Showing papers in "Deep-sea Research Part Ii-topical Studies in Oceanography in 1995"

Export production of particles to the interior of the equatorial Pacific Ocean during the 1992 EqPac experiment

Abstract: Twenty-four time-series, moored sediment traps were deployed between 2/2/92 and 1/27/93 along 140°W at 9°N, 5°N, 2°N, 0°, 2°S, 5°S and 12°S at water depths of approximately 1200 m and 2200 m, and 700 m above the bottom. The opening/closing of the traps was synchronized at 17-day periods, for 21 events, covering a total of 357 days. The average annual particle flux in the ocean's interior (2.2 to 4.4 km deep) from 5°N to 5°S was 28.5 g m−2 year−1, with 34.8 g−2 year−1 the maximum annual flux at the equator. Sixty-six per cent of settling particles were carbonate; 24% biogenic SiO2 and 5% organic carbon. The onset of tropical instability waves, marking the year's El Nino/post-El Nino boundary, was associated with a succession of intervals with greater organic carbon and opal at 5°N, 2°S and 5°S that occurred synchronously with a meridional oscillation of instability waves, while net carbon flux during El Nino and post-El Nino periods did not change. Although organic carbon flux increased at 5°N, 2°S and 5°S during the post-El Nino period, it was counterbalanced by decreases at the upwelling stations (2°N and the equator), resulting in no net carbon flux increase across the 5°N to 5°S region. In February/March 1992, only 0.34% of the organic carbon fixed by primary production over the 5°N to 5°S zone arrived in the ocean's interior. In August/September that year, zonal average of organic carbon flux increased slightly to 0.5% of primary production. Very little carbon reached the interior depths of the upwelling stations; however, the fraction of export was higher at the 5°N, 2°S and 5°S stations. The pattern of variability of particle flux at the shallow depths was observed also in deeper traps, without temporal offsets, suggesting a settling particle residence time shorter than the 17-day timeseries resolution during most of this experiment.

Bacterial mediation of carbon fluxes during a diatom bloom in a mesocosm

Abstract: Bacteria-diatom interactions were studied during a diatom bloom produced in a mesocosm, in the absence of metazoan grazers, in order to examine the significance of bacterial hydrolytic ectoenzymes in mediating carbon fluxes and influencing diatom aggregation. The abundances of bacteria and protozoa, the production rates and hydrolytic ectoenzyme activities (protease, a and B glucosidase and chitobiase) of attached and free bacteria, were followed as well as the dynamics of the dissolved organic carbon (DOC) pool. An intense diatom bloom occurred with chlorophyll a (chl a) concentrations reaching 132yg liter-' prior to aggregation. The diatoms were colonized by bacteria early on in the bloom and remained colonized throughout the bloom, yet they grew rapidly (>l day-'). Attached bacteria were numerically a small fraction of the total. but they also grew very rapidly & = 4-16 day-') and were generally responsible for the majority of bacterial carbon demand, BCD, (46-92%) and hydrolytic enzyme activities (41-990/o). BCD accounted for an estimated 4060% of the total carbon fixed during the bloom; thus, roughly one- half of the primary production was channeled. via the DOC pool, into bacteria. The high ectohydrolase activities of bacteria attached to the surface of diatoms suggests that the hydrolysis of diatom surface mucus could be responsible for a major flux into the DOC pool making it a significant. but previously unrecognized, mechanism of DOM production. Enzymatic hydrolysis of surface mucus may also have inhibited diatom aggregation. Addition of purified glucosidase and proteasc to samples from the mesocosm inhibited diatom aggregation in cxpcriments designed to induct aggregation. It is hypothesized that the action of bacterial ectoenzyme on diatom surfaces inhibited diatom aggregation by reducing stickiness, thus prolonging the bloom and allowing the accumulation of extremely high chl a levels prior to aggregation. Future studies should consider bacterial hydrolytic ectoenzymes as a potentially important variable influencing carbon Rux pathways, particle aggregation, and the size and duration of diatom blooms in the ocean.

The role of surface-active carbohydrates in the flocculation of a diatom bloom in a mesocosm

Abstract: A study was undertaken to evaluate the role of exocellular polysaccharides in the flocculation of a marine diatom bloom in a large tank mesocosm. Surface-active organic matter was extracted from 1.0 μ-filtered tank water by bubble adsorption each day for 7 days of the experiment. In agreement with past studies, particles (3–51 urn equivalent spherical diameter) were readily formed by bubbling and became concentrated in the foam. At the beginning of the bubbling (0–0.5 h), both particles and surface-active carbohydrates were extracted at high rates; however, these rates dropped off steeply after about 0.5 h of bubbling. The rate of particle formation by bubbling could be modeled fairly well by second order kinetics. The extracted, surface-active material was enriched in deoxysugars and galactose, while the residual material was enriched in glucose. Extracted surface-active carbohydrates reached a maximum of 33% of the total dissolved sugars ( 0.99) with particle stickiness (alpha). In addition, the concentration of surface-active carbohydrates was well correlated (2 = 0.91) with the concentration of transparent exopolymer particles (TEP) in the tank, and it was demonstrated that TEP could be copiously formed by bubbling of 1.0 μm-filered seawater. The finding of a highly surface-active, deoxysugar-rich polysaccharide material that can be rapidly (<0.5 h) and selectively extracted by bubble adsorption is significant, as it is apparent that this material played important roles in particle stickiness and TEP formation in the tank, and thus it may, at times, play similar roles in particle aggregation in the sea.

Dissolved organic carbon in the upper ocean of the central equatorial Pacific Ocean, 1992: Daily and finescale vertical variations

Abstract: Detailed depth profiles of dissolved organic carbon (DOC) were determined at the JGOFS Equatorial Pacific Time Series site at 0°, 140°W in March/April and October 1992. DOC concentrations in the top 200 m, determined with the high temperature combustion (HTC) method, showed no elevated values over previous wet chemical estimates and ranged from an average of 67 μM C in the surface waters to 46 μM C at 200 m. The average integrated DOC concentration over 200 m was 11.4 mol C m−2 in March/April and 11.0 mol C m−2 in October. Variability frequency (i.e. seasonal) was minimal despite large changes in many biogeochemical parameters. Vertical distribution of DOC was highly correlated with physical properties and remained relatively independent of biological properties. High frequency variability appeared to be governed by physical processes. DOC production and consumption in the equatorial Pacific were tightly coupled, resulting in little accumulation and export of labile DOC.

Regional estimates of the export flux of particulate organic carbon derived from thorium-234 during the JGOFS EqPac program

Abstract: The upper ocean 234Th activity distribution at 77 stations was measured between 12°N and 10°S, and 95°W and 170°W in the spring and autumn of 1992. A regional scavenging model was used to estimate vertical export of particulate 234Th. Given the relatively high upwelling rates in this region, particularly at equatorial latitudes near 140°W, it was necessary to include upwelling of 234Th in our model in order to quantify particulate export. Using this export flux and the measured organic C or N to 234Th ratio on particles, one can empirically determine POC and PON fluxes for this region. The estimated particulate organic C flux varies spatially and temporally within this region, ranging from 1 to 7 mmol C m−2 day−1, with enhanced export occurring over the equator. Fluxes are also enhanced along 95°W coincident with a low temperature/high nutrient peak at 4°S. Along 140°W, particulate organic C export from the upper 100 m is on the order of 2 mmol C m−2 day−1 at latitudes beyond 4°N and 4°S, with an equatorial peak of 3–5 mmol C m−2 day−1 in both spring and fall. These results suggest that a relatively small per cent of the total production is exported locally on sinking particles (particle export/primary production C 234 Th ratios. Given the measured C N ratio, particulate N fluxes from the upper 100 m would be 6 times lower than for POC.

Rapid formation and sedimentation of large aggregates is predictable from coagulation rates (half-lives) of transparent exopolymer particles (TEP)

Abstract: Two hypotheses have been proposed to account for the precipitous formation of large, rapidly settling aggregates at the termination of phytoplankton blooms in nature; aggregation due primarily to cell-cell collisions, and aggregation resulting from the presence of abundant transparent exopolymer particles (TEP), a recently discovered class of particles formed from polysaccharides excreted by phytoplankton. The hypothesis of TEP-driven coagulation in three disparate systems, a freshwater lake, a coastal ocean, and a saltwater mesocosm was evaluated, by comparing TEP abundance to several related factors including phytoplankton concentrations, measured sediment fluxes, and abundances of large aggregates. The timing of large aggregate formation and sedimentation events was related to coagulation rates expressed in terms of particle half-lives, t12, calculated as the time for TEP or phytoplankton to decrease to half their concentration through shear coagulation. While TEP have been previously investigated only in marine systems, it is reported here that TEP also can be present in high concentrations (860 ml−1) in freshwater lakes (Lake Constance, Germany) and that high fluxes of particulate organic matter at depth coincide with the disappearance of abundant TEP from overlying waters. The half-lives of TEP in the three different systems indicate that large aggregate formation and massive sedimentation events following diatom blooms occur when the TEP half-life decreases to less than a few days. By comparing TEP and phytoplankton half-lives in these systems, it is concluded that the formation of rapidly sinking aggregates following blooms of mucous-producing diatoms is primarily controlled by concentrations of TEP, not phytoplankton.

Aggregation of a diatom bloom in a mesocosm: The role of transparent exopolymer particles (TEP)

Abstract: The role of TEP (Transparent Exopolymer Particles) in the flocculation of a diatom bloom was studied under controlled conditions in a mesocosm. The concentration of TEP increased exponentially during growth, flocculation and senescence of the bloom. Aggregation began dominating the particle dynamics of TEP during the early growth phase of the bloom, several days prior to the appearance of large flocs and nutrient depletion. TEP aggregated with themselves and with phytoplankton due to the high stickiness of TEP, but phytoplankton was not observed to aggregrate with itself. The production of TEP, estimated from changes in concentration, did not increase after nutrients were depleted. The concentration of TEP was a linear function of chl a and particulate organic carbon (POC), indicating that production of TEP was linked to growth rather than standing stocks of phytoplankton. The ratio between TEP and phytoplankton appeared to be one of the factors determining the onset of the flocculation of the bloom. The concentration of TEP may have been decreased by bacterial degradation. Bacterial degradation of TEP may explain the low TEP to chl a values, the decrease in stickiness of particles as the bloom progressed, and the retarded onset of flocculation.

Microzooplankton grazing in the central equatorial Pacific during February and August, 1992

Abstract: Dilution studies were conducted on EqPac cruises in the central equatorial Pacific (2°N to 2°S, 140°W) during February–March and August–September 1992 to determine phytoplankton growth rates and mortality rates attributable to microzooplankton grazing. Instantaneous growth rates (μ) based on bulk chlorophyll measurements were highly variable from day-to-day, but averaged 0.83 day−1 for the upper (10–20 m), 0.34 day−1 for the mid (40–50 m) and 0.22 day−1 (70–80 m) for the lower euphotic zone on the first cruise. Corresponding rate estimates for microzooplankton grazing (m) were 0.72, 0.22 and 0.21 day−1, respectively. During the second cruise, growth estimates strongly exceeded grazing estimates for the two upper (μ = 0.98 and 1.00 day−1; m = 0.57 and 0.42 day−1), but not the lower depth strata (μ = 0.32 day−1; m = 0.27 day−1). Grazing losses accounted for about 83% of depth-integrated phytoplankton growth in February–March experiments and only about 55% in August–September experiments. In addition, growth rates in the presence of added nutrients (including iron) showed evidence of limitation in February–March, which coincided with a major El Nino event, but not in August–September, which was more representative of climatological mean conditions. Differences in growth rates, implied nutrient limitation, and the balance of phytoplankton growth and microzooplankton grazing were consistent with a greater abundance of large diatoms in August–September. Despite the disparity between chlorophyll-based estimates of growth and grazing rates for this cruise, flow-cytometric analyses of specific populations (Prochlorococcus, Synechococcus, and autotrophic nanoeukaryotes) in a subset of experiments conducted in August demonstrated that microzooplankton grazing was still sufficient to balance growth rates of the smaller components of the phytoplankton assemblage.

Hydrographic structure and variability of the Kara Sea: Implications for pollutant distribution

Abstract: Nuclear activity on land and dumping of waste in the Siberian shelf seas mean that the Kara Sea is most likely to experience inputs of radioactivity. Industrial and other anthropogenic activities in the expansive Ob' and Yenisey watersheds also contribute organochlorines, heavy metals and oil to this region. Contaminant fate is influenced by the distribution of the river discharge and processes associated with ice formation and ocean currents. Although average conditions are important in the transport of pollutants, events such as storms and iceberg gouging may be critical in deciding the ultimate fate of dumped and released contaminants.

A U.S. JGOFS process study in the equatorial Pacific (EqPac): Introduction

Abstract: This special issue contains data and scientific results from the JGOFS Process Study in the equatorial Pacific. Most of the contributions are from the U.S. JGOFS Process Study (EqPac) but the French and Australian results are represented as well. The equatorial Pacific plays a major role in the oceanic and atmospheric carbon cycles, and these studies are the first step for synthesizing the cycle of carbon and related elements in this region.

Mass aggregation of diatom blooms: Insights from a mesocosm study

Abstract: While the aggregation and mass settlement of diatoms at the termination of blooms results in significant export of carbon from the surface ocean, the mechanisms of bloom aggregation have been poorly understood. The aggregation of a multispecies diatom bloom was investigated under controlled conditions in a 1200 liter, nutrient-enriched, laboratory mesocosm in order to elucidate the parameters sufficient to accurately predict bloom aggregation. A diverse bloom of diatoms dominated by several species of Chaetoceros and Thalassiosira progressed through a classic pattern of exponential, stationary, and senescent phases in the mesocosm. Aggregates larger than 0.5 mm became detectable on the eighth day after inoculation, and aggregates >1 mm increased exponentially from Day 10 onward producing the appearance of a mass aggregation event late on Day 10. The bloom aggregated sequentially with Thalassiosira dominating early aggregates and Chaetoceros dominating later ones. Chaetoceros resting spores formed only in aggregates. Aggregation was not linked to nutrient depletion or to the physiological state of the cells since the onset of aggregation and the mass aggregation event occurred 1 to 3 days prior to nutrient depletion and while carbon:nitrogen ratios of cells were still very low and growth rates high. Moreover, visible aggregates did not form in the mesocosm until cell abundances were considerably higher than abundances observed to aggregate in nature, suggesting that aggregation was not strongly linked to phytoplankton cell concentration. Complementary studies in this volume clarify the role of non-phytoplankton particles in aggregation of the mesocosm bloom. The mesocosm approach proved highly effective in producing an aggregating diatom bloom under controlled conditions.

The transfer of reprocessing wastes from north-west Europe to the Arctic

Abstract: The discharge of radioactive waste, from nuclear fuel reprocessing facilities, into the coastal waters of north-west Europe has resulted in a significant increase in the inventories of a number of artificial radionuclides in the North Atlantic. Radiocaesium, 90Sr and 99Tc, which behave conservatively in seawater, have been used widely as tracers of water movement through the North Sea, Norwegian Coastal Current, Barents Sea, Greenland Sea, Fram Strait, Eurasian Basin, East Greenland Current and Denmark Strait overflow. These studies are summarised in the present paper. It has been estimated that 22% of the 137Cs Sellafield discharge has passed into the Barents Sea, en route to the Nansen Basin, via the Bjomoya-Fugloya Section, with another 13% passing through the Fram Strait. This amounts to 14 PBq 137Cs. Quantifying the influx of other radionuclides has been more problematic. The inflowing Atlantic water now appears to be diluting waters in the Arctic Basin, which were contaminated in the late 1970s and early 1980s as a result of the substantial decrease in the discharge of reprocessing wastes. Sellafield (U.K.) has dominated the supply of 134Cs, 137Cs, 90Sr, 99Tc and Pu, whereas La Hague (France) has contributed a larger proportion of 129I and 125Sb.

The 1991–1993 El Niño in the central Pacific

Abstract: The 1991–1993 El Nino event is described using data from the TOGA-TAO buoy network, concentrating on variability at 140°W where a full suite of temperature, current, and surface meteorological observations were made The daily time series furnished by the buoy array brings out the conspicuous importance of remotely forced intraseasonal variability in the form of equatorial baroclinic Kelvin waves during the evolution of the 1991–1993 El Nino Notable variations along 140°W included a major weakening of the Equatorial Undercurrent in late 1991 to early 1992, and a reduction in intensity of monthly period tropical instability waves during the latter part of 1991 compared to previous non-El Nino years The North Equatorial Countercurrent showed no major signal due to this El Nino, in contrast to earlier warm events Although anomalies of the South Equatorial Current spanning the equator were in an eastward sense during the height of the event, the result of these changes was that near-surface flow across 140°W between 5°S and 5°N was close to zero, so there was apparently no large eastward transport of surface water past 140°W into the eastern equatorial Pacific The relative phasing of anomalies of thermocline depth, equatorial undercurrent speed and SST during the warm event of 1991–1992 was somewhat similar to that seen during the 1986–1987 El Nino, although the earlier event was followed by a strong cold (La Nina) event whereas the recent one was not Uniquely among modern El Nino events, after the 1991–1992 episode appeared to end with a reappearance of the equatorial cold tongue in mid-1992, a second SST warming in the eastern Pacific occurred in early 1993 The present data set is inadequate to fully diagnose the mechanisms of SST change at 140°W, but time series of insolation, air-sea temperature difference and humidity show that local air-sea heat flux variations (either radiative or turbulent) were probably not the primary cause of the SST changes during the El Nino Similarly, although horizontal advective mechanisms were important contributors to SST signals at certain times, these alone did not account for the major warming and cooling events Instead, the largest SST variations of the 1991–1992 El Nino can be ascribed to upwelling variations, and a simple parameterization for this process is presented The major warming and cooling events in the central/eastern equatorial Pacific occurred nearly simultaneously over a wide longitudinal range, indicating that oceanic wave processes could not have been the sole source of the changes

Mesozooplankton grazing and metabolism at the equator in the central Pacific: Implications for carbon and nitrogen fluxes

Abstract: Primary productivity and chlorophyll in the equatorial Pacific are lower than expected based on ambient nutrient concentrations. We tested the hypothesis that these conditions are due to a balance between phytoplankton growth and mesozooplankton grazing. Grazing rates and biomasses of three size classes of zooplankton in the size range of 200-200 μm were measured during March-April and October of 1992 at the equator (140°W). El Nino conditions prevailed in March–April, whereas in October a tropical instability wave (TIW) passed through the study area. Weight-specific pigment ingestion rates of mesozooplankton tended to be higher in March–April than in October while the opposite was the case for biomass. There were no discernible temporal trends in mesozooplankton community grazing rates in March–April, but there were changes in October associated with the passage of the TIW. Mesozooplankton grazing removed 1–9% day−1 of the total chlorophyll and 1–12% of the primary production within the euphotic zone. Therefore, the grazing hypothesis as stated above is rejected, although a simple chlorophyll budget suggests that grazing (mostly by microzooplankton) and phytoplankton growth are roughly in balance. Most of the phytoplankton was 2-μm phytoplankton, we estimate removal rates of the > 2-μm chlorophyll standing stock of up to 27% day−1. Another question of biogeochemical importance is the rate of the production of large diatoms. Mesozooplankton grazing could have balanced growth of the large (> 20-μm) diatoms in March–April, but not in October. From estimates of respiration and excretion based on water temperature and body size, we infer that: (1) > 80% of the carbon ingested by mesozooplankton is not phytoplankton; (2) mesozooplankton excretion supports < 7% of the nitrogen demands of phytoplankton; (3) the flux of carbon passing through the mesozooplankton would be equivalent to 23% of the primary production; (4) mesozooplankton fecal carbon could account for 100% of the sinking POC flux in this region. Finally, a simple food chain analysis suggests that a significant fraction of the microzooplankton production probably passes through mesozooplankton.

The oceanic mixed-layer pump

Abstract: Diel variations in beam attenuation profiles in the upper water column were observed during Joint Global Ocean Flux Study (JGOFS) programs in the North Atlantic (North Atlantic Bloom Experiment-NABE) and Equatorial Pacific (EqPac). Beam attenuation due to particles in surface waters increased during the day by as much as 70% and decreased at night in both studies. In the equatorial Pacific the magnitude of the variations was larger during cool non-El Nino conditions (October, 1992) than during El Nino conditions (March/April 1992). However, the percent increase from the mean beam attenuation value for each time period was similar. The daytime increases result primarily from primary production, with possible contributions from changing optical effects of living cells. Nocturnal decreases in beam attenuation, reflecting particle loss, could be caused by grazing, remineralization, and the production of large particles (aggregates, fecal pellets) with subsequent settling. The nocturnal decreases also were associated with increases in the surface mixed layer depth and increases in nutrient concentrations. Part of these diel changes could result from mixing upward particle-depleted, nutrient-enriched water from below the daytime mixed layer. Since many biological and chemical species have strong gradients in the upper 100 m, regular oscillations in the depth of mixing can be an important forcing function for vertical exchange in surface waters. When it occurs, this “mixed-layer pump” is important in sustaining new primary production and in removing particles from surface waters, and should be included in models of oceanic surface mixing.

Early diagenesis of biogenic opal: Dissolution rates, kinetics, and paleoceanographic implications

Abstract: A study was undertaken to measure the rate of biogenic opal dissolution in equatorial Pacific sediments along the equator between 103 and 140°W, and across the equator between 12°S and 9°N. Along the equator, benthic incubation chamber measurements indicate a gradient in the opal dissolution rate, with rates decreasing from ∼0.7 mmol m−2 day−1 at 103°W to 0.4 mol m−2 day−1 at 140°W. Across the equator at 140°W, the pattern of opal dissolution is symmetrical, with dissolution rates of ∼0.4 mmol m−2 day−1 from 2°S to 2°N, decreasing to ∼0.1 mmol m−2 day−1 at the ends of the transect. Benthic fluxes calculated from pore water profiles of silicic acid are in good agreement with incubation chamber measurements. Each pore water profile fits with a function that exponentially approaches a constant value with depth (Cd), and Cd co-varies with the dissolution flux. At least three previously published models can explain this relationship: one in which Cd is regulated by the solubility of the opal present in the sediments; a second in which Cd depends on the availability of easily dissolvable opal; and the sediment mixing rate and a third rate in which Cd is controlled by the development of surface coatings. If the first model is correct, the data demonstrate that opal solubility varies spatially and that solubility is positively correlated with the opal rain rate, although the rate at which pore waters become saturated varies little among the stations between 5°N and 5°S. The implication of this model is that the opal burial rate depends on dissolution kinetics and sediment accumulation rate. If the second model is correct, fits to the pore water data and knowing the sediment mixing rate indicate that at least three types of solid phase opal must be present in the equatorial Pacific region, one that is essentially unreactive, one that has a dissolution rate constant between 0.27 ± 0.09 and 0.05 ± 0.02 year−1 , and another that has a dissolution rate constant of 6 ± 4 × 10−4 year−1. The more reactive phase dominates the dissolution flux between 5°S and 5°N, whereas the less reactive phase dominates the flux at the high latitude extremes of the transect. The implication of this second model is that sedimentary opal in equatorial Pacific sediments provides a record of only the non-reactive opal supply. If the third model is correct, surface coating development and opal preservation may depend upon the kinetics of the opal surface aging process or on the concentration of the coating material within the sediments. Storage experiments suggest that this third model may be the most realistic, but the implications of this model cannot be explored until the factors regulating coating growth are identified.

Microzooplankton and their role in controlling phytoplankton growth in the marginal ice zone of the Bellingshausen Sea

Abstract: Microzooplankton and their herbivorous activity were quantified on a transect in the Bellingshausen Sea along the 85°W meridian between 70°15'S and 67°30'S. The transect was worked during the Austral spring period of November-December 1992, with stations in pack-ice (Gertie/Herbie), ice-edge (Isolde), and open-waters (Jules and Katie) of the marginal ice zone (MIZ). Microzooplankton varied in their abundance in the surface mixed layer within the MIZ transect by more than an order of magnitude. In the waters beneath the pack-ice, microzooplankton concentrations were low (ca 850 organisms I−1); they increased at the ice-edge station (2110 organisms l−1) and reached a maximum (ca 17,000 organisms l−1) at the open water situated ca 200 km north of the ice-edge. The community was dominated by Protozoa, which contributed >97% of the numerical abundance of microzooplankton and >89% of the standing stocks. Protozoan communities were dominated by naked oligotrich ciliates and heterotrophic dinoflagellates. Microzooplankton biomass in the mixed layer of the MIZ varied between 0.2 and 54 μgC l−1) with the minimum situated at depth under the ice and maximum in open surface waters, respectively. Microzooplankton biomass was strongly correlated to ambient phytoplankton concentrations and averaged 24% of their biomass. Mixed layer standing stocks ranged between 50 and 2021 mgC m−2 at the southern and northern stations, respectively. Microzooplankton herbivorous activity also showed strong latitudinal gradients across the MIZ. Microzooplankton herbivory, investigated using dilution experiments within surface mixed layers, was generally vigorous. Microzooplankton turned over between 3 and 40% of the phytoplankton daily, thereby consuming between 0.04 and 31.21μgC l−1) day−1 in these experiments. The magnitude of this activity was positively related to sea-water temperature, to phytoplankton concentration, and to the ambient microzooplankton concentrations. A multiple regression model of microzooplankton herbivory was developed to produce an integrated analysis of their impact on phytogenic carbon flux. Microzooplankton in the mixed layer were estimated to graze 21 and 3260 mg phytoplankton-C m−2 day− at the southern pack-ice and northern open-water stations, respectively. This grazing flux accounted for 21 and 271% of the concurrent daily phytoplankton production, respectively. Microzooplankton are clearly important in determnining the fate of phytoplankton during the ice-melt period in the Bellingshausen Sea MIZ.

Coagulation efficiency, organic-matter glues and the dynamics of particles during a phytoplankton bloom in a mesocosm study

Abstract: The combination of phytoplankton aggregate formation and subsequent sedimentation has been proposed as a mechanism for termination of phytoplankton blooms. To test this hypothesis, the evolution of a phytoplankton bloom was studied in a mesocosm. During a seven-day period, the concentration and size of small particles (

Biomass and biomass production of heterotrophic bacteria along 140°W in the equatorial Pacific: Effect of temperature on the microbial loop

Abstract: Biomass and biomass production of heterotrophic bacteria were measured from 12°N to 12°S along 140°W during the El Nifio-affected months of February and March and also in August and September 1992. Although particulate primary production was highest in equatorial waters (2°N to 2°S) even during El Nifio, bacterial production was not significantly higher in these waters compared to elsewhere along the transect. Consequently, the ratio of bacterial production to primary production (BP: PP) varied from a maximum of 0.3 at 12°N and 12°S to a minimum of about 0.12 in equatorial waters. Relatively low bacterial production at the equator seemed to be due to low bacterial biomass, not low growth rates. In August-September, bacterial growth rates were significantly higher in equatorial waters than elsewhere along the transect whereas bacterial numbers and biovolumes were somewhat lower. In contrast to the lack of correlation between bacteria and phytoplankton along the transect, bacterial production increased with primary production, even though water temperatures decreased when the 1992 El Nifio momentarily waned in August and September. Temperature affected bacterial production in short-term bottle experiments, and temperature effects on production should have been measurable in situ, but increases in primary production, not temperature, accounted for the increase in bacterial production from February-March to August-September. These data suggest that large scale changes in bacterial production are controlled by the supply of dissolved organic matter which is coupled somehow to primary production and that primary production, not temperature, is most important in determining the movement of carbon and energy through the microbial loop.

The role of the large-scale Arctic Ocean circulation in the transport of contaminants

Abstract: The key features of the large-scale circulation of the Arctic Ocean are reviewed based on distributions of hydrographic parameters and natural and anthropogenic trace substances. Salinity and mass balances, as well as a combination of the tracers tritium and δ 18 O, suggest a mean residence time of the shelf waters in the Siberian seas of about 3 years. Potential pathways of pollutants released to the Siberian shelf seas from the dumpsites or from river runoff are inferred from the distributions of δ 18 O and salinity. Transit times needed for dissolved contaminants to cross the central Arctic basins (several years to one or two decades in near-surface waters) and mean residence times of contaminants in the intermediate (several decades) and deep waters (several centuries) are estimated from the distribution of transient tracers (tritium and its radioactive decay product, 3 He) and “steady-state” tracers ( 14 C and 39 Ar).

In-situ characteristics of particles settling within a deep-water estuary

Abstract: A Floc Camera Assembly is used to obtain the in-situ size, shape, concentration and settling velocity of marine particles, providing insight into the in-situ excess-density, porosity and mass of flocs, their population characteristics, and aggregation and sedimentation rate. Data highlight the time-dependent character of marine snow, including near-instantaneous, daily and seasonal influences for a mid-latitude estuary, Halifax Inlet. Methods are developed for estimating the mean size of the constituent particles comprising flocs, for determining the number spectra of non-flocculated and flocculated particles, and for determining the mass concentration and mass flux spectra for suspended flocs. The most active aggregation level is just below the estuarine surface layer. The largest aggregation rates involve the finest ( Floc settling velocity varies with floc size, and as a seasonally-variable power function. A floc's excess density also varies with floc size, but the relationship changes daily. Floes account for between 36% and 100% of the total suspended mass within Halifax Inlet, and >90% of the mass below the surface layer. Floc concentrations may change by a factor of four in just a few minutes, affected by the flux of larger floes. When river input of suspended load is high, floc concentration is also high, but the portion of total particulate matter composed of floc is low. Mean floc size varies considerably between seasons and sometimes between estuarine location. Small floes form when primary production is low and river input is high. Large flocs form when river input is very low. Suspended stringers are highest during the spring freshet and associated plankton bloom.

The distribution of dimethyl sulphide and dimethylsulphoniopropionate in Antarctic waters and sea ice

Abstract: Dimethyl sulphide (DMS) and particulate and dissolved fractions of dimethylsulphoniopropionate (DMSPp and DMSPd) were measured on two simultaneous cruises through the Drake Passage to the Bellingshausen Sea, Antarctica, during October to December 1992. Overall, average surface water concentrations for DMS and DMSPp were 2 nM (0.15–27 nM) and 25.6 nM (2–69 nM), respectively. Higher concentrations (DMS 5.8 and DMSPp 41.2 nM) were observed in a phytoplankton bloom forming a band between 67.2° and 68.2°S, associated with the Southern Polar Front. Concentrations of DMS + DMSP in ice were very high (up to 546 nM) but in the underlying water were 20 to 500 times lower. Comparison of sulphur data, pigments and phytoplankton species counts showed correlation of DMSPp with 19'-hexanoyloxyfucoxanthin (Hex) and Phaeocystis sp. A seasonal cycle for DMS in Antarctic surface waters, based on all available literature data, is discussed in terms of the probable Antarctic Ocean contribution to the global ocean DMS flux.

Water column and sea-ice primary production during Austral spring in the Bellingshausen Sea

Abstract: The findings of a cruise to study the phytoplankton bloom dynamics associated with the marginal ice zone (MIZ) in the Bellingshausen Sea during Austral spring (November-December) 1992 are reported. Biomass and rate process measurements were carried out at stations located in the ice, ice edge and open water along the 85°W meridian in order to establish the productivity of the microalgae associated with sea-ice and in the water column. In addition, a series of transects along 85°W from sea-ice to open water conditions enabled an assessment of the development of phytoplankton populations. Low phytoplankton biomass and production were noted at ice-covered and ice-edge stations and in the open water close to the ice edge. Observations from the transects indicated no development of a classical ice edge bloom despite evidence that sea-ice had retreated more than 100 km during the study period. Survey data along the 85°W line revealed a region of high chlorophyll, centred on 67.5°S, which was initially observed during brash ice conditions. This feature, which remained geographically consistent, persisted for at least 25 days and was thought to be associated with a frontal region. Water column primary production ( 14 C) in this high chlorophyll region was ca 0.8 g C m −2 day − , more than 8 times higher than noted in the MIZ. Phytoplankton photosynthetic characteristics within this region indicated that cells were adapted to a low light regime. A critical depth of 80 m, estimated directly from oxygen flux measurements, was sufficient to permit the initiation and net growth of phytoplankton standing stocks in a mixed layer of ca 70 m. A modelling approach using 14 C observations suggested that phytoplankton growth was less than the sum of the algal loss terms within this feature. An advective supply of cells therefore would be required to sustain the observed high and constant algal biomass. In addition, although this high chlorophyll feature was initially observed during brash-ice conditions, the available data suggest that it was initiated under open water conditions.

Zooplankton variability on the equator at 140°W during the JGOFS EqPac study

Abstract: The vertical distributions of zooplankton biomass and community composition were measured on the equator at 140°W during March/April (19 days) and October 1992 (21 days). El Nino conditions prevailed during the March/April time series. The average integrated (200 m) zooplankton (>64 μm) biomass was 32 mM C m−2 in March/April and 41 mM C m−2 in October. The overall cruise means were not significantly different; however during October zooplankton were more aggregated in the upper euphotic zone, there were more copepods >200 μm, and there were more day/night differences in the vertical distribution of zooplankton biomass as a result of vertical migration behavior. Cyclopoid copepods were more abundant during the El Nino conditions of March compared to October. There were no apparent trends in integrated zooplankton biomass during the March/April time series. However, the species composition of the zooplankton community changed towards the end of the time series when meridional currents flowed to the south. Zooplankton biomass during the October cruise increased nearly five-fold with the passage of a tropical instability wave. A lag in the increase of zooplankton compared to nitrate and chlorophyll as well as changes in the zooplankton community composition over the time series reflect the response times of zooplankton to upwelling events.

Plankton productivity and biomass in the western equatorial Pacific: Biological and physical controls

Abstract: During October 1990 in the western equatorial Pacific, from 5°S to 5°N along 155°E, the isothermal and isopycnal lines (near 150 m) domed upwards, surface temperatures were high (29.5–30.5°C), surface salinites were low (33.8–34.2), SST anomalies were <0.5°C between 120°E and 160°E, and the westward flowing SEC was well developed. These conditions are typical of non-ENSO conditions. An eastward flowing surface current was observed that was probably the result of a westerly wind burst. Between 5°S and 5°N, the barrier layer was much thicker south of the equator and the thickness of the barrier layer varied from 88 m at 2°S to 6 m at 1°N. Nitrate and nitrite were not detected in surface waters, but phosphate and silicate were present at concentrations of about 0.15 μM and 1.5 μM, respectively. Nitrate was always present at the deep chlorophyll maximum (DCM), which varied in depth from about 74 to 96 m. The distribution of chlorophyll a (plus divinyl-chlorophyll a) about the equator was distinctly bimodal, with the highest values (0.4 μg 1−1) occurring 2 degrees either side of the equator. Depth integrated values ranged from 23 to 32 mg m−2, and despite the very different oceanographic conditions these values are similar to those found in the nutrient-rich waters of the central and eastern equatorial Pacific. However, primary production estimates ranged from 96 to 241 mg C m−2 day−1(8.0 to 20.1 mmol C m−2 day−1), and these are much lower than previous rates reported for the central and eastern equatorial Pacific. The vertical distribution of cyanobacteria varied markedly from 5°S to 5°N. North of the equator, the concentrations were fairly high and uniform in the upper water column whereas, south of the equator, surface concentrations were low with a pronounced subsurface maximum. This distribution of cyanobacteria suggests that there was transient enrichment north of the equator but not to the south. The low salinity surface layer was highly oligotrophic and appeared to be effectively isolated from the subsurface fluxes of nitrate or dissolved inorganic carbon (DIC). Productivity in the surface layer seemed to be limited by the supply of nutrients, although there were indications that transient nutrient enrichment of surface waters may have occurred north of the equator. In the DCM it appeared to be limited by light. Integrated primary production and cyanobacterial cell counts were elevated north of the equator where the barrier layer was thin. The isolation of the surface waters from the deeper waters that were enriched in nutrients and DIC caused the pCO2 values in surface waters to be in near equilibrium with the atmosphere. Despite a strong N-S gradient in the thickness of the barrier layer, the nitracline and upper boundary of the deep chlorophyll maximum were independent of latitude and occurred at ≈60–70 m, close to the depth of 10% light penetration.

Comparing observed changes in particle size spectra with those predicted using coagulation theory

Abstract: The increasing appreciation of the importance of aggregates in transport of material through the ocean makes understanding their formation and movement crucial. Coagulation theory has the potential to explain key aspects of oceanic particle dynamics. Unfortunately, there are uncertainties about the best way to formulate coagulation theory in marine systems. A mesocosm experiment in which members of the SIGMA (Significant Interactions Generating Marine Aggregates) program grew marine phytoplankton and observed the changes in particle size spectra provides information needed to test different formulations of coagulation rates. Computer simulations incorporating coagulation theory were able to predict aspects of the particle size spectral evolution. The rectilinear formulation of coagulation kernels over-predicted coagulation rates, particularly because of high interaction rates between particles of much different sizes. The curvilinear formulation worked better, especially if used in conjunction with a disaggregation model. Disaggregation was a very important process regulating particle size spectra that was necessary to include in the simulations. The derived disaggregation rates were consistent with past observations of marine snow disaggregation. The simulations in which coagulation and disaggregation predicted the observed results best were those for which the dominant aggregation was between similar sized particles. The largest discrepancies between observed and predicted particle size spectra occurred when theory predicted that dominant interactions were between widely different particle sizes. While coagulation theory was useful in predicting particle size spectra for this experiment, there are still improvements needed in the theory.

Response of the copepod community to a spring bloom in the Bellingshausen Sea

Abstract: During Austral spring 1992, R.R.S. James Clark Ross worked a five station transect in the Bellingshausen Sea. The transect spanned unproductive waters under solid pack ice to an open-water bloom in the north, three weeks later. This paper addresses the ontogenetic development of the copepod community, and from grazing experiments on females of five species investigates their trophic response to a spring bloom. Copepods dominated the mesozooplankton in both numbers and biomass. Their mean biomass in the top 600 m was low (0.85–1.5 mg drymass m−3), which is similar to other high latitude oceanic localities in the Southern Ocean. Almost all the major copepod species underwent an ontogenetic seasonal ascent, from mainly below 250 m under the ice to the top 250 m at the open-water bloom stations. Based on the timings of migration, feeding and reproduction, the species appeared to fall into two broad categories. Firstly, the pronounced seasonal migrants, Calanoides acutus and Rhincalanus gigas, ascended from below 250 m into the top 50 m to coincide with the bloom. The few individuals of C. acutus in the surface layers prior to the bloom do not appear to have been feeding. Reproduction of R. gigas was later than that of C. acutus. Production of the second group, namely Calanus propinquus, Oithona spp. and possibly Metridia gerlachei, appeared to have been less keyed to the bloom. Their seasonal migration was less, and individuals were actively feeding prior to the bloom, albeit at rates about half of those measured during the bloom. Mass specific feeding rates of the species in this group were greater than those of C. acutus and R. gigas. In the top 250 m, carbon:nitrogen ratios of C. propinquus and M. gerlachei were less than those of R. gigas and C. acutus, which suggests less reliance on depot lipids at this time of year. Despite the cold temperatures, the mass-specific feeding rates of the five species measured were broadly comparable to summer values from more northern regions of the Southern Ocean. However, the estimated grazing impact of the copepod community at all the stations was negligible, rising to a maximum of only 8.4% of daily primary production at the most northerly bloom station. These low values result from the very low numbers of copepods in the epipelagic, particularly under the ice. Of the copepods measured, grazing was mainly by the adult female population of Oithona spp. before the bloom, and appeared to be mainly by Oithona spp. and C. acutus during the bloom.

Latitudinal variations in mesozooplankton grazing and metabolism in the central tropical Pacific during the U.S. JGOFS EqPac study

Abstract: Pigment ingestion rates by three size classes of mesozooplankton (200–500 μm, 500–1000 μm and 1000–2000 μm) within the euphotic zone were measured during the Survey l (February/March) and Survey 2 (August/September) cruises of the 1992 United States Joint Global Ocean Flux Study (U.S. JGOFS) in the central equatorial Pacific (EqPac). Survey 1 was characterized by El Nino conditions while Survey 2 was characterized by typical climatological conditions. The small animals (200–500 μm) contributed more than 50% (range: 34–80%) to the total mesozooplankton grazing reported here. Mesozooplankton grazing was higher within the equatorial region (5°S–5°N) than at higher latitudes (5°S–12°S, 5°N–12°N). The carbon-specific ingestion rates of both the large and small animals tended to be higher during Survey 1 than during Survey 2. In contrast to the carbon-specific ingestion rates, the mesozooplankton biomass during Survey 1 was lower than that during Survey 2. Thus, the higher ingestion rates during Survey 1 were offset by lower biomass and the mesozooplankton grazing was quite similar during Surveys 1 and 2. Mesozooplankton removal of chi a was higher within the high-phytoplankton-biomass equatorial region than at higher latitudes during Survey 1, but regional differences were not significant during Survey 2. Mesozooplankton community grazing was equivalent to an average daily removal of 3.0% (range: 0.5–7.7%) and 2.2% (range: 0.8–3.5%) of the total chi a standing stock within the euphotic zone during Surveys 1 and 2, respectively. However, mesozooplankton grazing was equivalent to a daily removal of 47% (range: 15–91%) and 36% (range: 23–66%) of the > 5μm chl a standing stock within the euphotic zone during Surveys 1 and 2, respectively. Assuming a carbon-to-chi a ratio of 58, we estimate that mesozooplankton grazing removed an average of 6% (range: 2–12%) of 14 C primary production during Survey 1, and 5% (range: 2–10%) during Survey 2. Removal rates were not significantly different between the equatorial region and the higher latitudes. Rates of respiration and ammonium excretion were estimated from empirical models based on animal weight and water temperature. Phytoplankton ingestion could not satisfy the estimated daily maintenance-carbon demands of mesozooplankton. The shortages were more pronounced in the large size fraction than in the small size fraction. The estimated ammonium excretion by the mesozooplankton could support 4–15% and 3–17% of 14 C primary production during Surveys 1 and 2, respectively.

CO2 distributions in the equatorial Pacific during the 1991–1992 ENSO event

Abstract: As part of the U.S. JGOFS Equatorial Pacific Process Study, measurements of CO2 species concentrations were made in the atmosphere and in the surface waters of the central and eastern equatorial Pacific during the boreal spring and autumn of 1992. Surface water fCO2 data indicate significant differences between the springtime El Nino conditions and the autumn post-El Nino conditions. The autumn fugacity (ΔfCO2) maxima were approximately 15–55 μatm higher than in the spring. The lower surface ΔfCO2 values in the spring data set were the result of: (i) advection of CO2-depleted water from the west at the equator near 170°W; and (ii) reduced upwelling and lower ΔfCO2 distributions as consequence of lighter zonal winds in the eastern Pacific from 140°W to 110°W. Assuming the springtime data are representative of the El Nino conditions and the autumn data are representative of the post-El Nino conditions, it is estimated that the net annual C02 flux during the 1991–1992 ENSO period is 0.3 Gt C. Over 60% of this flux occurred during the 4-month period in the autumn when ΔfCO2 values were close to normal. The net annual reduction of the ocean-atmosphere CO2 flux during the 1991–1992 El Nino is estimated to be on the order of 0.5–0.7 Gt C.