Marine benthic predator-prey interactions and global change.

Anthropogenic stressors such as habitat loss, extreme weather events, and acidification can change predator-prey interactions. An understanding of the mechanisms by which these stressors impact predator-prey interactions may elucidate the fate of bivalves in the face of global change. My dissertation research informs management of marine resources in Chesapeake Bay, which has experienced substantial seagrass and oyster reef loss, increased storm activity, and combined estuarine and atmospheric CO2 acidification. In my dissertation, I used field survey data, field caging experiments, laboratory mesocosm experiments, time-series analysis, and density-dependent mathematical models to assess the role of habitat, major storm events, acidification, and predators on bivalve distribution in lower Chesapeake Bay, with a special focus on the commercially important, thin-shelled clam species Mya arenaria, which has declined significantly in the past few decades.;In field surveys, seagrass supported one additional bivalve functional group (based on bivalve morphology and feeding mode) than all other habitat types, and bivalve diversity was 2754% higher in seagrass than in shell hash, oyster shell, coarse sand, and detrital mud habitats. The odds of finding M arenaria were higher in seagrass than in all other habitats. Predators likely consumed seasonal pulses of juveniles each year. In field caging experiments, blue crabs Callinectes sapidus were likely responsible for most of the mortality of juvenile M arenaria, which was 76.6% higher for caged juveniles than for uncaged individuals over 5 d. In mesocosm feeding trials, M arenaria maintained a low-density refuge from predation by blue crabs, and had higher survival in oyster shell or shell habitats as compared to sand or seagrass habitats. Time series analysis suggested M arenaria was subjected to a storm-driven phase shift to low abundance in 1972, which has been maintained by blue crab predation. Density-dependent predator-prey models parameterized with data from laboratory and field experiments confirmed the presence of a coexistence steady state at low densities of M arenaria, providing the theoretical proof-of-concept that M arenaria can exist in a low-density stable state in the face of blue crab predation. Acidification altered behavior of both predator (C. sapidus) and prey (M arenaria), resulting in no net change in proportional mortality of clams between acidified and control feeding trials.;My dissertation examined multiple lines of evidence to address the importance of structured habitat, extreme weather events, and acidification in the mediation of predator-prey dynamics. For the crab-bivalve predator-prey interactions examined here, predation exacerbated the effects of some anthropogenic stressors (habitat loss, extreme weather events) and ameliorated the impacts of other stressors (acidification) on bivalve prey. An understanding of density-dependent predation is a necessary component of an adaptive management strategy that can cope with climate change

Role of habitat and predators in maintaining functional diversity of estuarine bivalves

Habitat loss is occurring rapidly in coastal systems worldwide. In Chesapeake Bay, USA, most historical oyster reefs have been decimated, and seagrass loss is expected to worsen due to climate warming and nutrient pollution. This loss of habitat may result in declining diversity, but whether diversity loss will equate to loss in ecosystem function is unknown. A bivalve survey was conducted in a variety of habitat types (seagrass, oyster shell, shell hash, coarse sand, detrital mud) in 3 lower Chesapeake Bay sub-estuaries from spring 2012 through summer 2013 to examine the correlation between bivalve densities, habitat type, habitat volume (of material retained on 3 mm mesh), and predator density. Bivalves were analyzed as functional groups based on feeding mode, living position, and predator defense strategy. On average, seagrass supported one additional functional group, and diversity was increased 68-94%, in seagrass compared to the other habitats examined. Species richness and functional group richness were positively correlated with habitat volume. The greatest densities of deposit-feeding bivalves were in detrital mud habitats, the greatest densities of thin-shelled and surface-dwelling bivalves were in seagrass habitats, and the greatest densities of armored bivalves were in oyster shell habitats. Small, thin-shelled bivalves were negatively correlated with densities of predators, including blue crabs Callinectes sapidus and cownose rays Rhinoptera bonasus. Overall, bivalve diversity was as - sociated with habitat type, habitat volume, and predator densities. These results suggest that all habitats, and particularly seagrass, play a role in maximizing bivalve functional diversity in Chesapeake Bay

Effects Of Estuarine Acidification On An Oyster-Associated Community In New South Wales, Australia

Many of the features that make estuaries among the most productive natural systems on earth also make them prone to acidification. Understanding the effects of estuarine acidification on different components of an ecological community is an important step in identifying indicators of ecosystem degradation. This study examined the impact of estuarine acidification, as a result of acid sulfate soil runoff, on wild Sydney rock oysters Saccostrea glomerata and their associated epifaunal communities in estuaries experiencing acid sulfate soil runoff in New South Wales, Australia. The responses of oysters and their invertebrate epifaunal communities to chronic acidification (greater than 6 mo; represented by oyster source site conditions) were assessed by examining the differences in oyster communities associated with moderate acidification (3.5 km from the source of acidification) or low acidification (8.2 km from the source). Oysters from moderate-or low-acidification sites were transplanted to a site with high exposure to acidification (less than 3 km from the source) or back to their original source sites (control) to mimic episodic acidification (2 wk). Epifaunal mussels Xenostrobus securis and limpets Patelloida mimula showed a negative association with oyster mortality, suggesting that these communities are closely tied to oyster survival. Oyster-associated epifaunal communities exposed to both chronic and episodic acidification were significantly different from communities with low exposure. Epifaunal communities exposed to episodic acidification were significantly less diverse than the control. Spionid and syllid worms were significantly less abundant and the mussel X. securis was significantly more abundant on oysters with moderate exposure or chronic exposure to acidification, as compared with communities from areas with low exposure to acidification. The mussel X. securis and the snail Bembicium auratum were significantly less abundant in oyster communities that were exposed to episodic acidification, as compared with the control. In systems where community composition depends on a single habitat-forming species, maintaining resistance may rely on the ability of that species to persist in the face of environmental stress

Are predator−prey model predictions supported by empirical data? Evidence for a storm-driven shift toan alternative stable state in a crab−clam system

A dynamic systems approach can predict steady states in predator−prey interactions,but there are very few examples of predictions from predator−prey models conforming to empirical data. Here, we examined the evidence for the low-density steady state predicted by a Lotka-Volterra model of a crab−clam predator−prey system using data from long-term monitoring, and data from a previously published field survey and field predation experiment. Changepoint analysis of time series data indicate that a shift to low density occurred for the soft-shell clam Mya arenaria in 1972, the year of Tropical Storm Agnes. A possible mechanism for the shift is that Agnes altered predator−prey dynamics between M. arenaria and the blue crab Callinectes sapidus, shifting from a system controlled from the bottom up by prey resources, to a system controlled from the top down by predation pressure on bivalves, which is supported by a correlation analysis of time series data. Predator−prey ordinary differential equation models with these 2 species were analyzed for steady states, and low-density steady states were similar to previously published clam densities and mortality rates, consistent with the idea that C. sapidusis a major driver of M. arenaria population dynamics. Relatively simple models can predict shifts to alternative stable states,as shown by agreement between model predictions (this study) and published field data in this system. The preponderance of multispecies interactions exhibiting nonlinear dynamics indicatesthat this may be a general phenomenon

Restored freshwater flow and estuarine benthic communities in the northern Gulf of Mexico: research trends and future needs

Restoring river connectivity to rebuild and sustain land is a promising restoration strategy in coastal areas experiencing rapid land loss, such as the Mississippi river delta. Results of these large-scale hydrologic changes are preliminary, and there exists limited empirical evidence regarding how benthic communities will respond, specifically in Barataria Bay and Breton Sound in southeast Louisiana. In this review, the body of existing research in this geographic region pertaining to the drivers of benthic community response that are related to restored freshwater flow and sediment deposition is examined. Overall trends include (1) potential displacement of some species down-estuary due to reduced salinities; (2) temporary lower diversity in areas closest to the inflow; (3) increased benthic production along the marsh edge, and in tidal bayous, as a result of nutrient loading; (4) more habitat coverage in the form of submerged aquatic vegetation; and (5) reduced predation pressure from large and/or salinity-restricted predators. These trends highlight opportunities for future research that should be conducted before large-scale hydrologic changes take place

Acidification alters predator-prey interactions of blue crab Callinectes sapidus and soft-shell clam Mya arenaria

Acidification due to anthropogenic CO2 pollution, along with episodic or persistent acidification that occurs in coastal environments, will likely result in severe seasonal acidification in estuarine environments. Acidification decreases the fitness of individual species, but the degree to which predator-prey interactions will be impacted is largely unknown. This mesocosm study examined the effect of CO2 acidification on crab-bivalve predator-prey interactions involving two commercially important Chesapeake Bay species, the blue crab Callinectes sapidus and the soft-shell clam Mya arenaria. In particular, the direct effects of CO2 acidification on clam growth and behavior, and the indirect effects of CO2 acidification on interactions between crabs and clams were examined. Mya arenaria were grown in CO2-acidified water (pH 7.2) or ambient conditions (pH 7.8) for 30 days. To determine the effect of acidification on clam responsiveness to mechanical disturbance, a probe was slowly moved towards clams until they ceased pumping (a behavior to avoid detection by predators), and the distance between the probe and the clam's siphon was noted. Clams were exposed to predation by C. sapidus, which were held under acidified or ambient conditions for 48 h. Callinectes sapidus handling time, search time, and encounter rate were measured from video. Acidified clams had lighter shells than ambient clams, indicating that shell dissolution occurred. Acidification reduced the responsiveness of M. arenaria to a mechanical disturbance that simulated an approaching predator. As compared to ambient trials, crabs in acidified trials had higher encounter rates; however, this was offset by crabs taking longer to find the first clam in trials, and by increased occurrence of crabs eating only a portion of the prey available. As a result, there was no net change in predation-related clam mortality in acidified trials as compared to ambient conditions. Understanding how acidification will impact food webs in productive estuarine environments requires an examination of the direct impacts of acidification on organism behavior and physiology, as well as indirect effects of acidification mediated through predator-prey interactions

Variability in estuarine habitat use of a threatened species in the northern Gulf of Mexico: implications for coastal restoration

Coastal restoration projects often require sediment resources borrowed from estuarine or marine ecosystems. A generalized additive modeling (GAM) framework was used to characterize fish-habitat relationships and predict spatiotemporal and ontogenetic shifts of Gulf sturgeon distribution within an impacted estuary in the northern Gulf of Mexico, Lake Pontchartrain. Habitat use and movement of Gulf sturgeon (n = 103) was monitored using an array of acoustic receivers (n = 81) from 2016 to 2019. Telemetry data was paired with environmental conditions hindcast from a high-resolution hydrodynamic model to develop GAMs for three life stages of Gulf sturgeon: juvenile, sub-adult, and adult. Model results demonstrated clear spatial, temporal, and ontogenetic shifts in habitat use for Gulf sturgeon in the Lake Pontchartrain estuary. The importance of spatial and temporal variables in the final models of all life stages suggests that distribution of Gulf sturgeon within the estuary is more likely driven by biology (prey distribution) rather than environment. Interestingly, dependence of Gulf sturgeon on the estuary for overwintering habitat declined with ontogeny. The northeastern shoreline of Lake Pontchartrain was identified as potential nursery habitat for overwintering juvenile sturgeon from the Pearl River, while eastern portions of the estuary and associated channels provide critical staging habitat and corridors of connectivity for migrating sub-adult and adult sturgeon. Given the clear need to prioritize conservation of Gulf sturgeon while restoring coastal marshes, findings from this study provide needed information to shape future coastal restoration and inform the siting and timing of future dredging operations in Lake Pontchartrain

A Meta-Analytical Review of the Effects of Environmental and Ecological Drivers On the Abundance of Red Snapper (\u3ci\u3eLutjanus campechanus\u3c/i\u3e) In the U.S. Gulf of Mexico

We conducted a meta-analysis to summarize current knowledge on the effects of environmental and ecological drivers on the abundance of red snapper (Lutjanus campechanus) within the U.S. Gulf of Mexico. We reviewed 1252 published research articles and extracted or calculated effect sizes for 12 drivers from 82 independent studies within 26 articles that met our inclusion criteria. We used a fixed-effect model to calculate the absolute value of the mean effect size of each driver by age class studied and pooled across age classes to estimate effects on the overall abundance of red snapper. Habitat complexity and intra-specific competition had large effects on overall abundance and juvenile abundance, while habitat type and protection from predators showed medium to large effects on age 0 recruits and juvenile red snapper, and the mean effect of all drivers studied for adult red snapper were small or had no effect on abundance. Our results provide systematic support for the role of density-dependent mechanisms (habitat quality and availability, competition, predation) in shaping the regional abundance of red snapper, particularly during the juvenile stages. Sensitivity analyses indicated that issues with non-independence (e.g. within-article correlation), between-study heterogeneity, and publication bias influenced the magnitude and certainty of effect size estimates in a subset of drivers. Thus, our meta-analytical review also highlights the need for more empirical research on certain drivers (e.g. temperature, hypoxia) to improve our understanding of the factors that shape the regional abundance of red snapper