Human activity has greatly increased the amount of nutrients, such as nitrogen and phosphorus, applied to the global landscape. Much of this nutrient load is transported to coastal areas by streams and rivers. However, streams are not passive conduits but are sites of significant nutrient transformation. Nutrients may enter streams in dissolved inorganic forms, are transported a certain distance by water flow, and are eventually transformed into organic forms by stream organisms, which need these chemicals for maintenance and growth. This research will focus on the final stage of this cycle where stream organisms, such as bacteria, fungi, and insects, return excess nutrients to the water. Specifically, this research will determine the effect of increased nutrient input on nutrient transformations by stream organisms and their ability to influence the transport of nutrients downstream. The investigators will use a stable isotope of nitrogen to measure several nutrient processing metrics in both field and laboratory settings. This combined approach will allow for the description of broad patterns across time and space as well as the determination of specific mechanisms responsible for the observed patterns.
The increased nutrient loading occurring around the globe is causing a suite of environmental problems including declines in biodiversity, contamination of drinking water, and anoxia of coastal waters. The role of streams in transporting these excess nutrients from terrestrial to marine systems makes understanding the fate of nutrients within streams vital to policy makers attempting to mitigate the negative effects of nutrient loading. The temporal and spatial scope of this research will help fill a gap in the understanding of stream nutrient cycling and will add to the body of knowledge available to policy makers. This research also provides opportunities for undergraduate students to participate in the scientific process and contribute to the understanding of a relevant environmental issue.
Freshwater ecosystems provide many services including organic matter decomposition and nutrient transport and transformation. The organisms responsible for these services include bacteria and fungi. Responses of these organisms to changes in the environment, including increased nitrogen deposition, affect how well they continue to provide these services. This study investigated several factors that influence microbial function in stream ecosystems. Leaves that fall into stream are colonized by fungi and bacteria which use the carbon in the leaves to grow. This causes the leaves to decompose. These microbes also require other elements such as nitrogen to support growth. Our results show that microbes decomposing leaves in streams get their nitrogen from both the water and from the leaf itself and that the relative importance of these two nitrogen sources changes depending on how long the leaves have been in the water. We have also shown that animals that eat these microbes can affect how fast they decompose leaves. These results suggest that factors such as nutrient availability and predators can influence the rate at which carbon breaks down in stream ecosystems. Our project also generated empirical data that can be used to generate simulations of nitrogen and carbon transport through ecosystems. The design and implementation of this project provided several graduate and undergraduate students with hands-on experience in scientific research. At least two of the undergraduate students involved in this research have continued their education in ecology by attending graduate school.
