Landscapes are shaped by mechanical and chemical weathering, and the erosion, transport, and deposition of resulting particulate and dissolved material, all of which processes may be influenced by biota. These biogeomorphic interactions may be particularly important in karst landscapes, where biota may exert strong control on both weathering and transport of highly soluble bedrock.
We use Big Cypress National Preserve (BICY; S. Florida, USA) as a case study to understand how coupled hydrologic, ecological, and biogeochemical processes influence karst landscape evolution. BICY is a large, flat (2cm/km), rain-fed mosaic of pine uplands, hammocks, cypress strands, and isolated wetland dominated by cypress. The landscape is characterized by isolated, regularly-sized and -spaced wetland depressions. The specific questions we address in this project are (1) what biotic and physical feedbacks, if any, constrain the stable equilibrium size of the wetland depressions? (2) How do boundary conditions surface and groundwater drainage shape the morphology of wetland basins?
To address these questions we construct a process-based model, which includes (1) hydrological fluxes (i.e., ET, groundwater drainage, surface drainage, and local groundwater exchange), (2) geochemical reactions (i.e., dissolution of calcite), and (3) biological processes (i.e., respiration, producing of biological acidity, plant growth, and transpiration).
Results/Conclusions
In the absence of biological processes, the fate of karst depressions is either to expand indefinitely or, under some hydrologic conditions, to recede toward upland elevations. Only when evapotranspiration and biological acid production are included do individual basins can reach stable sizes. Rather than a unique stable size for all the basins, our model suggests that basins stabilize can stabilize across a range of radius and depth combinations.
Biological processes stabilize the basins via several negative feedbacks: (1) the nonlinear relationship between biomass accumulation and basin depth, i.e., reduced growth rate and increased mortality when the basin depth exceeds a certain value, and (2) the soil development by organic matter accumulation within the basin limits the basin’s capacity to export Ca2+ via surface drainage, a primary outlet for Ca2+export from a wetland basin.
Lastly, we find that the relative rates of surface drainage, groundwater drainage, and ET control stable basin sizes. Rapid groundwater drainage leads basins to stabilize at a deeper level. When ET is rapid and exceeds the rate of Ca2+ export to groundwater, the system stabilizes at a shallower depth.