Marine pelagic ecosystem models are fundamentally similar to those that are employed in freshwater systems, that is, they are constructed as a series of ordinary differential equations with source and sink terms that can be used to determine time-dependent changes in both living and nonliving components of marine ecosystems, like nutrients, phytoplankton, zooplankton, and detritus. These models vary tremendously in complexity and they employ a variety of different linear and nonlinear functions to model source terms (e.g., nutrient uptake and light-response of phytoplankton) and sink terms (e.g., zooplankton grazing impacts, export, and top-down control). Higher trophic levels are often parametrized in these models, that is, they are not represented explicitly, although there are examples of marine ecosystem models that include and even focus on higher trophic level interactions. These higher trophic level models have been used to study, for example, impacts of top-down control and multispecies interactions for marine resource management applications. At the other end of the size spectrum, representations of bacteria, DOM cycling, and virus impacts have been incorporated into marine ecosystem models, but these are still highly simplified. There have also been recent advances in the representation of biogeochemical functional groups, but these are still incomplete, focusing on autotrophs and only a few representative species. Benthic processes and seston/sediment loads have a strong influence on ecosystem dynamics and biogeochemical cycles in shallow coastal and estuarine systems. A variety of models have been developed that can account for these processes, but incorporating dynamic models of seston and sediment transport is still a significant challenge. 3D-coupled ecosystem-hydrodynamic models have been applied over scales ranging from small estuaries to global applications. Some of these models have become very complex. It appears that complex ecosystem models can provide improved skills as long as they are properly constrained with data, but there may be limits to the amount of complexity that can be gainfully added. Regardless, it is important to keep in mind in 3D modeling applications that the physical model provides first-order controls on ecosystem model response, yet very few studies have been conducted to assess the impact of different physical modeling approaches on ecosystem solutions.

See also: Hydrodynamic Models; Lake Models.

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