Temporal Dynamics and Spatial Patterns of C, N, P in a Forested First Order Watershed

Headwater streams are important conduits linking terrestrial and aquatic ecosystems, and moving water, nutrients, and energy to downstream surface waters. In the glacial drift landscapes of northern Michigan, headwater streams are much more abundant than large rivers and groundwater-dominated first order streams transfer important elements from forests to surface waters. Given the significance of these streams to inland lakes, we sought to improve our understanding of terrestrial-aquatic linkages within headwater watersheds by exploring the temporal and spatial patterns of carbon and dissolved matter dynamics. During 2015-2016, we measured stream flow and water chemistry at three locations along a 1.3 km headwater stream draining a 120 ha of forested watershed, and complemented this dataset with measurements of shallow groundwater chemistry in 20 wells (1-3 m deep) across the watershed in 2016-2017. The highest dissolved organic carbon (DOC) concentrations were observed during periods of high stream flow, notably spring snow melt and large summer storms. Annual mean DOC loads ranged from 154 kg C/year at the uppermost part of the stream to 3,392 kg C/year at the mouth. We observed a dilution of nitrate (NO3) during high flow times, although phosphate (PO4) concentrations were relatively consistent throughout the year. The DOC δ13C signatures ranged from -24.79‰ to -27.33‰ and were more depleted during high flow periods, suggesting the stream is receiving water that has been in contact with less decomposed organic matter sources and shallow soil horizons during these periods, possibly from a larger contributing area. Data from the three locations along the stream suggest that export to the adjacent 7,000 ha lake is heavily influenced by the lower portion of the watershed, while the influence of water from the upper stream reach is minimal. Differences in DOC δ13C along the stream course and seasonal patterns of DOC loads indicate that year-round hydrologic measurements at multiple points along a headwater stream can provide valuable insights into stream chemistry dynamics.