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The density of Bosmina longirostris (0. F. Mueller) declined loo-fold between 25 June and 20 August 1985 in Lake Michigan; a similar pattern of changing abundances was observed in 1986 but not in 1987 or 1988. Bosmina collected from periods before and during the decline in 1985 and 1986 were examined for size at maturity and clutch size. Population size-frequency distributions suggest that some animals began to mature at smaller sizes during the decline in 1985. In some cases, the average clutch size for Bosmina of similar body lengths was significantly reduced during the decline in 1985 and 1986. These data suggest that food limitation was a factor in the species decline in both years. Gut content analyses indicate that of seven potential predators, Leptodora kindtii (Focke) was probably most responsible for Bosmina mortality. There is ample evidence that Leptodora does not consume strictly fluid from its prey: juvenile Leptodoru ate mainly smaller prey that included Bosmina and the rotifer Conochilus; adults ate a broader size range of prey that included mainly Bosmina, Daphnia, and copepods. In 1987 and 1988, Bythotrephes cederstroemii Schoedler was abundant in Lake Michigan and was contemporaneous with collapse ofthe Leptodora population and concurrent increase of the Bosmina population in August and September. Predation by the large copepods Epischuru lucustris S. A. Forbes and Limnoculanus macrurus Sars did not appear to have any significant effect on the population dynamics of Bosmina. Lake Michigan has long been regarded as an ecosystem in which visual planktivory by fish exerts a dominant structuring influence. The exposition by Wells (1970) documenting changes in zooplankton populations in response to predation by alewife, Alosa pseudoharengus (Wilson), helped to establish size-selective predation as one of the paradigms of plankton community ecology. Wells observed that the successful invasion of alewife into Lake Michigan was associated with depressed densities of Daphnia, Leptodora, Limnocalanus, Epischura, and other large-bodied crustacean taxa and with increased densities of smaller taxa, particularly Bosmina longirostris.Although Wells (1970) argued convincingly that the large plankton were removed by selective planktivory, no rationale was advanced for the numerical increase in Bosmina. In retrospect, the taxon's success may have stemmed from relaxed competition for Acknowledgments We thank James Bowers, Douglass Burdette, Robert Gensemer, Robert Dorazio, Glenn Warren, and the captain and crew of the RV Laurentian for assistance with field sampling. Robert Black and Patricia ChowFraser provided suggestions for the manuscript.

Sedimentary and water column evidence from 45 boat-accessible and eight backcountry lakes was used to investigate the distribution of Bythotrephes longimanus in northeast Minnesota, USA, and adjacent Ontario, Canada. The results expand the documented range of Bythotrephes in Minnesota from Lake Superior, Island Lake, and Saganaga Lake to Flour Lake, Greenwood Lake, McFarland Lake, Pine Lake, and Caribou Lake as well as to Saganagons Lake in Ontario. The latter three lakes are located in roadless landscapes without motorized boat access. Results confirm that Bythotrephes is no longer present in Boulder Lake or Fish Lake (St. Louis County, Minnesota), providing the first evidence of range compression (extinction following introduction) of this species in North America. Distributional expansion was confined to a corridor along the international border between northeast Minnesota and Ontario. Lakes along the invasion corridor were deeper, more transparent, and had lower chlorophyll concentration, on average, compared to other lakes studied. The pattern of range expansion provided an opportunity to test the predictions of a forecasting model for Bythotrephes occurrence (MacIsaac et al. 2000 Archiv fu¨r Hydrobiologie 149: 1-21) based on habitat characteristics. The model predicted 51% of the surveyed inland lakes to be susceptible to invasion, however, only 13% were actually invaded, implying strong dispersal constraints. Application of the forecasting model to a broader set of 179 Minnesota lakes predicted that 41% may be vulnerable to establishment by Bythotrephes based on habitat characteristics, offering an estimate of the state's overall lake susceptibility (i.e., fundamental niche) to invasion. The results of this study provide evidence for the importance of a low-light refuge where Bythotrephes can minimize vulnerability to fish predation as a key habitat feature not considered by the forecasting model.

Mysis relicta is a species of omnivorous zooplankton that has been implicated over the last several decades in important food‐web processes such as the lengthening of food chains, the mobilization of benthic contaminants to the pelagic, and the restructuring of community dynamics. We used stable nitrogen isotopes (δ15N) to quantify ontogenetic diet shifting of M. relicta in five North American lakes. Analyses indicated that M. relicta measuring >1.5 cm were elevated, on average, 0.73 trophic levels above conspecifics measuring <1‐cm length, and 0.57 trophic levels above conspecifics measuring <1 but >1.5‐cm length. This pattern suggests a trend toward increasing carnivory with maturity, which is consistent with the results of previous conventional approaches to diet analysis in this species. Our results should be useful to modelers concerned with quantifying the effects of M. relicta on energy transfer and contaminant bioaccumulation in food chains.

Winter is an important season for many limnological processes, which can range from biogeochemical transformations to ecological interactions. Interest in the structure and function of lake ecosystems under ice is on the rise. Although limnologists working at polar latitudes have a long history of winter work, the required knowledge to successfully sample under winter conditions is not widely available and relatively few limnologists receive formal training. In particular, the deployment and operation of equipment in below 0 C temperatures pose considerable logistical and methodological challenges, as do the safety risks of sampling during the icecovered period. Here, we consolidate information on winter lake sampling and describe effective methods to measure physical, chemical, and biological variables in and under ice. We describe variation in snow and ice conditions and discuss implications for sampling logistics and safety. We outline commonly encountered methodological challenges and make recommendations for best practices to maximize safety and efficiency when sampling through ice or deploying instruments in ice-covered lakes. Application of such practices over a broad range of ice-covered lakes will contribute to a better understanding of the factors that regulate lakes during winter and how winter conditions affect the subsequent ice-free period.