Spatial and temporal variability of size-fractionated biomass and primary production in the Ross Sea (Antarctica) during austral spring and summer

The results of a study on the spatial and temporal dynamics of size-fractionated biomass and production of phytoplankton in the Ross Sea during the austral spring and summer are reported. The spring cruise took place in the offshore Ross Sea from 14 November to 14 December 1994. Sampling was carried out on a transect of 27 stations distributed from 76.5 to 72.0°S along 175°E, and covered the three main Antarctic environments of the polynya open waters, the marginal ice zone and the pack ice area. Three subsystems were identified. The subsystem of the polynya was characterised by the predominance of the micro- and nano-planktonic fractions, chlorophyll (Chl a) concentrations from 69.6 to 164.7 mg m⁻² and production rates from 0.68 to 1.14 g C m⁻² day⁻¹. The second subsystem, the marginal ice zone, showed a relative increase of the micro-planktonic fraction, high biomass levels (from 99.64 to 220 mg Chl m⁻²) and production rates from 0.99 to 2.7 g C m⁻² day⁻¹. The subsystem of the pack ice area had a phytoplankton community dominated by the pico-planktonic fraction and showed low biomasses (from 19.4 to 37.7 mg Chl m⁻²) and production rates (0.28 to 0.60 g C m⁻² day⁻¹). Selective grazing by krill is considered an important factor in determining the size structure of the phytoplankton communities. The summer study consisted of a time series carried out in inshore waters of Terra Nova Bay from 12 January to 8 February 1990. In a well stabilised water column and with high levels of PAR always available, the primary production rates of a community dominated by micro-plankton varied from 0.52 to 1.2 g C m⁻² day⁻¹ (average 0.84). A high P/B ratio, up to 3, and a remarkably elevated mean phaeopigment (Phaeo)/Chl a ratio of 2.4 indicated an active removal of biomass by grazing, confirmed by the presence of faecal pellets in quantities reaching 6000 m⁻³ in the upper 50 m. The peculiarities of the inshore versus offshore environments in terms of community size structure, production processes and their implications as regards the food web are discussed.     

Size-fractionated phytoplankton biomass and species composition in the Indian sector of the Southern Ocean during austral summer

During the late austral summer of 1994, Antarctic waters were characterized by low phytoplankton biomass. Along the 62°E meridian transect, between 49°S and 67°S, chlorophyll (Chl.) a concentration in the upper 150 m was on average 0.2 mg m⁻³. However, in the Seasonal Ice Zone (SIZ) chlorophyll a concentrations were higher, with a characteristic deep chlorophyll maximum. The highest value (0.6 mg Chl. a m⁻³) was measured at the Antarctic Divergence, 64°S, corresponding to the depth of the temperature minimum (100 m). This deep biomass maximum decreased from South to North, disappeared in the Permanently Open Ocean Zone (POOZ) and reappeared with less vigour in the vicinity of the Polar Front Zone (PFZ). In the SIZ, the upper mixed layer was shallow, biomass was higher and the >10 μm fraction was predominant. In this zone the >10 μm, 2–10 μm and <2 μm size fractions represented on the average 46%, 25.1% and 28.9% of the total integrated Chl. a stock in the upper 100 m, respectively. The phytoplankton assemblage was diverse, mainly composed of large diatoms and dinoflagellate cells which contributed 42.7% and 33.1% of the autotrophic carbon biomass, respectively. Moving northwards, in parallel with the decrease in biomass, the biomass of autotrophic pico- and nanoflagellates (mainly Cryptophytes) increased steadily. In the POOZ, the picoplanktonic size fraction contributed 47.4% of the total integrated Chl. a stock. A phytoplankton community structure with low biomass and picoplankton-dominated assemblage in the POOZ contrasted with the relatively rich, diverse and diatom-dominated assemblage in the SIZ. These differences reflect the spatial and temporal variations prevailing in the Southern Ocean pelagic ecosystem.     

Role of filtering and biodeposition by Adamussium colbecki in circulation of organic matter in Terra Nova Bay (Ross Sea, Antarctica)

At Terra Nova Bay, the scallop Adamussium colbecki (Smith, 1902) characterises the soft and hard bottoms from 20 to 80 m depth, constituting large beds and reaching high values of density (50–60 individuals/m²) and biomass (120 g/m² DW soft tissues). To assess its role in the organic matter recycling in the coastal ecosystem, its filtering and biodeposition rates were evaluated in laboratory experiments during the austral summer 1993/94. Filtration rates, measured in a flow-through system, were calculated from the difference in particulate organic carbon (POC), nitrogen (PON) and chlorophyll-a (Chl-a) concentration in inflow and outflow water. Experiments were performed using natural sea water with POC, PON and Chl-a concentrations of about 450 μg/l, 90 μg/l and 2 μg/l, respectively. The biodeposition rate and the biochemical composition of the biodeposits were studied in order to detect how the organic matter is transformed through feeding activity of A. colbecki. At +1°C temperature, the average filtering rate was about 1 l h⁻¹ g⁻¹ (DW soft tissues) in specimens ranging in body mass from 2 to 3 g (DW soft tissues) and 6–7 cm long. The biodeposition rate in 3–8 cm long specimens, ranging from 0.4 to 5.7 g (DW soft tissues), was about 5.65 mg DW/g DW/day, leading to an estimate of Corg flux, through biodeposition by A. colbecki, of about 21 mg C m⁻² day⁻¹ at in situ conditions. Comparison between the biochemical composition of seston and biodeposits shows a decrease of the labile compounds, of the Chl-a/phaeopigments ratio in the biodeposits. The recorded C/N ratio decrease suggests a microbial colonisation in the biodeposits. This study suggests that Adamussium colbecki plays an important role in coupling the material fluxes from the water column to the sea bed, processing about 14% of total Carbon flux from the water column to the sediments, with an assimilation efficiency of 36%.     

Seasonality of picophytoplankton chlorophyll a and biomass in the central Cantabrian Sea, southern Bay of Biscay

Seasonal changes in the abundance and biomass of cyanobacteria (Synechococcus and Prochlorococcus) and picoeukaryotes were studied by flow cytometry in the upper layers of the central Cantabrian Sea continental shelf, from April 2002 to April 2006. The study area displayed the typical hydrographic conditions of temperate coastal zones. A marked seasonality of the relative contribution of prokaryotes and eukaryotes was found. While cyanobacteria were generally more abundant for most of the year (up to 2.4 10⁵ cells mL^− 1), picoeukaryotes dominated the community (up to 10⁴ cells mL^− 1) from February to May. The disappearance of Prochlorococcus from spring through summer is likely related to shifts in the prevailing current regime. The maximum total abundance of picophytoplankton was consistently found in late summer–early autumn. Mean photic-layer picoplanktonic chlorophyll a ranged from 0.06 to 0.53 µg L^− 1 with a relatively high mean contribution to total values (33 ± 2% SE), showing maxima around autumn and minima in spring. Biomass (range 0.58–40.16 mg C m^− 3) was generally dominated by picoeukaryotes (mean ± SE, 4.28 ± 0.27 mg C m^− 3) with an average contribution of cyanobacteria of 30 ± 2%. Different seasonality of pigment and biomass values resulted in a clear temporal pattern of picophytoplanktonic carbon to chlorophyll a ratio, which ranged from 10 (winter) to 140 (summer). This study highlights the important contribution of picoplanktonic chlorophyll a and carbon biomass in this coastal ecosystem.     

Distribution of phytoplankton in the southern Black Sea in summer 1996, spring and autumn 1998

The species composition, abundance, and biomass of micro- (>15 μm) and nano- (<15 μm) phytoplankton were studied along the southern Black Sea during June–July 1996 and March–April and September 1998. A total of 150 species were identified, 50% of them being dinoflagellates. The average total phytoplankton abundance changed from 77×10³ cells l⁻¹ in spring to 110×10³ cells l⁻¹ in autumn and biomass from 250 μg l⁻¹ in summer to 1370 μg l⁻¹ in spring. Based on the extensive sampling grid from June–July 1996, phytoplankton seemed to have a rather homogeneous biomass distribution in the southern Black Sea. In all periods, the coccolithophorid Emiliania huxleyi was the most abundant species, its contribution to the total abundance ranging from 73% in autumn to 43% in spring. However, in terms of biomass, diatoms made up the bulk of phytoplankton in spring (97%, majority being Proboscia alata) and autumn (73%, majority being Pseudosolenia calcar-avis), and dinoflagellates in summer (74%, Gymnodinium sp.). There was a remarkable similarity in the dominant species between the western and eastern regions of the southern Black Sea, indicating transport of phytoplankton within the basin.     

The use of polycyclic aromatic hydrocarbons as a particulate tracer in the water column of Gaoping (Kaoping) Submarine Canyon

Time-series samples of settling particles were collected in the water column of Gaoping (formerly spelled Kaoping) Submarine Canyon (KPSC) with two sediment traps on taut-line moorings deployed at two different depths (60 and 280 m) between May 26 and June 27, 2004. Average total polycyclic aromatic hydrocarbon (PAH) concentrations of upper and lower trap array samples were 310 ± 61 ng g^− 1 dw (range: 200–440) and 240 ± 36 ng g^− 1 dw (range: 180–290), respectively. Principal component analysis results suggest that PAH sources in the trap-collected particles included diesel vehicle/coal burning, diagenetic sources, and petroleum release. PAH downward fluxes based on settling particles were estimated to be 12–44 μg m^− 2 d^− 1. These values are higher than those reported in the literature for most coastal areas. During the sampling period, both traps were significantly tilted by tidal current and fluctuated vertically. The upper traps experienced greater vertical movements, thus their particle characteristics (e.g., POC, particle mass, and fine particle fraction) varied more than those of the lower traps. Hourly depth variations of the tilted sediment trap array were echoed by the corresponding total PAH concentrations. Moreover, the PAH composition of the collected particles was related to the flow direction and speed. These observations suggest that PAHs can be used as an effective chemical tracer for the transport of terrestrial and marine particulates in a complex aquatic environment like Gaoping (Kaoping) Submarine Canyon.     

Marine molluscs as biomonitors for heavy metal levels in the Gulf of Suez, Red Sea

Levels of the heavy metals Copper (Cu), Zinc (Zn), Lead (Pb), Cadmium (Cd), Chromium (Cr), Nickel (Ni), Iron (Fe) and Manganese (Mn) were determined in coastal water, sediments and soft tissues of the gastropod limpet, Patella caerulea, and the bivalve, Barbatus barbatus, from seven different stations in the western coast of the Gulf of Suez. The concentrations of heavy metals in water ranged between 3.37–4.78, 18.83–21.46, 2.75–3.17, 0.22–0.27, 0.99–1.21, 2.69–3.65, 3.75–4.56 μg L^− 1 and 23.82–32.78 mg g^− 1 for Cu, Zn, Pb, Cd, Cr, Ni, Mn and Fe, respectively. The corresponding concentration values in the sediments were 8.65–12.16, 51.78–58.06, 36.52–42.15, 3.23–3.98, 9.03–12.75, 34.31–49.63, 3.28–4.56 and 64.20–70.22 μg g^− 1 for Cu, Zn, Pb, Cd, Cr, Ni, Mn and Fe, respectively. The highest accumulated metals were Fe, Zn and Mn in both P. caerulea and B. barbatus, while the lowest one was Cd. The accumulation of metals was more pronounced in P. caerulea than B. barbatus. The highest concentrations of all metals in water, sediments and mollusca were recorded at Adabiya harbour north of the Gulf, while the lowest concentrations were recorded at Gabal El-Zeit and Hurghada. Land based activities and ships awaiting berth are the main source of metal pollution in the northern part of the Gulf.     

A box model approach for a long-term assessment of estuarine eutrophication, Szczecin Lagoon, southern Baltic

We develop a layered “box model” to evaluate the major effects of estuarine eutrophication of the Szczecin lagoon which can be compared with integrating measures (chlorophyll a (Chl a), sediment burial, sediment oxygen consumption (SOC), input and output of total nutrient loads) and use it to hindcast the period 1950–1996 (the years when major increase in nutrient discharges by the Oder River took place). The following state variables are used to describe the cycling of the limiting nutrients (nitrogen and phosphorus): phytoplankton (Phy), labile and refractory detritus (D_N, D_Nref, D_P, D_Pref), dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and oxygen (O₂). The three layers of the model include two water layers and one sediment layer. Decrease of the carrying capacity with respect to the increased supply of organic matter of the system with advancing eutrophication over the period studied is parameterized by an exponential decrease of the sediment nitrogen fluxes with increasing burial, simulating changing properties from moderate to high accumulating sediments. The seasonal variation as well as the order of magnitude of nutrient concentrations and phytoplankton stocks in the water column remains in agreement with recent observations. Calculated annual mean values of nutrient burial of 193 mmol N m⁻² a⁻¹ and 23 mmol P m⁻² a⁻¹ are supported by observed values from geological sediment records. Estimated DIN remineralization in the sediments between 100 and 550 mmol N m⁻² a⁻¹ corresponds to SOC measurements. Simulated DIP release up to 60 mmol P m⁻² a⁻¹ corresponds to recent measurements. The conceptual framework presented here can be used for a sequential box model approach connecting small estuaries like the Szczecin lagoon and the open sea, and might also be connected with river box models.     

Production of Acartia omorii (Copepoda: Calanoida) in Ilkwang Bay, southeastern coast of Korea

Production of the marine calanoid copepod Acartia omorii was measured from 2 October 1991 to 8 October 1992 at a station in Ilkwang Bay on the southeastern coast of Korea. A. omorii (nauplii + copepodites + adults) were present in the plankton throughout the year, with seasonal variation in abundance. Biomass of A. omorii was averaged at 0.44 mgC m^− 3, with peaks in February and July, and relatively low biomass in late summer and fall. Egg production rate ranged from 2.4 to 151.9 μgC m^− 3 day^− 1, which was equivalent to 95–6075 eggs m^− 3 day^− 1. Fecundity of an adult female was averaged at 38 eggs female^− 1 day^− 1. Instantaneous growth rates of copepodites were higher than those of nauplii stages. Annual production of A. omorii ranged from 33.5 mgC m^− 3 year^− 1 to 221 mgC m^− 2 year^− 1, showing a seasonal variation of daily production rate with peaks in February and July. The daily production rate of A. omorii was significantly correlated with chlorophyll a concentration. These results suggest that standing stocks and/or productivity of phytoplankton are the major influencing factors, rather than water temperature for the seasonal variation of production of A. omorii in Ilkwang Bay.     

The trophic role of the tunicate Salpa thompsoni in the Antarctic marine ecosystem

During a repeat grid survey and drogue study carried out in austral summer 1994/95, the abundance and feeding activity of salps were estimated in the Lazarev Sea region from net tows and in situ measurements of gut fluorescence. Throughout the survey area, Salpa thompsoni accounted for >95% of the total salp stock while Ihlea racovitzai was consistently represented in very low abundances. Maximum densities of S. thompsoni, with ≈4000 ind. 1000 m⁻³, were recorded in the Marginal Ice Zone (MIZ) in December when chlorophyll-a concentrations were well below 1 mg m⁻³. A dramatic decrease in salp stock was observed at the beginning of January, when S. thompsoni virtually disappeared from the most productive area of the MIZ where chlorophyll-a concentrations had by then reached bloom levels of 1.5–3 mg (Chl-a) m⁻³. In situ grazing measurements showed that throughout the cruise S. thompsoni exhibited the highest ingestion rates per individual of any of the most abundant components of the grazing pelagic community, with maxima of ≈160 μg (pigm) ind. ⁻¹ d⁻¹. These feeding rates are 3 to 5 times higher than those previously obtained using in vitro incubations. The total daily consumption of the population of S. thompsoni varied from 0.3 to 108% of daily primary production. We suggest that competitive removal of food by S. thompsoni, rather than direct predation, is responsible for the low krill abundances generally associated with salp swarms.     

Appendicularian ecophysiology I: Food concentration dependent clearance rate, assimilation efficiency, growth and reproduction of Oikopleura dioica

Three aspects of the appendicularian O. dioica's ecophysiology were measured here: 1) morphological parameters over a wide range of appendicularian sizes, including mature animals in order to document the morphological characteristics inducing reproduction; 2) clearance rate and assimilation efficiency using feeding incubations with different algal concentrations and 3) the effect of food concentration on growth, mortality and reproduction.The relationship between the body carbon weight and the clearance rate follows a power function, with an exponent of 0.91 (± 0.07). The rate of particles retention increases with the food concentration following a Michaelis–Menten relationship (half-saturation constant = 151 ± 22 µg C l^− 1, maximum clearance rate = 12 ± 1 µg C µg C^− 1 d^− 1). The carbon assimilation efficiency decreases with the increasing food concentration. As a result, appendicularian growth which is limited in concentrations lower than 50 µg C l^− 1 is saturated above 100 µg C l^− 1.In immature animals the gonad represents less than 30% of the body volume whereas in mature individuals, its volume varies between 50% and 87% (mean 63%) suggesting that gonad/total volume ratio can be used as indicator of the maturation stages. The gonad weight in mature animals represents 70.3 (± 4.6)% of the total body carbon weight. Two major maturity stages can explain the changes in energy allocation: i) the somatic growth, when less energy is invested in gonad growth when compared to the rest of the body and ii) the maturation phase where most of the assimilated matter is invested in gonad maturation. This process is rapid, lasting only few hours. For this reason we measured completely mature organisms that are generally not measured during the experimental work with appendicularians. In food-limited conditions, the gonad maturation process starts with smaller individuals and ends with smaller reproductive animals having the same gonad to total volume ratio than in unlimited food conditions. The results obtained in this study were used to model the life cycle of O. dioica (see Lombard, F., Sciandra, A. and Gorsky, G., 2009-this volume. Appendicularian ecophysiology. II. Modeling nutrition, metabolism, growth and reproduction of the appendicularian Oikopleura dioica.).     

Th-derived particulate organic carbon fluxes in the northern Barents Sea with comparison to drifting sediment trap fluxes

Large-volume sampling of ²³⁴Th was conducted to estimate particulate organic carbon (POC) export in conjunction with drifting sediment trap deployments in the northern Barents Sea in July 2003 and May 2005. ²³⁴Th-derived POC fluxes averaged 42.3 ± 39.7 mmol C m^− 2 d^− 1 in 2003 and 47.1 ± 30.6 mmol C m^− 2 d^− 1 in 2005. Sediment trap POC fluxes averaged 13.1 ± 8.2 mmol C m^− 2 d^− 1 in 2003 and 17.3 ± 11.4 mmol C m^− 2 d^− 1 in 2005, but better reflected the transient bloom conditions that were observed at each station within a season. Although ²³⁴Th fluxes agreed within a factor 2 at most stations and depths sampled, sediment trap POC fluxes were lower than large-volume POC flux estimates at almost every station. This may represent an under-collection of POC by the drifting sediment traps or, conversely, an over-collection of POC by the large-volume sampling of ²³⁴Th. It is hypothesized that the offset between the two methods is partly due to the presence of the prymnesiophyte Phaeocystis pouchetii, which potentially causes a large variation in > 53-μm POC/²³⁴Th ratios. Due to the large proportion of dissolved carbon or mucilage released by P. pouchetii, and because it is thought that P. pouchetii does not contribute significantly to the vertical export of biogenic matter in the Barents Sea, the application of large-volume sampling of ²³⁴Th may yield relatively high, and possibly inaccurate POC/²³⁴Th ratios. Hence, POC fluxes derived from ²³⁴Th sampling may be inappropriate and drifting sediment traps might be a more reliable method to measure the vertical export of biogenic matter in regions that have recurrent P. pouchetii blooms, such as the Barents Sea.     

Seasonal variability of fractionated phytoplankton, biomass and primary production in the Straits of Magellan

The results on the distribution of phytoplankton biomass (expressed as Chla) and primary production (¹⁴C assimilation), during three oceanographic cruises carried out during Austral spring and at the end of the summer and the autumn in the Straits of Magellan, suggest a strong variability of trophic levels for this ecosystem.Seasonal evolution of the biomass concentration goes from the spring maximum of 2.33 μg/l through a sharp decrease, 0.49 μg/l, observed at the end of summer, until the minimum of 0.24 μg/l measured during the autumn.The trophic conditions are dependent on hydrographic, meteo-climatic and geo-morphological characteristics: at the Atlantic entrance and between the two Angosturas the strong mixing of water column limit the development of phytoplankton; at the Western opening and along the Pacific arm the complex exchange mechanisms with the ocean, the glacio-fluvial contribution and the presence of a thermohaline front near the Isla Carlos III influence both biomass and primary production distributions. The maximum values are reached in the Central Zone (Paso Ancho) characterized by high stability of the water column.Primary production ranged from a minimum of 12.3 to a maximum of 125.9 mgC m⁻² h⁻¹. The overall trend seems to be a progressive and simultaneous increase from the Pacific and Atlantic openings to the Central Zone of Paso Ancho where the maximum value was reached. In general, biomass and primary production distributions correspond quite well except for the area of Isla Carlos III where biological and chemico-physical causes tend to limit ¹⁴C assimilation.Contribution of pico-phytoplankton (< 2 μm) to total biomass appears to be time dependent: in the blooms observed during spring a very modest incidence (< 6%) was observed whereas became more (> 50%) during the summer-autumn seasons when total biomass was decreasing.Within the Straits, at the end of summer, the contribution of pico-phytoplankton primary production is 59%, whereas nano and microplankton contribute 39% and 2%, respectively. At the oceanic external stations the photosynthetic activity of the bigger size-fraction (> 2 μm) is predominant (> 50%).These findings support the hypothesis that the pico-phytoplankton ( < 2 μm) is substantially constant, whereas temporal variations are due to the larger (> 10 μm) cells only.     

Vertical profiles of particulate organic matter and its relationship with chlorophyll-a in the upper layer of the NE Mediterranean Sea

Particulate organic matter (POM), nutrients, chlorophyll-a (CHL) and primary production measurements were performed in the upper layer of three different regions (cyclonic, anticyclonic and frontal+peripherial) of the NE Mediterranean Sea in 1991–1994. Depth profiles of bulk POM exhibited a subsurface maximum, coinciding with the deep chlorophyll maximum (DCM) established near the base of the euphotic zone of the Rhodes cyclone and its periphery, where the nutricline was situated just below the euphotic zone for most of the year. Moreover, the POM peaks were broader and situated at shallower depths in late winter–early spring as compared to its position in the summer–autumn period. Under prolonged winter conditions, as experienced in March 1992, the characteristic POM feature disappeared in the center of the Rhodes cyclone, where the upper layer was entirely occupied by nutrient-rich Levantine deep water. Deep convective processes in the cyclonic gyre led to the formation of vertically uniform POM profiles with low concentrations of particulate organic carbon (POC) (2.1 μM), nitrogen (0.21 μM), total particulate phosphorus (PP) (0.02 μM) and chlorophyll-a (0.5 μg/L) in the euphotic zone. Though the Levantine deep waters ascended up to the surface layer with the nitrate/phosphate molar ratios (28–29) in March 1992, the N/P molar ratio of bulk POM in the upper layer was low as 10–12, indicating luxury consumption of phosphate during algal production. Depth-integrated primary production in the euphotic zone ranged from 38.5 for oligotrophic autumn to 457 mg C m⁻² day⁻¹ for moderately mesotrophic cool winter conditions.     

Microbial community structure in the marginal ice zone of the Bellingshausen Sea

Phytoplankton, bacteria and microzooplankton were investigated on a transect in the Bellingshausen Sea during the ice melt period in November–December 1992. The transect along the 85°W meridian comprised seven stations that progressed from solid pack-ice (70°S), through melting ice into open water (67°S). The abundance, biomass and taxonomic composition were determined for each component of the microbial community. The phytoplankton was mostly dominated by diatoms, particularly small (<20 μm) species. Diatom abundance ranged from 66 000 cells l⁻¹ under the ice to 410 000 cells l⁻¹ in open water. Phytoplankton biomass varied from <1 to 167 mg C m⁻³, with diatoms comprising 89–95% of the total biomass in open water and autotrophic nanoflagellates comprising 57% under the ice. The standing stocks of autotrophs in the mixed layer ranged from 95 mg C m⁻² under the pack-ice to 9478 mg C m⁻² in open waters. Bacterial abundance in ice-covered and open water stations varied from 1.1 to 5.5×10⁸ cells l⁻¹. Bacterial biomass ranged from 2.4 mg C m⁻³ under pack-ice to an average of 14 mg C m⁻³ in open water. The microzooplankton consisted mainly of aloricate oligotrich ciliates and heterotrophic dinoflagellates and these were most abundant in open waters. Their biomass varied between 0.2 and 54 mg C m⁻³ with a minimum at depth under the ice and maximum in open surface waters. Microheterotrophic standing stocks varied between 396 mg C m⁻² under pack-ice and 3677 mg C m⁻² in the open waters. The standing stocks of the total microbial community increased consistently from 491 mg C m⁻² at the ice station to 13 155 mg C m⁻² in open waters, reflecting the productive response of the community to ice-melt. The composition of the microbial community also shifted markedly from one dominated by heterotrophs (82% of microbial stocks) at the ice station to one dominated by autotrophs (73% of microbial stocks) in the open water. Our estimates suggest that the microbial community comprised >100% of the total particulate organic carbon (POC) under the ice and 62–66% of the measured POC in the open waters.     

Siliceous plankton dominate primary and new productivity during the onset of El Niño conditions in the Santa Barbara Basin, California

The potential for carbon export and the role of siliceous plankton in the cycling of C and N was assessed in natural plankton assemblages in the Santa Barbara Basin, California, by examining uptake rates of inorganic carbon, nitrate and silicic acid. In April–August 1997, the concentrations of chlorophyll a, particulate organic carbon, particulate organic nitrogen and biogenic silica were measured twice monthly, and results revealed the occurrence of at least three blooms, the largest in June. Particulate elemental ratios of C, N and Si were similar to ratios of nutrient-replete diatoms, suggesting that they dominated this bloom. Mean integrated rates of carbon, nitrate and silicon uptake during the 4-month study period are similar to other productive coastal and upwelling regions (103, 8.3 and 13 mmol m⁻² day⁻¹, respectively). New production rates were twice as high as previously reported in this region and indicate that high rates of new production along eastern boundary currents are not confined to the major coastal upwelling regions. C/NO₃⁻, Si/NO₃⁻ and Si/C uptake ratios varied widely, and mean integrated ratios were 14±5.4, 1.6±1.0 and 0.12±0.07 (S.D.), respectively. That mean C/NO₃⁻ uptake ratio corresponds to an f-ratio of about 0.5 indicating a large potential for particulate export. Based on the average Si/NO₃⁻ and Si/C uptake ratios, diatoms could perform all of the primary production and nitrate uptake that occurred during the study; these rates also suggest that export is controlled by diatoms in this system. The mean Si/C biomass ratio was lower than the mean Si/C uptake ratio, consistent with the preferential export of Si relative to C observed in sediment traps in the basin. The study took place during a period of surface-water warming, with nitrate and silicic acid concentrations decreasing throughout the onset of the 1997–1998 El Niño conditions. Although diatoms contributed less to particulate biomass during the low nutrient conditions, high f-ratios (0.33–0.66) were maintained.     

Atmospheric CO2 measurements and error analysis on seasonal air–sea CO2 fluxes in the Bay of Biscay

Atmospheric molar fraction of CO₂ (xCO₂^atm) measurements obtained on board of ships of opportunity are used to parameterize the seasonal cycle of atmospheric xCO₂ (xCO₂^atm) in three regions of the eastern North Atlantic (Galician and French offshore and Bay of Biscay). Three selection criteria are established to eliminate spurious values and identify xCO₂^atm data representative of atmospheric background values. The filtered data set is fitted to seasonal curve, consisting of an annual trend plus a seasonal cycle. Although the fitted curves are consistent with the seasonal evolution of xCO₂^atm data series from land meteorological stations, only ship-board measurements can report the presence of winter xCO₂^atm minimum on Bay of Biscay. Weekly air–sea CO₂ flux differences (mmol C·m^− 2 day^− 1) produced by the several options of xCO₂^atm usually used (ship-board measurements, data from land meteorological stations and annually averaged values) were calculated in Bay of Biscay throughout 2003. Flux error using fitted seasonal curve relative to on board measurements was minimal, whereas land stations and annual means yielded random (− 0.2 ± 0.3 mmol C·m^− 2·day^− 1) and systematic (− 0.1 ± 0.4 mmol C·m^− 2 day^− 1), respectively. The effect of different available sources of sea level pressure, wind speed and transfer velocity were also evaluated. Wind speed and transfer velocity parameters are found as the most critical choice in the estimate of CO₂ fluxes reaching a flux uncertainty of 7 mmol C·m^− 2·day^− 1 during springtime. The atmospheric pressure shows a notable relative effect during summertime although its influence is quantitatively slight on annual scale (0.3 ± 0.2 mmol C·m^− 2·day^− 1). All results confirms the role of the Bay of Biscay as CO₂ sink for the 2003 with an annual mean CO₂ flux around − 5 ± 5 mmol C m^− 2 day^− 1.     

A numerical investigation of phytoplankton and Pseudocalanus elongatus dynamics in the spring bloom time in the Gda sk Gulf

A nutrient–phytoplankton–zooplankton–detritus (1D-NPZD) ‘phytoplankton {Phyt} and Pseudocalanus elongatus {Zoop} dynamics in the spring bloom time in the Gda sk Gulf. The 1D-NPZD model consists of three coupled, partial second-order differential equations of the diffusion type for phytoplankton {Phyt}, zooplankton {Zoop}, nutrients {Nutr} and one ordinary first-order differential equation for benthic detritus pool {Detr}, together with initial and boundary conditions. In this model, the {Zoop} is presented by only one species of copepod (P. elongatus) and {Zoop} is composed of six cohorts of copepods with weights (W_i) and numbers (Z_i); where . The calculations were made for 90 days (March, April, May) for two stations at Gda sk Gulf with a vertical space step of 0.5m and a time step of 900 s. The flow field and water temperature used as the inputs in the biological model 1D-NPZD were reproduced by the prognostic numerical simulation technique using hydrographic climatological data. The results of the numerical investigations described here were compared with the mean observed values of surface chlorophyll-a and depth integrated P. elongatus biomass for 10 years, 1980–1990. The slight differences between the calculated and mean observed values of surface chlorophyll-a and zooplankton biomass are ca. 10–60 mg C m⁻³ and ca. 5–23 mg C m⁻², respectively, depending on the location of the hydrographic station. The 1D-NPZD model with a high-resolution zooplankton module for P. elongatus can be used to describe the temporal patterns for phytoplankton biomass and P. elongatus in the centre of the Gda sk Gulf.     

Inter-annual variability of hypoxic conditions in a shallow estuary

Water quality data from two monitoring programs in the Pamlico River Estuary (PRE) were analyzed for dissolved oxygen (DO), salinity, temperature, and nutrient concentrations. Data were collected bi-weekly at 8 stations from 1997 to 2003 by East Carolina University and continuously at three stations from 1999 to 2003 by the U.S. Geological Survey. Hypoxic conditions were observed mostly in the upper to middle estuary, but the frequency of hypoxic events varied between years. During June to October in 1997–1999 (referred to as the oxic summers) bottom water hypoxia (DO < 2 mg l^− 1) was found in 8.7% of the observations. By contrast, during June to October in 2001–2003 (referred to as the hypoxic summers), 37.9% of the total measurements had DO concentrations less than 2 mg l^− 1. The more frequent and/or prolonged hypoxic conditions during the hypoxic summers were closely associated with stronger salinity stratification and greater loadings of nutrient and particulate matter.Salinity stratification appeared to be governed by patterns of freshwater discharge, and frequency of wind mixing events. The “oxic” summers were characterized by continuous low freshwater inflow (except one extremely high flow event due to hurricanes), stronger northeastward wind, and more frequent wind mixing events. In contrast, the hypoxic summers were characterized by frequent moderate freshwater inflow events, and fewer wind mixing events.The greater loadings of nutrient (nitrate, ammonium, and phosphate) and particulate matter during the hypoxic summers were primarily due to higher river discharges. At the head of the PRE, no significant differences were found in concentrations of nutrient and particulate nitrogen between the oxic and the hypoxic summers. In addition, chlorophyll a concentrations were averaged above 30 μg l^− 1 (maximum 167 μg l^− 1) during the hypoxic summers, significantly higher than those during the oxic summers (averaged around 15 μg l^− 1).