65% was stored in sediments and the remaining 35% primarily r ecycled 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 remineralizatio n, corresponding to an increase in the degree of heterotrophy of the global coastal zone. This process, along with the release of CO2 from the formati on of CaCO3, counteracts the invasion of CO2 from the atmosphere to coastal waters that is driven by the rise in atmospheric CO2 concentrations. An an alysis of a possible reduction or full collapse of the oceanic thermohaline circulation, as believed to have occurred in the past and a possibility fo r 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 attribu table 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 em issions to the atmosphere, changes in land-use practices, and sewage discha rges have been the three main factors affecting the carbon cycle in the glo bal coastal zone. The latter inputs from land have apparently produced a sl ight increase in the heterotrophy of the global coastal zone. However, incr eases in the inputs of nutrient nitrogen and phosphorus from land to the co astal 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 CO,. The direction of future change in net ecosystem production in the coastal z one strongly depends on changes in the relative magnitudes of organic carbo n and nutrient N and P fluxes to the coastal zone via rivers, provided the upwelling fluxes remain constant. (C) 1999 Elsevier Science B.V. All rights reserved.