Wetlands as Metabolic Gates

Abstract

The wetlands and littoral flora of lakes and the land-water interface regions of rivers must be viewed as metabolic regulators of nutrient and organic matter loading to recipient waters. The interface communities of emergent and submersed macrophytes function as an integrated ecosystem of high organic productivity that supports intensive metabolism by attached microbial communities. The latter microbiota operate as an integrated community to conserve nutrients by intensive recycling. Nutrients imported via external loading are incorporated to support new microbial and plant growth within spatial and temporal constraints on community development. Nutrient excesses are exported downgradient. Organic matter released from the interface ecosystem is largely recalcitrant dissolved organic matter that is slowly degraded in recipient waters. An important variable of wetlands in the Great Lakes is marked, short-term changes in water level. These hydrodynamic oscillations modify loading rates of nutrients and organic matter from wetlands. Long-term water level changes can alter the extent and development of land-water interface regions. Hydrology of the ecosystem overrides metabolic capacities at high discharge, but often can be effectively managed to maximize system retentive capacities. Wetlands occupy a very small proportion of the drainage basins of the upper Great Lakes with deep, open pelagic waters. The regulatory influences of wetlands to nutrient loading and as a source of DOM increase in the lower Great Lakes but are relatively small in comparison to those of smaller inland waters.