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The Interface Between Biological and Physical Processes Mark Abbott* New approaches to ecosystem modeling The present structure of our nitrogen/phytoplankton/zooplankton (NPZ) models has been unchanged for over 60 years. Although we have added more components, the basic model assumes that everything can be based on a single element (nitrogen) and that the basic interactions are analogous to chemical reactions where the components can be repre- sented as continuous fields of reactants. The models are basically plumb- ing with various reservoirs and complex functions that represent the flow of nitrogen from one reservoir to another. As our knowledge of eco- systems has improved, we have added more reservoirs (e.g., adding the microbial loop, accounting for fixation of nitrogen), but once we establish the basic structure of the ecosystem, then it is difficult to model how the system will adapt to changes in environmental forcing. Doney et al. (2004) proposed that a new class of models be developed that are based on eco- system functions rather than trophic levels. Such functions could include processes such as nitrogen fixation, recycling, etc., where the structure would resemble models of genetic regulation that link environmental conditions with the expressions of particular genes (or in this case, func- tions). For example, could we express the export of surface carbon as a function that would be triggered by spring bloom conditions? If so, what *âCollege of Oceanic and Atmospheric Sciences, Oregon State University 121
122 OCEANOGRAPHY IN 2025 are the environmental processes that initiate blooms? Such new modeling approaches could lead us beyond the problems of parameter estimation and model tuning which are inhibiting the development of models that represent the complexity of ocean ecosystems, especially in the context of climate change. Coupling of vertical velocities to nutrient fluxes Most circulation models have difficulties simulating vertical velocity. Because of a range of factors, model estimates tend to be smoother than the observations as conditions of strong vertical velocity are restricted to small scales in time and space. For calculations of physical quanti- ties such as heat and momentum fluxes, highly smoothed estimates are likely acceptable. However, for biogeochemical processes such as nutri- ent uptake and photosynthesis, the nonlinear nature of these processes amplifies the response, such that small changes in the predicted light field have significant biological impact. Thus errors in vertical velocities that may be small and inconsequential in the context of physical processes may have enormous impacts on biogeochemical and ecological models. Regions such as the Polar Front are dominated by localized upwelling and downwelling, and incorporating these processes in coupled models will be a significant challenge over the next 20 years. Uptake of CO2 by the ocean Most biogeochemical models assume that the ocean will continue to take up and sequester about two gigatons/year of atmospheric CO 2. However, ocean CO2 uptake may diminish in response to changes in ocean pH and in climate forcing. For example, phytoplankton blooms might be less frequent in a warmer ocean, slowing down an important pathway for the downward flux of organic carbon. Because of the com- plexity of the relationships between climate forcing, biogeochemistry, and ecosystem response, this question will be an important issue for the next 20 years. Mechanisms underlying climate oscillations Coupled models still have difficulties reproducing major oscillations in the ocean/atmosphere system such as the ENSO, the Pacific Decadal Oscillation (PDO), and the North Atlantic Oscillation (NAO). As these processes can serve as natural laboratories to observe responses of ocean ecosystems to changes in physical forcing, it is important that our models begin to capture these important components of the Earth system.
Mark Abbott 123 The role of ârareâ species In any sample of surface ocean water, the phytoplankton biomass is dominated by a handful of species with the rest of the sample composed of species that are represented by only a few individuals. These species never dominate the phytoplankton community, instead only occurring in small numbers. We do not understand their role in the ecosystem or bio- geochemical cycling. Moreover, we do not understand how they persist in oligotrophic environments where there may be significant competition for nutrients. Impacts of geoengineering With increasing economic and political pressures for carbon seques- tration (e.g., cap and trade for carbon) as well as climate change mitiga- tion, there may be both government and private sector efforts to fer- tilize the upper ocean through iron additions, artificial upwelling, etc. At-sea aquaculture could also be considered a type of geoengineering. The point is that the oceans will become more âmanagedâ rather than simply exploited. Reference Doney, S.C., M.R. Abbott, J.J. Cullen, D.M. Karl, and L. Rothstein. 2004. From Genes to Ecosystems: The Oceanâs New Frontier. Frontiers in Ecology and the Environment. 2: 457-466.