Pre-Recorded Oral Presentation as part of National Virtual Conference AFSS Conference 2020

Interactive effects of light and nutrients on stream algal growth modified by forest management in boreal landscapes (#105)

Ryan M Burrows 1 2 , Micael Jonsson 3 , Emma Fältström 4 5 , Jannika Andersson 3 , Ryan A Sponseller 3
  1. Australian Rivers Institute, Griffith University, Brisbane, QLD, Australia
  2. School of Ecosystem and Forest Sciences, University of Melbourne, Melbourne, Victoria, Australia
  3. Department of Ecology and Environmental Sciences, Umeå University, Umeå, Västerbotten, Sweden
  4. Environmental Technology and Management, Department of Management and Engineering, Linköping University, Linköping, Sweden
  5. Sweden Water Research AB, Lund, Sweden

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 streams with variable catchment and chemical properties, and with site-level variation in canopy openness. We found that benthic algal growth 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 environments, it was not an important predictor of algal growth in our study despite a large DOC concentration gradient across sites (5 – 32 mg L-1). Overall, experimental nutrient additions supported the role of N as a key limiting nutrient, but also underscored both spatial and seasonal factors that modulate the effects of altered nutrient availability. In the context of forestry in the Fennoscandian landscape, our results suggest that differences in how light regimes and nutrient loading respond to clear-cutting can generate small-scale variation in the primary controls over stream productivity – and drive shifts in the relative importance of primary producers at the time scale of a forest rotation.