Zebra mussels are prolific benthic colonizers and can consume a large proportion of pelagic production, depositing nitrogen-rich feces and pseudofeces at the sediment surface. Previous research suggests sediments underlying zebra mussels can be hotspots for microbial nitrogen transformations. We examined the effects of zebra mussels on sediment nitrification and denitrification rates bimonthly for 1 year in Gull Lake. Eighteen shallow trays were filled with lake sediment and rocks, with half receiving rocks colonized by zebra mussels. We incubated trays in the littoral zone, and collected zebra mussels and underlying sediment samples from trays every 2 months. We measured sediment nitrification and denitrification in lab assays. Denitrification rates were highest in summer and winter (4.2±0.3 and 3.7±0.9 mg N d^-1 gAFDM^-1 respectively, p<0.02) and shifted from N-limited to C-limited in the presence of zebra mussels. Denitrification rates were higher in the presence of zebra mussels in spring and winter (p<0.01) and trended this way in summer. Nitrification was higher in the presence of zebra mussels in spring (p<0.01). Changes induced by zebra mussels in sediment organic and N loading translate into new environmental conditions for the sediment microbial community, which may lead to new patterns of N cycling in Gull Lake.