Factors affecting rotifer assemblages along a nearshore to offshore transect in southeastern Lake Michigan

Rotifer assemblages have been widely used as indicators of trophic conditions. Our goal was to evaluate whether differences in rotifer assemblages along a nearshore to offshore transect in southeastern Lake Michigan were consistent with productivity differences among sites. Sampling took place in March-December during 2013–2019 at a nearshore (17-m deep), mid-depth (45-m deep), and offshore site (110-m deep). Conochilus and Keratella were present in most samples (84–100%) and were the most abundant taxa overall at each site, combining for around 64–69% by density of the rotifers. Although the productivity gradient was not large between sites, there was a progression with the highest contributions of Keratella, Conochilus, and Kellicottia occurring at the nearshore, mid-depth, and offshore sites, respectively, which is consistent with a gradient of decreasing productivity from nearshore to offshore. The rotifer community also demonstrated strong seasonal patterns. In particular, there was a progression from dominance by illoricate forms, particularly Synchaeta, early in the season, to loricate and colonial forms in the summer and fall. Redundancy analysis indicated that in addition to productivity (i.e., total phosphorus), predatory factors (e.g., cyclopoid copepods, Cercopagis, Bythotrephes) had a strong influence on the rotifer community and seasonal patterns. Although productivity was important in structuring rotifer assemblages along our nearshore to offshore gradient, other factors, especially those related to predation, are difficult to separate from productivity effects. This complexity somewhat limits the usefulness of rotifers as an indicator group for productivity among sites that do not have extreme differences in trophy within a lake.