Μm Size Class Research Articles

A decade of change in the Skidaway River estuary II. Particulate organic carbon, nitrogen, and chlorophylla

A sampling program was initiated in 1986 in the Skidaway River estuary, a tidally dominated subtropical estuary in the southeastern USA. Hydrography, nutrients, particulate organic matter (POM), and microbial and plankton abundance and composition were measured at weekly intervals at high and low tide on the same day at a single site. Hydrographic and nutrient data during 1986–1996 were given in Verity (2002); particulate organic carbon (POC), nitrogen (PON) and chlorophylla (chla) are presented here; plankton data will be presented elsewhere. Chla was fractionated into 8 μm size classes. All classes of POM exhibited distinct seasonal patterns superimposed upon significant long-term increases during the study period. Total chla, 8 μm chla increased 36%, 61%, and 18%, respectively, however the fraction of total biomass attributable to small phytoplankton (<8 μm) increased 25%. The annual amplitude between minimum and maximum stock sizes increased significantly, meaning that bloom events became larger. POC and PON also increased 16% over the decade and, as observed with patterns in chla, exhibited increases in annual amplitude. The C:N ratio was typically 6.4–6.6 (wt:wt) and did not change significantly, while the annual mean C:Chla ratio decreased 19% from 165 to 140. These characteristics indicated highly labile POM composed of significant amounts of detritus, but which became increasingly autotrophic with time. Averaged over the decade, temperature explained 45–50% of the variance in POM. Nutrients were even better predictors of POM, as 60–75% of the variance in chla, POC, and PON were explained by ambient concentrations of DIN, or PO4. Combined with significant increases in NO3, NH4, PO4, Si(OH)4, and DON during 1986–1996, these data strongly suggest that anthropogenic activities contributed to increased loading of dissolved nutrients, which became incorporated into living and nonliving particulate organic matter.

A time series study of the carbon isotopic composition of deep‐sea benthic foraminifera

Variation of the δ13C of living (Rose Bengal stained) deep‐sea benthic foraminifera is documented from two deep‐water sites (∼2430 and ∼3010 m) from a northwest Atlantic Ocean study area 275 km south of Nantucket Island. The carbon isotopic data of Hoeglundina elegans and Uvigerina peregrina from five sets of Multicorer and Soutar Box Core samples taken over a 10‐month interval (March, May, July, and October 1996 and January 1997) are compared with an 11.5 month time series of organic carbon flux to assess the effect of organic carbon flux on the carbon isotopic composition of dominant taxa. Carbon isotopic data of Hoeglundina elegans at 3010 m show 0.3‰ lower mean values following an organic carbon flux maximum resulting from a spring phytoplankton bloom. This δ13C change following the spring bloom is suggested to be due to the presence of a phytodetritus layer on the seafloor and the subsequent depletion of δ13C in the pore waters within the phytodetritus and overlying the sediment surface. Carbon isotopic data of H. elegans from the 2430 m site show an opposite pattern to that found at 3010 m with a δ13C enrichment following the spring bloom. This different pattern may be due to spatial variation in phytodetritus deposition and resuspension or to a limited number of specimens recovered from the March 1996 cruise. The δ13C of Uvigerina peregrina at 2430 m shows variation over the 10 month interval, but an analysis of variance shows that the variability is more consistent with core and subcore variability than with seasonal changes. The isotopic analyses are grouped into 100 μm size classes on the basis of length measurements of individual specimens to evaluate δ13C ontogenetic changes of each species. The data show no consistent patterns between size classes in the δ13C of either H. elegans or U. peregrina. These results suggest that variation in organic carbon flux does not preferentially affect particular size classes, nor do δ13C ontogenetic changes exist within the &gt;250 to &gt;750 μm size range for these species at this locality. On the basis of the lack of ontogenetic changes a range of sizes of specimens from a sample can be used to reconstruct δ13C in paleoceanographic studies. The prediction standard deviation, which is composed of cruise, core, subcore, and residual (replicate) variability, provides an estimate of the magnitude of variability in fossil δ13C data; it is 0.27‰ for H. elegans at 3010 m and 0.4‰ for U. peregrina at the 2430 m site. Since these standard deviations are based on living specimens, they should be regarded as minimum estimates of variability for fossil data based on single specimen analyses. Most paleoceanographic reconstructions are based on the analysis of multiple specimens, and as a result, the standard error would be expected to be reduced for any particular sample. The reduced standard error resulting from the analysis of multiple specimens would result in the seasonal and spatial variability observed in this study having little impact on carbon isotopic records.

Insights into Southern Ocean carbon export from the δ13C of particles and dissolved inorganic carbon during the SOIREE iron release experiment

Abstract 13 C contents of total dissolved inorganic carbon ( δ 13 C-DIC) and particulate organic matter ( δ 13 C org ) were determined to examine the factors influencing phytoplankton 13 C contents and carbon export from the SOIREE iron-induced algal bloom. Suspended particles sieved into 200, 70, 20, 5, and 1 micrometer (μm) size classes displayed an extremely large range in δ 13 C org of 8‰. δ 13 C org values increased from −28‰ for the 1–5 μm class to a maximum of −20‰ for the 20–70 μm class, which was dominated by the large pennate diatom Fragilariopsis kerguelensis . Larger particles (70–200 and >200 μm) had similar δ 13 C org to the smaller (1–5 and 5–20 μm) particles, reflecting both the presence of long narrow Thalassiothrix antarctica diatoms and zooplankton that grazed on small phytoplankton. Comparison of results inside and outside the bloom identified cell surface/volume ratio (mainly reflecting cell size) as the dominant control of phytoplankton δ 13 C org , with subsidiary roles for growth rate and seawater [CO 2 ] aq . The SOIREE iron fertilization provoked an increase in the proportion of large (>20 μm) diatoms. This increased the δ 13 C org of the bulk suspended particles within the mixed layer, but there was minimal increase in the δ 13 C org of sub-surface suspended particles and negligible change in the δ 13 C org of particles obtained with sediment traps suspended below the bloom. This suggests that there was no increase in carbon export over the ∼13 day observation period. However, comparison to δ 13 C org results from previous voyages, and to vertical changes in δ 13 C-DIC, suggests that large diatoms control carbon export from the Antarctic Zone over the summer growth season. This result must be viewed with great caution as it is based on very sparse data and involves several assumptions.

Biologically active soil organic matter fractions in sustainable cropping systems

We sampled corn ( Zea mays L.), soybean ( Glycine max L.), and wheat ( Triticum aestivum L.) rotations, corn monocultures, and plant successional experiments in Southwest MI over a 2-year period to study the effects of alternative management practices on microbial biomass and particulate organic matter (POM) C and N in the top 20 or 25 cm of soil. Microbial biomass was measured using the chloroform fumigation, incubation method. POM was defined as the organic C and N content of primary soil particles in the 53–2000 μm size class. Microbial C decreased during a drought in 1994, and was greater in treatments receiving compost rather than inorganic fertilizer in 1995, but was only weakly correlated with N mineralization in aerobic laboratory incubations. Microbial biomass accounted for an average of 2.6% of soil C and 4.9% of soil N in all treatments sampled in 1995. Microbial C:N ratio was lower in July and November (6.0) than in April and September (7.3), and lower in successional treatments without tillage (5.2) than in agronomic treatments (6.7). Changes in microbial N were large enough to affect short term N availability, but tended to be transitory. POM increased after compost additions, and was greater in successional plots without tillage than in tilled treatments, but did not increase immediately after the incorporation of plant residues. POM accounted for 19.7% of C and 14.8% of N in 1995. Its C:N ratio was 20.8 in a never tilled successional treatment and 16.0 in all other soils. The C:N ratio was 17.0 on average in the 250–2000 μm size fraction, compared to 15.5 in the 53–250 μm fraction. There was a strong correlation between POM and N mineralization in 70- and 150-day aerobic incubations, but there was greater N mineralization per unit POM in April and November than in September or October in both years of the study. These results suggest that POM could be used to estimate N mineralization if combined with information about recently deposited plant residues.

Relative efficiencies of carbon transfer from bacteria and algae to zooplankton in a subtropical lake

Direct measurements were made of carbon transfer efficiency from bacteria and algae to zooplankton in subtropical Lake Okeechobee, Florida, USA. A radio-tracer method was used to determine carbon transfer efficiency to the >200 μm size class, comparing results from treatments where the label was added as 14C glucose or 14C-bicarbonate. The studies were done at a near-shore and an offshore site, and repeated on five occasions between July 1998 and 1999. The results displayed little variation with location, month, or type of tracer used. After 4 h incubations, between 0.1 and 1.0% of the particulate activity occurred in the >200 μm size class. Low efficiencies suggest a considerable loss of carbon from the copepod- and cyanobacteria-dominated food web; this seems to be typical of highly eutrophic lakes.

Dynamics and size structure of phytoplankton in the coastal waters of Singapore

In this paper, a study on phytoplankton dynamics and size in the tropical coastal waters of Singapore is presented. Chlorophyll-a levels in the Singapore Strait generally range from 0.4 to 10 µg l -1 , with an average value of 1.7 µg l -1 . In general, higher chlorophyll concentrations are typically found off the east coast of Singapore, as well as in the shallower, more protected waters of the Johor Strait (maximum ~78 µg l -1 , average = 21.5 µg l -1 ). Seasonal trends in chlorophyll and nutrients are also apparent, with higher chlorophyll generally observed during the south-west monsoon compared with the wetter north-east monsoon. Size-fractionated chlorophyll measure- ments of the phytoplankton community show that for low standing stocks of chlorophyll less than 3µ g l -1 , approximately 40% of the phytoplankton lies in the pico- and small nanoplankton (<8 µm) size range. As total chlorophyll levels in the system increase, the relative contribution from the larger size classes (i.e. 8-20 and 20-100 µm) increases, with a subsequent decrease in the smaller size classes. For total chlorophyll greater than 40 µg l -1 , it was found that the 20-100 µm size class contributed 60% of total chlorophyll. In contrast, the <8 µm size class contributed only about 5% of total chloro- phyll for these waters.

PLANKTIC FORAMINIFERS IN THE OUTER LAPTEV SEA AND THE FRAM STRAIT--MODERN DISTRIBUTION AND ECOLOGY

Planktic foraminifers were studied in the Arctic Ocean in the outer Laptev Sea (76–80°N, 100–150°E) in 1995, in the Fram Strait (81–82°N, 10°W–15°E) and in the western Barents Sea (76°N, 33°E) in 1997. Five depth intervals were sampled vertically between 500 m water depth and the sea surface. In the 125–250 μm size class, highest abundances of living individuals were present in the western Fram Strait (17 ind/m3) and the Barents Sea (14 ind/m3), decreasing to 4 ind/m3 near Severnaya Zemlya and less than 2 ind/m3 at the eastern Laptev Sea continental margin. The most common species Neogloboquadrina pachyderma (sinistral coiling) occured in cold Polar water masses between 50 and 100 m water depth, where they accounted for >70% of all shells. A synodic lunar reproductive cycle is demonstrated in all sampled areas, where reproduction takes place around full moon. In warmer Atlantic water masses of the West Spitzbergen Current and the Barents Sea, Turborotalita auinqueloba contributes >85% to the total assemblage. Most individuals were obtained between 50 and 200 m water depth. In the western Fram Strait affected by the East Greenland Current and at the outer Laptev Sea relative abundances decrease to 2–10%. All other species combined make up only 5% of the total fauna. In general, empty tests are most abundant in water depths deeper than 200 m as a result of gametogenesis and due to high juvenile mortality. The strong gradient in absolute abundances seems to be determined by the availability of food, low salinity due to freshwater discharge from the Siberian rivers and the ice coverage of the Arctic Ocean. The relative abundances, however, reflect the complex hydrographic interaction between relatively warm Atlantic water inflow and cold polar water export in the Arctic Ocean.

Nuclear configuration of bovine oocytes derived from fresh and in vitro-cultured preantral and early antral ovarian follicles

The aim of this experiment was to characterize the growth and nuclear configuration of oocytes isolated from late preantral and early antral bovine ovarian follicles immediately after recovery and after the in vitro culture. Individual follicles were isolated by microdissection from slices of the ovarian cortex. Follicles were sorted by diameter into 175 to 224, 225 to 274 and 275 to 325 µm-size classes. The follicles selected for in vitro culture were placed singly into 40 µL droplets of medium (TCM 199 enriched with FCS, insulin, transferrin, sodium selenite, sodium pyruvate, 1-glutamine, hypoxanthine, FSH and estradiol- 17β) and cultured for 6, 8, 11, 14 or 17 d. The sizes of follicles and oocytes were related to the duration of culture and gradually increased as culture duration was prolonged. The analysis of the relationship between mean diameters of oocytes at the time of recovery and after the in vitro culture, has shown significant differences after culture lasting 8 d (76.9±9.9 vs. 86.1±11.1 µm; P< 0.05), 11 d (77.0±9.9 vs. 91.9±17.5 µm; P< 0.01), 14 d (80.0±9.5 vs. 97.9±16.5 µm; P< 0.01) and 17 d (82.6±6.6 vs. 97.2±11.5 µm; P< 0.01). No statistical differences were shown among oocytes in the 5 pre-culture groups (79.5±8.8; 76.9±9.9; 77.1±9.9; 80.1±9.5 and 82.6±6.6 µm). Meiotic arrest was preserved in 71.9 % of oocytes in our culture system up to 14 d. Frequency of the germinal vesicle (GV) stage did not significantly differ among oocytes evaluated “fresh” or cultured for 6, 8, 11 or 14 d. No relationship was observed between the size class of follicles and the frequency of the GV-stage. Prolonging the culture period to 17 d drastically decreased the percentage of oocytes in the GV-stage (18.7 %) and increased the percentage of oocytes having premature initiation of meiosis (GVBD; 46.3 %) and degeneration (25.0 %). These results suggest that out of all culture periods used in our experiment, Day 14 was found to be the longest culture time allowing for both oocyte growth and maintenance of nuclear configuration at the GV-stage.

Grazing by mesozooplankton from Kiel Bight, Baltic Sea, on different sized algae and natural seston size fractions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 199:43-53 (2000) - doi:10.3354/meps199043 Grazing by mesozooplankton from Kiel Bight, Baltic Sea, on different sized algae and natural seston size fractions F. Sommer1,*, H. Stibor2, U. Sommer3, B. Velimirov1 1Institut für Medizinische Biologie, Universität Wien, Abteilung Allgemeine Mikrobiologie, Währinger Straße 10, 1090 Wien, Austria 2Zoologisches Institut, Ludwig-Maximilians-Universität, Abteilung Aquatische Ökologie, Karlstraße 23-25, 80333 München, Germany 3Institut für Meereskunde, Christian-Albrechts-Universität, Abteilung Meeresbotanik, Düsternbrooker Weg 20, 24105 Kiel, Germany *E-mail: a9101292@unet.univie.ac.at ABSTRACT: Grazing experiments were conducted with natural mesozooplankton from Kiel Bight, Germany, using radioactive labelled phytoplankton cultures and seston size fractions. The results of experiments using phytoplankton cultures indicated that bivalve veligers performed highest clearance of particles within a size range of 4.7 to 6.3 µm, whereas optimum particle size for copepods was 15 µm. The results of experiments using labelled natural seston size fractions identified bivalve veligers and appendicularians as those responsible for the removal of particles within the smallest size class (<2 µm). Seston size fractions larger than 5 µm were mainly cleared by copepods and nauplii. As particle size increased, the contribution of copepod clearance to total zooplankton clearance within size classes increased from 57% (<5 µm size class) to more than 81% (30 to 100 µm size class). When the nauplii clearance rates were included, the total copepod clearance accounted for 90 to 97.6% of the total volume cleared of particles bigger than 10 µm. Despite low abundances of bivalve veligers and appendicularians in Kiel Bight at the time of the experiment, we calculated that approximately 10 and 8.5%, respectively, of the carbon ingested by total mesozooplankton was due to veliger and appendicularian grazing. The importance of bivalve veligers might be seen in their grazing on seston particles that escape predation by copepods and on the amount of energy that is therefore directed from the water column to the benthos when larvae settle. KEY WORDS: Bivalve veligers · Copepods · Size-selectivity · Community grazing Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 199. Publication date: June 26, 2000 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 2000 Inter-Research.

Size-fractionated biological iron and carbon uptake along a coastal to offshore transect in the NE Pacific

Iron has been shown to limit phytoplankton growth in high-nutrient low-chlorophyll (HNLC) regions such as the NE subarctic Pacific. We report size-fractionated Fe-uptake rates by the entire plankton community in short (6–8 h) light and dark incubations along an E–W transect from P04 (a coastal ocean station) to OSP (an open-ocean HNLC station) during August–September 1997. Size-fractionated primary productivity and chl a were measured to monitor algal Fe : C uptake ratios and Fe-uptake relative to phytoplankton biomass. The >5.0 μm size-class, which consisted mostly of large diatoms, had the highest Fe-uptake rate at nearshore stations (P04 and P8), but Fe-uptake rates for this size class decreased despite increases in biomass and primary productivity when transecting westwards to HNLC waters. Fe-uptake rates of the small size class (0.2–1.0 μm, including heterotrophic bacteria and autotrophs) were inversely related to the >5.0 μm size-class uptake rates, in that stations with high dissolved Fe (DFe) concentrations had relatively low uptake rates compared to those in the low-Fe offshore region. The 1.0–5.0 μm size-class Fe-uptake rates were low, relatively invariant along the transect, and differed little between light and dark incubations. Dark Fe-uptake rates averaged 10–20% less than those in the light for the >5.0 μm size class. Dark uptake rates however, were higher than light uptake rates for the 0.2–1.0 μm size class at all stations. Fe : C uptake ratios were high for all size classes at P04, but decreased as DFe concentrations decreased offshore. The prokaryote-dominated 0.2–1.0 μm size class had the highest Fe : C uptake ratios at all stations. These data suggest that prokaryotic organisms make an important contribution to biological Fe uptake in this region. Our experiments support the results of previous culture work, suggesting higher Fe : C ratios in coastal phytoplankton compared to open-ocean species, and demonstrate that light can have a large effect on Fe partitioning between size classes in subarctic Pacific HNLC waters.

Large-Scale and Mesoscale Distribution of Plankton Biomass and Metabolic Activity in the Northeastern Central Atlantic

Plankton biomass and indices of metabolism and growth [electron transport system (ETS), glutamate dehydrogenase (GDH) and aspartate transcarbamylase (ATC) activities] were studied over a 2,800 km east-west section of the tropical North Atlantic Ocean (21°N) in ;500 µm size classes. On the large-scale, zooplankton (>;200 µm) enzymatic activities increased westward in the study section, where large cyanobacteria chains (Trichodesmium spp.) were observed. Parallel to it, an increase in medium calanoids (1–2 mm length) was observed towards the western part of the transect, whereas small calanoids (<1 mm) were dominant throughout the boundary area of the subtropical gyre. Microplankton ETS and mesoplankton ETS and ATC activities seemed to match the wave length of low frequency waves. Our results suggest that such waves are related to the observed enhancement of metabolic activity of micro- and mesoplankton. The large-scale and mesoscale variability observed give evidence of the inadequacy of assuming a steady-state picture of the euphotic zone of tropical and subtropical waters.

Stable isotope and ecological habitat of planktonic foraminifera adjacent to the ice edge in the western Weddell Sea

Planktonic foraminifera collected from the entire 200 m water column in the western Weddell Sea using vertically-towed nets consisted largely ofNeogloboquadrina pachyderma (s) andGlobigerina quinqueloba. Peak abundance of planktonic foraminifera occurred in low-nutrient waters adjacent to the sea ice margin. Nearly all specimens ofN. pachyderma (s) in the > 250 μm size class possessed a kummerform final chamber, but less than 20% added gametogenic calcite to the exterior of their shells. The frequent occurrence of δ18O values ofN. pachyderma (s) lighter than the δ18O in equilibrium with ambient surface waters presumably indicates that significant growth occurred either out of isotopic equilibrium or in water beneath the ice shelf containing18O-depleted snow melt. We suggest herein that the variation between δ18O and δ18O is most likely due to secretion of shell carbonate in water influenced by snow melt or glacier melt.

Changes in Phytoplankton Size-class Abundance and Species Composition Coinciding with Changes in Water Chemistry and Zooplankton Community Structure of Lake Michigan, 1983 to 1992

Phytoplankton and zooplankton were collected at offshore sites of Lake Michigan during 37 cruises in the spring and summer from 1983 to 1992. For the period, 39 common phytoplankton species accounted for 96.0% of the total abundance and 85.6% of the biomass. Over the 10-year study, the lake-wide average spring and summer phytoplankton biomass in the pelagic waters ranged from 0.27 to 1.2 g/m 3 (mean ± S.E. = 0.54 ± 0.03 g/m 3) and phytoplankton abundance ranged from 5,132 to 39,780 cells/mL (mean ± S.E. = 18,291 ± 822 cells/mL). Mesotrophic diatoms accounted for 47.2% of the total phytoplankton biomass. The lack of a trend in the ratio of mesotrophic to eutrophic diatom indicator species suggested that no change in trophic status of the pelagic region occurred during the 1983 to 92 period; that is, the water quality of the offshore of Lake Michigan did not change. A year-to-year shift in dominance from one mesotrophic diatom species to another was evident. Over the 10-year period, flagellates accounted for 52.2% of the summer phytoplankton biomass. The data support the concept of a shift in summer species composition away from blue-green algae dominance to flagellates prior to 1982. However after 1987, the relative importance of the > 70 μm size class increased to over 21% of the phytoplankton abundance in the spring and over 55% in the summer. In particular, the colonial blue-greens, Anacystis and Aphanothece, became substantially more prevalent during the summer, while the flagellates, Chromulina and Ochromonas, decreased in abundance. The changes in the relative abundance of phytoplankton size classes and species composition were examined to determine if they correlated with either the top-down mediated changes in the zooplankton community during the 1980s and 1990s or with any bottom-up variability in nutrient chemistry. Canonical correspondence analysis (CCA) suggested a significant portion of the variability of the spring and summer phytoplankton size classes from 1983–1992 (88.0% and 99.2%, respectively) was explained by environmental axis 1. Primary determinants of relative abundance of phytoplankton size class included nutrients (silica, total phosphorus, and N:P ratio) and abundance of some species of zooplankton. Specifically, Diaptomus minutus, D. ashlandi, D. sicilis, and Daphnia galeata mendotae were inversely correlated with the 0 to 10 μm size class, which we interpret as a grazing effect on phytoplankton size structure, and positively correlated with an increase in abundance of the > 70 μm size class of phytoplankton. CCA suggested that top-down and bottom-up effects were affecting phytoplankton size composition and abundance simultaneously.

Abundance and condition of larval cod (Gadus morhua) at a convergent front on Western Bank, Scotian Shelf

In November and December 1992, Atlantic cod (Gadus morhua) larvae were most abundant at a convergent front located at the periphery of a well-mixed gyrelike water mass rotating near the crest of Western Bank (outer Scotian Shelf). Zooplankton wet biomass and plankton abundance (272 and 529 µm size-classes) were also higher in the frontal region relative to the adjacent water masses. We used the frontal feature to test the hypothesis that larvae in frontal regions are in better condition than larvae elsewhere. No significant differences in triacylglycerol content (an index of nutritional condition), Fulton's K condition index, nor in the daylight feeding ratio were found between larvae in the frontal region and those in the adjacent waters. The convergent front acted as a larval collector, but exchange with other water masses eliminated measurable differences in larval condition. Our observations indicate that physically driven retention, not differential mortality (approximated by condition), was responsible for high abundances of cod larvae at this front.

Size‐fractionated phytoplankton standing stocks and primary production during austral winter and spring 1993 in the Subtropical Convergence region near New Zealand

Size‐fractionated phytoplankton standing stocks and potential primary production (PPP) off the west and east coasts of South Island, New Zealand, were evaluated in austral winter and spring 1993. These are the first size‐fractionated primary production data to be reported from the southern Subtropical Convergence (STC) and in oceanic New Zealand waters. Picophytoplankton (< 2 μm) formed > 30% of integrated chlorophyll a and daily PPP in most water types and seasons, except when the 20–200 μm size class dominated in west coast waters in spring and in the STC in winter and spring. In subtropical and STC waters, PPP was 30% higher than at similar latitudes in the North Atlantic Ocean. Interpretation of whether or not there was accumulation of chlorophyll a in the various water types and seasons depends on microzooplankton grazing, because mesozooplankton grazing and the proportion of chlorophyllous particulate matter sinking out of the upper water column was low. In the STC in spring, phytoplankton growth was slowing down (potential doubling times were at most 1.01 per day at 10 m and PPP per unit biomass was lower than in water masses to the north and south), probably because of decreasing concentrations of dissolved reactive silica. A sedimentation event of diatoms is postulated to have occurred in the STC soon after our spring sampling dates. The balance between algal growth and loss factors in the south‐west Pacific depends on the unique combination of physical and chemical environmental conditions experienced there. The implications of the distribution of phytoplankton biomass and primary production by size‐fraction for carbon export is discussed.

Net zooplankton and the biological pump: a comparison between the oligotrophic and mesotrophic equatorial Pacific

Structure and functioning of the zooplankton community and their consequences on the export fluxes of carbon, nitrogen and phosphorus were studied in two contrasting systems of the equatorial Pacific: the oligotrophic TTS (Typical Tropical Structure) to the west, and the mesotrophic HNLC (High Nutrient-Low Chlorophyll) in the central Pacific. Data were collected during the FLUPAC cruise equatorial transect (September-October 1994) of R.V. L'Atalante and four 6–8-day long time-series stations made between 165°E and 150°W. Along the equator, a sharp 2.5-fold increase in mesozooplankton biomass (200–2000 pm) was observed between 174° and 172°W, with a simultaneous change in surface salinity and chlorophyll concentration, corresponding to the shift between the TTS and HNLC systems. No significant zonal trend was observed within the two systems. Compared with TTS, HNLC presented a significantly greater contribution of the (500–2000 μm) size class to total mesozooplankton biomass, less diel variations, and a shallower vertical distribution. Lower metabolic rates in HNLC were accounted for by different taxonomic compositions in the two areas. Microzooplankton (35–200 gm) had a rather uniform biomass in TTS and HNLC, presented no significant diel variations in the 0–100 or 0–200 m layers, and displayed a shallower vertical distribution than the mesozooplankton. Consequences of such zooplanktonic features on the “biological pump” are assessed. Dissolved nitrogen (DN) and phosphorus (DP) active fluxes, resulting from excretion of interzonal mesozooplankton migrants were 2.1 times higher in the TTS than in the HNLC. However, dissolved inorganic carbon active fluxes were equal in the TTS and HNLC systems, due to differences in C, N, P metabolisms. Combined mesozooplankton and estimated micronekton nitrogen active fluxes represented 40% of the passive flux as measured by sediment traps in the TTS, and 9% in the HNLC. Estimates of mesozooplankton fecal production in the pbotic zone lead to a 2-fold increment between the oligotrophic and mesotrophic stations, and a larger contribution of the fecal production to the sinking flux. It is, therefore, concluded that the mesozooplankton role in the biological pump is mainly passive in the HNLC system, in contrast to the TTS site.

Heterotrophic protists in the Central Arctic Ocean

Distribution, general composition and activity of heterotrophic protists, as well as the distribution of bacteria, were assessed in the upper water column of the central Arctic Ocean during the Arctic Ocean Section, July–September 1994. Bacterial biomass varied from 5 to > 25 mg C 1−1, with the highest values occurring in the Chukchi Sea. Protist biomass was highest (5–107 mg Cl−1) in the upper 50 m of the water column. Higher integrated (0–50 m) protist biomass values (average 910±250 mg C m−2, range 580–1370 mg C m−2) were found in the Chukchi Sea, compared to the central Arctic Ocean (average 480±320 mg C m−2, range 120–1120 mg C m−2). Heterotrophic dinoflagellates were more abundant than ciliates in the >20 μm size class at all stations. In the central Arctic Ocean, the <20 μm size class was numerically composed of dinoflagellates (16%), choanoflagellates (4%) and other flagellates (80%). Choanoflagellates were slightly more abundant in the Chukchi Sea (9% of cell numbers), but were a large component of the flagellate assemblage (55% of cell numbers) at only one station, in the Nansen Basin. Bacterivory estimated via uptake of added fluorescently labeled bacteria ranged from 1·2 × 103 to 46 × 103 bacteria ml−1 day−1; the highest rate was found at the station with a high choanoflagellate abundance. Observation of food vacuole contents showed that all size classes and taxonomic types of protists ingested phytoplankton. Choanoflagellates, and monads as small as 1·5 μm in size, ingested picoplanktonic eukaryotic phytoplankton, which were abundant (103−104 cells ml−1) in the upper 50 m. Larger protists ingested cryptomonads and diatoms, as well as pico-autotrophs. Clearance rates of 10–100 μm sized ciliates and dinoflagellates, based on the uptake of 1–5 μm fluorescent microspheres, were similar to rates reported for herbivorous protists in temperate waters. In terms of ecosystem carbon flow, we infer that phagotrophic protists in the Arctic Ocean are important consumers of phytoplankton and bacteria, and may represent a significant food resource for zooplankton.

Regeneration, recycling, and trophic transfer of trace metals by microbial food‐web organisms in the pelagic surface waters of Lake Erie

Rapid regeneration of 109Cd and 65Zn from their picoplankton prey into the dissolved phase by microzooplankton was observed in water sampled from the pelagic surface waters of Lake Erie (summer 1994 and 1995). Trace metals were added to grazing (lake water &lt;210 µm) and control (lake water &lt;0.2 µm) treatments in the form of radiolabeled Synechococcus. Picoplankton (0.2–3 µm) were grazed heavily by consumers in the nanoplankton (3–20 µm) and microplankton (20–210 µm) size classes (collectively referred to as microzooplankton) as confirmed by dilution assays used to independently measure grazing activity. Most consumed trace metals were regenerated into the dissolved phase (&lt;0.2 µm), but some trophic transfer of 109Cd and 65Zn from radiolabeled prey into the nanoplankton and microplankton did occur: 65Zn was transferred 2.5 times more efficiently into the microplankton and 2.9 times more efficiently into the nanoplankton than was 109Cd. Recycling o:f regenerated 109Cd back into plankton biomass was greater than that for 65Zn. Grazing by microzooplankton influenced the molecular size distribution of regenerated trace metal in the dissolved phase (77±6% 109Cd &lt;5,000 MW; 8 ± 24% 65Zn &lt;5,000 MW). These results show that microzooplankton grazing tends to prolong the residence times of metals such as Cd and Zn in the pelagic surface waters of large lakes.

Coupling between physical and biological fields in the North Atlantic subtropical front southeast of the Azores

The physical and biological properties of the North Atlantic subtropical region southeast of the Azores were studied during an oceanographic cruise carried out in March 1992. The main patterns of physical variability were defined by the presence of (a) a thermohaline front (subtropical front; STF) located at 34°N–35°N and extending from 15°W to 28°W, which separates warmer more saline Western Atlantic Water from colder and fresher Eastern Atlantic Water, and (b) a strong eastward flow running along the subtropical front (Azores Current; AC) extending down to at least 250 m and showing velocities of 50–70 cm s −1 at the core of the current. Overall a close linkage was observed between the STF-AC physical feature and high levels of chlorophyll a. A high resolution survey showed chlorophyll a fluorescence associated with the southern frontal boundary (consisting of chain forming diatoms and flagellates) and with the AC (made up of cells in the less than 2 μm size-class), respectively. Primary production rates measured in the frontal high-chlorophyll region (> 1 mg C m −3 h −1) were much higher than previous measurements carred out in the same area in late spring and summer and about 2 times higher than modelling estimates for the region. The large spatial extension of the biological signature associated with the STF-AC system suggests that carbon fixation within the frontal structure could be significant for regional carbon budgets of the subtropical northeast Atlantic.

Formation and stability of iron(II) oxidation products under natural concentrations of dissolved silica

Model solutions were used to evaluate the effect of dissolved silica on the mineralogy and stability of Fe(II) oxidation products. Mineralogy was evaluated with scanning electron microscopy (SEM) and X-ray diffraction. Stability was evaluated by measuring the decrease in turbidity of colloidal suspensions with time. At Si Fe molar ratios of 0.1 or less, oxidation of Fe(II) produced lepidocrocite, a moderately crystalline oxide that settled rapidly out of solution. At Si Fe molar ratios of 0.36 or higher, ferrihydrite formed from the oxidation of Fe(II). The ferrihydrite consisted of 0.1 μm spherical particles that were poorly crystalline and increasingly stable with higher Si Fe molar ratios. The ferrihydrite was similar in structure and composition to Fe oxide colloids isolated from two natural samples: (1) a reduced groundwater sample that was allowed to oxidize in the laboratory and (2) the colloidal particles in the < 1.0 μm size class of surface water samples from the Tualatin River watershed in northwest Oregon. The similarity of the natural and synthetic colloids is evidence that Fe oxide colloids in natural waters can result from the oxidation of Fe(II) and that dissolved silica may contribute to the stability of the colloids.