Effects of water level fluctuations on phosphorus, iron, sulfur, and nitrogen cycling in shallow freshwater ecosystems

EFFECTS OF WATER LEVEL FLUCTUATIONS ON PHOSPHORUS, IRON, SULFUR, AND NITROGEN CYCLING IN SHALLOW FRESHWATER ECOSYSTEMS By Lauren Kinsman-Costello Wetlands are often conserved, enhanced, restored, and constructed to provide ecosystem services, particularly water quality improvement and biodiversity support. Although wetlands are effective at removing some pollutants, like excess nitrogen, wetland sediments vary in their capacity to retain phosphorus (P). In addition, natural chemical stressors that tend to accumulate in wetland sediment pore waters, at times to toxic concentrations, may limit a restored wetland’s ability to support biodiversity. I investigated how the variable hydrology of shallow freshwater ecosystems influences their functioning, specifically P exchange between sediments and water, and concentrations of natural stressors. In recent decades, re-flooding historically drained areas has become common in an attempt to regain lost wetland habitat and services. In a case study in southwest Michigan, restoring wetland hydrology to historically drained land caused rapid release of large amounts of inorganic P from sediments to surface waters. Prolific growth of filamentous algae and duckweed ensued, even after available P concentrations had become lower. These observations demonstrate that when restoring wetlands by reflooding historically drained areas, managers should consider the potential for sediment P release to jeopardize restoration goals. Net sediment-water P exchange is controlled by several biogeochemically distinct processes, all of which are controlled to varying degrees by sediment moisture and oxygen conditions. To better understand how hydrology and sediment biogeochemistry interact to influence net sediment-water P exchange in sediments from 16 biogeochemically diverse ecosystems, we temporarily desiccated and re-flooded sediment-water microcosms and compared sediment P release to continuously flooded controls. The effects of hydrologic regime on both the direction and magnitude of P exchange depended significantly on sediment identity, and treatment effects on P release to pore and surface waters differed. Ten of the 16 temporarily desiccated sediments released more P into pore and/or surface waters than continuously inundated references of the same sediment types. Potentially toxic levels of three naturally occurring chemical stressors (sulfide, ammonia, and iron) are prevalent in freshwater sediments, yet their roles in shaping ecosystem structure and function may be overlooked. To assess the prevalence of toxic levels of sulfide, ammonia, and iron, we sampled sediments, pore waters, and surface waters from 42 locations across 24 shallow (< 2 m deep) freshwater ecosystems in southwest Michigan and compared our measured concentrations to water quality criteria established by the U. S. Environmental Protection Agency (EPA) and to toxic thresholds in the published literature. The benthic environment of almost every freshwater ecosystem we measured was theoretically toxic or stressful to some component of aquatic life in some area or at some time. Organismal tolerances to chemical stressors vary, so the toxicant concentrations that we measured are likely shaping benthic ecological communities and influencing rates of ecosystem function.