CHANGES IN THE FUNCTIONING OF WETLANDS ALONG ENVIRONMENTAL GRADIENTS

One of the prevalent gradients in wetlands is the continuum of depth a nd frequency of flooding. While much emphasis has been placed on the i mportance of hydrology as a driving force for wetlands, few other pers pectives 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 transitio ns that occur within a wetland or group of similar wetland types and ( 2) resource-based transitions that allow comparisons of the flow of wa ter and processing of nutrients among very different wetland types. La ndscape-based continua include the transition from upstream to downstr eam in riverine wetlands and between aquatic and terrestrial ecosystem s within a wetland. Along the upstream-downstream continuum, sources o f flood-water delivery change dominance from ground-water discharge an d overland runoff, as in low order streams, to dominance by overbank f looding, as in high order streams. With increasing size, properties re lated to the aquatic-to-terrestrial transition are replaced by propert ies related to wetland-atmospheric exchanges and by landscape maintena nce, the latter not normally acknowledged as a wetland function. Resou rce-based continua include the extremes of (1) sources of water to wet lands (precipitation, overland flow, and ground water) and (2) the var iation in inflows and outflows of nutrients and sediments. Emphasis on water source forces consideration of controls beyond the wetland's bo undaries. A broader view of biogeochemical functioning is gained by ca tegorizing wetlands into groups based on the exchange of nutrients and sediments among landscape units rather than on serving as a sink or s ource for a particular element. Based on this analysis, the less frequ ently flooded or saturated portions of wetlands are no less functional ly active than wetter portions; the functions are simply different. Ef forts to classify wetlands according to their hydroperiod do little to reveal their fundamental properties.