Thu, Aug 05, 2021:On Demand
Background/Question/Methods
Climate change will likely increase the frequency of droughts in parts of the world. A 13-week drawdown at Alabama’s Gantt Reservoir, required for hydroelectric dam maintenance, was used as a natural experiment to study the consequences of dewatering on mussels. The most abundant species, Elliptio pullata, was used a surrogate for the species of conservation concern. The study area was divided into three depth zones and two reaches (upstream vs downstream). Each week, four 50 × 50 cm soil quadrats (our statistical unit) were selected at random from each zone × reach combination (among the sites known to have mussels) and excavated. Each mussel was classified as either freshly dead or alive. We also collected a set of 11 variables that could affect mussel survival, including the time in weeks after drawdown, mussel length, the temperature and moisture of the quadrat, underwater depth of the quadrat before the drawdown, whether the mussel was buried in the sediment or exposed, and whether natural or artificial cover was present in the quadrat. We used generalized linear mixed models to evaluate the relationship between the predictors and mussel survival and used the Akaike Information Criterion to identify the best model.
Results/Conclusions We sampled a total of 875 live or freshly dead mussels in 228 unique quadrats. Survival declined sharply over time, with ~ 80% live mussels on week 1 and only ~ 10% at the end. Residual variation (not explained by time since drawdown) was best explained by protection against desiccation, as evidenced by the findings that: 1) when mussels were buried in the sediment, as opposed to being found on the surface (everything else being equal), the estimated difference in probability of survival (effect size) was comprised between 6 and 35% over the 13-week period; 2) when cover/shading was present, the effect size was comprised between 11 and 26%; 3) survival probability increased monotonically with quadrat moisture; at the 2.5th percentile, the effect size ranged between -28 and -6 % while at the 97.5th percentile, the effect size ranged between 13 and 36 %. For mussel length and quadrat depth, however, survival was maximal at or near the average value, and was minimal towards both ends of the distribution. Our study points towards actions, such as adding sources of natural and/or artificial cover, that could increase the survival of federally endangered mussel species during periodic drawdowns and droughts.
Results/Conclusions We sampled a total of 875 live or freshly dead mussels in 228 unique quadrats. Survival declined sharply over time, with ~ 80% live mussels on week 1 and only ~ 10% at the end. Residual variation (not explained by time since drawdown) was best explained by protection against desiccation, as evidenced by the findings that: 1) when mussels were buried in the sediment, as opposed to being found on the surface (everything else being equal), the estimated difference in probability of survival (effect size) was comprised between 6 and 35% over the 13-week period; 2) when cover/shading was present, the effect size was comprised between 11 and 26%; 3) survival probability increased monotonically with quadrat moisture; at the 2.5th percentile, the effect size ranged between -28 and -6 % while at the 97.5th percentile, the effect size ranged between 13 and 36 %. For mussel length and quadrat depth, however, survival was maximal at or near the average value, and was minimal towards both ends of the distribution. Our study points towards actions, such as adding sources of natural and/or artificial cover, that could increase the survival of federally endangered mussel species during periodic drawdowns and droughts.