Interactive effects of light and nutrients on stream algal growth modified by forest management in boreal landscapes

Boreal forests account for 30% of the world?s total forest cover and in many places are subject to intensive forest management, which often involves complete removal of overstory vegetation by clear-cutting. However, we still do not fully understand how forest management affects aquatic ecosystems in these landscapes. Here we asked how forest management-induced changes in environmental conditions, such as incident light and nutrient availability, affect benthic algal growth and nutrient limitation in boreal headwater streams of northern Sweden. We answered this question using a combination of correlative and experimental approaches across a range of forested streams spanning a gradient of site (e.g. canopy openness and water chemistry) and catchment-level (e. g. age of forest regrowth) parameters, with variation among the study streams influenced by different forest management histories and underlying natural variation. We found that benthic algal growth in these forested streams was largely driven by local interactions between dissolved inorganic nitrogen (N) availability and incident light reaching benthic surfaces. Greater water temperature and shallower depths were also associated with greater algal growth. Although high dissolved organic carbon (DOC) concentrations often play a role in reducing light availability to autotrophs in boreal aquatic systems, it was not an important predictor of algal growth in small forested streams despite a large DOC concentration gradient (5 ? 32 mg/L). Results from experimental nutrient additions supported the role of N as a key limiting nutrient, but also revealed both spatial and seasonal factors that modulate the effects of altered nutrient availability. Overall, our results suggest that differences in how light regimes and nutrient loading respond to forest management generate small-scale variation in the controls over stream primary productivity, which likely shift in relative importance at the time scale of a forest rotation (60 to 100 years).