Release of phosphorus (P) from aquatic sediments is critical in regulating the growth of algae and aquatic plants. Sediment release of P is referred to as internal eutrophication because of the pivotal role of P as a limiting nutrient. Recent research has suggested that internal eutrophication involves complex biogeochemical processes and is accelerated by moderate concentrations of sulfate, a pollutant on the increase in many areas. Yet interestingly, the effects of sulfate can be reversed if sediments are allowed to dry periodically. This research seeks to understand the biogeochemistry of sediment P release, with an emphasis on how it is affected by water level stabilization vs. fluctuation. New linkages between sediment P release, the forms of iron and sulfur in sediments, and the role of periodic desiccation will be explored, focusing on a lake system regulated by a weir.
This research has broader impacts because internal eutrophication is fundamental to managing lakes and wetlands: it can cause eutrophication, stymie restoration efforts, and delay recovery after nutrient loading is reduced. Water level regimes are altered by a myriad of human actions including impoundments, augmentation wells, agricultural land drainage, water withdrawals, and climate change. It is important to understand how these changes in water level regimes impact eutrophication, and thus affect ecosystem services and values provided by lakes and wetlands.
Phosphorus is a nutrient required by all living things, but its widespread use in fertilizers and other products has increased its occurrence to polluting levels in many agricultural and urban watersheds. Excess phosphorus in water bodies often causes eutrophication – undesirably high growth of algae and aquatic plants. Eutrophication is a pervasive problem in populated or agricultural regions such as the U.S. and Europe, and can impact aquatic ecosystems through blooms of toxic phytoplankton, reduced water clarity, changes in plant communities, and fish kills. Due to the ability of their sediments to accumulate phosphorus, wetlands often retain phosphorus and prevent its entry into downstream ecosystems. However, the ability of wetland sediments to bind phosphorus varies, and at times wetlands can become sources of, rather than sinks for, phosphorus. The ways in which phosphorus binds to and is released from sediments has been studied extensively in deep lakes, but less so in shallow wetlands. Shallow wetlands differ from lakes in that they often dry and reflood and tend to have sediments with more organic matter (decomposing plant material). This study investigated how water level fluctuations influence phosphorus binding and release in wetland sediments of varying characteristics. Experimental wetting and drying of sediments from a variety of wetlands and shallow lakes in southwestern Michigan, in combination with surveys of sediment and water chemistry, showed the variable behavior of sediment phosphorus release. This behavior can be predicted to some extent from the chemistry of the sediments, which in turn is related to local geology and hydrology (i.e., the sources and fluctuations of water supplied to a water body). Two created wetlands were also studied: 1) a stormwater treatment wetland in an urban area that has unexpectedly released phosphorus rather than retaining it; and 2) a wetland restoration project on military land that entailed reflooding historically drained land that had once been used for farming. In both cases the change in flooding regime invoked large releases of phosphorus to the water column. A literature review revealed a wide array of studies, many conducted in Europe as well as in the U.S., that show how sediment phosphorus storage can represent an important reservoir, potentially releasing phosphorus to water bodies for decades after new inputs are reduced. The findings of this research help inform understanding of how sediment-water exchanges of phosphorus control algal and plant growth in freshwater ecosystems, which has direct applications to management of many freshwater ecosystems. Improving our ability to predict sediment phosphorus retention and release would help managers who manipulate lake and wetland water levels avoid undesirable phosphorus release and/or encourage greater phosphorus retention, thereby protecting downstream waters from eutrophication. In addition to management implications of the research, there have been educational benefits as well. Two graduate students and a postdoctoral research associate have been trained, and a number of undergraduate students have participated in summer research experiences. The graduate students have developed K-12 curricula aimed at teaching the definition, ecology, and functional roles of wetlands and wetland sediment. Lesson plans have been developed through a NSF GK-12 project, in partnership with teachers at Lawton Community Schools in Southwest Michigan, and are being taught to students there with direct assessment of student learning. Assessments of student learning from lesson plans helped refine them and the ultimate goal is their publication in education literature.
