Interaction of hydrology and nutrient limitation in the Ridge and Slough landscape of the southern Everglades

Abstract

Extensive portions of the southern Everglades are characterized by series of elongated, raised peat ridges and tree islands oriented parallel to the predominant flow direction, separated by intervening sloughs. Tall herbs or woody species are associated with higher elevations and shorter emergent or floating species are associated with lower elevations. The organic soils in this ''Ridge-and-Slough'' landscape have been stable over millennia in many locations, but degrade over decades under altered hydrologic conditions. We examined soil, pore water, and leaf phosphorus (P) and nitrogen (N) distributions in six Ridge and Slough communities in Shark Slough, Everglades National Park. We found P enrichment to increase and N to decrease monotonically along a gradient from the most persistently flooded sloughs to rarely flooded ridge environments, with the most dramatic change associated with the transition from marsh to forest. Leaf N:P ratios indicated that the marsh communities were strongly P-limited, while data from several forest types suggested either N-limitation or co-limitation by N and P. Ground water stage in forests exhibited a daytime decrease and partial nighttime recovery during periods of surface exposure. The recovery phase suggested re-supply from adjacent flooded marshes or the underlying aquifer, and a strong hydrologic connection between ridge and slough. We therefore developed a simple steady-state model to explore a mechanism by which a phosphorus conveyor belt driven by both evapotranspiration and the regional flow gradient can contribute to the characteristic Ridge and Slough pattern. The model demonstrated that evapotranspiration sinks at higher elevations can draw in low concentration marsh waters, raising local soil and water P concentrations. Focusing of flow and nutrients at the evapotranspiration zone is not strong enough to overcome the regional gradient entirely, allowing the nutrient to spread downstream and creating an elongated concentration plume in the direction of flow. Our analyses suggest that autogenic processes involving the effects of initially small differences in topography, via their interactions with hydrology and nutrient availability, can produce persistent physiographic patterns in the organic sediments of the Everglades.

Across-scale patterning of plant-soil–water interactions surrounding tree islands in Southern Everglades landscapes

“…Recent research supports the assumption that N and P resource islands play a sink/source role in landscape-scale biogeochemical cycling (e.g. Jayachandran et al 2004;Ross et al 2006;Wetzel et al 2005Wetzel et al , 2009Givnish et al 2008), but the spatial patterns of resource island gradients have not yet been fully described.…”

“…Landscape features within the Ridge and Slough peatlands are arranged parallel to the predominant historical direction of hydrologic flow (Ross et al 2006). Lower slough communities have relatively long hydroperiods, inundated approximately 11 months per year, and are therefore the main conduits for water flow through the Everglades.…”

“…Previous studies have characterized nutrient dynamics within Everglades communities and/or landscapes (e.g. Orem et al 2002;Jayachandran et al 2004;Ross et al 2006;Wetzel et al 2009), but few have employed a multi-scale approach. In this study we aimed to analyze the spatial variability and patterning of nutrients in relation to cross-scale site parameters within and among two hydrologically defined Everglades landscapes; the freshwater Marl Prairie and the Ridge and Slough.…”

Coupled local facilitation and global hydrologic inhibition drive landscape geometry in a patterned peatland

“…Moreover, it has been shown elsewhere that different combinations of processes may generate similar patterns (Eppinga et al, 2009), making inference of a single dominant mechanism challenging. Models of ridge-slough pattern genesis have invoked differential sediment transport Lago et al, 2010), nutrient redistribution (Ross et al, 2006), and biased subsurface flow induced by anisotropic hydraulic conductivity (Cheng et al, 2011) as driving mechanisms. Using a process-based numerical model, Ross et al (2006) showed that differential evapotranspiration rates in higherelevation soils may lead to the concentration of dissolved nutrients (particularly phosphorus), suggesting that this nutrient redistribution, alone or in combination with sediment and nutrient transport, may generate the ridge-slough pattern in the Everglades.…”

Nutrients and Hydrology Indicate the Driving Mechanisms of Peatland Surface Patterning

“…In arid ecosystems, similar patterns are called labyrinths (Rietkerk et al 2002). Peatland maze patterning can be induced by nutrient accumulation under ridges, which is driven by increased evapotranspiration rates by vascular plants (especially shrubs and trees) that grow on these ridges (Rietkerk et al 2004a;Wetzel et al 2005;Ross et al 2006). This structuring mechanism would imply that because of higher evapotranspiration rates, there is a net flow of water and dissolved nutrients toward ridges.…”