G. Plaia

Rapid subduction of organic matter by maldanid polychaetes on the North Carolina slope

In situ tracer experiments conducted on the North Carolina continental slope reveal that tube-building worms (Polychaeta: Maldanidae) can, without ingestion, rapidly subduct freshly deposited, algal carbon ( 13 C-labeled diatoms) and inorganic materials (slope sediment and glass beads) to depths of 10 cm or more in the sediment column. Transport over 1.5 days appears to be nonselective but spatially patchy, creating localized, deep hotspots. As a result of this transport, relatively fresh organic matter becomes available soon after deposition to deep-dwelling microbes and other infauna, and both aerobic and anaerobic processes may be enhanced. Comparison of tracer subduction with estimates from a diffusive mixing model using 234 Th-based coefficients, suggests that maldanid subduction activities, within 1.5 d of particle deposition, could account for 25-100% of the mixing below 5 cm that occurs on 100-day time scales. Comparisons of community data from the North Carolina slope for different places and times indicate a correlation between the abundance of deep-dwelling maldanids and the abundance and the dwelling depth in the sediment column of other infauna. Pulsed inputs of organic matter occur frequently in margin environments and maldanid polychaetes are a common component of continental slope macrobenthos. Thus, the activities we observe are likely to be widespread and significant for chemical cycling (natural and anthropogenic materials) on the slope. We propose that species like maldanids, that rapidly redistribute labile organic matter within the seabed, probably function as keystone resource modifiers. They may exert a disproportionately strong influence (relative to their abundance) on the structure of infaunal communities and on the timing, location and nature of organic matter diagenesis and burial in continental margin sediments.

Demographic Responses of Estuarine Polychaetes to Pollutants: Life Table Response Experiments

Capitella sp. I and Streblospio benedicti are infaunal, deposit-feeding polychaetes that occur in estuaries and littoral wetlands throughout much of the United States. Life table response experiments (sensu Caswell 1989a) were carried out in the laboratory to compare the demographic responses of these species to three common sources of estuarine contamination or enrichment: sewage (Milorganite), blue-green algae (Spirulina sp.), and hydrocarbons (No. 2 fuel oil). Life table data were used to generate two population projection models (a fully age-classified model and a simple two-stage model) for each species in each treatment and in a salt marsh sediment control. These models were used to quantify the effects of treatments on survival, reproduction, and age at maturity, and hence on population growth rate. For both species, survival was high in all treatments except the blue-green algae treatment, where oxygen depletion (to <1 mL/L) occurred. Treatments had dramatic effects on age at maturity, fertility, and generation time, which differed between species and among contaminants. Population growth rates (X) were higher in Capitella sp. I than in S. benedicti for all treatments, primarily due to earlier maturation and a fertility advantage exhibited by Capitella during the first few weeks of reproduction. In Capitella sp. I, explosive increases in X were seen in the sewage (X = 5.31) and algae (X = 2.81) enrichments relative to the control (X = 1.86) and the hydrocarbon treatments (X = 1.67). Reduced maturation time and increases in age-specific fertility associated with rapid growth and large body size were responsible. Hydrocarbons reduced X primarily through delayed maturation and reduced age-specific fertility. Population growth rates of S. benedicti in the hydrocarbon treatment (X = 1.11) and algae treatment (X = 1.09) were reduced relative to the control (X = 1.46) and sewage treatments (X = 1.41). The hydrocarbon reduction resulted from delayed maturity and reduced fertility, whereas the algal effects were caused by reductions in both juvenile survival and fertility. Our analyses revealed that Capitella sp. Is population growth rate was less sensitive than that of S. benedicti to these three common forms of estuarine contamination, that different sources of organic enrichment (sewage and blue-green algae) introduced at the same C and N levels could have varying demographic effects, and that when two contaminants (hydrocarbons and blue-green algae) caused similar reductions in population growth rate in a species (Streblospio), the underlying mechanisms may have differed. For both species all demographically important effects of contaminants occurred early in life, suggesting a need to focus on juveniles and young adults in field and laboratory testing. The experiments performed here demonstrated the sensitivity of polychaete demo- graphic properties to the condition of estuarine sediments. This sensitivity may be exploited to evaluate organic enrichment and hydrocarbon contamination in field settings.

Effects of sediment amended with sewage, algae, or hydrocarbons on growth and reproduction in two opportunistic polychaetes

Abstract The effects of organic- (sewage and blue-green algae) and hydrocarbon- (no. 2 fuel oil) amended sediments on growth schedules, age and size at maturity, fecundity, and reproductive output were examined for the opportunistic polychaetes Streblospio benedicti Webster and Capitella sp. I. The two species responded very differently to the amended sediments. For S. benedicti, asymptotic size was reduced and age at first reproduction occurred later in the algae and hydrocarbon treatments compared to the marsh mud only and sewage treatments. Organic- and hydrocarbon- amendedsediments did not affect per brood measures of fecundity or C and N investment in S. benedicti. In contrast, Capitella sp. I exhibited strong, positive responses to the organically amended sediments; this was seen in terms of more rapid growth (2 × ), younger age at first reproduction (50%), larger asymptotic size (6 × ), and higher per brood fecundity and C and N investment (4 × ). Reproductive output, a relative measure of reproductive investment, was not directly affected by treatments in either species. For Capitella sp. I in organically enriched settings, the benefits of larger body size appear to include higher per brood fecundity without increasing the relative cost (in terms of reproductive output) of producing a brood of young. The population explosions of Capitella sp. I in response to organic enrichment are the result of earlier reproduction and increased body size and fecundity. Positive population-level responses of S. benedicti to contaminated sediments may be the result of its ability to tolerate conditions that other members of a community do not.

The remineralization of organic carbon on the North Carolina continental slope

The sources and fates of metabolizable organic carbon were examined at three sites on the North Carolina slope positioned offshore of Cape Fear, Cape Lookout and Cape Hatteras. The13C/12C compositions (δ13C) of the solid phase organic matter, and the dissolved inorganic carbon (ΣCO2) produced during its oxidation, suggested that the labile fraction was predominantly marine in origin. The ΣCO2 concentration gradient across the sediment-water interface, and by inference the ΣCO2 flux and production rate, increased northward from Cape Fear to Cape Hatteras. Methane distributions and ΣCO2 δ13C values suggest that the rate of anaerobic diagenesis increased northward as well. The differences in sedimentary biogeochemistry are most likely driven by an along-slope gradient of reactive organic carbon flux to the seabed. This trend in reactive organic carbon flux correlates well with macrofaunal densities previously observed at the three sites. Proximity to the shelf and the transport of particulate material by surface boundary currents may control the deposition of metabolizable material on the Carolina slope. Evidence for methanogenesis was found only on the Cape Hatteras slope. The methane, which was produced at a depth of approximately 1 m in the seabed, was consumed nearly quantitatively in the biologically mixed layer as it diffused upward. Irrigation of the sediments by infauna may have provided the necessary oxidant for the consumption of the methane.

Pilot Evaluation of Early Juvenile Blue Crab Stock Enhancement Using a Replicated BACI Design

We quantified whether local populations of early juvenile blue crabs (J1–2) could be enhanced through the translocation of crabs to underutilized nursery habitats, and if enhancement success, survival, and potential impacts of stocked crabs on their benthic prey varied in a density-dependent manner. Using plankton nets, ∼143,000 blue crab megalopae were collected as they ingressed into Pamlico Sound, NC. Of these, ∼13,800 early juvenile blue crabs (J1–2 stages) were then stocked at potential nursery sites relatively far removed (32–70 km) from their initial settlement areas using a replicated before-after control impact (BACI) experimental design. On average, there was negative enhancement success (−34%) five weeks after local crab enhancement, and no evidence of density-dependent enhancement success, mortality, or impact on potential crab prey. Poor stocking success was likely due to pelagic emigration from enhancement sites relative to controls. Attempts to assess the feasibility of stocking blue crabs at local scales of small coves should (i) probably not consider J1–2 stages because of their apparent propensity to emigrate from these areas, or (ii) further assess the effects of geomorphology and wind fetch of release sites on density-dependent emigration.

Timing and route of migration of mature female blue crabs in a tidal estuary

Information on migration patterns is critical to using no-take migratory corridors and marine reserves to protect the spawning stock of commercially exploited species. Both active and passive acoustic tracking methods quantified movement of commercially and ecologically important blue crabs in the White Oak River estuary, NC, USA. We targeted post-mating female crabs migrating down-estuary to oceanic spawning grounds. Crabs travelled approximately 14.1 km mainly in deeper channels and over 12–26 days from mating areas to spawning grounds. No crabs were detected migrating down-estuary in the autumn and only 30% were detected migrating down-estuary in spring. None of the crabs detected near spawning grounds were detected or recaptured back up-estuary, suggesting that they either (i) do not return to the estuary after a one to two week period in the spawning area or (ii) were captured by fishermen. The results from this study demonstrate that (1) acoustic transmitters coupled with passive acoustic receivers provided reliable and valuable data on migration patterns of mature female blue crabs and (2) mature female blue crabs are capable of migrating primarily within deep channels to spawning grounds shortly after insemination.