Changes in nitrogen loading to the Northeast Creek Estuary, Bar Harbor, Maine, 2000 to 2010

Since 1999, the U.S. Geological Survey and the National Park Service have been monitoring land use and nitrogen loading in a 26.3-square-kilometer (10-square-mile) estuarine watershed at Acadia National Park, Mount Desert Island, Maine. The initial study linking land use and nitrogen loads entering the Northeast Creek estuary was completed in 2000, and findings were used to develop simulations of nitrogen loading to the estuary, thereby helping to inform local land-use planning decisions. At that time, the amount of nitrogen entering the Northeast Creek estuary was relatively small, and no evidence of nutrient-related degradation was observed in the Ruppia-dominated estuarine ecosystem. A new round of water-quality monitoring and streamflow measurements was conducted to determine nitrogen loading from 2008 to 2011 as a means to evaluate the effects of increased rural residential housing within the watershed since 2000. On the basis of a 2.6-percent increase in residential-housing land use in the watershed from 2000 to 2010, simulations of nitrogen export predicted a 7-percent increase in nitrogen loading to Northeast Creek. The measurement-based loads estimated for the Northeast Creek tributaries, however, increased much more than predicted, from 1.89 kilograms per hectare per year (kg/ha/yr) in 2000 to 3.12 kg/ha/yr in the time period centered on 2010—a 66-percent increase. This increase is likely primarily a result of the prevalence of much wetter conditions during the 2008–11 sampling period than during the earlier sampling period. In addition to increasing the physical transport of nitrogen in the watershed, wet climatic conditions have been shown in other studies to increase the rates of biotic and abiotic processes that control nitrogen export from northern-latitude forested watersheds. The new loading estimates, however, also support the possibility that some portion of the increase in nitrogen loading results from the observed land-use changes, and that the increase in residential housing has, in fact, contributed to the observed increase in nitrogen loading.