The role of epibenthic predators in structuring marine soft-bottom communities along an estuarine gradient

A unifying theory of community regulation in soft-bottom systems remains elusive, despite extensive field studies on factors controlling community structure. Here, I have (1) reviewed models of community regulation, (2) examined the role of predation in controlling benthic diversity along a salinity gradient, (3) examined effects of predation upon an abundant bivalve, Macoma balthica, and (4) revised a model of community regulation in an estuarine soft-bottom system. The Menge and Sutherland (MS) consumer stress model posits that consumers feed ineffectively in harsh environments, and the importance of physical disturbance, competition and predation varies with recruitment, environmental conditions, and trophic position. In this model, competition for resources depends directly upon the level of recruitment. I have revised the model to fit soft-bottom systems by changing the recruitment axis to a recruitment: resource ratio. Hence, the impact of a given level of recruitment depends upon resource availability. According to the MS model, predation is most important in determining community structure when environmental conditions are not severe. I investigated the applicability of the MS model in a soft-bottom estuarine community. I quantified predator abundance, prey abundance and diversity, and the differential effect of predation on species diversity and survival of an abundant prey species, Macoma balthica, along an estuarine gradient in two tributaries of Chesapeake Bay. Benthic diversity was lower in upriver high-stress habitats than downriver low-stress habitats, in agreement with predictions of the MS model. However, the following findings are inconsistent with model predictions: (1) predator abundance was greater upriver, (2) predation intensity and its impact on benthic diversity were greater upriver, and (3) predation-induced mortality of transplanted Macoma balthica clams, and natural mortality of clams were higher upriver. An alternative community regulation model applies to this system because higher predator abundance and predation intensity in higher environmental stress is contrary to the MS model predictions. Predators aggregated upriver where carbon production was increased, and prey were abundant. Hence, a more suitable model for this soft-bottom system is one that incorporates the effects of production and predation along with recruitment, competition and environmental stress

Quantifying finfish and blue crab use of created oyster reefs in the lower Chesapeake Bay

Structurally complex reefs created by the eastern oyster Crassostrea virginica provide a host of ecosystem services yet have experienced significant declines, prompting extensive restoration efforts. We investigate the use of created oyster reefs in the lower Bay by mobile finfish and blue crabs with field surveys and diet analysis. The results of this study provide insight into how restoration activities influence estuarine community dynamics and the provision of ecosystem services

Landscape-Level Impacts of Shoreline Development on Chesapeake Bay Benthos and Their Predators

Within the coastal zone, waterfront development has caused severe loss of shallow-water habitats such as salt marshes and seagrass beds. Little is known about the impact of habitat degradation and ecological value of subtidal shallow-water habitats, despite their prevalence. In coastal habitats, bivalves are dominant benthic organisms that can comprise over 50% of benthic prey biomass and are indicative of benthic production. We examined the effects of shoreline alteration in shallow habitats by contrasting the benthos of the subtidal areas adjacent to natural marsh, riprap, and bulkhead shorelines in three Chesapeake Bay subestuaries that differ in the level of shoreline development. In all cases, benthic abundance and diversity were higher in subtidal habitats near natural marsh than those near bulkhead shorelines; however, abundance and diversity were intermediate near riprap shorelines, and appeared to depend on landscape features. In heavily impacted systems such as the Elizabeth-Lafayette system, benthos adjacent to riprap was depauperate, whereas in less-developed tributaries (York River and Broad Bay), benthos near riprap was abundant and was similar to that near natural marsh shorelines. Furthermore, predator density and diversity were highest adjacent to natural marsh, intermediate near riprap, and low near bulkhead shorelines. There is thus a crucial link between natural marshes, benthic infaunal prey in subtidal habitats, and predator abundance. Restoration of living shoreline habitats is likely to have benefits for adjacent benthos and their predators. Protection and restoration of marsh habitats may be essential to the maintenance of high benthic production and consumer biomass in Chesapeake Bay. Moreover, the collective impacts of the system-wide, landscape-level features are felt from the benthos through higher trophic levels.https://scholarworks.wm.edu/vimsbooks/1124/thumbnail.jp

Loss of seagrass results in changes to benthic infaunal community structure and decreased secondary production

Seagrass beds have decreased in abundance and areal coverage over the past several decades. Although previous studies have examined the importance of seagrass for benthic community assemblages and abundances, the effect of seagrass on deep-dwelling, large (high-biomass) infauna and the importance for benthic secondary production in Chesapeake Bay have not been addressed. Using benthic suctions and push cores, we compared density, diversity, and secondary productivity of benthic communities in seagrass to those in other shallow-water habitats and estimated benthic secondary productivity lost in the York River due to loss of seagrass from 1971 to 2016. We examined four habitat types in the York River: unvegetated, Gracilaria spp., mixed seagrass (multiple seagrass species), and Zostera marina L. seagrass. Physical characteristics of habitat types and biomass of organisms were assessed, and annual secondary productivity was calculated using biomass and production-to-biomass ratios. Benthic density, diversity, secondary productivity, sedimentary chlorophyll a, and percent sand were all highest in seagrass beds with Z. marina alone. Approximately 35% of benthic secondary productivity, or 1.51 × 108 g C yr–1, was lost in the York River in 1971–2016 due to the loss of seagrass beds to unvegetated substrate. The loss of seagrass in the York River over time and the associated decrease in benthic secondary productivity that we estimated could have negative consequences for the productivity of epibenthic predators. Our data emphasize the importance of conservation and restoration of seagrass

Direct and indirect impacts of shoreline development on shallow-water benthic communities in a depauperate estuarine system

Modification of natural coastlines is prevalent as human coastal populations swell and effects of global climate change become clearer. We investigated effects of shoreline hardening and environmental factors on benthic infauna and trophic structure in the Patuxent River, Maryland, a stressed mesohaline Chesapeake Bay tributary. We characterized differences in density, diversity, biomass, and trophic structure for large (\u3e3 mm) and small (\u3e500 μm) infauna adjacent to natural marsh, riprap, and bulkhead (i.e., seawall) shores throughout the river. Akaike information criterion model comparisons were used to assess the evidence for differences in benthic infaunal structure using primary (shoreline type) and secondary (e.g., sediment grain size, predator abundance) variables. There was strong evidence for secondary factors to explain reduced biomass of infauna adjacent to developed shorelines. For large infauna, evidence suggested that shorelines with riprap had reduced diversity, and with bulkhead had increased diversity. Increased wave energy and chlorophyll-a were associated with high densities for both size fractions riprap shorelines. Trends suggested high biomass and more carnivores, omnivores, and deposit feeders adjacent to natural marshes, compared to low biomass and more filter feeders at developed shorelines. While similar studies in lower Chesapeake Bay systems have shown clear effects of shoreline type on benthic communities, the extensive development in the Patuxent River may contribute to larger-scale stress, yet some shoreline-specific effects were detected. Non-parametric tests revealed differences in infaunal communities by shoreline type and river zone. Thus, the benthic community in this estuary is driven by local shoreline effects, as well as large-scale physical and biotic factors

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

Saved by the shell: Oyster reefs can shield juvenile blue crabs Callinectes sapidus

Juvenile blue crabs Callinectes sapidus use seagrass and other structured habitats as refuges from predation. Oyster reef habitats provide structural complexity that may offer refuge, but the value of these habitats for juvenile blue crabs has not been examined. We quantified survival of juvenile C. sapidus in structured oyster reef habitat versus unstructured soft-bottom habitat. In a field tethering experiment in the York River, lower Chesapeake Bay (USA), juvenile C.sapidus (10−50 mm carapace width [CW]) were tethered in sand (n = 40) or oyster reef (n = 39)habitats at subtidal sites 1−2 m deep. An underwater camera system was used to record predation activity during 24 h trials. Juvenile crab survival was significantly higher on the oyster reef habitat (53.8%) than on bare sand (15.0%), and tended to increase with crab CW in both habitats. The main successful predators on juvenile blue crabs were northern pufferfish Sphoeroides maculatus in the oyster reef habitat and adult blue crabs in the sand habitat. The high survival rate of juvenile C. sapidus in oyster reef habitats suggests that oyster reefs include physical habitat complexity that may offer refuge from predators. Restored and natural oyster reefs could provide an alternative nursery habitat for juvenile blue crabs, expanding the ecosystem services provided by restored oyster reefs

Individual, population, and ecosystem effects of hypoxia on a dominant benthic bivalve in Chesapeake Bay

Hypoxia is an environmental stressor that affects abundance, biomass,diversity, and ecosystem function of benthic assemblages worldwide, yet its collective impact at individual, population, and ecosystem levels has rarely been investigated. We examined the effects of hypoxia on the biomass-dominant clam,Macoma balthica, in the York and Rappahannock Rivers (Chesapeake Bay, USA). We (1) surveyed the M. balthica populationsin both rivers in 2003 and 2004, (2) determined the effects of low dissolved oxygen (DO) on M.balthica fecundity in a laboratory experiment, and (3) employed a predator-exclusion fieldexperiment to establish the effects of hypoxia and prey density on predation upon M. balthica.The resultant data were used to parameterize a matrix model, which was analyzed to define potential effects of hypoxia at the population level. In both rivers, hypoxia decreased individual clam growth and caused local extinction of populations. Hypoxia reduced egg production of M. balthicaby 40%and increased protein investment per egg. In the predator-exclusion field experiment, hypoxia magnified predation rates threefold and altered the functional response of predators toM. balthicafrom a stabilizing type III functional response to a destabilizing type II functional response. In a density-independent matrix model, hypoxia resulted in coupled source–sink metapopulation dynamics, with hypoxic areas acting as black-hole sinks. Increases in the spatial and temporal extent of hypoxia caused the populations to decline toward extinction. In a second model that incorporated density dependence, under mild hypoxic conditions trophic transfer from M. balthica to predators increased, but at increased spatial or temporal extent of hypoxia trophic transfer decreased. The major declinein trophic transfer to predators under severe hypoxia resulted from diversion of M. balthica biomass into the microbial loop. Our model predicted that there are multiple stable states forM. balthic apopulations (high and very low densities), such that the saddle point (threshold at which the population switches from one state to the other) increased and resilience decreased with the spatial extent of hypoxia. We underscore how effects of a stressor at the individual level can combine to have substantial population and ecosystem-level effect

Efficacy of blue crab spawning sanctuaries in Chesapeake Bay

Sanctuaries can potentially protect a significant fraction of the spawning stock, and thereby sustain heavily exploited populations. Despite the worldwide use of marine and estuarine spawning sanctuaries, the effectiveness of such sanctuaries remains untested. We therefore attempted to quantify the effectiveness of the spawning sanctuaries for adult female blue crabs (Callinectes sapidus) in Chesapeake Bay. We used baywide winter dredge survey data to estimate the potential spawning stock prior to the major exploitation period, and summer trawl survey data to estimate spawning stock abundance within the Lower Bay Spawning Sanctuary and adjacent Bayside Eastern Shore Sanctuary during the reproductive period. Hence, we were able to approximate the percentage of the potential spawning stock that was protected by both sanctuaries after exploitation. On average, approximately 16% of the potential spawning stock survived to reach the Lower Bay Spawning Sanctuary and Bayside Eastern Shore Sanctuary. Even under a best-case scenario (i.e., crab residence time of 2 weeks), the sanctuaries only protected an estimated 22% of the potential spawning stock, which is well below the percentage recommended by recent stock assessments for sustainable exploitation (28%). In the worst case, a mere 11% of the potential spawning stock survived to reach the spawning sanctuaries. Hence, we recommend a substantial expansion of the spawning sanctuaries, as well as the complementary protection of other life stages in critical habitats, such as nursery grounds and dispersal corridors. Furthermore, traditional fisheries management measures (e.g., effort control) should be used in concert with sanctuaries to thwart impediments to effective implementation of the sanctuaries, such as redirected fishing effort.https://scholarworks.wm.edu/vimsbooks/1084/thumbnail.jp