Microbial community and functional shifts across agricultural and urban landscapes within a Lake Erie watershed

The role of sediment microbial communities in regulating the loss and retention of nutrients in aquatic ecosystems has been increasingly recognised. However, in the Great Lakes, where nutrient mitigation focuses on harmful algal blooms, there are limited studies examining the fundamental role of water/sediment microbes in nutrient biogeochemical cycling. Little is understood in this regard considering the increase in anthropogenic pressure on in-stream biological processes impacting nutrient flux to lakes. In this study, metagenomic and metatranscriptomic approaches were used to investigate the microbial community and gene regulation. The study focused on nitrogen (N) metabolism in a nutrient-polluted watershed of Lake Erie in southwestern Ontario, Canada. Nutrients and microbial analyses of water and sediments were collected in 2020 and 2021 from Sturgeon Creek headwaters to the nearshore of Lake Erie. Results showed no significant shifts in community structure with nutrient concentrations or land use. Metabolically, active genes involved in denitrification (consisting of 32-53% of N metabolic transcripts) showed the highest expression within agricultural and wetland dominant locations. Based on active gene expression patterns, the urbanised location coinciding with peak nitrate (NO₃^-) concentrations showed the greatest potential for nitrous oxide (N₂O) emission and nitrogen loss along this transect. In contrast to denitrification, direct nitrification (5-21% of N metabolic transcripts) increased two-fold approaching downstream and nearshore lake locations. Across this river-lake corridor, expression of key functional genes associated with N transformation showed strong correlation with the change in concentrations of aqueous NO₃^- and nitrite (NO₂^-) and the ratio of NO₂^-/NO₃^-. Our findings demonstrated a clear link between sediment microbial metabolism and overlying water chemistry in this lotic system. We suggest that future studies assessing nutrient mitigation consider sediment biogeochemical processes and N-metabolising bacteria, and their fundamental role and cooperative relationship with nutrient and hydrological dynamics of overlying waters.