From landscapes to streams : the pattern and function of labile dissolved organic carbon.

Dissolved organic carbon (DOC) is a broadly important component of stream water chemistry, yet the patterns and the roles of the labile fraction of the DOC pool remain understudied at appropriate scales. I used a whole stream labile DOC addition on the Kenai Peninsula, Alaska, to address how relevant quantities of labile DOC influence stream ecosystem function, from microbes to fish. I additionally quantified the labile DOC pool in streams that spanned a gradient of anthropogenic land covers, during contrasting hydrologic periods. I found that relevant quantities of labile DOC can strongly alter basal ecosystem function. Labile DOC increased ecosystem respiration by ~50%, and generally increased nitrogen uptake. Labile DOC also increased bacterial biomass production, which served as an additional food source for stream consumers. Benthic invertebrate counts were ~9x greater and juvenile salmonid abundances were ~2x greater nearest the labile DOC source. Initially larger juvenile salmonids also grew faster than similar sized reference reach individuals. Labile DOC patterns in the Ozark Highlands were strongly influenced by anthropogenic DOC sources. Sites receiving municipal wastewater treatment plant (WWTP) effluent had consistently elevated labile DOC concentrations. Developed land use positively influenced labile DOC concentrations in sites without WWTP inputs, but only when there was significant hydrologic connectivity to the landscape, suggesting that anthropogenic patterns of labile DOC are strongly tied to hydrology. Overall, my results suggest that 1) labile DOC can be a critically important component to ecosystem function, and 2) anthropogenic influences on labile DOC can be complex and dependent on meteorological factors. Thus, understanding stream ecosystems, especially in the “anthropocene,” may necessitate adding labile DOC to the currently nutrient-centric paradigm of bottom-up control on stream ecosystem function.