DEVELOPMENT OF A GENERAL ECOSYSTEM MODEL FOR A RANGE OF SCALES AND ECOSYSTEMS

We have developed a General Ecosystem Model (GEM) that is designed to simulate a variety of ecosystem types using a fixed model structure. D riven largely by hydrologic algorithms for upland, wetland and shallow -water habitats, the model captures the response of macrophyte and alg al communities to simulated levels of nutrients, water, and environmen tal inputs. It explicitly incorporates ecological processes that deter mine water levels, plant production, nutrient cycling associated with organic matter decomposition, consumer dynamics, and fire. While the m odel may be used to simulate ecosystem dynamics for a single homogenou s habitat, our primary objective is to replicate it as a ''unit'' mode l in heterogeneous, grid-based dynamic spatial models using different parameter sets for each habitat. Thus, we constrained the process (i.e ., computational) complexity, yet targeted a level of disaggregation t hat would effectively capture the feedbacks among important ecosystem processes. A basic version was used to simulate the response of sedge and hardwood communities to varying hydrologic regimes and associated water quality. Sensitivity analyses provided examples of the model dyn amics, showing the varying response of macrophyte production to differ ent nutrient requirements, with subsequent changes in the sediment wat er nutrient concentrations and total water head. Changes in the macrop hyte canopy structure resulted in differences in transpiration, and th us the total water levels and macrophyte production. The GEM's modular design facilitates understanding the model structure and objectives, inviting variants of the basic version for other research goals. Impor tantly, we hope that the generic nature of the model will help allevia te the ''reinventing-the-wheel'' syndrome of model development, and we are implementing it in a variety of systems to help understand their basic dynamics.