Relative rates of sexual versus asexual reproduction are fundamental to evolutionary potential and ecological functioning in aquatic plant populations. These rates determine amounts and structure of genetic diversity at multiple scales, potential for dispersal, and potential for resilience to disturbances via acclimation or adaptation. Vallisneria americana, a dioecious aquatic plant native to eastern North America, is capable of both sexual and asexual reproduction. Informal observations in 2015 indicated low sexual reproduction in the Hudson River, NY. In summer 2018, mode of reproduction was assessed at 14 sites along the salinity gradient of the Hudson. Three Chesapeake Bay sites were sampled for comparison. Flowering was predicted to be positively correlated with large plant size, high genotypic diversity, high bed density, and low salinity. All sites were sampled twice. 20-25 plant samples were taken along a transect line, and male and female inflorescences were counted and the size of each plant was recorded. Then female flowers and fruits on the water surface were counted and presence of male flowers was noted along longer transect lines. Potential for pollination was quantified by counting the number of samples and transect segments with both male and female flowers.
Results/Conclusions
Chesapeake Bay sites had an average of 312.3 inflorescences and a mean female flower density of 0.97 per 1000m2, whereas Hudson River sites averaged only 15.6 inflorescences and a female flower density of 0.19. Plants with long leaves and many ramets were more likely to flower; Chesapeake plants had longer leaves and Hudson plants had more ramets. However, we suggest that the most important driver of regional flowering differences is temperature- driven phenology. Long-term (1984-2018) temperature data from monitoring stations revealed the Hudson is ~ 5 °C cooler and ~1 month later throughout spring and early summer. Spatial isolation of sexes and skewed sex ratios were pronounced in the Hudson at the scale of the river and transect. The results help explain why the Hudson River is low in genetic diversity and lead us to predict that Hudson populations are characterized by chronic low sexual reproduction due to delayed flower production. Little to no fruit set, followed by repeated clonal growth increases spatial isolation of males and females and perpetuates populations with low genetic diversity and skewed sex ratios. This study lays the groundwork for testing the implications of a self-reinforcing cycle of low genetic diversity in the Hudson River.