Changes in Land Use and Water Cycle Influence Nutrient Export in Northeastern Streams

Human changes on the land can have profound impacts on nearby waterways. Specifically, land use and land cover (LULC) changes alter the flow of water across landscapes and can increase nutrient runoff into surface water (lakes and rivers) which has negative effects on water quality. Excess nutrients, such as phosphorus and nitrogen, can cause eutrophication of freshwaters and can stimulate excessive algal growth, stripping the water of oxygen and harming other aquatic life. Human alterations to the land and climate have changed the distribution and flow of nutrients into waterways, in some cases exacerbating negative effects on water quality.

Land use and land cover changes impact the supply and transport of nutrients, affecting the biogeochemistry of waterways by altering the magnitude, timing, and form of exported nutrients. With the onset of climate change, the northeastern United States is expected to experience higher temperatures, more extreme precipitation events, less snow, and more winter rain, all of which will affect the regions’ water cycle and the supply and transport of nutrients to watersheds.

University of Vermont and Vermont EPSCoR Post-Doctoral Associate Erin Seybold and colleagues conducted a two-year study in seven watersheds throughout the northeastern U.S. to investigate the export of two nutrients, dissolved organic carbon and nitrate. The research team used electronic sensors in streams in three different land use types (forested, agricultural, and urban watersheds) to study the impact of LULC and hydrologic variability on dissolved organic carbon and nitrate export.

Researchers studied seven small streams with varying LULC (forested, agricultural, and urban) and climate in Delaware, Rhode Island, and Vermont. Data used for this particular study was collected from 2015-2016 using the in-stream sensors to monitor dissolved organic carbon and nitrate concentrations, stage (water level), and discharge, or stream flow. Precipitation data was also monitored using in-stream sensors or preexisting devices at the sites.

In Delaware, the most temperate location, researchers monitored one forested stream. In Rhode Island, where sites received an even distribution of precipitation throughout the year, they monitored one forested stream, one agricultural, and one urban. In Vermont, where sites were most impacted by snowfall, they monitored three streams within the Lake Champlain watershed: one forested (Wade Brook), one agricultural (Hungerford Brook), and one urban (Potash Brook).

Seybold and colleagues found that at six out of the seven sites, 2015 was a wetter year than 2016. This variation in rainfall was important for explaining interannual variation in the cumulative volume of runoff, nitrate, and dissolved organic carbon from all sites. Simply put, export in wet years was greater than export in dry years due to enhanced transport of nutrients to the stream network. They also found that, in general, the greatest amount of dissolved organic carbon and nitrate export came from agricultural watersheds, followed by urban and then forested watersheds, highlighting a strong influence of land use on nutrient export.

Across all three states, the timing of nutrient export in forested watersheds was similar and tended to correspond to early season fluxes prior to the emergence of leaves, particularly for nitrate export. In Delaware and Vermont, after the spring flux of nitrate, a plateau in nitrate export seemed to correspond to the date of leaf emergence. Export patterns differed slightly in Rhode Island, with mid-season fluxes influencing dissolved organic carbon and nitrate yield in 2015, and late-season fluxes corresponding with leaf-fall having more of an impact in 2016.

Nitrate and dissolved organic carbon export from urban sites in Rhode Island and Vermont were not affected by season, and nutrient export was distributed evenly throughout the year. Researchers suggest this is due to the dynamics of engineered urban systems superseding natural variation in nutrient cycling. Agricultural sites in these states had the greatest interannual and across-site variability. Sites in Rhode Island exhibited greater fluxes of dissolved organic carbon and nitrate in the fall after harvesting and tilling, whereas sites in Vermont displayed greater fluxes of dissolved organic carbon and nitrate in the late spring and early summer. Nutrient export from agricultural sites closely aligned to cumulative water export, and researchers suggest this could be due to altered landscapes preventing soils from holding water as effectively as natural systems and allowing for nutrient export with water runoff.

Overall, this study demonstrates that LULC and hydrologic variability both have great influence on the supply and transport of nutrient export in watersheds. Those landscapes with greater human alterations are those that are the greatest sources of dissolved organic carbon and nitrate to downstream waters. Additionally, due to the influence of hydrologic drivers on nutrient export, this research suggests greater mobilization of nutrients with climate change, particularly in the northeastern U.S. where the region will face increased intense precipitation events, warmer weather patterns, less snowfall, and more winter rainfall.