Multiple regime shifts in a subtropical peatland: community-specific thresholds to eutrophication

Abstract

Ecosystems have a natural resilience to perturbations, where resilience is the magnitude of a disturbance that an ecosystem can resist before changes in structure, function, and services result in a regime shift. The Everglades region of Florida, USA, has been detrimentally impacted by phosphorus (P) enrichment and a regime shift from Cladium (sawgrass) to Typha (cattail) marsh has been described. We examine another facet of the low-nutrient Everglades stability regime, open-water sloughs, to determine if eutrophication leads to similar regime shifts. We analyzed surface water P and soil P as controlling variables that, once a critical threshold is surpassed, alter ecosystem state variables. Nonlinear relationships between P and vegetation were observed along a northern Everglades eutrophication gradient. In addition to the Cladium-Typha regime shift, a second independent regime shift, slough-Typha, was identified. Synoptic surveys of 49 sloughs within the boundary between the slough and Typha regime revealed that surface water total phosphorus (TP) and the benthic algal floc layer (BAFL) were the controlling variables, with critical thresholds of 11 mu g/L and 412 mg/kg, respectively. The slough regime below these thresholds was characterized by calcareous periphyton (BAFL TP 298 mg P/kg; BAFL calcium 149 g Ca/kg). Above the TP thresholds, vegetation composition shifted to open-marsh species with significantly higher BAFL TP (700 mg P/kg) and total organic carbon (TOC) ( 350 g C/kg). A second BAFL TP threshold occurred at 712 mg P/kg, above which Nymphaea dominated and BAFL TP ( 1034 mg P/kg) and TOC (417 g C/kg) significantly increased. Nymphaea sloughs transitioned to the Typha regime. The boundary reflects the loss of ecosystem resilience due to eutrophication. Both low-nutrient stability regimes (slough and Cladium) lie precariously close to the P critical threshold but differ in how eutrophication is absorbed and resisted. The slough regime transitions rapidly through a series of ecosystem state changes linked to positive feedback loops that affect P dynamics, whereas the Cladium regime does not. An adaptive management strategy has been implemented to address the surface water TP threshold; however, to ensure successful restoration of the Everglades, the BAFL and soil TP thresholds also need to be considered.