Progressive enrichment of benthic primary producer and dreissenid δ¹⁵N with depth in Lakes Erie and Ontario

Invasive quagga mussels (Dreissena rostriformis bugensis) have physically, and potentially biogeochemically, transformed the bottom of the Laurentian Great Lakes by colonizing vast expanses of soft sediment habitat. We analyzed the δ¹³C and δ¹⁵N of quagga mussels, the epizoic algal biofilms on their shells, and epipelic algae on sediments in Lake Erie and Lake Ontario. Spatial patterns in isotopic values of these basal organisms were broadly similar in the two lakes: the δ¹³C of mussels were consistent with a phytoplankton diet and exhibited lower spatial variation than benthic algae. Decreases in epipelic and epizoic algal δ¹³C with depth were consistent with decreasing benthic algal photosynthesis. Thus, mussels and their associated biofilms provide baseline δ¹³C for phytoplankton and benthic algae, respectively, refining our ability to track autochthonous C through lake food webs. However, the δ¹⁵N of mussels and epizoic algae were significantly higher than epipelic algae, and increased markedly with depth. The depth-specific increases in mussel and epizoic algal δ¹⁵N strongly suggests that invasive mussels influence denitrification at the sediment–water interface. The spatial variation in δ¹³C and δ¹⁵N of basal resources must be incorporated into isotope-based food web models that include highly mobile predators, such as fish.