Distribution and relative importance of jellyfish in a region of hydrothermal venting

Net sampling to 3000 m depth at Endeavour Ridge in the northeast Pacific in July 1991–1994 shows that medusae in the immediate vicinity of the hydrothermal vent fields often make up a larger proportion of the total zooplankton abundance and biomass from mesopelagic to bathypelagic depths than in the surrounding waters. This was particularly evident in the dominant Trachymedusae, and least evident in the siphonophores. In addition, the large red Scyphomedusa Stygiomedusa gigantea was a major biomass component in the region of the deep (1000–1800 m depth) migrating scattering layers at the vent field, but was not found in any net tows greater than 10 km away from vents. There is no concurrent increase in relative or percent biomass of fish or chaetognaths, which are the other major predators in the community. We hypothesize that predaceous medusae respond opportunistically to the enhanced zooplankton biomass throughout the water column around vents in spring to early summer, in a way that other predators do not.     

Ascending and descending particle flux from hydrothermal plumes at Endeavour Segment,Juan de Fuca Ridge

Bio-acoustic surveys and associated zooplankton net tows have documented anomalously high concentrations of zooplankton within a 100 m layer above the hydrothermal plumes at Endeavour Segment, Juan de Fuca Ridge. These and other data suggest that congregating epi-plume zooplankton are exploiting a food substrate associated with the hydrothermal plume. Ascending, organic-rich particles could provide a connection. Consequently, two paired sequentially sampling ascending and descending particle flux traps and a current meter were deployed on each of three moorings from July 1994 to May 1995. Mooring sites included an on-axis site (OAS; 47°57.0′N, 129°05.7′W) near the main Endeavour vent field, a “down-current” site 3 km west of the main vent field (WS), and a third background station 43 km northeast of the vent field (ES). Significant ascending and descending particle fluxes were measured at all sites and depths. Lipid analyses indicated that ascending POC was derived from mid-depth and deep zooplankton whereas descending POC also contained a component of photosynthetically derived products from the sea surface. Highest ascending POC fluxes were found at the hydrothermal plume-swept sites (OAS and WS). The limited data available, however, precludes an unequivocal conclusion that hydrothermal processes contribute to the ascending flux of organic carbon at each site. Highest ascending to descending POC flux ratios were also found at WS. Observed trends in POC, PMn/PTi, and PFe/PTi clearly support a hydrothermal component to the descending flux at the plume-swept WS site (no descending data was recovered at OAS) but not at the background ES site. Alternative explanations for ascending particle data are discussed. First-order calculations for the organic carbon input (5–22 mg C m⁻² d⁻¹) required to sustain observed epi-plume zooplankton anomalies at Endeavour are comparable both to measured total POC flux to epi-plume depths (2–5 mg C m⁻² d⁻¹: combined hydrothermal and surface derived organic carbon) and to estimates of the total potential in situ organic carbon production (2–9 mg C m⁻² d⁻¹) from microbial oxidation of hydrothermal plume H₂, CH₄ and NH₄⁺.     

Modelled effects of primary and secondary production enhancement by seamounts on local fish stocks

This paper addresses how large aggregations of fish found on many seamounts are sustained. We used a generic seamount ecosystem model from the Northeast Atlantic to examine the impact of a potential increase of local primary production on higher trophic levels, to quantify the immigration of allochthonous micronekton that would be required to maintain a “typical” seamount community, and to quantify if the necessary immigration ratios could be supported by local oceanographic conditions. Our simulation predictions indicate a lack of autochthonous resources in the system to support large amounts of seamount aggregating fish. In other words, autochthonous seamount production may be responsible for sustaining only a small amount of its total biomass. Additionally, our study supports the idea that enhancement of primary productivity also cannot sustain large aggregations of seamount fish. Our seamount model, which took into account high abundances of fish, marine mammals, seabirds and tuna, required a total immigration of allochthonous micronekton of 95.2 t km⁻² yr⁻¹ less than the potential available biomass after considering the immigration of prey based upon average current velocities and prey standing stocks in oceanic waters. Our model predicted that the horizontal flux of prey would be sufficient to sustain the rich communities living on seamounts.     

Greenland halibut diet in the Northwest Atlantic from 1978 to 2003 as an indicator of ecosystem change

The Northwest Atlantic marine community underwent dramatic changes during the last 30 years, including the collapse of many groundfish stocks and an increase in shrimp populations. Greenland halibut Reinhardtius hippoglossoides is an important commercial species and one of the top fish predators in this system. It is a large, wide-ranging flatfish that is found at depths up to 2200 m and it has an opportunistic diet which makes it a potential candidate for an ecosystem indicator. Analysis of stomach contents of Greenland halibut between 1978 and 2003 indicates that diet composition reflects the major changes in community structure. Over the entire period there was a clear increase in the importance of invertebrates, particularly after 1992. This change was associated with a higher importance of Pandalus shrimp and Gonatus squid and a protracted reliance on zooplankton by predators under 25 cm length. Capelin Mallotus villosus was the dominant prey between 1978 and 1992 for predators in the 12–63 cm range, but its importance dropped off drastically in the mid 1990s. Levels of main prey in the diet of Greenland halibut correlated well with fishery-independent surveys. Greenland halibut sample capelin well, compared to bottom trawl surveys and acoustic surveys. Greenland halibut consumed small shrimp which are not routinely caught by surveys and may be important in deriving information on year classes and growth of shrimp. Our results suggest that Greenland halibut's diet is a useful tracker of ecosystem change.     

Heat,volume and chemical fluxes from submarine venting: A synthesis of results from the Rainbow hydrothermal field, 36°N MAR

High-temperature hydrothermal activity occurs in all ocean basins and along ridge crests of all spreading rates. While it has long been recognized that the fluxes associated with such venting are large, precise quantification of their impact on ocean biogeochemistry has proved elusive. Here, we report a comprehensive study of heat, fluid and chemical fluxes from a single submarine hydrothermal field. To achieve this, we have exploited the integrating nature of the non-buoyant plume dispersing above the Rainbow hydrothermal field, a long-lived and tectonically hosted high-temperature vent site on the Mid-Atlantic Ridge. Our calculations yield heat and volume fluxes for high-temperature fluids exiting the seafloor of ～0.5 GW and 450 L s^?1, together with accompanying chemical fluxes, for Fe, Mn and CH₄ of ～10, ～1 and ～1 mol s^?1, respectively. Accompanying fluxes for 25 additional chemical species that are associated with Fe-rich plume particles have also been calculated as they are transported away from the Rainbow vent site before settling to the seabed. High-temperature venting has been found to recur at least once every ～100 km along all slow-spreading ridges investigated to-date, with half of all known sites on the Mid-Atlantic Ridge occurring as long-lived and tectonically hosted systems. If these patterns persist along all slow- and ultraslow-spreading ridges, high-temperature venting of the kind reported here could account for ～50% of the on-axis hydrothermal heat flux along ～30,000 km of the ～55,000 km global ridge crest.     

Trophic relationships at intrannual spatial and temporal scales of macro and megafauna around a submarine canyon off the Catalonian coast (western Mediterranean)

The spatial and temporal changes of near-bottom macrofauna (suprabenthos and macroplankton) and the trophic relationships of megabenthic decapod crustaceans were analyzed off the Catalonian coasts (western Mediterranean) around Berenguera submarine canyon in four periods (April and December 1991, March and July 1992) and four zones (within Berenguera Canyon at ca. 450 m, and on adjacent slope at ca. 400, 600 m and 1200 m). In March 1992, we found the highest macrofauna abundance and the smallest sizes in the canyon, suggesting a positive effect of river discharges on suprabenthos recruitment. By contrast, macroplankton (decapods, fishes and euphausiids) did not show higher recruitment into canyons. After analyzing the diet of 23 decapod crustaceans, we found a significant segregation between guilds feeding on zooplankton and on benthos. Zooplankton (euphausiids and Pasiphaeidae) and infauna (polychaetes, Calocaris macandreae and ophiuoroids) were consistently the main prey exploited by decapod crustaceans around Berenguera Canyon. We also found some macrophyte (Posidonia oceanica) consumption, which was higher in periods of water column homogeneity (winter–spring and late autumn). Positive correlations between decapods' gut fullness (F) and decapod abundance indicate feeding aggregations, while positive correlations were also found between F and Llobregat River (situated ca. 18 km from Berenguera head) flow 1 to 2 months before sampling. Increases in F were delayed only 1 month when zooplankton feeders were analyzed alone, while benthos feeders did not show significant relationships with any environmental variables. That indicates that the response of megabenthic decapods feeding on benthos to environmental shifts is slower than that of zooplankton feeders. The importance of river flows in enhancing food supply of macro- and megabenthos dwelling close to submarine canyons was apparent, with a delay in the fauna response of 0–2 months after river flow peaks.     

Seasonal patterns in the fish and epibenthic crustaceans community of an intertidal zone with particular reference to the population dynamics of plaice and brown shrimp

The intertidal zone of a sandy beach located on the French coast of the Eastern Channel, was sampled during spring and summer 2000 to analyse the community structure of fish and epibenthic crustaceans. The presence of many juvenile fish (mainly O-group) and crustaceans indicated the important role played by the intertidal zone as a nursery ground. The brown shrimp, Crangon crangon and O-group plaice, Pleuronectes platessa are the two most abundant species of the intertidal ecosystem.Plaice settlement period extended from mid-March to early June. Over the survey period, densities increased to maximum numbers of about 27 ind. 10 m⁻² in mid-April. In the following week, density rapidly decreased due to mortality and migration into deeper waters. The mortality was attributed mainly to predation by brown shrimp (C. crangon) and to a lesser extent by the shore crab (Carcinus maenas). The mean size of 0-group plaice increased from 19 mm in mid-April to 58 mm in July. Growth of juvenile 0-group plaice is described by an exponential equation: total length (TL, mm) = 12.602 e^{0.022 (post-settlement age)}. Post-settlement growth rates, estimated by otolith microstructure analysis, were 0.38 mm d⁻¹ for plaice ≤30 mm and 0.61 mm d⁻¹ for plaice >30 mm. Settlement of juvenile brown shrimp started in mid-April, peaked in early June (93 ind. 10 m⁻²) and continued with fluctuating intensity throughout the summer. Growth rate of juvenile C. crangon, estimated after the settlement peak, was 0.163 mm d⁻¹. Growth conditions of juvenile plaice and C. crangon were analysed by comparing estimated growth in the field with predicted maximum growth according to temperature-growth rate models from experimental studies of growth with unlimited food supply. For plaice, the estimated growth rate was lower (plaice ≤30 mm) but similar (plaice >30 mm) to the predicted maximum growth suggesting a food limitation only for newly settled individual. The observed increase in mean length of juvenile C. crangon was lower than the maximum possible growth. The function of the intertidal zone in the early juvenile stages of marine species is discussed.     

Geochemistry and stable isotope constraints on high-temperature activity from sediment cores of the Saldanha hydrothermal field

The Saldanha hydrothermal field is hosted atop a mafic–ultramafic seamount, located at a non-transform offset on the Mid-Atlantic Ridge. Previous observations revealed a field where transparent low-temperature fluids discharge through centimeter-sized vents without the formation of chimney structures. We present geochemical and stable isotope (O and C) analyses from sediment samples collected at this field, both at and far from the vent area. Most sediments, including some directly adjacent to orifice vents, are pelagic oozes with only a weak hydrothermal overprinting. Hydrothermal precipitates are characterized by Fe–Mn oxyhydroxides and a minor amount of Cu–Zn sulphide minerals. However, one of the cores (SCD7) collected at the vent area shows a much stronger hydrothermal signature. This core is composed of a matrix of serpentine + talc ± chlorite with high porosity, where calcite + chalcopyrite + sphalerite/wurtzite ± pyrite–pyrrhotite were precipitated. In this core, metal enrichments, REE patterns, and the oxygen and carbon isotope composition of calcites indicate that mineralization must have occurred in the subsurface by high-temperature fluids, with minor mixing with seawater and with a significant magmatic contribution. Thus, while most samples confirm previous findings indicating that Saldanha hydrothermal fluid discharge is mainly diffuse and of low temperature, data from core SCD7 suggest that areas of high-temperature hydrothermal activity also occur, where temperatures of the fluids could reach > 260 °C and maximum temperatures of 330 °C. We suggest that fluids can flow through faults at the top of the mount and discharge in a more focused way through vent orifices, producing intense hydrothermal alteration of the sediments. At these locations complex hydrothermal processes occur, including reactions of the hydrothermal fluids with mafic and ultramafic rocks and magma degassing, as suggested by the carbon isotope composition of hydrothermal calcites. The high temperature of the fluid inferred from the geochemistry of the hydrothermal minerals requires a significant heat input to the system, suggesting an additional magmatic heat source to the already proposed exothermic serpentinization reactions.     

High resolution vertical distribution of the copepod Calanus chilensis in relation to the shallow oxygen minimum zone off northern Peru using LOKI,a new plankton imaging system

The vertical distribution of copepodite stage V and adult Calanus chilensis was studied on two transects across the Humboldt Current System off northern Peru using the LOKI system. LOKI is an optical plankton recorder, which simultaneously collects images of zooplankton and environmental data such as temperature, salinity, oxygen, and fluorescence. Image quality allowed determination of CV, females and males and identification of C. chilensis from 3 co-occurring Calanid copepods. C. chilensis was inhabiting the upper 250 m. Highest abundances with a maximum of ca. 44.000 Ind. m⁻² were observed in a narrow band within Cold Coastal Water at stations closest to the coast, coinciding with the Poleward Undercurrent. This raises questions for the life cycle closure within the Humboldt Current system. In contrast to observations in the southern part of the Humboldt Current System, the three stages studied were most abundant in hypoxic waters at oxygen concentrations between 5 and 50 µM. Thus C. chilensis seems to be the only species of the family Calanidae where not only a resting stage can tolerate hypoxia, but also both adult stages. This impacts availability to predators, as despite a locally high biomass only part of the population is available to anchovy and other important fish species which are restricted to waters with higher oxygen concentrations.     

Diel variations of the bathymetric distribution of zooplankton groups and biomass in Cap-Ferret Canyon,France

The bathymetric distribution, abundance and diel vertical migrations (DVM) of zooplankton were investigated along the axis of the Cap-Ferret Canyon (Bay of Biscay, French Atlantic coast) by a consecutive series of synchronous net hauls that sampled the whole water column (0–2000 m in depth) during a diel cycle. The distribution of appendicularians (maximum 189 individuals m⁻³), cladocerans (maximum 287 individuals m⁻³), copepods (copepods<4 mm, maximum 773 individuals m⁻³, copepods>4 mm, maximum 13 individuals m⁻³), ostracods (maximum 8 individuals m⁻³), siphonophores (maximum >2 individuals m⁻³) and peracarids (maximum >600 individuals 1000 m⁻³) were analysed and represented by isoline diagrams. The biomass of total zooplankton (maximum 18419 μg C m⁻³, 3780 μg N m⁻³) and large copepods (>4 mm maximum 2256 μg C m⁻³, 425 μg N m⁻³) also were determined. Vertical migration was absent or affected only the epipelagic zone for appendicularians, cladocerans, small copepods and siphonophores. Average amplitude of vertical migration was about 400–500 m for ostracods, some hyperiids and mysids, and large copepods, which were often present in the epipelagic, mesopelagic, and bathypelagic zones. Large copepods can constitute more than 80% of the biomass corresponding to total zooplankton. They may play an important role in the active vertical transfer of carbon and nitrogen.     

Megafaunal distribution and assessment of total methane and sulfide consumption by mussel beds at Menez Gwen hydrothermal vent,based on geo-referenced photomosaics

The Menez Gwen hydrothermal vents, located on the flanks of a small young volcanic structure in the axial valley of the Menez Gwen seamount, are the shallowest known vent systems on the Mid-Atlantic Ridge that host chemosynthetic communities. Although visited several times by research cruises, very few images have been published of the active sites, and their spatial dimensions and morphologies remain difficult to comprehend. We visited the vents on the eastern flank of the small Menez Gwen volcano during cruises with RV Poseidon (POS402, 2010) and RV Meteor (M82/3, 2010), and used new bathymetry and imagery data to provide first detailed information on the extents, surface morphologies, spatial patterns of the hydrothermal discharge and the distribution of dominant megafauna of five active sites. The investigated sites were mostly covered by soft sediments and abundant white precipitates, and bordered by basaltic pillows. The hydrothermally-influenced areas of the sites ranged from 59 to 200 m². Geo-referenced photomosaics and video data revealed that the symbiotic mussel Bathymodiolus azoricus was the dominant species and present at all sites. Using literature data on average body sizes and biomasses of Menez Gwen B. azoricus, we estimated that the B. azoricus populations inhabiting the eastern flank sites of the small volcano range between 28,640 and 50,120 individuals with a total biomass of 50 to 380 kg wet weight. Based on modeled rates of chemical consumption by the symbionts, the annual methane and sulfide consumption by B. azoricus could reach 1760 mol CH₄ yr⁻¹ and 11,060 mol H₂S yr⁻¹. We propose that the chemical consumption by B. azoricus over at the Menez Gwen sites is low compared to the natural release of methane and sulfide via venting fluids.     

Zooplankton metabolism and carbon demand at two seamounts in the NE Atlantic

Zooplankton metabolic rates, determined from electron transfer system (ETS) activity, were studied at two seamounts (Seine: 34°N, 14°W, summit depth ∼170 m; Sedlo: 40°N, 27°W, summit depth ∼750 m) in the northeast (NE) Atlantic during three cruises in November 2003, April 2004 and July 2004. ETS activity and respiratory carbon demand were measured for samples taken at seamount and open-ocean locations in order to probe the hypothesis of locally enhanced seamount productivity. ETS activity and biomass revealed no consistent diel patterns of feeding activity and vertical migration at Seine and Sedlo Seamounts. Spatial differences of biomass-specific ETS activity were observed at both seamounts and coincided with differences in food abundance and quality. At Seine Seamount in April 2004, biomass-specific ETS activity was on average higher at the seamount locations compared to the open ocean, though the enhancement was of a lower magnitude than spatial and temporal variability and had no apparent influence on zooplankton respiratory carbon demand or biomass. A persistent pattern of reduced zooplankton biomass above the summit location at Seine Seamount in April 2004 and July 2004 resulted in a local reduction of respiratory carbon demand. At Sedlo Seamount in November 2003, large spatial differences in biomass-specific ETS activity observed at the seamount locations resulted in a large range of respiratory carbon demand at the seamount, but were not reflected in zooplankton biomass. The depth-integrated (0–150 m) median respiratory carbon demand of the zooplankton community estimated from day and night hauls was 2.1 mg C m⁻² d⁻¹ at Seine Seamount (range: 0.3–6.3) and 2.9 mg C m⁻² d⁻¹ at Sedlo Seamount (range: 1.6–12.0). The sporadic nature and low magnitude of locally higher zooplankton respiration rates at the seamounts, which did not result in locally higher zooplankton standing stock biomass, lead us to reject the hypothesis that locally enhanced seamount productivity provides an autochthonous food supply to the resident faunas at Seine and Sedlo Seamounts. Instead, we conclude that the faunas at both seamounts are more likely supported by advection of food from the surrounding ocean.     

Zooplankton distribution across Fram Strait in autumn: Are small copepods and protozooplankton important?

We investigated zooplankton distribution in September 2006/2007 at eight stations across Fram Strait in contrasting water masses ranging from cold Polar water to warm Atlantic water. Our main objectives were: (1) to describe the plankton community in the upper 200 m during autumn, and (2) to investigate the importance of small-sized copepods and protozooplankton in an arctic ecosystem when the majority of the large Calanus species had entered diapause. We sampled both with a WP-2 net and Go-Flo bottle and show that small copepods <1 mm are significantly undersampled using a WP-2 net with 90 μm mesh.Small copepods and protozooplankton made a significant contribution both in terms of abundance and total zooplankton biomass at all stations in September, when the large calanoid copepods had left the upper 200 m. The dominating group in the upper 60 m at all stations was Oithona spp. nauplii and their daily estimated grazing potential on the <10 μm phytoplankton ranged from 0.1% to 82% of the standing stock. Both Oithona copepodites and nauplii biomass showed a significantly positive relation with temperature, but not with potential food. Heterotrophic protozooplankton, on the other hand, were most likely bottom-up regulated by the availability of phytoplankton <10 μm. We hypothesise that Oithona nauplii and protozooplankton compete for food and conclude that there was a strong link between the zooplankton community and the microbial food web in Fram Strait.     

Egg production of turbot,Scophthalmus maximus,in the Baltic Sea

In the brackish water Baltic Sea turbot spawn at ~ 6–9 psu along the coast and on offshore banks in ICES SD 24–29, with salinity influencing the reproductive success. The potential fecundity (the stock of vitellogenic oocytes in the pre-spawning ovary), egg size (diameter and dry weight of artificially fertilized 1-day-old eggs) and gonad dry weight were assessed for fish sampled in SD 25 and SD 28. Multiple regression analysis identified somatic weight, or total length in combination with Fulton's condition factor, as main predictors of fecundity and gonad dry weight with stage of maturity (oocyte packing density or leading cohort) as an additional predictor. For egg size, somatic weight was identified as main predictor while otolith weight (proxy for age) was an additional predictor. Univariate analysis using GLM revealed significantly higher fecundity and gonad dry weight for turbot from SD 28 (3378–3474 oocytes/g somatic weight) compared to those from SD 25 (2343 oocytes/g somatic weight), with no difference in egg size (1.05 ± 0.03 mm diameter and 46.8 ± 6.5 μg dry weight; mean ± sd). The difference in egg production matched egg survival probabilities in relation to salinity conditions suggesting selection for higher fecundity as a consequence of poorer reproductive success at lower salinities. This supports the hypothesis of higher size-specific fecundity towards the limit of the distribution of a species as an adaptation to harsher environmental conditions and lower offspring survival probabilities. Within SD 28 comparisons were made between two major fishing areas targeting spawning aggregations and a marine protected area without fishing. The outcome was inconclusive and is discussed with respect to potential fishery induced effects, effects of the salinity gradient, effects of specific year-classes, and effects of maturation status of sampled fish.     

Aggregation of the Arctic copepod Calanus hyperboreus over the ocean floor of the Greenland Sea

According to combined observations from vertical plankton tows, dredging with epibenthic nets 1 m above the ocean floor, video recordings and acoustic data from a scanning sonar obtained during descent and during deployment on the ocean floor, the calanoid copepod Calanus hyperboreus was aggregated in high concentrations near the ocean floor of the Greenland Sea between 2300 and 2500 m during late July and August. Concentrations were highest very close to the ocean floor and decreased rapidly further upward. These nearly mono-specific aggregations were apparently drifting in cloud-like formations with a horizontal extension of ca. 270 m with the near-bottom currents. Maximum abundances observed were up to 2 orders of magnitude higher than in the water column. The biomass in the bottom 20 m layer was around 18% of the biomass in the rest of the water column. Stage composition, reduced metabolic rates and insensibility to mechanical stimuli indicate that these C. hyperboreus were representing the resting population. The fact that high concentrations were observed during deployments lasting >1 d and in 3 years suggests that aggregation near the ocean floor is a regular, rather than an extraordinary, pattern in the life history of C. hyperboreus in the Greenland Sea, but there is need for comparison with other seas and eventually other Calanus species.     

Dispersion of a tracer on the East Pacific Rise (9°N to 10°N), including the influence of hydrothermal plumes

On 12 November 2006, 3 kg of sulfur hexafluoride were released in a 1.2 km long streak in the axial summit trough of the East Pacific Rise at 9°30′N to study how circulation and mixing affect larval dispersion. The first half of a tracer survey performed approximately 40 days after the injection found a small percentage of the tracer on the ridge axis between 9°30′N and 10°10′N, with the main concentration near 9°50′N, a site of many active hydrothermal vents. These observations provide evidence of larval connectivity between vent sites on the ridge. The latter half of the survey detected the primary patch of tracer west of the ridge and just south of the Lamont Seamounts, as a majority of the tracer had been transported off the ridge. However, by the end of the survey, the eastern edge of this patch was transported back to within 10 km of the ridge crest at 9°50′N by a reversal in the subinertial flow, suggesting another pathway for larvae between points along the ridge. Both the horizontal and vertical distributions of the tracer were complex and were likely heavily influenced by topography and vents in the area. Elevated tracer concentrations within the axial summit trough and an adjacent depression on the upper ridge flank suggest that tracers may be detained in such depressions. Correlated tracer/turbidity profiles provide direct evidence of entrainment of the tracer into vent plumes from 9°30′N to 10°N. A comparison of the vertical tracer inventory with neutral density vent-plume observations suggests that on the order of 10% of the tracer injected was entrained into vent plumes near the injection site. The results imply that effluent from diffuse hydrothermal sources and larvae of hydrothermal vent fauna can be entrained in significant quantities into plumes from discrete sources and dispersed in the neutrally buoyant plumes.     

Zooplankton and micronekton biovolume at the Mid-Atlantic Ridge and Charlie–Gibbs Fracture Zone estimated by multi-frequency acoustic survey

To examine the potential influence of the Mid-Atlantic Ridge and Charlie–Gibbs Fracture Zone on zooplankton and micronekton biovolume in the upper 200 m of the water column, multi-frequency acoustic data (18, 38, 70, 120 and 200 kHz) were acquired at four study sites from the RRS James Cook using hull-mounted scientific echosounders. Multi-frequency inversion techniques were employed to classify each 20 m depth×500 m along-track region of the water column to a zooplankton or micronekton acoustic scatterering class, such as copepod or euphausiid, and to estimate biovolume. We found a highly significant north–south (across fracture zone) difference in areal biovolume (p-value=0.01) but no significant east–west (across ridge) difference (p-value=0.07). Areal biovolume at all sites was dominated by the acoustic scatter class ‘euphausiid’, with higher biovolumes occurring in the southern stations. Our acoustic observations suggest the existence of different pelagic communities to the north and south of the SPF, with the southern community having a greater proportion of fish.     

Habitat,growth and physiological ecology of a basaltic community of Ridgeia piscesae from the Juan de Fuca Ridge

The vestimentiferan tubeworm Ridgeia piscesae is an ecosystem-structuring organism in the hydrothermal vent environments of the Northeast Pacific. During this study, a single representative aggregation of the long-skinny morphotype of R. piscesae from the main endeavor segment was monitored for 3 yr before being collected in its entirety with a hydraulically actuated collection device manipulated in situ by a research vehicle. Vestimentiferan growth rates in this aggregation were determined by staining the exterior of the tubes and measuring newly deposited tube sections. The average growth rate of R. piscesae in this aggregation was very low in both years of the growth study (3.2 mm yr⁻¹). Although the incidence of plume damage from partial predation was very high (>95%), mortality was very low (<4% yr⁻¹). The distribution and the very tight clustering of recently recruited individuals indicated gregarious settlement behavior that is hypothesized to be partly due to biotic cues from settled larvae. Coupled measurements of vent fluid sulfide concentration and temperature were used to calculate the exposure of the vestimentiferans to sulfide from short- and long-term temperature monitoring. Plume-level temperature records indicate that most of the time individuals in this aggregation were exposed to extremely low levels of vent fluid, and therefore sulfide (<0.1 μM), while their posterior sections were consistently exposed to sulfide concentrations in the 100 μM range. A rootball-like structure formed the common base of the aggregation. In contrast to the anterior sections of the tubeworm tubes, the portions of the tubes within the “rootball” were freely permeable to sulfide. The results of this study show that R. piscesae, unlike vestimentiferans from the East Pacific Rise, can survive and grow in areas of low diffuse vent flow with very low plume-level exposure to sulfide. We propose that this morphotype of R. piscesae has the ability to acquire sulfide from sources near their posterior ends, similar to some species of cold seep vestimentiferans from the Gulf of Mexico. The ability of this single species of vestimentiferan to survive low exposure to vent flow with low mortality coupled with sulfide uptake across posterior tube sections may help explain the occurrence of a single vent vestimentiferan species in a wide variety of habitat conditions at hydrothermal vent sites in the Northeast Pacific.     

Bioaccumulation and trophic transfer of mercury in striped bass (Morone saxatilis) and tautog (Tautoga onitis) from the Narragansett Bay (Rhode Island,USA)

We examined the bioaccumulation and trophic transfer of mercury in two marine finfish species, striped bass (Morone saxatilis) and tautog (Tautoga onitis), collected from the Narragansett Bay (Rhode Island, USA). For each of these target fish, white muscle tissue was analyzed for total mercury (Hg) and results were evaluated relative to fish age, body size, and Hg content of preferred prey. Dietary and stable isotope analysis was also used to elucidate the effect of trophic processes on Hg concentrations in fish. The Hg content of muscle tissue was positively correlated with fish age and length for both species, although striped bass accumulated Hg faster than tautog. Accelerated Hg bioaccumulation in striped bass is consistent with its high trophic level (trophic level = 4.07) and Hg-enriched prey (forage fish and macrocrustaceans; mean Hg content = 0.03 mg Hg kg wet wt^?1). In contrast, tautog maintain a lower trophic status (trophic level = 3.51) and consume prey with lower Hg levels (mussels and crabs; mean Hg content = 0.02 mg Hg kg wet wt^?1). Despite differences in Hg bioaccumulation between target fish, the mean Hg concentration of tautog exceeded levels in striped bass (0.24 and 0.16 mg Hg kg wet wt^?1, respectively) due to a disparity in age-at-catch between sampled groups (mean age of tautog and bass = 11.3 and 4.3 yr, respectively). Taking into account legal minimum catch lengths further revealed that 75.0% of legal-size striped bass (>70.2 cm TL; n = 4) and 44.8% of tautog (>40.6 cm TL; n = 29) had Hg levels beyond the US EPA regulatory threshold of 0.3 mg Hg kg wet wt^?1. Moreover, Hg-length relationships suggest that each target fish meets this threshold near their minimum legal catch length. Our findings reiterate the value of species ecology to improve predictions of fish Hg and permit better management of human contamination by this important dietary source.