Investigating biotic, chemical, and physical drivers of periphyton and invertebrate biomass in boreal streams supporting juvenile Atlantic salmon

1. Atlantic salmon (Salmo salar) are an anadromous fish with a broad distribution from eastern North America to western Europe, with ecological, economic, and cultural significance. Despite their expansive range, wild populations across the North Atlantic have experienced decline. Survival to reproductive age is a management concern, with a focus on identifying factors affecting the juvenile freshwater life stage. Specifically, research is needed to identify potential bottlenecks limiting freshwater productivity. 2. We addressed this research gap with a case study on the Exploits River, Newfoundland - a watershed that receives one of the largest anadromous Atlantic salmon returns in North America. We aimed to identify biotic, chemical, and physical predictors of food resources for juvenile salmon (i.e., invertebrates) and food resources for invertebrates (i.e., periphyton). To test our hypotheses, we collected data on a suite of biotic (e.g., invertebrates and periphyton), chemical (e.g., total dissolved N), and physical (e.g., wetted width) variables at 42 sites within and around this watershed. 3. We observed evidence for relationships between invertebrate biomass and biotic and temporal variables. Specifically, invertebrate biomass was higher at sites with high periphyton biomass that were sampled later in the season. In addition, we observed that periphyton biomass was related to biotic, chemical, and physical variables. Here, we present evidence for bottom-up control, with increased periphyton biomass supporting increased invertebrate biomass. 4. We expect knock-on effects where increased invertebrate biomass results in increased juvenile Atlantic salmon biomass. Our large sample size revealed some predictors of invertebrate biomass and periphyton biomass. However, we found no evidence of correlation between invertebrate biomass and chemical or physical variables. The low productivity of our maritime boreal forest system could be an outlier in expected biotic, chemical, and physical relationships in streams. 5. Our findings provide an important step forward in furthering boreal stream ecology, producing novel invertebrate and periphyton knowledge for watershed managers.