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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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