Local and regional stressors interact to drive a salinization-induced outbreak of predators on oyster reefs

Oyster Restoration to Recover Ecosystem Services

Oyster reef loss represents one of the most dramatic declines of a foundation species worldwide. Oysters provide valuable ecosystem services (ES), including habitat provisioning, water filtration, and shoreline protection. Since the 1990s, a global community of science and practice has organized around oyster restoration with the goal of restoring these valuable services. We highlight ES-based approaches throughout the restoration process, consider applications of emerging technologies, and review knowledge gaps about the life histories and ES provisioning of underrepresented species. Climate change will increasingly affect oyster populations, and we assess how restoration practices can adapt to these changes. Considering ES throughout the restoration process supports adaptive management. For a rapidly growing restoration practice, we highlight the importance of early community engagement, long-term monitoring, and adapting actions to local conditions to achieve desired outcomes.

Dynamic disparities in inorganic nitrogen and phosphorus fluxes into estuarine systems under different flow regimes and streamflow droughts

Elongated periods of low flow conditions, which can be termed as streamflow droughts, influence the nutrient (e.g., nitrogen and phosphorus) balance in estuarine systems. Analyzing temporal trends of nutrient fluxes into such systems under different streamflow regimes can complement the understanding about the dynamic evolution of streamflow droughts and their impacts on nutrient levels. The objective of this paper was to evaluate how dynamic evolution of streamflow droughts (from low flow conditions) affects the inorganic nutrient flux in a tropical estuarine system. We analyzed a 20-year time series of streamflow data together with the concentrations of two nutrient parameters-dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN)-in the Lower Apalachicola River that drains into Apalachicola Bay in northeastern Gulf of Mexico, Florida. Our findings revealed that droughts affect the seasonal patterns and fluxes of both DIP and DIN. We also observed post-drought flushing patterns in DIP and contrasting changes in DIP and DIN fluxes in the long-term (20 years here) under different streamflow conditions. Dynamically changing correlations between the streamflow and the fluxes were found throughout different phases of droughts. In the long-term (from 2003 to 2021), the DIP flux in high flows increased by 35.3%, while the flux decreased by 15.7% in low flows. Conversely, DIN flux in high flows showed a decrease of <1.2%, but an increase of <23.7% in low flows after droughts end. The insights from this study highlighted the need for effective regulation plans such as proper nutrient management against streamflow droughts to mitigate negative ecological consequences in estuarine systems such as harmful algal blooms.

Climate change impacts on oyster aquaculture - Part I: Identification of key factors

Oysters are enriched with high-quality protein and are widely known for their exquisite taste. The production of oysters plays an important role in the local economies of coastal communities in many countries, including Atlantic Canada, because of their high economic value. However, because of the changing climatic conditions in recent years, oyster aquaculture faces potentially negative impacts, such as increasing water acidification, rising water temperatures, high salinity, invasive species, algal blooms, and other environmental factors. Although a few isolated effects of climate change on oyster aquaculture have been reported in recent years, it is not well understood how climate change will affect oyster aquaculture from a systematic perspective. In the first part of this study, we present a systematic review of the impacts of climate change and some key environmental factors affecting oyster production on a global scale. The study also identifies knowledge gaps and challenges. In addition, we present key research directions that will facilitate future investigations.

A global synthesis of predation on bivalves

Predation is a dominant structuring force in ecological communities. In aquatic environments, predation on bivalves has long been an important focal interaction for ecological study because bivalves have central roles as ecosystem engineers, basal components of food webs, and commercial commodities. Studies of bivalves are common, not only because of bivalves' central roles, but also due to the relative ease of studying predatory effects on this taxonomic group. To understand patterns in the interactions of bivalves and their predators we synthesised data from 52 years of peer-reviewed studies on bivalve predation. Using a systematic search, we compiled 1334 studies from 75 countries, comprising 61 bivalve families (N = 2259), dominated by Mytilidae (29% of bivalves), Veneridae (14%), Ostreidae (8%), Unionidae (7%), and Dreissenidae and Tellinidae (6% each). A total of 2036 predators were studied, with crustaceans the most studied predator group (34% of predators), followed by fishes (24%), molluscs (17%), echinoderms (10%) and birds (6%). The majority of studies (86%) were conducted in marine systems, in part driven by the high commercial value of marine bivalves. Studies in freshwater ecosystems were dominated by non-native bivalves and non-native predator species, which probably reflects the important role of biological invasions affecting freshwater biodiversity. In fact, while 81% of the studied marine bivalve species were native, only 50% of the freshwater species were native to the system. In terms of approach, most studies used predation trials, visual analysis of digested contents and exclusion experiments to assess the effects of predation. These studies reflect that many factors influence bivalve predation depending on the species studied, including (i) species traits (e.g. behaviour, morphology, defence mechanisms), (ii) other biotic interactions (e.g. presence of competitors, parasites or diseases), and (iii) environmental context (e.g. temperature, current velocity, beach exposure, habitat complexity). There is a lack of research on the effects of bivalve predation at the population and community and ecosystem levels (only 7% and 0.5% of studies respectively examined impacts at these levels). At the population level, the available studies demonstrate that predation can decrease bivalve density through consumption or the reduction of recruitment. At the community and ecosystem level, predation can trigger effects that cascade through trophic levels or effects that alter the ecological functions bivalves perform. Given the conservation and commercial importance of many bivalve species, studies of predation should be pursued in the context of global change, particularly climate change, acidification and biological invasions.

Persistence of fish populations to longer, more intense, and more frequent mass mortality events

Over the last decades, mass mortality events have become increasingly common across taxa with sometimes devastating effects on population biomass. In the aquatic environment, fish are sensitive to mass mortality events, particularly at the early life stages that are crucial for population dynamics. However, it has recently been shown for fish, that a single mass mortality event in early life typically does not lead to population collapse. Moreover, the frequency and intensity of extreme events that can cause mass mortality, such as marine heatwaves, are increasing. Here, we show that increasing frequency and intensity of mass mortality events may lead to population collapse. Since the drivers of mass mortality events are diverse, and often linked to climate change, it is challenging to predict the frequency and severity of future mass mortality events. As an alternative, we quantify the probability of population collapse depending on the frequency and intensity as well as the duration of mass mortality events. Based on 39 fish species, we show that the probability of collapse typically increases with increasing frequency, intensity, and duration of the mortality events. In addition, we show that the collapse depends on key traits such as natural mortality, recruitment variation, and density dependence. The presented framework provides quantitative estimates of the sensitivity of fish species to these increasingly common extreme events, which paves the way for potential mitigation actions to alleviate adverse impacts on harvested fish populations across the globe.

Meta-analysis reveals drivers of restoration success for oysters and reef community

Restoration aims to reverse global declines of foundation species, but it is unclear how project attributes, the physical setting, and antecedent conditions affect restoration success. In coastal seas worldwide, oyster reef restoration is increasing to counter historic habitat destruction and associated declines in fisheries production and biodiversity. Yet, restoration outcomes are highly variable and the factors that enhance oyster production and nekton abundance and diversity on restored reefs are unresolved. To quantify the drivers of oyster restoration success, we used meta-analysis to synthesize data from 158 restored reefs paired with unstructured habitats along the U.S. Gulf and Atlantic coasts. The average recovery of oyster production was 65% greater in subtidal (vs. intertidal) zones, 173% greater in polyhaline (vs. mesohaline) environments and increased with tidal range, demonstrating that physical conditions can strongly influence the restoration success of foundation species. Additionally, restoration increased the relative abundance and richness of nektonic fishes and invertebrates over time as reefs aged (at least 8 years post-construction). Thus, the restoration benefits for provisioning habitat and enhancing biodiversity accrue over time, highlighting that restoration projects need multiple years to maximize ecosystem functions. Furthermore, long-term monitoring of restored and control sites is needed to assess restoration outcomes and associated drivers. Lastly, our work reveals data constraints for several potential drivers of restoration outcomes, including reef construction material, reef dimensions, harvest pressure and disease prevalence. More experimental and observational studies are needed to target these factors and measure them with consistent methods across studies. Our findings indicate that the assisted recovery of foundation species yields several enhancements to ecosystem services, but such benefits are mediated by time and environmental conditions.

Shifting linguistic patterns in oyster restoration news articles surrounding the Deepwater Horizon disaster

Populations of the eastern oyster Crassostrea virginica are declining globally. With the loss of oysters, sustainable provision of natural resources and ecosystem services are also threatened. In 2010, the Deepwater Horizon (DWH) oil spill became the largest marine oil spill in history, imperiling coastal and marine habitats in the Gulf of Mexico. Whereas ecological restoration serves as an important tool in natural resource management, its success depends on achieving ecological objectives and meeting public expectations. However, little is known about how the public perceives ecological restoration—even less in the context of disasters. It has long been understood that mass media messaging helps shape public understanding. Documenting patterned representation of oyster restoration in mass media texts can help set goals, improve stakeholder communication, and ensure required support for restoration activities. To address this goal, this study asks, does newspaper language on the topic of oyster reef restoration change in relation to an environmental disaster? If so, how? A 1.1 million-word Deepwater Horizon Oyster Restoration (DHORN) Corpus—a comprehensive body of newspaper articles about oyster restoration from 3 national and 18 gulf-state newspapers—was developed for the period April 2008–April 2014. The distribution and deployment of collocates of oyster* across three DHORN subcorpora delimited by time (pre-, during, and post-Deepwater Horizon oil spill) were compared through iterative quantitative and qualitative analysis. Examination of salient collocates in use over time indicated an increase in the representation of oysters as quantifiable entities during/post-DWH; at the same time, there was a decrease in the representation of the roles of oyster* in the Gulf ecosystem. Furthermore, multiple propositions associating oysters and oyster restoration activity with DWH and oil spills were introduced into language use by the disaster and persisted for years afterwards. This association was not present pre-DWH. Understanding shifts in linguistic patterns of oyster restoration in news articles before, during, and after Deepwater Horizon can be used to deliberately refine communication between the conservation community and both journalists and policymakers to promote conservation initiatives.

Medium-term monitoring reveals effects of El Niño Southern Oscillation climate variability on local salinity and faunal dynamics on a restored oyster reef

Human activities and regional-scale climate variability drive changes in the ecology of coastal and marine ecosystems. Ecological restoration has emerged as a best-management practice to combat habitat degradation and restore lost ecological functions. However, relatively short project monitoring timeframes have limited our understanding of the effects of interannual climate cycles on water quality and restoration dynamics. We collected measurements on a 23-ha oyster reef constructed in the Gulf of Mexico to determine the relationship between El Niño Southern Oscillation (ENSO)-driven climate variability and local salinity patterns, and to evaluate the effects of this climate variability and salinity on oyster population dynamics and faunal community composition over a medium-term (five-year) timeframe. The role of ENSO-driven climate variability on local salinity patterns (primarily from changes in precipitation and evaporation) and faunal dynamics was investigated using the Oceanic Niño Index (ONI). Salinity was negatively correlated with ONI with an approximately 4-month lag. Higher ONI values (El Niño periods) were followed by reductions in salinity, increases in oyster recruitment and density, and reductions in resident motile fauna density and species richness. Lower ONI values (La Niña periods) had higher and less variable salinities, and higher areal coverage of restoration substrates by large oysters. ENSO-driven salinity reductions in the second year after reef construction coincided with a shift in resident motile faunal community composition that was maintained despite a second strong salinity reduction in year 5. Our results indicate that it is important to expand the typical monitoring timeframes to at least five years so that resource managers and restoration practitioners can better understand how both short-term environmental variability and longer-term climate cycles can affect the outcomes of restoration actions.

Romain SJ, Pedersen EI, Siddiqui S, Chappell PE, White JW, Armbrust KL, Brander SM. Salinity Alters Toxicity of Commonly Used Pesticides in a Model Euryhaline Fish Species (Menidia beryllina)

Changing salinity in estuaries due to sea level rise and altered rainfall patterns, as a result of climate change, has the potential to influence the interactions of aquatic pollutants as well as to alter their toxicity. From a chemical property point of view, ionic concentration can increase the octanol–water partition coefficient and thus decrease the water solubility of a compound. Biologically, organism physiology and enzyme metabolism are also altered at different salinities with implications for drug metabolism and toxic effects. This highlights the need to understand the influence of salinity on pesticide toxicity when assessing risk to estuarine and marine fishes, particularly considering that climate change is predicted to alter salinity regimes globally and many risk assessments and regulatory decisions are made using freshwater studies. Therefore, we exposed the Inland Silverside (Menidia beryllina) at an early life stage to seven commonly used pesticides at two salinities relevant to estuarine waters (5 PSU and 15 PSU). Triadimefon was the only compound to show a statistically significant increase in toxicity at the 15 PSU LC₅₀. However, all compounds showed a decrease in LC₅₀ values at the higher salinity, and all but one showed a decrease in the LC₁₀ value. Many organisms rely on estuaries as nurseries and increased toxicity at higher salinities may mean that organisms in critical life stages of development are at risk of experiencing adverse, toxic effects. The differences in toxicity demonstrated here have important implications for organisms living within estuarine and marine ecosystems in the Anthropocene as climate change alters estuarine salinity regimes globally.

Environmental forcing and predator consumption outweigh the nonconsumptive effects of multiple predators on oyster reefs

The ability to predict how predators structure ecosystems has been shown to depend on identifying both consumptive effects (CEs) and nonconsumptive effects (NCEs) of predators on prey fitness. Prey populations may also be affected by interactions between multiple predators across life stages of the prey and by environmental factors such as disturbance. However, the intersection of these multiple drivers of prey dynamics has yet to be empirically evaluated. We addressed this knowledge gap using eastern oysters (Crassostrea virginica), a species known to suffer NCEs, as the focal prey. Over 4 months, we manipulated orthogonally the life stage (none, juvenile, adult, or both) at which oysters experienced simulated predation (CE) and exposure to olfactory cues of a juvenile oyster predator (crab), adult predator (conch), sequentially the crab and then the conch, or none. We replicated this experiment at three sites along an environmental gradient in a Florida (USA) estuary. For both juvenile and adult oysters, survival was reduced solely by CEs, and variation in growth was best explained by among-site variation in water flow, with a much smaller and negative effect of predator cue. Adults exposed to conch cue exhibited reduced growth (an NCE), but this effect was outweighed by a positive CE on growth: Surviving oysters grew faster at lower densities. Finally, conch cue reduced larval settlement (another NCE), but this was swamped by among-site variation in larval supply. This research highlights how strong environmental gradients and predator CEs may outweigh the influence of NCEs, even in prey known to respond to predator cues. These findings serve as a cautionary tale for the importance of evaluating NCE processes over temporal scales and across environmental gradients relevant to prey demography.

Success of concrete and crab traps in facilitating Eastern oyster recruitment and reef development

BACKGROUND

Abundance of the commercially and ecologically important Eastern oyster, Crassostrea virginica, has declined across the US Eastern and Gulf coasts in recent decades, spurring substantial efforts to restore oyster reefs. These efforts are widely constrained by the availability, cost, and suitability of substrates to support oyster settlement and reef establishment. In particular, oyster shell is often the preferred substrate but is relatively scarce and increasingly expensive. Thus, there is a need for alternative oyster restoration materials that are cost-effective, abundant, and durable.

METHODS

We tested the viability of two low-cost substrates-concrete and recycled blue crab (Callinectes sapidus) traps-in facilitating oyster recovery in a replicated 22-month field experiment at historically productive but now degraded intertidal oyster grounds on northwestern Florida's Nature Coast. Throughout the trial, we monitored areal oyster cover on each substrate; at the end of the trial, we measured the densities of oysters by size class (spat, juvenile, and market-size) and the biomass and volume of each reef.

RESULTS

Oysters colonized the concrete structures more quickly than the crab traps, as evidenced by significantly higher oyster cover during the first year of the experiment. By the end of the experiment, the concrete structures hosted higher densities of spat and juveniles, while the density of market-size oysters was relatively low and similar between treatments. The open structure of the crab traps led to the development of larger-volume reefs, while oyster biomass per unit area was similar between treatments. In addition, substrates positioned at lower elevations (relative to mean sea level) supported higher oyster abundance, size, and biomass than those less frequently inundated at higher elevations.

DISCUSSION

Together, these findings indicate that both concrete and crab traps are viable substrates for oyster reef restoration, especially when placed at lower intertidal elevations conducive to oyster settlement and reef development.

Short-term low salinity mitigates effects of oil and dispersant on juvenile eastern oysters: A laboratory experiment with implications for oil spill response activities

Following the Deepwater Horizon oil spill, eastern oyster (Crassostrea virginica) reefs in the northern Gulf of Mexico were exposed to oil and various associated clean-up activities that may have compromised oyster reef health. Included in the exposure was oil, dispersant, and in some locales, atypical salinity regimes. Oil and dispersants can be detrimental to oysters and the effects of salinity depend on the level. In addition to these extrinsic factors, genetic diversity of oyster populations may help the oysters respond to stressors, as demonstrated in other systems. We used a 3×3×2 factorial design to experimentally examine the effects of oil/dispersed oil, intraspecific genetic diversity, and salinity on juvenile (ca. 25 mm shell height) oyster survivorship and growth during a 21-d exposure in a closed, recirculating system. The genetic effect was weak overall, oil and dispersed oil negatively affected juvenile oyster survivorship, and low salinity mitigated mortality in oil and dispersed oil treatments. Survivorship was about 40% greater in low-salinity than in mesohaline water for both oil and dispersed oil treatments, bringing survivorship in low salinity oil-only treatments to a similar level with low salinity controls (no oil). Oyster growth was minimal after 21 d but appeared to be negatively affected by oil and dispersed oil, and had a significant interaction with salinity. Our results may be informative for future decisions regarding oil spill response activities and suggest that a pulse of low salinity water may be a viable short-term mitigation option for oysters if filtration characteristics, exposure time, and water temperatures are all considered, in addition to weighing the costs and benefits of this type of response on other organisms and habitats.