Presentations:

The limited co-existence of zebra mussels and native mussels in the Richelieu River over the past 25 years by Kennedy Zwarych, McGill University

North American unionid mussel populations have experienced significant mortality due to competition and fouling by the Eurasian zebra mussel Dreissena polymorpha. Habitats whose water chemistry is suboptimal for the zebra mussel could plausibly serve as refugia in which unionids and zebra mussels co-exist. The Richelieu River, invaded by the zebra mussel in the mid-1990s, has a mean calcium concentration (∼18 mg/L) and is suggested to be sub-optimal for supporting a dense population of zebra mussels capable of severely damaging unionids. Using a 25-year dataset of SCUBA surveys, we tested how unionid diversity and abundance have changed along with zebra mussel fouling levels in the river over time. Despite predictions by some risk assessments, unionid diversity and abundance in the Richelieu River have eroded to a similar extent and followed a similar timeline of population decline as has been observed in invaded calcium-rich habitats. This presentation will explore the results of this dataset and hypothesize explanations for the patterns observed.

Multiple invasions decimate the most imperiled freshwater invertebrates in by Alexander Y Karatayev, Buffalo State College

Invaders can have devastating impacts on freshwater ecosystems, but these impacts can subside over time as ecosystems “adapt” to the invasion of new species. We analyzed changes in species composition and density of molluscs in Oneida Lake (New York, USA), one of the best studied North American lakes based on detailed surveys conducted in 1915-17, 1967-68, 1992-95, 2012, and 2022-23, and on annual benthic surveys from 2009 through 2023. Eutrophication and habitat alteration after 1920 resulted in a 25% decline in species richness and a 95% decline in the density of native gastropods by 1967, while species richness of unionids did not change. The arrival of zebra mussels in 1991 and quagga mussels in 2005 was associated with an increase in species richness and density of native gastropods and an extirpation of unionids by 1995. However, an invasion by the round goby in 2013 led to a significant decline across all gastropod families, disproportionately impacting soft-shelled and shallow-dwelling species, while other species, including invasive dreissenids, partially recovered 3 to 7 years after the goby invasion. This mollusc recovery was depth-related and was limited to deeper areas. Altogether, molluscan communities were sensitive to ecosystem change and invasives species, with some invaders offsetting the impacts of eutrophication and habitat alterations. While individual stressors have taxon-specific and sometimes positive impacts, eutrophication and species invasions have collectively decimated the native mollusc community over the past century.