Evaluating the potential resiliency of Vallisneria americana across multiple rivers along the Atlantic coast (USA) using individual-based networks of genetic distances
Large-scale losses from nutrient and sediment loading, competition with non-native species, and loss of habitat connectivity cause concern for long-term persistence of submersed aquatic vegetation (SAV) and the essential ecosystem services they provide. Extensive, connected habitats are more resilient due to higher probabilities of supporting large, genetically diverse populations that can tolerate, acclimate, or adapt to environmental changes. The aquatic angiosperm Vallisneria americana (wild celery) has large, extensive patches of habitat scattered throughout fresh to oligohaline waters across the Atlantic coast of North America. The extensive range and intermittent local abundance of V. americana suggests potentially high resilience to perturbations. Resiliency, and its effect on future persistence, is increasingly important for SAV because current stresses, including pollution and nutrient loading, are not abating and large-scale disturbances are expected to become common as storm events increase in frequency and intensity in the face of climate change. However, because V. americana reproduces both sexually and clonally, currently occupied habitat may not support sufficient genetic diversity to ensure resiliency. We quantified genetic diversity of V. americana collected from three regions (MD, NY, and ME) and used individual-based networks of genetic distances to compare patterns in the structure of genetic diversity across watersheds.
We genotyped samples collected along tidal and non-tidal portions of the Potomac River, MD (n=723 across 29 sites) as well as samples collected along freshwater tidal regions of the Hudson River, NY (n=140 across 5 sites) and the Kennebec River, ME (n=150 across 5 sites). We identified 401 multilocus genotypes (MLGs) in the Potomac and 115 MLGs in the Hudson. Most sites within the Potomac were composed of a few dominate MLGs, while the other regions largely consisted of unique MLGs. Comparing across regions, allelic and genotypic diversity increased in a downstream direction. Genetic distances among all MLGs were used in network analysis to quantify connectivity. Networks revealed that relatedness among MLGs was higher in non-tidal/upriver regions. Breaks in geneflow were also observed and might be associated with changes in dispersal regime (unidirectional vs. radial) or differences in selective environmental forces. Overall, prospects for resiliency at first appear to warrant optimism because several sites across all regions had high allelic and genotypic diversity. However, many of the distinct genotypes we detected are highly related, which may have negative consequences on resiliency if it is associated with, or leads to, increased inbreeding.