A model of depressional wetland formation in low-relief karst landscapes

Abstract:

Karst landscapes are formed by the self-reinforcing dissolution of limestone and other soluble rocks, and these positive feedbacks can create a variety of landforms depending on initial topography, climate, bedrock characteristics, and potentially, the activity of biota. In Big Cypress National Preserve (BICY), a low-relief karst landscape in southwestern FL (USA), depressional wetlands, are interspersed within an upland matrix in a regular pattern. This landscape is characterized by over-dispersion of wetland patches, periodic variation in bedrock depth and soil thickness, and distinct bi-modality of these and other soil properties. We hypothesize that the structure of the BICY landscape reflects the concurrent effects of local positive feedbacks among hydroperiod, vegetation productivity and bedrock dissolution; these local processes may ultimately be constrained by landscape scale limitations of water volume. We further hypothesize that low relief and shallow water tables are essential boundary conditions for the emergence of regular patterning of wetlands. To explore these hypotheses, we have developed a quasi-spatial model of a single nascent wetland and its catchment, where the expansion of the wetland basin is driven by acidity associated with belowground root production and aquatic metabolism and their effects on carbonate mineral dissolution, and by the lateral and vertical discharge of water between wetlands and bedrock porosity. This model can, depending on boundary conditions, recreate a range of karst features, including vertical dissolution holes, extensive wetlands that overtake the entire basin, or smaller wetlands whose size equilibrates at a small proportion of the catchment area. This last endpoint, a landform similar to those observed in BICY, occurs only in response to relatively shallow water tables, limited hydrologic inputs, and strong positive feedbacks of biotic activity on dissolution.