Changes in the functioning of wetlands along environmental gradients

Abstract

One of the prevalent gradients in wetlands is the continuum of depth and frequency of flooding. While much emphasis has been placed on the importance of hydrology as a driving force for wetlands, few other perspectives have emerged to demonstrate unifying patterns and principles. In contrast to the wetness continuum, the functioning of wetlands can be separated into two broad categories: (1) landscape-based transitions that occur within a wetland or group of similar wetland types and (2) resource-based transitions that allow comparisons of the flow of water and processing of nutrients among very different wetland types. Landscape-based continua include the transition from upstream to downstream in riverine wetlands and between aquatic and terrestrial ecosystems within a wetland. Along the upstream-downstream continuum, sources of flood-water delivery change dominance from ground-water discharge and overland runoff, as in low order streams, to dominance by overbank flooding, as in high order streams. With increasing size, properties related to the aquatic-to-terrestrial transition are replaced by properties related to wetland-atmospheric exchanges and by landscape maintenance, the latter not normally acknowledged as a wetland function. Resource-based continua include the extremes of (1) sources of water to wetlands (precipitation, overland flow, and ground water) and (2) the variation in inflows and outflows of nutrients and sediments. Emphasis on water source forces consideration of controls beyond the wetland's boundaries. A broader view of biogeochemical functioning is gained by categorizing wetlands into groups based on the exchange of nutrients and sediments among landscape units rather than on serving as a sink or source for a particular element. Based on this analysis, the less frequently flooded or saturated portions of wetlands are no less functionally active than wetter portions; the functions are simply different. Efforts to classify wetlands according to their hydroperiod do little to reveal their fundamental properties. © 1993 Society of Wetland Scientists.