The legacy effects of seagrass genetic diversity on productivity
The effects of genotypic richness on population, community, and ecosystem processes have been demonstrated across a range of species and systems, yet we have little understanding of how these relationships vary temporally and spatially in natural populations. Genotypic richness may have “legacy” effects on ecological processes, persisting across seasons or generations, even as genetic diversity itself changes. We examined the longevity of diversity effects in the seagrass Zostera marina – a marine foundation species for which the positive effects of genetic diversity on productivity and resistance to disturbance have been well demonstrated experimentally. On three sampling dates from fall 2013 to summer 2014, we measured seagrass genetic diversity, productivity (shoot density and percent cover), shoot morphology, above-/below-ground biomass, and depth at seven sites in Nantucket Harbor, MA. We used 12 microsatellite loci to quantify genotypic richness on each sampling date. Using model selection, we tested alternative models to examine whether seagrass genetic diversity predicted productivity or morphology and if so, which time point of diversity was most informative.
In fall 2013, there was a significant positive relationship between fall seagrass genotypic richness and seagrass productivity and morphology. In spring 2014, spring genotypic richness was a strong predictor of eelgrass morphology, but not of eelgrass productivity. Instead, genotypic richness from the previous fall best predicted spring productivity, including shoot density and percent cover. Fall 2013 genotypic richness continued to explain variation in productivity in summer 2014. Similarly, fall 2013 genotypic richness was a better predictor of summer 2014 aboveground biomass than current (summer 2014) or recent (spring 2014) genetic diversity. In addition, depth explained significant variation in spring and summer eelgrass metrics, with decreased productivity at greater depths. Our results demonstrate that genotypic richness may have prolonged effects on ecological processes and that a better understanding of these lag effects is critical to assessing the importance of genetic diversity in natural populations.