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Source-Sink Dynamics in Marine Systems: Linking Recruitment, Dispersal, and Post-Settlement Processes in Space and Time
Source-sink dynamics may define the patterns in distribution and abundance of many marine and estuarine species.  In source habitats there is a demographic surplus (births + emigration > deaths + immigration), whereas in sinks a demographic deficit (deaths + immigration > births + emigration) leads to local extinction, without immigration from sources.  Moreover, total production in linked sources and sinks may be higher than in either alone, due to subsidies to sinks by emigrants from sources.  Hypotheses based on source-sink dynamics implicitly incorporate recruitment processes, physical transport mechanisms, dispersal behavior, post-settlement demography and habitat heterogeneity across various scales of space and time.  Many of the current paradigms of marine ecology can be subsumed or applied within the conceptual framework of source-sink dynamics.  In this project, we use extensive population data and a stage-structured, spatially-explicit matrix population model with dispersal to test hypotheses concerning survival, growth, reproduction and dispersal between subpopulations in putative source and sink habitats for the Baltic clam Macoma balthica.  The sources and sinks have been tentatively identified for the Macoma in two independent, spatially separated and environmentally different locations in Chesapeake Bay--the Rhode and York Rivers.  We hypothesize that there exists a system of vital source habitats (shallow mud flats) and linked sink habitats (adjacent sand flats; detrital muds) for Macoma which dictate its persistence and population fluctuations.  The nominal sources are hypothesized to contribute significantly more to the recruit pool than sinks; recruitment into sinks from the recruit pool is disproportionately higher than their reproductive output.  Food limitation may be the primary mechanism stimulating redistribution of recruits (postlarvae and young juveniles) between sources and sinks.  If emigrants from sinks reinvade source habitats, then total production in the population is enhanced through source-sink dynamics.  We posit that this represents the first comprehensive test of the existence and consequences of source-sink dynamics in a marine system.  This research is supported by the National Science Foundation (OCE9810624) and is a collaborative research effort with Anson Hines at the Smithsonian Environmental Research Center.


Spatial Dynamics and the Protection of Critical Habitats for the Blue Crab
Protection of critical spawning, mating, feeding, and nursery habitats is a key element of management strategies that seek to conserve exploited marine species, such that recruitment overfishing and population collapse are averted.  The use of marine protected areas (sanctuaries, corridors) is a potentially powerful tool for the conservation of critical habitats and heavily exploited species through effort control.  Although essential habitats have been readily identified for many exploited marine species, determination of the optimum quantity and spatial distribution (i.e., seascape) of essential habitats requiring protection to conserve spawning stock and recruitment remains largely theoretical.  Using the blue crab, a species with wide dispersal that supports the world’s second largest crab fishery, we examine the influence of recruitment processes, habitat quality, food availability, environmental stress, exploitation, and spatial distribution of protected critical habitats to the conservation and enhancement of spawning stock and recruitment.  My key role in this multi-investigator project is in evaluating the relative roles of food and refuge in determining the value of essential habitats to the distribution and abundance of crabs in nursery habitats and dispersal corridors.  Distribution of the blue crab can be affected by several factors including abundance of food, habitat type or complexity, and proximity to favorable currents.  Larger blue crabs are not likely to be controlled by top-down factors in many habitats (i.e., predation), since they obtain a size refuge at about 90 mm carapace width.  Alternatively, bottom-up factors (i.e., food availability) may affect crab abundance.  Specifically, bivalves comprise approximately 50% of the blue crab diet, although crabs secondarily consume conspecifics, polychaetes, amphipods, and other benthic prey.  Recent work indicates that blue crab densities are higher in areas with elevated clam densities, suggesting that habitats with abundant food are essential and should be protected.  This research is supported by NOAA - National Sea Grant.


Impacts of Low Dissolved Oxygen on Food-Web Dynamics in Benthic Communities of Chesapeake Bay
The role of environmental perturbations, such as hypoxia (<2 mg O2/L) and anoxia (0 mg O2/L), in determining the outcome of food-web dynamics is poorly known.  Hypoxic zones in marine systems are increasing in areal extent and duration due to heightened anthropogenic stresses such as eutrophication, particularly in estuarine systems such as Long Island Sound, Chesapeake Bay and the Gulf of Mexico.  Worldwide, there are over 50 dead zones due to hypoxia. Further worsening of environmental conditions due to global warming or escalating anthropogenic insults may alter the productivity base for food webs and their respective fisheries because many oxygen-stressed systems appear close to a threshold.
Hypoxia is generally thought to be detrimental because of the observed reductions in benthic faunal abundance associated with persistent severe hypoxia.  However, transfer of benthic production to higher trophic levels may be facilitated in hypoxic areas because of the vertical migration of infauna to shallower depths where they are more susceptible to epibenthic predators when hypoxia is not severe.  In contrast, where hypoxia is chronic and severe, epibenthic predators such as fish and crabs may not be able to enter hypoxic areas to exploit benthic prey. We are currently measuring dissolved oxygen levels and faunal responses at fine spatial scales across a gradient encompassing normoxic to anoxic conditions (shallow shoals to deep channels); these are compared with responses at normoxic control sites.  The study is conducted in two tributaries (York and Patuxent Rivers) of Chesapeake Bay that differ fundamentally in the severity and duration of hypoxia, and are therefore expected to have contrasting impacts upon trophic dynamics.  Impacts on a major epibenthic predator (blue crab, Callinectes sapidus) and its chief prey (Baltic clam, Macoma balthica), as well as other infaunal prey such as polychaetes, are quantified concurrently.  A demonstration of the impact of hypoxia on trophic transfer within the Chesapeake Bay benthic system can serve as a model for the other estuarine systems worldwide.  This work is supported by Maryland Sea Grant.


Predator-prey dynamics and evolutionary defense tactics for marine bivalves 
Persistence of prey species when faced with intense predation pressure is fostered by density-dependent survival and habitat features such as architecturally complex grasses or algae that furnish refugia from predation.  Infaunal bivalve molluscs, such as the thin-shelled clams Mya arenaria and Macoma balthica, are dominant species in soft-bottom estuarine and marsh systems and suffer heavy losses to epibenthic predators including the blue crab, Callinectes sapidus, and various demersal fish in Chesapeake Bay.  Using this predator-prey system, we are conducting a series of experiments that test habitat-specific and density-dependent mortality for subtidal, soft-bottom, deep-burrowing prey, and thereby enable development of a conceptual model linking predator-prey dynamics, habitat structure and evolutionary defense tactics for marine benthos.  We are assimilating the most basic feature of predator-prey dynamics, the functional response into a mechanistic model that incorporates the role of habitat and predator-prey dynamics in determining the major evolutionary defense tactics--armor and avoidance--of marine bivalves.

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Community Ecology, c/o Rochelle Seitz
 Virginia Institute of Marine Science
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