Swimming against the tide: Patterns of genetic connectivity and discontinuity among Vallisneria americana populations of a periodically disturbed riverbed

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Magdalene Ngeve, Plant Science and Landscape Architecture, University of Maryland, College Park, MD, Katharina Engelhardt, Appalachian Laboratory, University of Maryland Center for Environmental Science, Frostburg, MD, Michelle Gray, University of Maryland, College Park, MD and Maile C. Neel, Plant Science & Landscape Architecture and Entomology, University of Maryland, College Park, MD

Background/Question/Methods
Disturbance and dispersal are two fundamental processes that drive the distribution of diversity from genes to ecosystems. But when disturbance magnitude and intensity exceed normal threshold, it could alter patterns of connectivity and the persistence of populations. Yet, impacts of disturbance are challenging to quantify because reference conditions are unknown and monitoring after a disturbance is often too short to assess impacts. Realizing these challenges, we evaluate the efficacy of using multiple population genetic analyses that work at different time scales to assess the impact of disturbance history on a species’ genetic architecture and patterns of connectivity even when samples are only available for a single time point. We sampled the aquatic foundation species, Vallisneria americana, from 25 sites within the Hudson River, New York, four years after Tropical Storms Lee and Irene destroyed over 90% of its submersed beds. 270 shoots were genotyped at 10 microsatellite loci to evaluate historical and contemporary effective population sizes and patterns of genetic connectivity and discontinuity.
Results/Conclusions
This one sample in time provided insight into major declines in effective sizes of populations, significant alterations in patterns of genetic connectivity, and identification of genetic barriers. Our results suggest both recurrent and chronic disturbance have contributed to structuring genetic variation in this riverscape. The isolation of proximate downstream sites suggests sea level rise-induced saltwater intrusion presents a significant barrier to connectivity. Upstream dispersal and connectivity of populations beyond tidal reach limits indicates that non-conventional dispersal agents, such as waterfowls, contribute to gene flow. Therefore, in the absence of reference genetic data, the impact of disturbance history on species may still be rigorously assessed with comprehensive sampling and multi-pronged genetic analyses to inform management and restoration decisions. Specifically, for the Hudson River, management that enhance connectivity and genetic diversity through data-driven efforts will be profitable.