Biogeochemical patchiness, geomorphic feedbacks, and flow connectivity in river-floodplain corridors

Abstract:

Ecogeomorphology is the understanding of the signature of life on landforms. The field has advanced primarily on the basis of studies of physical-biological feedbacks between vegetation, flow, and sediment transport. However, biogeochemistry also has the potential to exert strong feedback on life and landforms. This potential is particularly high in tropical and subtropical regions, where vegetation is often phosphorus (P)-limited. Aquatic landscapes are essentially closed systems for P—in contrast to nitrogen—and the transport of P primarily in particulate form can establish a tight feedback between biogeochemical and geomorphic processes in these regions. Here we examine mechanisms that can contribute to spatial patterning, or patchiness, in nutrient distributions and vegetated landforms.

We evaluated hypotheses for evapotranspiration focusing, differential hydrologic exchange, and particulate nutrient redistribution mechanisms to explain spatial patterns of P retention and function of the Everglades. Based on field measurements in sloughs and on slightly higher and more densely vegetated ridges and field-grounded mechanistic models, we quantified P fluxes attributable to the three mechanisms. Findings suggest that evapotranspiration focusing is not a driver of Everglades nutrient retention nor of ridge and slough patterning. Instead, differential hydrologic exchange, driven by different periods of groundwater-surface water connectivity across topographic elements, is the primary cause of elevated P concentrations on ridges and can completely explain interpatch differences in long-term P accumulation rates. With historical flow velocities, which were an order of magnitude higher than at present, particulate P redistribution would have further increased the interpatch difference in long-term P retention rates nearly twofold, with potential consequences for landscape pattern development. In conclusion, differential hydrologic exchange and particulate nutrient redistribution are the dominant drivers of nutrient patchiness in the Everglades and are hypothesized to be important in P-limited river and floodplain corridors globally.