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Round gobies and dreissenid mussels, exotic species in the North American Great Lakes basin, are euryhaline organisms whose geographic spread and ecological impacts in freshwaters may be limited by low levels of dissolved ions such as calcium (Ca). We measured source populations of these exotics in the St. Lawrence River and found population densities of dreissenids (range of ~1,000–6,400 individuals m−2) and round gobies (6–32 individuals m−2) similar to those in other Great Lake locations from which they have spread inland. However, we found little evidence for their secondary invasion of inland tributary rivers and lakes of northern New York State. Using natural waters collected from inland ecosystems, we ran laboratory bioassays of reproduction, growth, and survival of several life stages of zebra and quagga mussels as well as the round goby. We found little difference in the responses of zebra and quagga mussels, with each species showing moderate reproductive success, growth, and survival at Ca concentrations > 13 mg L−1 and dramatic improvements at >18 mg L−1. Round gobies showed moderate survival in waters with Ca concentrations > 8 mg L−1 and high survival > 18 mg L−1. These bioassays are the first such experiments for quagga mussels and round gobies and show how all three species may be similarly restricted in their ability to invade and permanently colonize significant geographic regions of New York State and perhaps the US.