Shelf Stations Research Articles

Seasonal dynamics of picoplankton in shelf waters of the southern Bay of Biscay

Seasonal variations in autotrophic and heterotrophic picoplankton community structure were studied by flow cytometry at 3 shelf stations in the central Cantabrian Sea (southern Bay of Biscay), from April 2002 to April 2003. A marked seasonality was found for autotrophs, with cyanobacteria (Synechococcus and Prochlorococcus) being more important in nutrient-depleted waters (June to October, 75 ± 3% of picophytoplanktonic cells) whereas picoeukaryotes clearly dom- inated in terms of biomass during the rest of the year (82 ± 2%). The absence of Prochlorococcus in spring and summer seems to be related to hydrology rather than to nutrient conditions. Under strati- fication the maximum abundance of cyanobacteria was found above the nutricline, in contrast to picoeukaryotes. Autotrophic groups cell size was strongly influenced by nutrient conditions while chlorophyll a fluorescence varied in parallel to irradiance. Heterotrophic bacteria were less variable than picophytoplankton and high nucleic acid content (HNA) cells dominated the community in win- ter and spring (64 ± 1%) but only at depth during the rest of the year. A similar response of low nucleic acid content (LNA) bacteria to environmental conditions suggests they are, at least partially, metabolically active, in contrast to the commonly held view that they are a homogeneous group of dead or dormant cells. A larger mean size of LNA compared with HNA bacteria was consistently observed in winter and spring. Regardless of variations in abundance and group composition, the great constancy of the picoplanktonic contribution to integrated algal biomass (17 ± 2%) over the seasonal cycle outlines the importance of picophytoplankton also in coastal zones.

Spatio-temporal changes in diversity and community structure of planktonic copepods in Sagami Bay, Japan

Seasonal changes in diversity and community structure of planktonic copepods at a shelf site in Sagami Bay, Japan was studied in relation to cross-shelf interaction of species components. Seasonal mesozooplankton samples were collected from the shelf station (St. M) of the north-west part of Sagami Bay from 1995 to 1997. Vertical multi-layered samples were collected near the center of Sagami Bay (St. P) in June 1996. A total 185 copepod species were identified from the two stations. We observed a clear seasonal succession in calanoid diversity and community structure at St. M from a simple shelf water community (>11 species) during spring blooming periods to highly diverse and mixed communities (ca 20–30 species) of shelf water species coupled with various Kuroshio Current species during late summer to autumn. Cluster and non-metric multidimensional scaling ordination analyses showed two distinct calanoid community groups. One group, which included samples of St. M and the surface layer of St. P, consisted of shelf water species, such as Calanus sinicus, Ctenocalanus vanus, Paracalanus spp., and Kuroshio species, such as, Canthocalanus pauper, Scolecithrix danae, etc. The other cluster was restricted to the samples collected from mid and deep layers at St. P, which consisted of meso- and bathypelagic species and Oyashio species (cold-current species, such as Neocalanus cristatus, Pseudocalanus spp., Eucalanus bungii and Metridia pacifica). In the mid and deep layers at St. P, the population of dormant copepodid stage V (CV) of Eucalanus californicus and C. sinicus were dominant. The deep CV population of C. sinicus might be ecologically discriminated from the surface and shelf water population due to their larger body length and dormant life cycle. E. californicus was also collected at the shelf site during each spring bloom period, whereas the population might descend into the mid- and deep-layers of the central bay before summer. Our results suggest that the seasonal fluctuation of community structure in the shelf water was controlled by both physical (Kuroshio Current) and biological factors, i.e., spring bloom and ontogenetic vertical migration of E. californicus. In particular, transport and diffusion processes of Kuroshio Current in Sagami Bay played a key role in controlling the shelf water calanoid community.

Composition and distribution of zooplankton across an upwelling front on the northern Portuguese coast during summer

Zooplankton was sampled during the summer of 1995 from a grid of 34 shelf stations located off the Ria de Aveiro (northwestern Portugal), which were arranged into cross-shore transects extending either to the shelf break or to the 50 m isobath. At each station, a bongo net was hauled obliquely from close to the bottom or a maximum depth of 50 m to the surface. STD casts were made also at each station and wind data were obtained from a land station. Analysis of salinity and temperature data detected the presence of strong thermohaline stratification and a lens of reduced salinity water at the surface. A well-defined front, located at a depth of 10 to 35 m, separated cold water close to shore from warmer offshore water. Isopycnals between the front and the shore were bent towards the surface. The structure of the thermohaline features was consistent with the upwelling-favourable winds measured during the sampling period. A classification and a multivariate analysis of variance of the more abundant species showed the presence of two distinct assemblages that were separated by the front, indicating that these formations are important features structuring the zooplanktonic community of upwelling areas. The data further indicate that the zooplankton was composed of Palaeartic Atlantic and Mediterranean neritic species, and that oceanic species were absent from the samples. The absence of oceanic species is interpreted as resulting from the offshore displacement of the slope poleward flow and of the Portugal Current that takes place during the summer, as a response to upwelling-favourable winds.

Impacts of water depth, sediment pigment concentration, and benthic macrofaunal biomass on sediment oxygen demand in the western Arctic Ocean

We investigated the impacts of water depth, sediment pigment concentration, and benthic macrofaunal biomass on sediment oxygen demand (SOD) during three cruises to the western Arctic Ocean. SOD values were similar to those of most studies from the Arctic and ranged from a high of 20.68 mmol O2·m–2·day–1 at a shallow shelf station to a low of 0.29 mmol O2·m–2·day–1 at the deepest basin station (3648 m). SOD was significantly greater at shallow sites (<500 m; mean = 7.39 mmol O2·m–2·day–1; standard deviation (SD) = ±5.38) than at deep sites (>500 m; mean = 1.39 mmol O2·m–2·day–1; SD = ±0.96). As hypothesized, SOD was negatively correlated with water depth and positively correlated with both surface-sediment pigment concentration and macrofaunal biomass, with macrofaunal biomass explaining approximately 74% of the variability in SOD. We propose that higher macrofauna-normalized respiration rates (i.e., SOD divided by macrofaunal biomass) in deep water indicate that microbial–meiofaunal respiration predominates in deep versus shallow water. Finally, deeper stations associated with Barrow Canyon had SODs, benthic macrofaunal biomass, and surface-sediment pigment concentrations that were similar to those of shallower shelf locations, suggesting down-canyon transport of organic material.

Acoustic observations of finescale zooplankton distributions in the Oregon upwelling region

Abstract We collected data on two GLOBEC cruises to investigate the ubiquity and character of small and finescale phytoplankton and zooplankton vertical distributions in continental shelf waters. Coincident data from a four-frequency echosounding system (38, 120, 200, and 420 kHz), CTD, 150 kHz ADCP, and a suite of high-resolution bio-optical instruments were collected at three continental shelf stations off the coast of Oregon in June and August of 2000. Temporally and spatially coherent finescale features were identified in the acoustic record at all sites throughout the upper water column and the dominant characteristics were observed. There was evidence of an association between the vertical structure of horizontal velocity and the finescale structure seen in the acoustic data. These observations support the hypothesis that discrete, finescale vertical distributions of phytoplankton and zooplankton may be common in productive upwelling areas and indicate that additional concurrent physical and biological measurements, at appropriate temporal and spatial scales, must be made to further characterize the ecological significance of these finescale distributions.

Phyto- and zooplankton community structure and production around South Georgia (Southern Ocean) during Summer 2001/02

During Austral summer 2001/02 the spatial distribution of phytoplankton and zooplankton communities and associated production processes were investigated in waters to the north of the island of South Georgia. Nearest neighbour cluster analysis of phytoplankton and zooplankton data sets indicated the presence of 3 major station groupings within each analysis that collectively had great geographic integrity. Thus a shelf station group was characterised by low phytoplankton biomass and primary production rates (median values 69 mg chl a m −2; 418 mg C m −2 d −1, respectively) and a dominance of nano- and pico-phytoplankton, on the one hand, and low overall mesozooplankton levels (median 50,135 ind. m −2), which was also coincident with elevated krill biomass, on the other. At the western end of the island high mesozooplankton abundance (median >250,000 ind. m −2) in oceanic waters was co-incident with a phytoplankton bloom that originated over the North West Georgia Rise (NWGR). These ‘bloom’ stations were characterised by high phytoplankton biomass and primary production rates (median 218 mg m −2 and 916 mg C m −2 d −1, respectively) and were dominated by microphytoplankton. This elevated production was associated with an anticyclonic eddy which was considered to have been initiated where the Southern Antarctic Circumpolar Current Front (SACCF) interacted with the bathymetry of the NWGR. SeaWiFS and drifter buoy data indicated that this production was subsequently entrained and transported cyclonically around the Georgia Basin downstream from the island. In contrast, at the eastern end of the island, in waters south of the SACCF, phytoplankton biomass and primary production rates were lower (median 40 mg m −2 and 214 mg C m −2 d −1, respectively) although median zooplankton abundance was intermediate (∼157,000 ind. m −2). Of the environmental variables measured in this study the proportion of microphytoplankton within samples, rather than for example, phytoplankton biomass, explained the greatest proportion of the variance associated with the carbon mass of individual copepod species stages, egg production rates and total zooplankton abundance. We conclude that stations in the western and eastern oceanic regions were dominated by bottom up controls, with physical forcing and adequate nutrient availability leading to bloom conditions of large diatoms in the west, whereas in the east, phytoplankton growth was lower (possibly due to micronutrient limitation) and zooplankton abundance was lower. Over the shelf we suggest that a higher average biomass of krill (∼80 g wet mass m −2) compared to offshelf (43 g wet mass m −2) may have exerted top down control and grazed out chl a and thus have been responsible for low mesozooplankton abundance.

Microbial abundance and activity in the seasonal upwelling area off Concepción (∼36°S), central Chile: a comparison of upwelling and non-upwelling conditions

Abstract The contribution of bacteria and nanoflagellates to marine carbon flux was studied in the upwelling area off Concepcion, central Chile (∼36°S), at coastal shelf and oceanic stations, during October 1998 (austral spring, upwelling season) and July 1999 (austral winter, non-upwelling season). During the spring cruise, euphotic-zone integrated primary production ranged from 1200 to 8740 and from 320 to 540 mg C m−2 d−1 and bacterial production from 238 to 760 and from 91 to 102 mg C m−2 d−1 at coastal shelf and adjacent oceanic stations, respectively. Bacterial growth in coastal shelf waters was higher (0.4–1.3 d−1) than at the oceanic stations (0.2 d−1). In the same station order, the abundance ranges for heterotrophic nanoflagellates were 0.7–33.9×105 and 0.2–56.9×105 cells L−1 and, for autotrophic nanoflagellates, 3.8–32.1×105 and 0–30.4×105 cells L−1, respectively. During the non-upwelling conditions, primary production was lower at the coastal shelf stations (481–1710 mg C m−2 d−1) and slightly higher (415–865 mg C m−2 d−1) at the oceanic stations. Bacterial production (

Ecosystem dynamics at six contrasting sites: a generic modelling study

A pelagic marine ecosystem simulation model ERSEM-2004, developed from the European Regional Seas Ecosystem Model (ERSEM II), is presented along with a parameter set applicable to six highly contrasting sites, ranging from a temperate mixed shelf station to a permanently stratified tropical deep-ocean station. The physical characteristics are simulated by direct coupling to a 1D vertically resolved turbulence model, parameterised for each site. A mathematical description of the pelagic ecosystem model is presented. Additions to ERSEM II's well resolved community and decoupling of gross production and ambient nutrient concentration include variable carbon to chlorophyll ratios, coupling of bacterial production to nutrient availability, improved resolution of the organic particulate and dissolved fractions and developments to the mesozooplankton description. Comparison of seasonally depth resolved and integrated properties illustrates that the model produces a wide range of community dynamics and structures that can be plausibly related to variations in mixing, temperature, irradiance and nutrient supply. The spatial–temporal variability in key environmental indicators only partially correlates with the spatial–temporal variability in community structure ( ρ<0.5). Thus we infer that the complexity of the model's trophic structure and hence that of the marine system is important in defining the ecological response to the environment. A physical description of a marine domain may not be an adequate indicator of marine community structure or function. Particularly, lysis and grazer response are identified as important processes that define ecosystem dynamics and community structure. There is a closer correlation ( ρ>0.75) between spatial–temporal variability in community structure (biomass) and function (production). ERSEM-2004 is shown to be a robust model that is capable of representing a range of systems commonly described in the marine system. Consequently, the model is proposed as a potential basis for an ecosystem-based management tool that may, with appropriate physical representation, be applied over large geographic and temporal scales with utility to both heuristic and predictive studies of the marine lower trophic levels.

Reproductive patterns of Calanus finmarchicus at the Norwegian midshelf in 1997

Gonad stage, mean clutch size, prosome length, oil sac volume and feeding activity of adult female Calanus finmarchicus were studied from preserved samples collected at two transects in the Norwegian Sea from February to October 1997. The pattern of co-ordinated changes in gonad maturity and egg clutch size suggested that two generations were produced annually in the study region. The main period of reproduction, judging from proportion of females with mature gonads and mean clutch size, occurred in April, followed by less pronounced peaks in June and July. Gonad stage and the oil sac volume were negatively correlated, suggesting that gonad development was, at least partly, fuelled by reserve lipids. Individual clutch size (estimated as the number of mature oocytes per female) was highly variable, ranging from 10 to 104 during the studied season. Feeding activity at the shelf stations was usually higher than at oceanic stations, and was higher at the southern transect than the northern, causing earlier maturation over the shelf, and in the south. The distinctly pronounced spring generation was probably synchronized by the termination of the overwintering phase, while the summer generation, which depends on the variable feeding conditions of the habitat, was less synchronized.

Distribution of DDT in sediments off the central California coast

A collaborative sampling cruise off the central California coast was conducted to evaluate contaminant transport pathways along and across the shelf in the spring of 2002. The area has a complex current structure and net transport routes are not known for sure. Sediment characteristics, and organic and trace metal contaminants were analyzed in sediments taken from locations near shore, out to the heads of several canyons. Relative to the continental shelf and Pioneer Canyon stations, DDT was found at higher concentrations in Ascension and Monterey Canyons. Monterey Bay still receives DDT from terrestrial runoff and may be the source of DDT found in Ascension Canyon. DDT concentrations in Monterey Bay biota indicate bioaccumulation is occurring at depth due to continuing input from the shore. Effects on the deep ocean benthic community is unknown.

Picophytoplankton and bacterioplankton in the Mississippi River plume and its adjacent waters

Picoplankton abundance and distribution in the Mississippi River plume and its adjacent waters were studied during two cruises in April (high discharge) and October (low discharge) 2000 using flow cytometry. Concentrations of photosynthetic picoplankton,Synechococcus and picoeukaryotes were low in the turbid plume water but high in the coastal waters—i.e., the green waters resulting from mixing of river and oceanic waters. In this region, three types ofSynechococcus, characterized by their phycoerythrin chromophore composition, were found:Synechococcus cells with a low phycourobilin to phycoerythrobilin ratio (PUB:PEB) occurred throughout the region and dominated the totalSymechococcus abundance during both seasons; high PUB:PEB cells, which are the dominant strains in the open or blue ocean, occurred only at the outer shelf stations; and PEB-onlySynechococcus were abundant in most of the surveyed area during april, but were not observed during October.Prochlorococcus cyanobacteria only occurred at the oceanic stations, but extended farther inshore in October compared to April. This was a consequence of the reduced discharge and plume size during October. Picophytoplankton were a less important component of total phytoplankton biomass in the turbid river water and more important in the oligotrophic Gulf water. Seasonally, the contribution of picophytoplankton to total phytoplankton biomass in the surveyed area was higher during low discharge in October than during high discharge in April, even though the spring 2000 river discharge was unusually low and might not present a typical high discharge scenario. The abundance of heterotrophic bacteria was weakly correlated to chlorophylla (chla) concentration, but better correlated to picophytoplankton biomass. A higher proportion of High DNA bacteria occurred in the river-impacted regions during both seasons, with the ratio of High DNA bacteria to Low DNA bacteria significantly higher in April.

Seasonal variations in upwelling and in the grazing impact of copepods on phytoplankton off A Coruña (Galicia, NW Spain)

The impact of grazing by copepods on phytoplankton was studied during a seasonal cycle on the Galician shelf off A Coruña (NW Spain). Grazing was estimated by measuring the chlorophyll gut content and the evacuation rates of copepods from three mesh-size classes: 200–500 (small), 500–1000 (medium), and 1000–2000 μm (large). Between February 1996 and June 1997, monthly measurements of water temperature, chlorophyll concentration, primary production rates, and copepod abundance, chlorophyll gut content, and evacuation rates were taken at an 80-m-deep, fixed shelf station. Additionally, the same measurements were collected daily during two bloom events in March and in July 1996. Small copepods were the most abundant through the seasonal cycle. The highest grazing impact, however, was due to the medium and large size classes. Grazing by small copepods exceeded grazing by medium and large copepods only during phytoplankton spring blooms. The impact of copepod grazing (considering all size fractions) was generally low. On average, 2% of the phytoplankton biomass and 6% of the primary production were removed daily by the copepod community. Maximum grazing impact values (9% of the phytoplankton biomass and 39% of the primary production) were found in mid-summer. These results suggest that most of the phytoplankton biomass would escape direct copepod grazing in this upwelling area.

228Ra and 226Ra in the Kara and Laptev seas

The surface water in the Transpolar Drift in the Arctic Ocean has a strong signature of 228Ra. In an earlier study of 228Ra in the open Arctic we showed that the major 228Ra source had to be in the Siberian shelf seas, but only a single shelf station was published so far. Here we investigate the sources of this signal on the Siberian shelves by measurements of 228Ra and 226Ra in surface waters of the Kara and Laptev Sea, including the Ob, Yenisey and Lena estuaries. In the Ob and Lena rivers we found an indication for a very strong and unexpected removal of both isotopes in the early stage of estuarine mixing, presumably related to flocculation of organic-rich material. Whereas 226Ra behaves conservatively on the shelf, the distribution of 228Ra is governed by large inputs on the shelves, although sources are highly variable. In the Kara Sea the maximum activity was found in the Baydaratskaya Bay, where tidal resonance and low freshwater supply favour 228Ra accumulation. The Laptev Sea is a stronger source for 228Ra than the Kara Sea. Since a large part of Kara Sea water flows through the Laptev Sea, the 228Ra signal in the Transpolar Drift can be described as originating on the Laptev shelf. The combined freshwater inputs from the Eurasian shelves thus produce a common radium signature with a 228Ra/ 226Ra activity ratio of 4.0 at 20% river water. The radium signals of the individual Siberian rivers and shelves cannot be separated, but their signal is significantly different from the signal produced on the Canadian shelf (Smith et al., in press). In this respect, the radium tracers add to the information given by Barium. Moreover, with the 5.8 year half-life of 228Ra, they have the potential to serve as a tracer for the age of a water mass since its contact with the shelves.

Importance of the large copepod Paraeuchaeta antarctica (Giesbrecht, 1902) in coastal waters and the diet of seabirds at Kerguelen, Southern Ocean

The importance of the euchaetid copepod Paraeuchaeta antarctica in the subantarctic pelagic ecosystem was quantified in the coastal waters of the Golfe du Morbihan at Kerguelen Islands by comparing food samples from two diving seabirds with concurrent net samples taken within the predator foraging area. Paraeuchaeta antarctica occurred in very high densities (up to 30 individuals m 3 and 96 mg dry weight m -3 ) in the water column, being more abundant in the deepest part of the gulf than in shallow waters or at the more offshore shelf stations. The common diving petrel feeds almost exclusively on crustaceans, its diet being dominated by the amphipod Themisto gaudichaudii (52% by number and 84% by reconstituted mass) and P. antarctica (33% and 16%, respectively). Rockhopper penguins preyed upon crustaceans and fish, with four taxa being important, namely T gaudichaudii (37% and 23%, respectively), Euphausia vallentini (24% and 41%), postlarval fish (10% and 24%), and P. antarctica (13% and 3%). Paraeuchaeta antarctica dominated numerically in 21% of the diving-petrel food samples and in 12% of penguin samples. The two bird species segregated by preying upon dfferent developmental stages of P antarctica, diving petrels fed equally on CV of both sexes and CVI♀, while penguins fed on CVI♀ only. Comparison of P. antarctica found in net and food samples indicated no prey selection by common diving petrels that caught the different copepod stages in proportion to their availability in the water column. On the other hand, the diving performance of penguins, which is better than that of the petrels (mean maximum dive depths 69 m for penguins versus 32 m in petrels), allows them to catch CVI♀ in deeper waters, probably near the bottom. Our study shows that P antarctica is a major component of the coastal macrozooplankton community and a significant prey for two species of diving seabirds inhabiting Kerguelen. This is also the first record of a copepod species as a prey for penguin, and the first to highlight P. antarctica in the food of austral seabirds.

The initial laparoscopic appearance can predict response to combination therapy in hepatitis C

Antarctica is the fifth-largest continent, covering 5.4 million square miles (14 million square km). The average winter temperature in Antarctica is −81°F (−63°C) and has reached −128.6°F (−89.2°C). About 98% of the Antarctic continent is covered by glacial ice that averages about 1.2 miles (1.9 km) in thickness. The total volume of ice in Antarctica is about 6.4 million cubic miles (26.5 million cubic km), comprising about 90% of the world's ice and about 70% of the world's fresh water. The average height of the ice sheet is approximately 10,000 ft above sea level. At its thickest point, the ice is 15,700 ft (4776 m) thick. The East Antarctic Ice Sheet is by far the biggest ice sheet, making up 92% of the total glacial ice, while the West Antarctic Ice Sheet makes up only about 8%.The Steig et al. analysis that all of Antarctica is warming was refuted by O'Donnell et al., who showed that their methodology was badly flawed. Using the same data as Steig et al., but with better technology, they produced a map showing cooling dominating most of the East Antarctic Ice Sheet with warming primarily constrained to the Antarctica Peninsula. Satellite and surface temperature measurements demonstrate that the Steig et al. contention that all of Antarctica is warming is clearly false. Antarctic satellite temperatures show no warming for 37 years. The Southern Ocean around Antarctica has cooled markedly since 2006. Sea ice has increased substantially, especially since 2012. Surface temperatures at 13 stations on or near the Antarctic Peninsula have been cooling sharply since 2006. Ocean temperatures have been plummeting since about 2007, sea ice has reached all-time highs, and temperatures have been cooling since 2000. The Larsen Ice Shelf Station has been cooling at an astonishing rate of 1.8°C per decade (18°C per century) since 1995.The thickness of most of the West Antarctic Ice Sheet is more than 6000 ft (2000 m) and large areas are more than 10,000 ft (3000 m) thick. The importance of ice thickness is that virtually all of the ice sheet is considerably thicker than the depth below sea level to bedrock, so the ice is grounded and will not float. If the West Antarctic Ice Sheet melts away, where the ice is 10,000 ft thick, the land beneath will rebound isostatically several 1000 ft, bringing most of the subglacial bed above sea level.The Pine Island and Thwaites outlet glaciers are only about 30 miles across, so draining 2.2 million km3 of ice through their narrow channels or sending sea water 1000 miles under the ice sheet is not plausible. Studies of subglacial geothermal heat flow show that the area under the Thwaites glacier is unusually high and is the most likely cause of subglacial melting, rather than ocean water. The West Antarctic Ice Sheet is not collapsing, the retreat of these small glaciers is not caused by global warming, and sea level is not going to rise 10 ft.

Trace metals and nitrogenous nutrition of Antarctic phytoplankton: experimental observations in the Ross Sea

As part of the US JGOFS Antarctic Environment Southern Ocean Process Study (AESOPS), shipboard incubation experiments were conducted to investigate the effects of iron and zinc addition on ammonium (NH 4 + ) and nitrate (NO 3 − ) uptake by natural assemblages of Antarctic phytoplankton. Samples were collected from the Ross Sea during austral summer (January–February, 1997) within the mixed layer at two stations: a continental shelf station—Orca, and a high NO 3 − , low chlorophyll (HNLC) station located further offshore—Blue. Twenty-liter bottle incubations were sampled repeatedly during an 8-day period at Orca, and a 13-day period at Blue for short-term 15 N-tracer uptake experiments. Ambient concentrations of NO 3 − were elevated at both locations. Biomass, measured as chlorophyll a (chl a ), was relatively high at Orca (chl a =2.31 μg l −1 ), whereas true HNLC conditions were observed at Blue (chl a =0.52 μg l −1 ). All Fe enrichments produced an acceleration in NO 3 − decline, and accumulation in biomass. Chl a increased 2–3-fold at Orca and 4–5-fold at Blue; these increases were directly correlated with increasing Fe enrichment. Unambiguous responses to zinc addition were not evident at Blue, whereas increased total biomass accumulation and nitrate drawdown were observed at Orca. Rates of biomass (particulate nitrogen) specific NO 3 − -uptake in the Fe-enriched samples were up to 2-fold greater than un-enriched controls at both sites. There were no significant changes in specific uptake rates of NH 4 + at the HNLC site, and only a 30–40% increase at Orca with the highest Fe enrichments. These results clearly indicate that Fe additions resulted in faster rates of NO 3 − consumption per unit phytoplankton biomass at both sites. The N-uptake response to zinc enrichment was not as evident as with Fe, presumably due in part to the relatively high dissolved concentrations of zinc (ambient concentrations=ca. 2 nM). However, during the later sampling periods, zinc addition resulted in a 40% increase in the specific uptake rates of NO 3 − but not NH 4 + , whereas the specific uptake rates of NO 3 − and NH 4 + increased by 4–16% and 18–49%, respectively, at Orca. Absolute uptake rates of NO 3 − and NH 4 + (corrected for isotopic dilution) were 4–5 and 2–3 times greater, respectively, than the controls at Blue, whereas at Orca both NO 3 − - and NH 4 + -uptake rates doubled as a result of Fe enrichment. Post-incubation, size-fractionation uptake experiments demonstrated that the higher rates of nitrogen uptake were primarily due to larger phytoplankton (>5 μm). The f -ratio [ f =NO 3 − -uptake/(NO 3 − +NH 4 + -uptake)] increased from ca. 0.7 to 0.8 as a result of Fe enrichment at the HNLC site, but declined from an average of 0.84 to 0.52–0.69 in both the controls and the Fe-amended samples collected further onshore at Orca . This decrease may be due to the inhibition of NO 3 − -uptake by elevated NH 4 + concentrations resulting from increased heterotrophic remineralization within the carboys over time. Based on the apparent half-saturation constant ( K s ) of 0.09 nM Fe estimated for community planktonic NO 3 − -uptake, the availability of dissolved Fe (ambient concentration=0.03–0.04 nM) limits the uptake of NO 3 − by phytoplankton at the HNLC and continental shelf regions of the western Ross Sea during austral summer.

Total dissolved zinc in the upper water column of the subarctic North East Pacific

Abstract Total dissolved Zn concentrations are reported for winter and summer along the E–W line P transect and for summer along S–N line Z transect extending from Ocean Station Papa (OSP; 50°N 145°W) to the respective shelf waters. Surface-water (10 m) concentrations ranged from 0.04 nM in the open ocean to 0.9 nM at the Canadian shelf station. A clear gradient is evident, with dissolved Zn concentrations decreasing with increasing distance from shore along the E–W transect in both summer and winter. No consistent trend was observed along the S–N transect. Low concentrations of Zn (0.05–0.07 nM) were observed in near-surface water at OSP in winter. The vertical distribution of dissolved Zn is oceanographically consistent, showing a silicon-like vertical profile. Concentrations of silicic acid and dissolved Zn at depth are higher than those at equivalent depths in the Atlantic, as expected from deep-water circulation patterns and build-up in concentration of these elements. Dissolved Zn/Si ratios in the upper 200 m indicate a trend of decreasing with distance from shore, which infers a decoupling between dissolved Zn and silicic acid in the upper ocean. We hypothesise that the silicon-like profile of dissolved Zn is a result of recycling from relatively biologically resistant organic particulate phases, that leads to profiles very similar to those of dissolved silicon. Other than at two stations where there is very high primary production, there is no apparent horizontal relationship between total dissolved Zn and chlorophyll a or production.

Trace metal distributions in shelf waters of the northwestern Black Sea

Measurements of dissolved and leachable particulate trace metals (Mn, Fe, Co, Pb, Cd, Zn, Cu and Ni) and total particulate Mn and Fe were made on seawater samples collected from the northwestern Black Sea during the EROS 2000 expedition conducted in July–August 1995. The investigation concentrated on waters of the shelf and shelf edge, but included one deeper water (1440 m) station. In the oxic layer of the deep station, the suspended particulate fractions of Mn and Fe were a major part of the total metal mass, consistent with the presence of the “Fine Particle Layer” which forms on the shelf and spreads all over the Black Sea with intensities decreasing from the coast. Dissolved and total particulate concentrations were, respectively, Mn, 0.69–9.6, 1.2–29; Fe, 0.79–3.03, 2.3–7.4 nM. Dissolved Cu and Ni concentrations were relatively high (1–8 and 8–12 nM, respectively), and did not show any depletion in surface oxic waters, possibly as a result of strong organic complexation. Dissolved Pb concentrations (100–200 pM) were higher than were generally found on the shelf. This was attributed to atmospheric inputs combined with less efficient scavenging of metals in these low SPM waters. The distribution of dissolved Co closely resembled that of dissolved Mn reflecting coupling through oxidation of Mn. Concentrations of dissolved Cd and Zn were low in surface water (0.07–0.09 and 0.9–2.0 nM, respectively), and increases in concentrations with depth were sharply reversed around the top of the redoxcline. For most metals (Mn, Fe, Co, Pb, Cu, Cd, Zn) dissolved concentrations were low in the anoxic layers as a result of solubility by formation of, or association with, solid sulphide phases. Dissolved Ni was not affected by sulphide precipitation. At most of the shelf stations there were clear enhancements of dissolved Mn and Fe in the deepest waters, consistent with other evidence that significant benthic fluxes of these metals arise through the redox conditions in the region of the sediment-water interface. In the shelf water column, dissolved Mn and Fe concentrations ranged between 1.2 and 1350 and between 0.4 and 181 nM, respectively; the highest concentrations were found near the bottom. Particulate concentrations of Mn and Fe were high, implying high oxidation rates of Mn(II) and Fe(II) and/or high supply rates from rivers. Total particulate concentrations of Mn and Fe were 0.7–1050 and 2.3–2650 nM, respectively; the highest concentrations were found in surface and bottom waters. The distributions of particulate Mn and Fe were consistent with the isopycnal transport of Mn and Fe oxyhydroxides from the shelf by the coastal circulation. Distributions of other trace metals (Co, Pb, Cu, Ni, Cd, Zn) were considerably influenced by riverine inputs. Relatively high dissolved and available particulate metal concentrations were generally found in surface waters at stations directly influenced by the Danube River. Some trace metals (Co, Ni, Cd and Zn) were influenced by Mn and Fe cycling and increases in their dissolved concentrations occurred at a number of stations near the sediment–water interface. Dissolved and available particulate metal concentrations (nM) at stations on the shelf were, respectively: 0.171–1.80, 0.003–0.437 (Co); 0.014–0.614, 0.010–1.48 (Pb); 7.6–28.8, 0.048–3.75 (Cu); 11.0–17.5, 0.018–2.10 (Ni); 0.033–0.161, 0.003–0.063 (Cd); 1.01–8.33, 0.135–7.58 (Zn).

Amphipod-based food web: Themisto gaudichaudii caught in nets and by seabirds in Kerguelen waters, southern Indian Ocean

Comparing food samples from diving and surface-feeding seabirds breeding in the Golfe du Morbihan at Kerguelen Islands to concurrent net samples caught within the predator foraging range, we evaluated the functional importance of the hyperiid amphipod Themisto gaudichaudii in the subantarctic pelagic ecosystem during the summer months. T. gaudichaudii occurred in high densities (up to 61 individuals m-3) in the water column, being more abundant within islands in the western part of the gulf than at open gulf and shelf stations. The amphipod was a major prey of all seabird species investigated except the South Georgian diving petrel, accounting for 39, 80, 68, 59 and 46% of the total number of prey of blue petrels, thin-billed prions, Antarctic prions, common diving petrels and southern rockhopper penguins, respectively. The length-frequency distribution of T. gaudichaudii was similar between the 2 diving species, which fed on 1 large size class of adult individuals, whereas the 3 surface-feeding seabirds preyed upon 2 size classes but in different proportions. Juveniles and adults T. gaudichaudii were equally important in the diet of blue petrels, whereas juveniles and adults predominated in the food of thin-billed and Antarctic prions, respectively. Comparison of T. gaudichaudii found in nets and food samples together with observations at sea indicated that common diving petrels and southern rockhopper penguins fed in the close vicinity of the colonies in the Golfe du Morbihan, whereas blue petrels, and thin-billed and Antarctic prions mainly preyed upon amphipods outside the sampled area. Our study shows that T. gaudichaudii is an important local component of the macrozooplankton community and the main prey for planktivorous seabirds inhabiting the Kerguelen archipelago. In certain areas of the subantarctic zone, it therefore has a trophic role similar to that of Antarctic krill Euphausia superba further south, in Antarctic waters.

Field comparison of an LHPR net sampling system and an Optical Plankton Counter (OPC) in the Southern Ocean

A field comparison of an optical plankton counter (OPC) and a Longhurst-Hardy Plankton Recorder (LHPR) was undertaken around the sub-Antarctic island of South Georgia during austral spring, 1997, to investigate the capacity of the OPC to accurately describe fine-scale plankton distributions. Initially, formalin-preserved developmental stages of representative zooplankton were passed through a benchtop OPC and a relationship established between maximum cross sectional area (CSA) of the plankton and the digital output value (DOV) of the OPC. The LHPR with an OPC attached to the net frame was then deployed at two stations representing contrasting plankton abundances. Outputs from two oblique hauls taken at a shelf station, and one at an oceanic station, were compared. Following corrections for volume swept by both instruments, and standardization of OPC particle and net plankton counts on the basis of sampler mouth area, the OPC was found to ‘overestimate' relative to the net by up to several orders of magnitude. However, following the correction for rate of flow though the OPC, and the application of a size threshold to the OPC data excluding the proportion of particles <0.25 mm2 CSA, based on the expected retention of plankton by the net there was a greatly improved and significant correlation. Overall there was no consistent over- or undersample by the OPC relative to the net, with mean OPC:net ratios being 1–1.8 for net plankton densities of up to 6000 ind. m–3. There was also a generally good agreement between the abundances of the rarer large biomass dominant calanoid copepods estimated by both samplers. An OPC overestimate of 2–3× relative to the net was seen on the ascent portion of the haul at the oceanic station, attributable to the presence of high abundances (estimated density ≥1600–4000 ind. l–1) of large centric diatoms which appeared to cause coincident counts. We conclude that with careful interpretation, OPC data can accurately describe small-scale plankton distributions in the Southern Ocean as well as order of magnitude changes.