First observations of tumor-like abnormalities (exophytic lesions) on Lake Michigan zooplankton

Tumor-like abnormalities (exophytic lesions) were found on a variety of planktonic calanoid copepods and cladocerans (Diaptomus spp., Epischura lacustris, Limnocalanus macrurus, Polyphemus pediculus, Diaphanosoma sp., and Daphnia galeata mendotae) collected from inshore (3-m depth contour) and offshore (100- to 110-m depth contours) stations of eastern Lake Michigan. The abnormalities, which were quite large relative to animal size and variable in shape, are documented in photographs. Abnormality incidences among species ranged between 0 and 72%. Predatory species of calanoids and cladocerans had higher incidences of tumors than herbivorous species. The abnormalities on some copepods were very similar to cysts described for calanoid copepods in Lago Maggiore, Italy, which like Lake Michigan is undergoing oligotrophication. The recent appearance of the lesions in Europe and North America may indicate an emerging global phenomenon that has a common cause.

A Reappraisal of Phenotypic Variation in Daphnia galeata mendotae: The Role of Interspecific Hybridization

Seasonal, within-lake, and among-lake variation in helmet size and shape has been reported for Daphnia galeata mendotae. The genetic component of this variation was assessed by an allozyme and morphological reanalysis of nine previously studied sites. Genetic associations with morphological variation were found, but all were attributed to D. galeata mendotae × Daphnia rosea hybridization or to taxonomic confusion with D. rosea. The existence of hybrids accounted for spatial polymorphism both within and among lakes. In addition, hybrids contributed to seasonal variation as they increased in abundance from summer to winter. Nevertheless, cyclomorphosis in D. galeata mendotae s.s was real and due largely to phenotypic plasticity. Local hybridization between D. galeata mendotae and D. rosea was widespread from the midwestern states to Vermont. The distributions and habitat associations of D. galeata mendotae and D. rosea indicated that ecological segregation at the lake level acts as an important reproductive isolating mechanism. Reinterpretation of previous studies, however, suggests that disturbance followed by one or a few hybridization events often leads to hybrid domination of a lake.

Acid and Aluminum Effects on Sodium Homeostasis and Survival of Acid-Sensitive and Acid-Tolerant Cladocera

Acid-sensitive (Daphnia galeata mendotae) and acid-tolerant (Bosmina longirostris) cladocerans were exposed to 0, 100, or 200 μg Al/L at pH 4.5, 6.0, or 7.5. Whole-body Na content and survivals were determined after 12- and 24-h exposures to the nine treatments. Daphnia galeata mendotae experienced significant decreases in body Na content and survival at pH 4.5. Aluminum effects were pH dependent. At pH 7.5 and 6.0, Na content and survival declined with increasing Al; at pH 4.5, the highest Al concentration enhanced Na content and prolonged survival. Bosmina longirostris Na content and survival were only slightly reduced at pH 4.5, and there were no significant Al or pH × Al interaction effects. The results support the view that (1) the extinction of D. galeata mendotae and the relative increase of B. longirostris during lake acidification are largely due to differential impacts of acid stress on osmoregulation and (2) Al toxicity might also be a factor responsible for D. galeata mendotae population declines, which are most pronounced near pH 6.0.

Changes in the Nearshore and Offshore Zooplankton Communities in Lake Ontario: 1981–88

We examined trends and factors influencing changes in nearshore and offshore zooplankton abundance and composition in Lake Ontario between 1981 and 1988. In the nearshore (southshore and eastern basin), zooplankton abundance decreased and shifts occurred in the relative abundances of Bosmina longirostris and Daphnia retrocurva (eastern basin) and Daphnia retrocurva and Daphnia galeata mendotae (southshore). These changes could have resulted from increased vertebrate predation or reduced food resources which intensified the effects of predation. In the offshore, the first appearance (FA) of the larger, less common cladoceran species occurred earlier in the season as of 1985. FA was correlated with cumulative epilimnetic temperature (CET) and the catch per unit effort (CPUE) of alewife (Alosa pseudoharengus) [Formula: see text] caught in U.S. waters in the spring. In 1987, when CET was high and CPUE of alewife [Formula: see text] was low, large populations of these cladocerans developed in June and July. Bythotrephes cederstroemi, a recent invader in the Great Lakes, was abundant only in 1987 when the CPUE of alewife was lowest. Changes in zooplankton abundance, development, and composition along the nearshore–offshore gradient reflected effects of temperature, habitat, and planktivory on the community.