Effects of disturbance on genetic diversity in the submersed aquatic plant Vallisneria americana

In summer 2011, Tropical storms Lee and Irene caused an estimated 90%
decline of the submersed aquatic plant Vallisneria americana Michx.
(Hydrocharitaceae) in the Hudson River, of New York (USA). To understand
the genetic impact of such large-scale demographic losses, we compared
diversity at 10 microsatellite loci in 135 samples collected from 5 sites
just before the storms with 239 shoots collected from 9 sites four years
after. Although 80% of beds sampled in 2011 lacked V. americana in 2015,
we found similar genotypic and genetic diversity and effective population
sizes in pre-storm versus post-storm sites. These similarities suggest
that despite local extirpations concentrated at the upstream end of the
sampling area, V. americana was regionally resistant to genetic losses.
Similar geographically based structure among sites in both sampling
periods suggested that cryptic local refugia at previously occupied sites
facilitated re-expansion after the storms. However, this apparent
resistance to disturbance may lead to a false sense of security. Low
effective population sizes and high clonality in both time periods suggest
that V. americana beds were already small and had high frequency of
asexual reproduction before the storms. Dispersal was not sufficient to
recolonize more isolated sites that had been extirpated. Chronic low
diversity and reliance on asexual reproduction for persistence can be
risky when more frequent and intense storms are paired with ongoing
anthropogenic stressors. Monitoring genetic diversity along with extent
and abundance of V. americana will give a more complete picture of
long-term potential for resilience.