A delicate balance : Ecohydrological feedbacks governing landscape morphology in a lotic peatland

The Everglades ridge and slough landscape is characterized by elevated sawgrass ridges regularly interspersed among lower and more open sloughs that are aligned parallel to the historic flow direction. Landscape degradation, characterized by topographic flattening, has coincided with a century of drainage, levee construction, nutrient enrichment, and flow reductions. Here we develop a conceptual model of Everglades landscape dynamics based on a literature synthesis and supported by the numerical model PeatAccrete. We propose that two feedback mechanisms govern landscape characteristics. The first, simulated with PeatAccrete, involves differential peat accretion governed by water level and phosphorus concentration, leading to the attainment of an equilibrium ridge elevation relative to slough. Differential peat accretion, however, cannot produce a characteristic ridge width or landscape wavelength. Instead, we propose that feedback between channel morphology and sediment mass transfer controls lateral and longitudinal topographic features, consistent with processes in anabranching rivers. This sediment transport feedback was critical in pattern initiation and evolution, and sediment redistribution from slough to ridge provides a plausible mechanism for preventing gradual ridge expansion. However, PeatAccrete model results show that, in the absence of sediment transport, ridges expand only on the order of meters per century. This result suggests that a combination of factors has driven the widespread disappearance of sloughs over the past century, including altered vertical peat accretion rates that lead to slough infilling. Sensitivity tests indicated that changes in duration and depth of surface water inundation, phosphorus supply, and redox potential have altered differential peat accretion rates in a way that favors topographic flattening. These factors are relatively well defined compared with the role of sediment transport, which requires further quantification. Because both positive and negative feedback processes interact in the Everglades, the trajectory of landscape evolution in time will depend upon current conditions, with areas of remnant ridge and slough topography being more likely than areas of degraded topography to respond to changes in water management in ways that enhance landscape heterogeneity over human timescales. Dual feedbacks between peat accretion and sediment transport are likely important controls on landscape evolution in low-gradient peatlands worldwide with pulsed, unidirectional flow.