MODELING PREINDUSTRIAL C-N-P-S BIOGEOCHEMICAL CYCLING IN THE LAND COASTAL MARGIN SYSTEM

Human activities were important forcing functions in the pre-industria l world and have become increasingly more important in the 21st centur y. Because of both natural temperature changes and pre-industrial anth ropogenic activities, the carbon and nitrogen cycles were not in stead y state prior to the Industrial Revolution and the beginning of import ant fossil fuel emissions to the atmosphere. In order to examine the r ole of the land-coastal margin system in global climate forcing and to assess changes in element cycling in response to natural and anthropo genic forcings, we have developed a conceptual model for the C-N-P-S b iogeochemical cycles in this critical ecological zone. To demonstrate the application of the model, we evaluate the behavior of the coupled CN-P cycles in the land-coastal margin system in response to a sustain ed global temperature perturbation on terrestrial denitrification. Mod el results show that the nutrient-limited coastal margin is extremely sensitive to changes in the dissolved and particulate organic matter l oading from land via the rivers. These results suggest that perturbati ons affecting the terrestrial denitrification flux are amplified rathe r than attenuated in the coastal margin. In a global warming scenario with enhanced terrestrial denitrification, the coastal margin becomes more autotrophic relative to its present heterotrophic status. In cont rast, in the case of a cooling scenario with decreased denitrification fluxes on land, the coastal margin becomes more heterotrophic relativ e to its initial status. For the +10 degrees to -10 degrees C range of temperature perturbations simulated by our model, net ecosystem metab olism of the coastal margin varied by a factor of 34, from about +0.5 x 10(12) to -17 x 10(12) moles of carbon per year. It is evident that the coastal margin trophic status is principally governed by changes i n the flux of organic matter from land. The effect of changes in the r iverine flux of inorganic nutrients to the coastal ocean, which change s in the same direction as that of the riverine organic flux, is small compared to that of terrestrial organic matter loading. We argue that the effects of temperature change and of certain land-use activities, such as deforestation, are similar with respect to the nitrogen cycle in the coupled land-coastal margin system. Natural global temperature variations over the last 6000 years of Earth's history, including the relative warmth of the Holocene Climatic Optimum and the Medieval War m Period and the coolness of the Little Ice Age, probably led to chang es in denitrification fluxes on land and transport of organic matter a nd inorganic nutrients to the coastal oceans by rivers and groundwater . Pre-industrial anthropogenic activities on land probably enhanced th e delivery of organic and inorganic nitrogen and organic matter to the oceans. These variations in the riverine flux led to continuous chang e in the trophic status of the global coastal margin and in the air-se a exchange of CO2. The global analysis provides some insight into the direction of change but does not provide quantitative estimates of the magnitude of change. However, there is little doubt that on a global scale, downstream reservoirs of shallow groundwater, rivers and coasta l margins can be affected relatively rapidly by changes in terrestrial denitrification fluxes brought about by natural temperature variation s and land-use activities. The land-coastal margin system must be view ed as an entity in considerations of global environmental change.