Sand aggradation alters biofilm standing crop and metabolism in a low-gradient Lake Superior tributary

Sediment deposition changes the physical characteristics of river beds, and may alter the production and/or processing of allochthonous and autochthonous organic matter, with potential consequences for stream food webs. We conducted a comparative study of biofilm standing crop and metabolism in the Salmon Trout River, a tributary of Lake Superior where watershed disturbances have led to 3-fold increases in streambed fine sediments, predominately sand, in the past decade. We compared biofilm standing crop and metabolism rates using light-dark chambers in reaches where substrate consisted of predominately exposed rock or sand substrates. Biofilm chlorophyll a was 4.2-fold greater on rock substrates, but ash-free dry mass was 8-fold greater on sand substrates. Rates of gross primary production were 2 to 25-fold greater on rock versus sand substrates, and differences persisted whether rates were scaled for area or biofilm standing crop. In contrast, rates of respiration were similar on rock and sand substrates when scaled per unit area but 7-13 times lower on sand when scaled for biofilm standing crop. Together, these results suggest that sand aggradation in this river alters organic matter processing during the summer from net production to net consumption and storage of organic matter. Sand aggradation may alter the availability and processing of both allochthonous and autochthonous food resources in this forested river, with potential far-reaching impacts on the food web.