Spatial and Temporal Variability of In-Stream Functioning within a Forested, Headwater Piedmont Watershed

Abstract

As anthropogenic nutrient loads threaten the health of the Chesapeake Bay, lotic processes throughout its headwaters may buffer increased nitrogen inputs by converting them to stable forms, ultimately through denitrification to N2 gas. However, the temporal environmental factors controlling baseflow nitrogen retention are poorly understood, particularly temperature, shading, and dissolved organic matter dynamics. This study therefore attempts to elucidate the effects of these environmental variables on nitrogen cycling within the Fair Hill Natural Resources Management Area (Fair Hill), a forested watershed within the Piedmont physiographic province of the Chesapeake Bay. As expected, groundwater and allochthonous organic matter inputs set the foundation for lotic biogeochemistry at Fair Hill, creating a nutrient-limited, heterotrophic reach. Within this setting, three temporal "hot-moments" of in-stream nutrient processing were observed: the release of ammonium and phosphate during the warm - but shaded - growing season; nitrate uptake during autumnal leaf-fall; and a unique spike of nitrate uptake and respiration-induced degradation of labile organic matter during a drought. Consequently, the baseflow capacity of this headwater stream to buffer nutrient exports to the Chesapeake Bay constantly varies throughout the year in response to light availability, temperature, and in-stream organic matter dynamics.