The Influence of Carbon Subsidies on Stream Chemistry and Function in Northern Michigan Streams

Abstract

Headwater streams are largely allochthonous, relying on subsidies to fuel productivity. Carbon inputs are important to streams not only because they fuel productivity but they also strongly affect many instream biological processes and physiochemical characteristics. Terrestrial ecosystems contribute a large component of carbon to streams which varies in quantity and quality, possibly affecting instream carbon concentrations and the biological uptake of that carbon. In addition, tributaries of the Great Lakes provide a unique opportunity to examine carbon sources and sinks more frequently associated with marine environments. For example, potomodromous fish which migrate between the Great Lakes and its tributaries to spawn may deliver a pulse of lake derived nutrients similar to the well-documented pulse of ocean-derived nutrients associated with anadromous fish moving between the ocean and rivers. The goal of this study was to examine linkages between these allochthonous carbon inputs and steam functioning in a remote largely undeveloped temperate forest. Terrestrial inputs were evaluated by examining dissolved organic carbon (DOC) concentration in 26 streams, and comparing them against riparian and watershed land cover. At the riparian level, forest (p<0.001), agriculture (p<0.001) and wetlands (p<0.001) all significantly influenced instream DOC concentrations. However, at the watershed level, only agriculture explained a significant amount of variation in DOC (p<0.001). Watershed land cover was also compared to carbon spiraling turnover length although no significant effects were detected. Fish derived inputs were evaluated via an evaluation of nutrient influences associated with the spawning migrations of longnose and white sucker in the Salmon Trout River. In total, 1,474

suckers were recorded swimming upstream in 2008. Whole stream metabolism was measured before and after the run, upstream and downstream of a barrier that prevented fish passage to distinguish between the effect of fish-derived carbon and possible temporal effects. Although not found to be significant, there was an obvious spike in metabolism at the downstream site during the run, which may indicate an effect of the sucker runs. The findings suggest that these two sources of allochthonous carbon are important to stream functioning and anthropogenic alterations of these inputs have the potential to affect the aquatic carbon cycle.