Long-term monitoring in the Lamprey River Hydrologic Observatory: two decades of stream chemistry that captures hydrologic extremes and human population growth

The Lamprey River watershed, located in southeastern New Hampshire (USA), has been developed into a hydrologic observatory that serves as a focal point for research, teaching and outreach efforts of the NH Water Resources Research Center. The Lamprey is the largest tributary to NH's most significant estuary, the Great Bay, which has experienced a significant loss in eelgrass and low dissolved oxygen levels that both threaten aquatic life. The watershed is predominantly forested (67%), contains significant wetland cover (12%) as well as developed areas (5%) and agriculture (7%). Development pressure is high in the Lamprey River Hydrologic Observatory (LRHO) and human population density increased from 46 to 60 people/km² between 1990 and 2010. Weekly monitoring began in 1999 at a site co-located with USGS gauging station 01073500 (our site LMP73; watershed area 479 km²). In 2004, monthly sampling began at 15 stream sites throughout the LRHO to capture areas of development along the main stem and major tributaries. Weekly precipitation chemistry monitoring began in 2003 at a site co-located with the Climate Reference Network station "NH Durham 2 SSW". Initial stream samples were analyzed for nutrients and organic matter. Beginning in 2002, all samples were also analyzed for major ions and silica. In situ sensors were installed 1 km upstream of LMP73 in 2012 to measure nitrate, florescent dissolved organic matter and other physical parameters at 15-minute intervals. Our monitoring program captured 100-year flood events in 2006 and 2007. Surprisingly, these two flood events had little, if any, effect on multiyear solute trends. Through 2014, we observed small decreases in concentrations and fluxes of salts and major ions, no significant trends in nutrients and a small decrease in dissolved organic carbon concentrations. Even though nutrients have not increased (or decreased) at LMP73 over our 15-year record in response to the increased human footprint, our data show a remarkably strong relationship between human population density and inorganic N flux among our sub-basins. These results underscore the importance of maintaining long-term records and utilizing multiple analytical approaches to understand the biogeochemistry of watersheds experiencing climate variability and land use change.