Multi-stable isotope tracing of elevated sulfate export from a forested headwater wetland following an induced flood pulse event

Headwater wetlands are significant in the conservation of water and water quality for downstream aquatic ecosystems. During flooding events following periods of drought in many headwater wetlands, significant quantities of sulfate (SO₄^2-) can be exported which impacts downstream surface water quality. Such periodic flood pulses of SO₄^2- are rarely studied in detail but are a suggested cause of prolonged freshwater eutrophication. Flood pulsing events are exacerbated by the increasing severity and frequency of summer droughts along with episodic flooding. Specifically, many wetlands are fed by water from upstream reservoirs which can control wetland-stream and groundwater-surface water interactions that can impact downstream water quality. In this study, an induced flood pulse event of a forested wetland (Beverly Swamp) was arranged by scheduling an upstream reservoir drawdown event (Valens Reservoir). This orchestrated flooding event exhibited a seven-fold increase (38 to 270 mg L^-1) in surface water SO₄^2- concentrations discharging from the wetland. To elucidate the source and release time lag of these elevated SO₄^2- concentrations, high resolution surface water sampling and discharge monitoring were coupled with stable isotopes of S (δ³⁴S-SO₄^2-) and water (δ¹⁸O-H₂O and δ²H-H₂O) along with scanning electron microscopy (SEM) analysis of the peat samples. Following the flood, a considerable increase in surface water SO₄^2- concentrations was observed at sampling locations with decreases in δ³⁴S-SO₄^2- values which is characteristic of oxidation following dissimilatory sulfate reduction (DSR). This indicates prior DSR resulted in the precipitation of mineral sulphides in the upper peat layers. During the pre-flood drought period, sulphides were re-oxidised to SO₄^2- and flushed from the wetland during flooding. δ¹⁸O-H₂O values of surface and sub-surface peat soil waters reflected the horizontal and vertical travel time of reservoir water flooding the wetland and flushing SO₄^2- from the system. Surface water acidity increased shortly after the SO₄^2- pulse, but within a short time rebounded due to carbonate buffering capacity within the peat. A conceptual model of the SO₄^2- budget indicated ∼ 68.7 kg SO₄^2- ha−1 d^-1 was released from Beverly Swamp during the post flood period. This study shows that Beverly Swamp contains significant stores of reduced sulfur (S), including iron sulfides. These reduced S stores will sustain recurrent release of particularly large quantities of SO₄^2- through redox cycling to the downstream littoral wetlands of Lake Ontario.