The influence of land use and potamodromous fish on ecosystem function in Lake Superior tributaries

Allochthonous nutrients and carbon are recognized as dominant controls on biogeochemistry of low-order streams. In some systems, potamodromous fish may provide a complementary source of material as they deliver lake-derived materials to spawning streams. This study examines nutrient and carbon inputs from terrestrial ecosystems and migratory fishes to streams in undeveloped watersheds in northern Michigan, USA. We compared watershed and riparian area, slope, and landcover to nutrient concentrations at 26 sites, as well as whole-stream metabolism at 5 sites. Despite low levels of agricultural land use (0–3%), agriculture had the largest influence on stream chemistry as indicated by higher dissolved organic carbon (DOC), ammonium, silica, and chloride concentrations at the watershed level, and increased DOC and chloride at the riparian level. Ecosystem respiration and net primary production increased with watershed and riparian area, and the proportion of managed forest. To quantify inputs from fish, we monitored the spawning migrations of white (Catostomus commersonii) and longnose (C. catostomus) suckers at one site, and measured nutrients and stream metabolism above and below an impassable dam. Nutrient concentrations were uniformly low and did not increase during the fish migration; however, temporal shifts in stream metabolism during sucker migration suggest that fish influenced respiration, presumably by providing high-quality carbon and bioavailable nutrients. We conclude that both watershed land use and fish migrations provide important sources of allochthonous material to these oligotrophic streams. Recognizing the bi-directional nature of allochthonous inputs is important for understanding controls on ecosystem functioning in low-order streams.