Fluctuating fortunes: Stressor synchronicity and fluctuating intensity influence biological impacts.

Ecosystems remain under enormous pressure from multiple anthropogenic stressors. Manipulative experiments evaluating stressor interactions and impacts mostly apply stressors under static conditions without considering how variable stressor intensity (i.e. fluctuations) and synchronicity (i.e. timing of fluctuations) affect biological responses. We ask how variable stressor intensity and synchronicity, and interaction type, can influence how multiple stressors affect seagrass. At the highest intensities, fluctuating stressors applied asynchronously reduced seagrass biomass 36% more than for static stressors, yet no such difference occurred for photosynthetic capacity. Testing three separate hypotheses to predict underlying drivers of differences in biological responses highlighted alternative modes of action dependent on how stressors fluctuated over time. Given that environmental conditions are constantly changing, assessing static stressors may lead to inaccurate predictions of cumulative effects. Translating multiple stressor experiments to the real world, therefore, requires considering variability in stressor intensity and the synchronicity of fluctuations.

Greater Consideration of Animals Will Enhance Coastal Restoration Outcomes.

As efforts to restore coastal habitats accelerate, it is critical that investments are targeted to most effectively mitigate and reverse habitat loss and its impacts on biodiversity. One likely but largely overlooked impediment to effective restoration of habitat-forming organisms is failing to explicitly consider non-habitat-forming animals in restoration planning, implementation, and monitoring. These animals can greatly enhance or degrade ecosystem function, persistence, and resilience. Bivalves, for instance, can reduce sulfide stress in seagrass habitats and increase drought tolerance of saltmarsh vegetation, whereas megaherbivores can detrimentally overgraze seagrass or improve seagrass seed germination, depending on the context. Therefore, understanding when, why, and how to directly manipulate or support animals can enhance coastal restoration outcomes. In support of this expanded restoration approach, we provide a conceptual framework, incorporating lessons from structured decision-making, and describe potential actions that could lead to better restoration outcomes using case studies to illustrate practical approaches.

Exogenous 20-hydroxyecdysone induces epidermal carbonic anhydrase but inhibits exoskeletal calcification in the post-ecdysial blue crab, Callinectes sapidus.

The crustacean exoskeleton is composed primarily of chitin, proteins and various inorganic compounds. It is the inorganic compounds, such as calcium and magnesium, that underlie the exoskeletal mineralization process following ecdysis. Little is known about the hormonal mechanism for this process in crustaceans. Carbonic anhydrase (CA) in the epidermis has been suggested to aid in deposition of calcium and magnesium carbonates to the exoskeleton in crustaceans by generating bicarbonate ions, resulting in mineralization. Given a similar pattern of fluctuation in prevalence between epidermal CA and ecdysteroids during the crab molting cycle, it has been proposed in a previous study that post-ecdysial epidermal CA and subsequent metal deposition to the exoskeleton are controlled by the ecdysteroid molting hormones in the blue crab, Callinectes sapidus. This study sought to acquire evidence to support such a proposition. Early postmolt and early intermolt blue crabs were used to quantify epidermal CA mRNA expression when exposed in vitro to three physiologically relevant concentrations of 10 nM, 100 nM and 1 µM 20-hydroxyecdysone (20-HE), using the epidermis-with-exoskeleton (EWE) tissue assay method. It was found that 100 nM 20-HE significantly induced the mRNA of CasCAg, a CA isoenzyme, in epidermal tissues of early intermolt crabs and that injection of 20-HE at a dose of 15 ng/g significantly elevated epidermal CA activity in vivo in early intermolt crabs. These two lines of evidence clearly show that post-ecdysial epidermal CA is influenced by the molting hormone. Interestingly, exoskeletal calcium content was significantly lower in the 20-HE treated crabs than in the control, whereas magnesium content was unchanged. The inhibition of calcification in the post-ecdysial exoskeleton by the exogenous molting hormone may implicate that sclerotization and mineralization of the new shell must be coordinated and that enhanced deposition of carbonate salts as a result of increased epidermal CA activity following the administration of exogenous molting hormone would be inhibited to avoid the formation of a structurally disrupted exoskeleton.

Stressor fluctuations alter mechanisms of seagrass community responses relative to static stressors.

Ecosystems are increasingly affected by multiple anthropogenic stressors that contribute to habitat degradation and loss. Natural ecosystems are highly dynamic, yet multiple stressor experiments often ignore variability in stressor intensity and do not consider how effects could be mediated across trophic levels, with implications for models that underpin stressor management. Here, we investigated the in situ effects of changes in stressor intensity (i.e., fluctuations) and synchronicity (i.e., timing of fluctuations) on a seagrass community, applying the stressors reduced light and physical disturbance to the sediment. We used structural equation models (SEMs) to identify causal effects of dynamic multiple stressors on seagrass shoot density and leaf surface area, and abundance of associated crustaceans. Responses depended on whether stressor intensities fluctuated or remained static. Relative to static stressor exposure at the end of the experiment, shoot density, leaf surface area, and crustacean abundance all declined under in-phase (synchronous; 17, 33, and 30 % less, respectively) and out-of-phase (asynchronous; 11, 28, and 39 % less, respectively) fluctuating treatments. Static treatment increased seagrass leaf surface area and crustacean abundance relative to the control group. We hypothesised that crustacean responses are mediated by changes in seagrass; however, causal analysis found only weak evidence for a mediation effect via leaf surface area. Changes in crustacean abundance, therefore, were primarily a direct response to stressors. Our results suggest that the mechanisms underpinning stress responses change when stressors fluctuate. For instance, increased leaf surface area under static stress could be caused by seagrass acclimating to low light, whereas no response under fluctuating stressors suggests an acclimation response was not triggered. The SEMs also revealed that community responses to the stressors can be independent of one another. Therefore, models based on static experiments may be representing ecological mechanisms not observed in natural ecosystems, and underestimating the impacts of stressors on ecosystems.

Evaluating multiple stressor research in coastal wetlands: A systematic review.

Multiple stressors are ubiquitous in coastal ecosystems as a result of increased human activity and development along coastlines. Accurately assessing multiple stressor effects is essential for predicting stressor impacts and informing management to efficiently and effectively mitigate potentially complex ecological responses. Extracting relevant information on multiple stressor studies conducted specifically within coastal wetlands is not possible from existing reviews, posing challenges in highlighting knowledge gaps and guiding future research. Here, we systematically review manipulative studies that assess multiple anthropogenic stressors within saltmarsh, mangrove, and seagrass ecosystems. In the past decade, there has been a rapid increase in publications, with seagrasses receiving the most attention (76 out of a total of 143 studies). Across all studies, nutrient loading and temperature were tested most often (N = 64 and N = 48, respectively), while the most common stressor combination was temperature with salinity (N = 12). Stressor application and study design varied across ecosystems. Studies are mostly conducted in highly controlled environments, without considering how natural variations in the physicochemical environment of coastal ecosystems may influence stressor intensity and timing under these conditions. This may result in vastly different ecological responses across levels of biological organisation. Shifting focus from univariate analytical approaches to multivariate, particularly path analysis, will help elucidate complex ecological relationships and highlight direct and indirect effects of multiple stressors in coastal ecosystems. There is a solid foundation of multiple stressor research in coastal wetlands. However, we recommend future research enhance ecological realism in experimental design by studying the effects of stressor combinations whilst accounting for spatiotemporal variability that reflects natural conditions of coastal ecosystems.