Latitudinal variation in growth of Crangon crangon (L.): Does counter-gradient growth compensation occur?

Length growth in relation to water temperature was studied for Crangon crangon (L.) from two populations at the northern and southern edges of its distributional range to determine whether counter-gradient growth compensation occurs. In crustaceans, growth rate depends on the time between moulting events (intermoult period) and the size increase at moult (moult increment). In this study, the period between moults was shorter at higher temperature, ranging respectively from about 11 days at 25 °C to 27 days at 10 °C at southern edge, and from 10 to 24 days at the same temperatures at the northern edge. Moult increment showed a large variability, from 1.5 to 2.7 mm with no clear trend with temperature at the northern edge; and decreasing from about 2.7 mm at 10 °C to about 1.5 mm at 25 °C at the southern edge. As a result, the temperature effect on the overall growth rate differed between shrimps from the north and those from the south, suggesting counter-gradient growth compensation. At the northern edge, mean growth increased from about 0.12 mm d^− 1 at 10 °C to about 0.23 mm d^− 1 at 25 °C, while at the southern edge, growth was lower, about 0.08 mm d^− 1 at 10 °C and increased to about 0.16 mm d^− 1 at 25 °C. Maximum observed growth rates of shrimps from the north were also higher and ranged from 0.17 mm d^− 1 at 10 °C to 0.89 mm d^− 1 at 25 °C, while shrimps from the south grew at a maximum of 0.08 to 0.75 mm d^− 1 respectively at 15 and 20 °C. Sex and size differences were also found, with males growing slower than females and at a decreasing growth rate with increasing size. Implications for the brown shrimp's life cycle are discussed.     

Bioenergetic model estimates of interannual and spatial patterns in consumption demand and growth potential of juvenile pink salmon (Oncorhynchus gorbuscha) in the Gulf of Alaska

A bioenergetic model of juvenile pink salmon (Oncorhynchus gorbuscha) was used to estimate daily prey consumption and growth potential of four ocean habitats in the Gulf of Alaska during 2001 and 2002. Growth potential was not significantly higher in 2002 than in 2001 at an alpha level of 0.05 (P=0.073). Average differences in growth potential across habitats were minimal (slope habitat=0.844 g d⁻¹, shelf habitat=0.806 g d⁻¹, offshore habitat=0.820 g d⁻¹, and nearshore habitat=0.703 g d⁻¹) and not significantly different (P=0.630). Consumption demand differed significantly between hatchery and wild stocks (P=0.035) when examined within year due to the interaction between hatchery verses wild origin and year. However, the overall effect of origin across years was not significant (P=0.705) due to similar total amounts of prey consumed by all juvenile pink salmon in both study years. We anticipated that years in which ocean survival was high would have had high growth potential, but this relationship did not prove to be true. Therefore, modeled growth potential may not be useful as a tool for forecasting survival of Prince William Sound hatchery pink salmon stocks. Significant differences in consumption demand and a two-fold difference in nearshore abundance during 2001 of hatchery and wild pink salmon confirmed the existence of strong and variable interannual competition and the importance of the nearshore region as being a potential competitive bottleneck.     

Molluscan assemblages of seagrass-covered and bare intertidal flats on the Banc d'Arguin,Mauritania, in relation to characteristics of sediment and organic matter

The Banc d'Arguin, a non-estuarine area of shallows and intertidal flats off the tropical Saharan coast of Mauritania, is characterised by extensive intertidal and subtidal seagrass beds. We examined the characteristics of intertidal seagrass (Zostera noltii) meadows and bare areas in terms of the presence and abundance of molluscs (gastropods and bivalves). To explain observed differences between molluscan assemblages in seagrass and bare patches, some aspects of the feeding habitat (top-5 mm of the sediment) and of food (organic materials) of molluscs were examined. The novelty of this study is that phytopigments were measured and identified to assess source and level of decay (freshness) of organic material in the sediment and to study their importance as an explanatory variable for the distribution of molluscs. Over an area of 36 km² of intertidal flats, at 12 sites, paired comparisons were made between seagrass-covered and nearby bare patches. Within seagrass meadows, dry mass of living seagrass was large and amounted to 180 ±10 g AFDM m^− 2 (range 75–240). Containing twice the amount of silt per unit dry sediment mass, seagrass sediments were muddier than bare areas; the relative amount of organic material was also larger. The total number of species of bivalves and gastropods amounted to 27, 14 of which were found only in seagrass areas, 4 only in bare and 9 in both types of habitat. Among the three numerically most abundant species, the bivalves Anadara senilis, Dosinia hepatica and Loripes lacteus, the first was numerically most abundant in bare and the other two in seagrass-covered areas. Bare intertidal areas had greater mean total biomass of molluscs (80.5 g AFDM m^− 2) than seagrass meadows (30.0 g AFDM m^− 2). In both habitats, the bulk of the biomass was made up by A. senilis. Excluding this species, bare mudflats contained on average only 3.1 g AFDM m^− 2 and seagrass meadows 6.9 g AFDM m^− 2. As compared to previous surveys in 1980–1986, the biomass of A. senilis had increased almost 10-fold and D. hepatica, previously found in very small numbers, had become the most numerous species. However, the total biomass excluding that of A. senilis was similar. Concentrations of phytopigments were similar to those observed at temperate mudflats, indicating that the Banc d'Arguin might not be as oligotrophic as previously thought. Per unit of dry sediment mass, smaller amounts of phytopigments were found in bare than in seagrass areas. Per unit of dry organic material, bare sediments contained most (fresh) phytopigments. This suggests that in seagrass-covered meadows the organic material is more degraded than in bare sediments. Overall, the composition of phytopigments, quite surprisingly, indicated a benthic-diatom-dominated trophic system. Multivariate statistics revealed that patterns of zoobenthic assemblages were correlated with patterns of a combination of four environmental parameters: grain size of the sediment, amount of fresh phytopigments and amounts of leaves and roots of seagrass.     

Basin-scale variability of phytoplankton biomass,production and growth in the Atlantic Ocean

The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 50°N to 50°S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75–125 mg Chl m⁻²) were found in North Atlantic temperate waters and in the upwelling region off NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m⁻². The estimated concentration of surface phytoplankton carbon (C) biomass was 5–15 mg C m⁻² in the oligotrophic regions and increased over 40 mg C m⁻² in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m⁻² d⁻¹ at the central gyres to 500–1000 mg C m⁻² d⁻¹ in upwelling and higher latitude regions, where faster growth rates (μ) of phytoplankton (>0.5 d⁻¹) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface μ averaged 0.21±0.02 d⁻¹ (mean±SE)], representing <20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in μ were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass specific rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.     

Population structure and life history of Hemimysis lamornae mediterranea (Malacostraca: Mysida) in the Ebro Delta (NW Mediterranean)

Hemimysis lamornae mediterranea Bacescu, 1936 has been recently reported in the Ebro Delta (Spain, NW Mediterranean). Little is known about the biology and ecology of this mysid and we provide the first information about its population biology. H. l. mediterranea were collected from Sant Carles de la Ràpita harbor from June 2010 to March 2012 at night. The H. l. mediterranea population was composed of two main individual size categories: larger-sized winter/spring individuals and smaller-sized spring/summer individuals. The overall sex ratio is highly skewed; mature females are 2.5 times more abundant than mature males. Reproductive activity was higher during late winter and spring but was almost continuous throughout the year. The intra-marsupial development and growth of juveniles has been studied in the laboratory. The mean duration of incubation period (intra-marsupial stages) in laboratory conditions was 11 days (20 °C) and the age at first maturity ranged from14 to 20 days. The growth rate was faster in early juveniles and declined with age, showing a maximum of 0.152 mm d^− 1. The laboratory results and demographic data suggest that H. l. mediterranea will produce several generations per year in the Ebro Delta. H. l. mediterranea was characterized by a combination of early maturation of individuals (short juvenile period), rapid growth, small adult size, a continuous reproduction all year round, iteroparous females, a relatively high fecundity and a high number of generations per year.     

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.     

Contrasting life-histories,secondary production,and trophic structure of Peracarid assemblages of the bathyal suprabenthos from the Bay of Biscay (NE Atlantic) and the Catalan Sea (NW Mediterranean)

The life-histories and the secondary production of four dominant peracarid crustaceans (the mysids Boreomysis arctica and Parapseudomma calloplura, the amphipod Rhachotropis caeca, and the isopod Ilyarachna longicornis) in bathyal depths of the Bay of Biscay (NE Atlantic; between 383 and 420 m) and the Catalan Sea (Northwestern Mediterranean; between 389 and 1355 m) were established. Both the Atlantic and the Mediterranean populations of the major part of the target-species had two generations/year with mean cohort-production intervals (CPI) ranging from 5.5 mo for Ilyarachna longicornis to 6.3 mo for Parapseudomma calloplura. The Hynes method showed secondary production to vary in the Bay of Biscay between 0.113 mg DW m⁻² yr⁻¹ for I. longirostris and 3.069 mg DW m⁻² yr⁻¹ for P. calloplura, with P/B ratios between 4.57 (I. longirostris) and 7.93 (Boreomysis arctica). In the Catalan Sea, production varied between 0.286 mg DW m⁻² yr⁻¹ for I. longirostris and 1.096 mg DW m⁻² yr⁻¹ for P. calloplura, with P/B between 5.72 (I. longirostris) and 6.66 (P. calloplura). Application of two different empiric models to the whole peracarid assemblage gave similar levels of secondary production in both study areas (between 29.26 and 32.14 mgDWm⁻² yr⁻¹ in the Bay of Biscay; between 26.23 and 26.54 mg DW m⁻² yr⁻¹ in the Catalan Sea). From the analysis of gut contents of 22 species the dominant species in each study area were assigned to two basic trophic levels, detritus feeders and predators. Also, cumulative curves of dominance showed high diversity (low dominance) for peracarid assemblages distributed at mid-bathyal depths (524–693 m) both in the Bay of Biscay off Arcachon and in the Catalan Sea off Barcelona. We also discuss and compare, both within and between areas, how environmental features may explain the observed diversity patterns, the trophic structure, and the production results obtained for the suprabenthos assemblages.     

Export fluxes of particulate organic carbon in the Chukchi Sea: A comparative study using 234Th/238U disequilibria and drifting sediment traps

Large-volume sampling of ²³⁴Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of ²³⁴Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. ²³⁴Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m^− 2 day^− 1 and 16.5 ± 6.5 mmol C m^− 2 day^− 1, respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m^− 2 day^− 1 and 19.5 ± 3.3 mmol C m^− 2 day^− 1, respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m^− 2 day^− 1 and 29.1 ± 14.2 mmol C m^− 2 day^− 1, respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m^− 2 day^− 1 and 13.0 ± 6.4 mmol C m^− 2 day^− 1, respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/²³⁴Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.     

Picophytoplankton dynamics and production in the Arabian Sea during the 1995 Southwest Monsoon

Phytoplankton community structure is expected to shift to larger cells (e.g., diatoms) with monsoonal forcing in the Arabian Sea, but recent studies suggest that small primary producers remain active and important, even in areas strongly influenced by coastal upwelling. To better understand the role of smaller phytoplankton in such systems, we investigated growth and grazing rates of picophytoplankton populations and their contributions to phytoplankton community biomass and primary productivity during the 1995 Southwest Monsoon (August–September). Environmental conditions at six study stations varied broadly from open-ocean oligotrophic to coastal eutrophic, with mixed-layer nitrate and chlorophyll concentrations ranging from 0.01 to 11.5 μM NO₃ and 0.16 to 1.5 μg Chl a. Picophytoplankton comprised up to 92% of phytoplankton carbon at the oceanic stations, 35% in the diatom-dominated coastal zone, and 26% in a declining Phaeocystis bloom. Concurrent in situ dilution and ¹⁴C-uptake experiments gave comparable ranges of community growth rates (0.53–1.05 d⁻¹ and 0.44–1.17 d⁻¹, to the 1% light level), but uncertainties in C:Chl a confounded agreement at individual stations. Microzooplankton grazing utilized 81% of community phytoplankton growth at the oligotrophic stations and 54% at high-nutrient coastal stations. Prochlorococcus (PRO) was present at two oligotrophic stations, where its maximum growth approached 1.4 d⁻¹ (two doublings per day) and depth-integrated growth varied from 0.2 to 0.8 d⁻¹. Synechococcus (SYN) growth ranged from 0.5 to 1.1 d⁻¹ at offshore stations and 0.6 to 0.7 d⁻¹ at coastal sites. Except for the most oligotrophic stations, growth rates of picoeukaryotic algae (PEUK) exceeded PRO and SYN, reaching 1.3 d⁻¹ offshore and decreasing to 0.8 d⁻¹ at the most coastal station. Microzooplankton grazing impact averaged 90, 70, and 86% of growth for PRO, SYN, and PEUK, respectively. Picoplankton as a group accounted for 64% of estimated gross carbon production for all stations, and 50% at high-nutrient, upwelling stations. Prokaryotes (PRO and SYN) contributed disproportionately to production relative to biomass at the most oligotrophic station, while PEUK were more important at the coastal stations. Even during intense monsoonal forcing in the Arabian Sea, picoeukaryotic algae appear to account for a large portion of primary production in the coastal upwelling regions, supporting an active community of protistan grazers and a high rate of carbon cycling in these areas.     

Isotopic and elemental indicators of nutrient sources and status of coastal habitats in the Caribbean Sea,Yucatan Peninsula,Mexico

Nutrient inputs associated with coastal population growth threaten the integrity of coastal ecosystems around the globe. In order to assess the threat posed by rapid growth in tourism, we analyzed the nutrient concentrations as well as the δ¹⁵N of NO₃⁻ and macrophytes to detect wastewater nitrogen (N) at 6 locations along a groundwater-dominated coastal seagrass bed on the Caribbean coast of Mexico. We predicted that locations with greater coastal development would have higher concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (P), as well as δ¹⁵N of NO₃⁻, reflecting wastewater sources of N. However, concentrations of NO₃⁻ were not significantly different between developed (3.3 ± 5.3 μM NO₃⁻) and undeveloped (1.1 ± 0.7 μM) marine embayments. The most important control on DIN concentration appeared to be mixing of fresh and salt water, with DIN concentrations negatively correlated with salinity. The δ¹⁵N of NO₃⁻ was elevated at an inland pond (7.0 ± 0.42‰) and a hydrologically-connected tide pool (7.6 ± 0.57‰) approximately 1 km downstream of the pond. The elevated δ¹⁵N of NO₃⁻ at the pond was paralleled by high δ¹⁵N values of Cladophora sp., a ubiquitous green alga (10 ± 1‰). We hypothesize that inputs of nitrogen rich (NO₃⁻ > 30 μM) groundwater, characterized by ¹⁵N enriched signatures, flow through localized submarine groundwater discharges (SGD) and contribute to the elevated δ¹⁵N signatures observed in many benthic macrophytes. However, changes in nitrogen concentrations and isotope values over the salinity gradient suggest that other processes (e.g. denitrification) could also be contributing to the ¹⁵N enrichments observed in primary producers. More measurements are needed to determine the relative importance of nitrogen transformation processes as a source of ¹⁵N to groundwaters; however, it is clear that continued inputs of anthropogenic N via SGD have the potential to severely impact ecologically and economically valuable seagrass meadows and coral reefs along the Caribbean coast of Mexico.     

Top-down control of spring surface phytoplankton blooms by microzooplankton in the Central Yellow Sea,China

Phytoplankton growth and microzooplankton grazing were studied during the 2007 spring bloom in Central Yellow Sea. The surveyed stations were divided to pre-bloom phase (Chl a concentration less than 2 μg L⁻¹), and bloom phase (Chl a concentration greater than 2 μg L⁻¹). Shipboard dilution incubation experiments were carried out at 19 stations to determine the phytoplankton specific growth rates and the specific grazing rates of microzooplankton on phytoplankton. Diatoms dominated in the phytoplankton community in surface waters at most stations. For microzooplankton, Myrionecta rubra and tintinnids were dominant, and heterotrophic dinoflagellate was also important in the community. Phytoplankton-specific growth rates, with an average of 0.60±0.19 d⁻¹, were higher at pre-bloom stations (average 0.62±0.17 d⁻¹), and lower at the bloom stations (average 0.59±0.21 d⁻¹), but the difference of growth rates between bloom and pre-bloom stations was not statistically significant (t test, p=0.77). The phytoplankton mortality rate by microzooplankton grazing averaged 0.41±0.23 d⁻¹ at pre-bloom stations, and 0.58±0.31 d⁻¹ during the blooms. In contrast to the growth rates, the statistic difference of grazing rates between bloom and pre-bloom stations was significant (after removal of outliers, t test, p=0.04), indicating the importance of the top-down control in the phytoplankton bloom processes. Average potential grazing efficiency on primary productivity was 66% at pre-bloom stations and 98% at bloom stations, respectively. Based on our results, the biomass maximum phase (bloom phase) was not the maximum growth rate phase. Both phytoplankton specific growth rate and net growth rate were higher in the pre-bloom phase than during the bloom phase. Microzooplankton grazing mortality rate was positively correlated with phytoplankton growth rate during both phases, but growth and grazing were highly coupled during the booming phase. There was no correlation between phytoplankton growth rate and cell size during the blooms, but they were positive correlated during the pre-bloom phase. Our results indicate that microzooplankton grazing is an important process controlling the growth of phytoplankton in spring bloom period in the Central Yellow Sea, particularly in the “blooming” phase.     

Kinetic study reveals weak Fe-binding ligand,which affects the solubility of Fe in the Scheldt estuary

The chemistry of dissolved Fe(III) was studied in the Scheldt estuary (The Netherlands). Two discrete size fractions of the dissolved bulk (< 0.2 μm and < 1 kDa) were considered at three salinities (S = 26, 10 and 0.3).Within the upper estuary, where fresh river water meets seawater, the dissolved Fe concentration decreases steeply with increasing salinity, for the fraction < 0.2 μm from 536 nM at S = 0.3 to 104 nM at S = 10 and for the fraction < 1 kDa from 102 nM to 36 nM Fe. Further downstream, in the middle and lower estuary, this decrease in the Fe concentration continues, but is far less pronounced. For all samples, the traditionally recognised dissolved strong organic Fe-binding ligand concentrations are lower than the dissolved Fe concentrations.Characteristics of dissolved Fe-binding ligands were determined by observing kinetic interactions with adsorptive cathodic stripping voltammetry. From these kinetic experiments we concluded that apart from the well-known strong Fe-binding organic ligands (L, logK′ = 19–22) also weak Fe-binding ligands (P) existed with an α value (binding potential = K′ · [P]) varying between 10^11.1 and 10^11.9. The presence of this relatively weak ligand explained the high concentrations of labile Fe present in both size fractions in the estuary. This weak ligand can retard or prevent a direct precipitation after an extra input of Fe.The dissociation rate constants of the weak ligand varied between 0.5 × 10^− 4 and 4.3 × 10^− 4 s^− 1. The rate constants of the strong organic ligand varied between k_d = 1.5 × 10^− 3–17 × 10^− 2 s^− 1 and k_f = 2.2 × 10⁸–2.7 × 10⁹ M^− 1 s^− 1. The dissociation rate constant of freshly amorphous Fe-hydroxide was found to be between 4.3 × 10^− 4 and 3.7 × 10^− 3 s^− 1, more labile or equal to the values found by Rose and Waite [Rose, A.L., Waite, T.D., 2003a. Kinetics of hydrolysis and precipitation of ferric iron in seawater. Environ. Sci. Technol., 37, 3897–3903.] for freshly precipitated Fe in seawater.Kinetic rate constants of Fe with the ligand TAC (2-(2-Thiazolylazo)-p-cresol) were also determined. The formation rate constant of Fe(TAC)₂ varied between 0.1 × 10⁸ and 3.6 × 10⁸ M^− 1 s^− 1, the dissociation rate constant between 0.2 × 10^− 5 and 17 × 10^− 5 s^− 1 for both S = 26 and S = 10. The conditional stability constant of Fe(TAC)₂ (β_Fe(TAC)2′) varied between 22 and 23.4 for S = 10 and S = 26 more or less equal to that known from the literature (logβ_Fe(TAC)2′ = 22.4; [Croot, P.L., Johansson, M., 2000. Determination of iron speciation by cathodic stripping voltammetry in seawater using the competing ligand 2-(2-Thiazolylazo)-p-cresol (TAC). Electroanalysis, 12, 565–576.]). However, at S = 0.3 the logβ_Fe(TAC)2′ was 25.3, three orders of magnitude higher. Apparently the application of TAC to samples of low salinity can only be done when the correct β_Fe(TAC)2′ is known.     

Commercially-cultured oysters (Crassostrea gigas) exert top-down control on intertidal pelagic resources in Willapa Bay,Washington, USA

The capacity of filter feeders to reduce seston and phytoplankton concentrations in the water column has important implications for restoration and management of coastal ecosystems. We directly measured changes in chlorophyll a concentration on commercially stocked intertidal oyster beds (Crassostrea gigas) in Willapa Bay, Washington, USA by recording water properties near small drifters as they tracked parcels of water across tide flats. Chlorophyll declined 9.6% per half hour in water passing on-bottom adult oysters and 41% for longline adult oysters, whereas chlorophyll concentrations increased as water flowed across tide flats without adult oysters. Field filtration rates, which were fit to exponential declines in chlorophyll and accounted for oyster density and water depth, averaged 0.35 L g^− 1 h^− 1 (shucked dry weight) for on-bottom aquaculture and 0.73 L g^− 1 h^− 1 for longline culture, compared to values of 2.5–12 L g^− 1 h^− 1 reported from laboratory studies of C. gigas. Field filtration rates may be lower than laboratory rates due to unfavorable field conditions (e.g., low initial chlorophyll concentrations) or masked by resuspension of benthic microalgae. In addition to distinctions among on-bottom, longline, and no-oyster habitats, Akaike's Information Criterion analysis showed temperature, initial chlorophyll concentration, and depth related to chlorophyll decline. This research corroborates mathematical models suggesting that benthic suspension feeders are exerting top-down control of pelagic production in this estuary, with strong patterns in chlorophyll emerging across extensive tideflats populated by C. gigas despite low field filtration rates.     

Community respiration/production and bacterial activity in the upper water column of the central Arctic Ocean

Community metabolism (respiration and production) and bacterial activity were assessed in the upper water column of the central Arctic Ocean during the SHEBA/JOIS ice camp experiment, October 1997–September 1998. In the upper 50 m, decrease in integrated dissolved oxygen (DO) stocks over a period of 124 d in mid-winter suggested a respiration rate of ∼3.3 nM O₂ h⁻¹ and a carbon demand of ∼4.5 gC m⁻². Increase in 0–50 m integrated stocks of DO during summer implied a net community production of ∼20 gC m⁻². Community respiration rates were directly measured via rate of decrease in DO in whole seawater during 72-h dark incubation experiments. Incubation-based respiration rates were on average 3-fold lower during winter (11.0±10.6 nM O₂ h⁻¹) compared to summer (35.3±24.8 nM O₂ h⁻¹). Bacterial heterotrophic activity responded strongly, without noticeable lag, to phytoplankton growth. Rate of leucine incorporation by bacteria (a proxy for protein synthesis and cell growth) increased ∼10-fold, and the cell-specific rate of leucine incorporation ∼5-fold, from winter to summer. Rates of production of bacterial biomass in the upper 50 m were, however, low compared to other oceanic regions, averaging 0.52±0.47 ngC l⁻¹ h⁻¹ during winter and 5.1±3.1 ngC l⁻¹ h⁻¹ during summer. Total carbon demand based on respiration experiments averaged 2.4±2.3 mgC m⁻³ d⁻¹ in winter and 7.8±5.5 mgC m⁻³ d⁻¹ in summer. Estimated bacterial carbon demand based on bacterial productivity and an assumed 10% gross growth efficiency was much lower, averaging about 0.12±0.12 mgC m⁻³ d⁻¹ in winter and 1.3±0.7 mgC m⁻³ d⁻¹ in summer. Our estimates of bacterial activity during summer were an order of magnitude less than rates reported from a summer 1994 study in the central Arctic Ocean, implying significant inter-annual variability of microbial processes in this region.     

Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption

A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO₃, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds.The in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4±0.6, 3.6±0.5 mmol m⁻² d⁻¹ at 1300 and 3100 m depth, respectively, and between 1.9±0.3 and 2.4±0.2 mmol m⁻² d⁻¹ at 4000 m depth. Diffusive oxygen uptakes (DOU) were 2.8±1.1, 2.3±0.8, 0.8±0.3 and 1.2±0.1 mmol m⁻² d⁻¹, respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel–levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5–8 times larger than the vertical flux recorded in traps.Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800 m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19 mm y⁻¹) found on this site. The Lobe region could receive as high as 19 mol C m⁻² y⁻¹, 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.     

The Manila clam population in Arcachon Bay (SW France): Can it be kept sustainable?

The venerid clam Ruditapes philippinarum is the most prominent suspension-feeding bivalve inhabiting muddy intertidal seagrass beds in Arcachon Bay (SW France). It is exploited by fishermen, and Arcachon Bay ranks number one in France in terms of production and total biomass of this species. Previous studies revealed a decrease in the standing stock of R. philippinarum since 2003 and unbalanced length–frequency distributions with a lack of juveniles and of adults > 40 mm. Consequently, the population dynamics of this bivalve were studied at four intertidal sites and one oceanic site in Arcachon Bay. As clam size structure did not allow classical dynamics computations, field monitoring was coupled with field experiments (tagging–recapture) over two years. Monitoring of condition index and gonadal maturation stages highlighted a high variability in spawning number and intensity between sites. Recruitment events in the exploited area varied spatially but with uniformly low values. Von Bertalanffy Growth Function (VBGF) parameters (K, L_∞) were determined using Appeldoorn and ELEFAN methods. In the exploited sites in the inner lagoon, K was relatively high (mean = 0.72 yr^? 1) but L_∞ was low (mean = 41.1 mm) resulting in a moderate growth performance index (Φ′ = 2.99). Growth parameters were not correlated with immersion time and L_∞ was different between sites. Comparison of mortality coefficients (Z) between cage experiments and field monitoring suggested that fishing accounts for 65–75% of total adult mortality. Low recruitment, a low growth rate and a normal mortality rate led to low somatic production (4.1 and 8.7 g Shell-Free Dry Weight (SFDW) m^? 2 yr^? 1) and an annual P/B ratio from 0.44 to 0.92 yr^? 1. Under current conditions, the possibility of a sustainable population in Arcachon Bay will strongly depend on recruitment success and fishing management.     

Microzooplankton grazing impact in the Western Arctic Ocean

Microzooplankton grazing impact on phytoplankton was assessed using the Landry–Hassett dilution technique in the Western Arctic Ocean during spring and summer 2002 and 2004. Forty experiments were completed in a region encompassing productive shelf regions of the Chukchi Sea, mesotrophic slope regions of the Beaufort Sea off the North Slope of Alaska, and oligotrophic deep-water sites in the Canada Basin. A variety of conditions were encountered, from heavy sea-ice cover during both spring cruises, moderate sea-ice cover during summer of 2002, and light to no sea ice during summer of 2004, with a concomitant range of trophic conditions, from low chlorophyll-a (Chl-a; <0.5 μg L⁻¹) during heavy ice cover in spring and in the open basin, to late spring and summer shelf and slope open-water diatom blooms with Chl-a >5 μg L⁻¹. The microzooplankton community was dominated by large naked ciliates and heterotrophic gymnodinoid dinoflagellates. Significant, but low, rates of microzooplankton herbivory were found in half of the experiments. The maximum grazing rate was 0.16 d⁻¹ and average grazing rate, including experiments with no significant grazing, was 0.04±0.06 d⁻¹. Phytoplankton intrinsic growth rates varied from the highest values of about 0.4 d⁻¹ to the lowest values of zero to slightly negative growth, on average 0.16±0.15 d⁻¹. Light limitation in spring and post-bloom senescence during summer were likely explanations of observed low phytoplankton growth rates. Microzooplankton grazing consumed 0–120% (average 22±26%) of phytoplankton daily growth. Grazing and growth rates found in this study were low compared to rates reported in another Arctic system, the Barents Sea, and in major geographic regions of the world ocean.     

Effects of mesoscale phytoplankton variability on the copepods Neocalanus flemingeri and N. plumchrus in the coastal Gulf of Alaska

The copepods Neocalanus flemingeri and N. plumchrus are major components of the mesozooplankton on the shelf of the Gulf of Alaska, where they feed, grow and develop during April–June, the period encompassing the spring phytoplankton bloom. Satellite imagery indicates high mesoscale variability in phytoplankton concentration during this time. Because copepod ingestion is related to food concentration, we hypothesized that phytoplankton ingestion by N. flemingeri and N. plumchrus would vary in response to mesoscale variability of phytoplankton. We proposed that copepods on the inner shelf, where the phytoplankton bloom is most pronounced, would be larger and have more lipid stores than animals collected from the outer shelf, where phytoplankton concentrations are typically low. Shipboard feeding experiments with both copepods were done in spring of 2001 and 2003 using natural water as food medium. Chlorophyll concentration ranged widely, between 0.32 and 11.44 μg l⁻¹ and ingestion rates varied accordingly, between 6.0 and 627.0 ng chl cop⁻¹ d⁻¹. At chlorophyll concentrations<0.50 μg l⁻¹, ingestion is always low, <40 ng cop⁻¹ d⁻¹. Intermediate ingestion rates were observed at chlorophyll concentrations between 0.5 and 1.5 μg l⁻¹, and maximum rates at chlorophyll concentrations>1.5 μg l⁻¹. Application of these feeding rates to the phytoplankton distribution on the shelf allowed locations and time periods of low, intermediate and high daily feeding to be calculated for 2001 and 2003. A detailed cross-shelf survey of body size and lipid store in these copepods, however, indicated they were indistinguishable regardless of collection site. Although the daily ingestion of phytoplankton by N. flemingeri and N. plumchrus varied widely because of mesoscale variability in phytoplankton, these daily differences did not result in differences in final body size or lipid storage of these copepods. These copepods efficiently dealt with small and mesoscale variations in their food environment such that mesoscale structure in phytoplankton did not affect their final body size.     

Biogenic fluxes in the Cariaco Basin: a combined study of sinking particulates and underlying sediments

The fluxes of total mass, organic carbon (OC), biogenic opal, calcite (CaCO₃) and long-chain C₃₇ alkenones (ΣAlk₃₇) were measured at three water depths (275, 455 and 930 m) in the Cariaco Basin (Venezuela) over three separate annual upwelling cycles (1996–1999) as part of the CARIACO sediment trap time-series. The strength and timing of both the primary and secondary upwelling events in the Cariaco Basin varied significantly during the study period, directly affecting the rates of primary productivity (PP) and the vertical transport of biogenic materials. OC fluxes showed a weak positive correlation (r²=0.3) with PP rates throughout the 3 years of the study. The fluxes of opal, CaCO₃ and ΣAlk₃₇ were strongly correlated (0.6<r²<0.8) with those of OC. The major exception was the lower than expected ΣAlk₃₇ fluxes measured during periods of strong upwelling. All sediment trap fluxes were significantly attenuated with depth, consistent with marked losses during vertical transport. Annually, strong upwelling conditions, such as those observed during 1996–1997, led to elevated opal fluxes (e.g., 35 g m⁻² yr⁻¹ at 275 m) and diminished ΣAlk₃₇ fluxes (e.g., 5 mg m⁻² yr⁻¹ at 275 m). The opposite trends were evident during the year of weakest upwelling (1998–1999), indicating that diatom and haptophyte productivity in the Cariaco Basin are inversely correlated depending on upwelling conditions.The analyses of the Cariaco Basin sediments collected via a gravity core showed that the rates of OC and opal burial (10–12 g m⁻² yr⁻¹) over the past 5500 years were generally similar to the average annual water column fluxes measured in the deeper traps (10–14 g m⁻² yr⁻¹) over the 1996–1999 study period. CaCO₃ burial fluxes (30–40 g m⁻² yr⁻¹), on the other hand, were considerably higher than the fluxes measured in the deep traps (∼10 g m⁻² yr⁻¹) but comparable to those obtained from the shallowest trap (i.e. 38 g m⁻² yr⁻¹ at 275 m). In contrast, the burial rates of ΣAlk₃₇ (0.4–1 mg m⁻² yr⁻¹) in Cariaco sediments were significantly lower than the water column fluxes measured at all depths (4–6 mg m⁻² yr⁻¹), indicating the large attenuation in the flux of these compounds at the sediment–water interface. The major trend throughout the core was the general decrease in all biogenic fluxes with depth, most likely due to post-depositional in situ degradation. The major exception was the relatively low opal fluxes (∼5 g m⁻² yr⁻¹) and elevated ΣAlk₃₇ fluxes (∼2 mg m⁻² yr⁻¹) measured in the sedimentary interval corresponding to 1600–2000 yr BP. Such compositions are consistent with a period of low diatom and high haptophyte productivity, which based on the trends observed from the sediment traps, is indicative of low upwelling conditions relative to the modern day.     

Nutrient and phytoplankton dynamics off the west coast of Vancouver Island during the 1997/98 ENSO event

Six research cruises were conducted off the west coast of Vancouver Island between April and October of 1997 and 1998 as part of the Canadian GLOBEC project to compare nutrient and phytoplankton dynamics between ENSO (1997) and non-ENSO (1998) years. Limited sampling also was conducted during three cruises in 1999. During the 1997 ENSO period, there was a shallow thermocline (∼10 m) that resulted in a shallower mixed layer, lower salinity and density, and stronger summer stratification. In general on the shelf, the 1997 growing season was characterized by higher nitrate (7.5 μM) and silicic acid (17 μM) concentrations, lower total chlorophyll (∼76 mg m⁻²), lower phytoplankton carbon biomass (0.2 mg C L⁻¹), and lower diatom abundance and biomass than in 1998. Phytoplankton assemblages were dominated by nanoplankton in 1997 and by diatoms in 1998. These results suggest that the 1997 ENSO was responsible for a reduction in the growth and biomass of larger phytoplankton cells. In mid-1998, the hydrographic characteristics off the west coast of Vancouver Island changed suddenly. The 1997 poleward transport of warm water reversed to an equatorward transport of coastal water in July 1998, which was accompanied by normal summer upwelling. During 1998, a large diatom bloom (mainly dominated by Chaetoceros debilis, Leptocylindrus danicus and to a lesser extent by Skeletomema and Pseudo-nitzschia sp.) was observed in July over the continental shelf. This large bloom resulted in chlorophyll concentrations of up to 400 mg m⁻², primary productivity of up to 11 g C m⁻² d⁻¹, and near undetectable dissolved nitrogen concentrations at some of the shelf stations in 1998. In contrast, during 1997, the sub-tropical waters that were advected over the slope, resulted in low chlorophyll a and primary productivity (generally <1 g C m⁻² d⁻¹). Therefore, there was a sharp contrast between the very high primary productivity on the shelf in July 1998, due to normal nutrient replenishment from summer upwelling and outflow from the Strait of Juan de Fuca, and the lower primary productivity during the 1997 ENSO year. During 1998, non-ENSO conditions resulted in phytoplankton biomass that was twice as high on the shelf as that measured in regions beyond the continental shelf of the west coast of Vancouver Island.