Highly Disturbed Populations of Seagrass Show Increased Resilience but Lower Genotypic Diversity

Abstract

The response of seagrass systems to a severe disturbance provides an opportunity to quantify the degree of resilience in different meadows, and subsequently to test whether there is a genetic basis to resilience. We used existing data on levels of long-standing disturbance from poor water quality, and the responses of seagrass (Zostera muelleri) after an extreme flood event in Moreton Bay, Queensland, Australia. Sites were grouped into high and low disturbance categories, in which seagrass showed high and low resilience, respectively, as determined by measuring rates of key feedback processes (nutrient removal, suppression of sediment resuspension, and algal grazing), and physiological and morphological traits. Theoretically, meadows with higher genotypic diversity would be expected to have greater resilience. However, because the more resilient meadows occur in areas historically exposed to high disturbance, the alternative is also possible, that selection will have resulted in a narrower, less diverse subset of genotypes than in less disturbed meadows. Levels of genotypic and genetic diversity (allelic richness) based on 11 microsatellite loci, were positively related (R2 = 0.58). Genotypic diversity was significantly lower at highly disturbed sites (R = 0.49) than at less disturbed sites (R = 0.61). Genotypic diversity also showed a negative trend with two morphological characteristics known to confer resilience on seagrass in Moreton Bay, leaf chlorophyll concentrations and seagrass biomass. Genetic diversity did not differ between disturbed and undisturbed sites. We postulate that the explanation for these results is historical selection for genotypes that confer protection against disturbance, reducing diversity in meadows that contemporarily show greater resilience.

Does Warming Enhance the Effects of Eutrophication in the Seagrass Posidonia oceanica?

“…Overall, our results suggest that ongoing eutrophic conditions in the Gulf of Pozzuoli are weakening the local P. oceanica meadow, since these plants showed the greatest percentage of mortality, particularly under increased temperature. Our findings do not seem to be a general rule for seagrasses, since Connolly et al (2018) found the opposite trend in Z. muelleri. In that study, plants growing in more disturbed sites were more resilient to further disturbance, although with a lower genotypic diversity, suggesting a genotypic selection.…”

Low genotypic diversity and long-term ecological decline in a spatially structured seagrass population

“…Seagrasses often exist as natural monocultures and, therefore, make ideal model systems to test ecological and population genetic hypotheses. This model has proved informative in both population dynamic 11,73,74 and population genetic studies 39,66 but these are rarely linked. In this study, we estimated a suite of common genetic and genotypic diversity parameters amongst Zostera marina populations exhibiting a range of long-term population trajectories.…”

“…Under extreme high levels of disturbance, the majority of genets can be removed from a location, leading to very low clonal richness 65 . Regular disturbance events have also been linked with low levels of clonal diversity in tropical seagrasses, whereas less frequent disturbance had no effect 66 , suggesting some threshold effect in a species’ ability to resist loss of genetic diversity (see also 39,40 ). Old Grimsby Harbour is clearly impacted by damage to the seabed, which provides external forcing of the spatial pattern of seagrass – essentially large holes in previously more uniform vegetation 59 .…”

“…Many studies into the role of genetic diversity on population fate have focused on snapshot indicators of population state; e.g., resilience. Metrics are typically recorded within a season 34,39 or, at best, over a couple of years 40 . While these short-term studies are valuable to test mechanistic hypotheses, natural and anthropogenic drivers may be chronic in nature, applying sustained forcing over multiple generations.…”

The influence of seagrass donor source on small‐scale transplant resilience

“…Here, shoalgrass meadows have varying levels of population connectivity due to the physical isolation of the shallow embayments and lagoonal habitats where most meadows are located (Handley, Altsman, & DeMay, 2007). Thus, the morphological attributes of the two populations could be related to the level of genetic diversity derived from a population's degree of isolation (Connolly et al, ; Diekmann et al, ). Future transplant experiments should include sampling of donor genetics to determine the relative influence of genetic diversity on any connection between morphological features and transplant resilience.…”