Going With the Flow? Relative Importance of Riverine Hydrologic Connectivity Versus Tidal Influence for Spatial Structure of Genetic Diversity and Relatedness in a Foundational Submersed Aquatic Plant

Genetic connectivity in rivers is generally high, and levels of genotypic and genetic diversity of riverine species are expected to accumulate in downstream locations. Genetic structure of marine and estuarine species is less predictable, even though hydrologic connectivity is also expected to be relatively high in those ecosystems. These observations have been generated across different species and locations such that our understanding of the effects of hydrologic connectivity in the same river, spanning tidal and nontidal habitats, remains incomplete. To control for species and location, we quantified diversity in 941 samples of <i>Vallisneria americana</i> Michx. (Hydrocharitaceae) collected from 36 sites along the species' entire distribution in the tidal and nontidal Potomac River of Maryland, Virginia, and the District of Columbia, USA. Using 10 microsatellite loci, we found 507 unique multilocus genotypes (MLGs) that were collapsed to 482 multilocus lineages (MLLs). Fifty-three MLLs were found multiple times across the riverscape, accounting for over 54% of the genotyped shoots. We found some evidence supporting connectivity throughout the river and stronger evidence that tidal regime drives genotypic and genetic structure within <i>V. americana</i>. Extensive clonality, including two MLLs spanning 230 and 152 km, limits diversity in the nontidal reaches and contrasts with very little evidence of clonal reproduction in tidal reaches. Genetic differentiation, structure, and pairwise relatedness of sampled shoots and MLLs also differed by tidal reach, with the nontidal Potomac having higher levels of relatedness, lower allelic diversity, and higher heterozygosity. The differences in spatial distribution of genetic diversity suggest very different outlooks for <i>V. americana</i> adaptation and acclimation to perturbations in tidal and nontidal regions of the Potomac, which lead to different recommendations for restoration of the same species in the same river.