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Phytoplankton biomass and rate of production were measured along a transect from 57.54°N to 37.01°N in the northeast Atlantic during July 1996 and at a series of stations over a 7-day period at 37°N 20°W. Surface nutrient concentrations ranged from 4 μmol l −1 NO 3 − , and 0.35 μmol l −1 PO 4 3− at 57.54°N to −1 NO 3 − and ∼10 nmol l −1 PO 4 3− at 37.01°N. The greatest phytoplankton biomass and production were measured in the vicinity of a frontal system at 50°N, and there was a general decline in total phytoplankton biomass and production to the south of the transect. Production was measured in three size fractions. At the station with the highest chlorophyll concentrations (50.34°N), phytoplankton cells larger than 5 μm dominated the assemblage, accounting for 72% of the chlorophyll concentration (22.9 mg m −2 ) and 51% of primary production (54.1 mmol Cm −2 d −1 ), but picophytoplankton production was also high (43%). At 57°N, carbon fixation by the >5 μm fraction accounted for 75% of the daily production of 60.75 mmol Cm −2 d −1 . At 37°N, picophytoplankton was the dominant group, accounting for ∼58% (10 mg m −2 ) of chlorophyll and ∼64% (46 mmol Cm −2 d −1 ), of primary production. Nitrate, ammonium and phosphate uptake rates also were determined. Although high nitrate uptake rates were measured in the surface water at ∼50°N, the greatest uptake rates of both depth-integrated nitrate and ammonium were at the south of the transect. At 37°N, a deep euphotic zone was present and light penetrated through the nitracline; total nitrate uptake was enhanced because of assimilation at the base of the euphotic zone. As a consequence, high values of depth-integrated f -ratio were measured in the oligotrophic waters at the south of the transect. Phosphate was predominantly incorporated into the picoplankton fraction, which included heterotrophic and autotrophic components, at all stations and a significant proportion of phosphate uptake occurred in the dark. The C : N : P assimilation ratios were variable throughout the region; phosphate uptake was generally greater than would be expected if nutrient assimilation were in proportion to the Redfield ratio.

Data are summarised for two Lagrangian experiments in the North Atlantic in early summer 1996. At 59°N 20°W, plankton dynamics was studied in an SF6 tracer release experiment within a mesoscale eddy over a 9-day period. At 37°N 20°W, a second experiment followed a drifting buoy for 7 days. The data obtained in these two experiments have been averaged for 3 depth strata; the euphotic zone, the surface mixed layer (SML), and the seasonal thermocline immediately beneath the surface mixed layer. At 59°N, the euphotic zone was only marginally deeper than the SML, but at 37°N the SML was ca 30 m and the euphotic depth was ca 110 m. At 37°N, nutrient concentrations in the SML were low but significant new production occurred in the thermocline because of light penetration into the nutricline. The particulate organic carbon (POC) concentration of the SML at 59°N was 13–15 μmol C kg−1, but at 37°N POC concentrations were 4 μmol C kg−1. These POC measurements include biota and detritus. As a way of investigating latitudinal differences in the plankton communities, estimates have been made of the carbon and nitrogen content of phytoplankton, bacterioplankton, microzooplankton and mesozooplankton. At both 59°N and 37°N, phytoplankton was the largest component, accounting for ca 50% of the planktonic biomass in the SML. At 59°N, microzooplankton was 16% of the planktonic carbon, but at 37°N this reduced to 8% of the total. Mesozooplankton was a relatively constant proportion (ca 20%) of the planktonic carbon in the SML at both 59°N and 37°N. Bacterioplankton was 14% of the biomass at 59°N, increasing to 24% in the microbial loop-dominated system at 37°N. Mean carbon fixation rate in the oligotrophic southern station was 24% of that at the north, with more carbon fixation below the SML at 37°N than at 59°N. Respiration rates showed little variation with latitude, and the rates at 37°N were 80% of those at 59°N. Nitrate and ammonium uptake rates were very low in the oligotrophic conditions in the SML at 37°N, but nitrate uptake in the euphotic zone was comparable to that at 59°N. Ammonium uptake by phytoplankton was also significantly greater at 37°N, in both the euphotic zone and thermocline, but uptake in the SML was only 20% of that in the SML at 59°N.

The UK PRIME cruise, June–July 1996 in the NE Atlantic, consisted of two legs. During the first, detailed chemical and biological observations were made in time-series mode adjacent to the centre of a cold-core eddy in the vicinity of 59°N 20°W using SF6 tracer techniques as the basis for the Lagrangian study. The eddy, which appeared to have been formed the previous winter, remained coherent over the 9 days of the survey and advected only slowly. The phytoplankton community in the eddy was dominated by the coccolithophorid Coccolithus huxleyi. High microzooplankton grazing rates indicated minimal export losses from the surface layer. Significant shifts in many, but not all, of the chemical and biological properties measured were observed over the course of the experiment, especially after the passage of a storm event, which resulted in considerable deepening of the mixed layer followed by a return to fully stratified conditions. The second leg consisted of a transect from 59°N 20°W to 37°N 19°W, with a further Lagrangian time-series study based on a drogue marker initiated at the southern end of the transect. Maximal biological activity was generally encountered in the region between two fronts located at 52.5°N and 48°N, while to the south of 48°N oligotrophic conditions prevailed. At the southern Lagrangian site, a deep chlorophyll maximum was present and high column new production was recorded as a result of the euphotic zone extending below the depth of the nutricline. Microzooplankton grazing rates were lower at this location than at the northern eddy site. The influx of a warm, saline water body into the upper layers during the southern survey led to a major shift in many of the biological and chemical properties being measured. At both the northern and southern Lagrangian sites, the biomass of the mesozooplankton exceeded that of the microzooplankton.

Chlorophyll a (chl a) concentrations and primary production by the 0.2–2, 2–18 and >18 μm phytoplankton size-fractions were estimated along a transect in the NW Indian Ocean extending from the coast of Oman to 8°N 68°E during the late SW monsoon and autumn intermonsoonal seasons in 1994. Primary production was estimated using the 14C technique with either in situ or simulated in situ incubations. During the late monsoon season, maximal chl a and production values were recorded in the coastal upwelling zone with values of 69 mg m-2 and 3800 mg C m-2 d-1, respectively. The maxima, which were distributed patchily in this region, were dominated by the >18 μm size-fraction. Over the remainder of the transect chl a concentrations and production averaged 30 mg m-2 and 1500 mg C m-2 d-1, respectively, with approximately equal contributions by the three size-fractions in the case of chl a at the majority of stations, but in general, with a maximum in production in the 0.2–2 μm fraction. Immediately following cessation of the SW monsoon wind, chl a and production values over the northern part of the transect decreased to values similar to those over the southern part of the transect at the time of the SW monsoon, with the contributions by the three size-fractions being approximately equal. During the following intermonsoonal season, both chl a concentrations and production across the section were dominated by the 0.2–2 μm size-fraction, with average chl a and production values of the order of 20 mg m-2 and 750 mg C m-2 d-1, respectively. Considerable variation in production values, however, was exhibited across the transect. A clearly defined subsurface chl a maximum was only recorded at the southernmost stations of the transect in oligotrophic waters: the feature did not develop universally across the transect during the intermonsoon.

Data collected during three expeditions to the Arabian Sea are examined to characterise the changes in phytoplankton properties between the intermonsoon and monsoon seasons. The results are used to elucidate the key changes that lead to the occurrence of phytoplankton blooms following the onset of the monsoon winds. Contrary to expectations, changes in the assimilation number and the initial slope of the photosynthesis-light curve decreased in the SW monsoon season, apparently in the wrong sense for them to account for the incidence of blooms in the Arabian Sea during this season. On the other hand, there were strong differences between the intermonsoon and monsoon values of the chlorophyll-specific absorption coefficient for the phytoplankton, probably arising from seasonal changes in cell size and community composition. The sense of these changes was such as to increase substantially the maximum quantum yield for photosynthesis during the monsoon season compared with the intermonsoon. When these results are applied in a simple expression for the daily, primary production of the water column, they can explain the rapid accumulation of phytoplankton observed in the Arabian Sea following the start of the monsoon season. Thus, the occurrence of seasonal blooms in the Arabian Sea depends on combined variations in both the optical and photosynthetic characteristics of phytoplankton in the region.

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.

Samples were taken from the open water region directly north of the ice-edge zone between 65.5 and 69.0°S, 83.5 and 88.0°W by R.R.S. Discovery between 23 November and 8 December 1992 for estimates of size-fractionated chlorophyll a concentrations and primary production and also for data on phytoplankton taxonomic distributions. The size-fractions (SF) considered were >18 μm, 2–18μm and 0.2–2 μm. The data confirmed the presence of a high chlorophyll zone (HCZ) immediately south of the surface boundary of the Southern Polar Front and also the absence of a classical stability induced ice-edge bloom, as inferred from a concurrent hydrographic survey. Chlorophyll a concentrations both within and without the HCZ were dominated by the >18 μm SF, with this fraction consistently comprising >75% of the total chlorophyll biomass within the HCZ. Although maximal productivity also was associated with the > 18μm SF, at the majority of locations its relative contribution to the total was generally less than for the comparable chlorophyll a samples, indicating a reduced photosynthetic efficiency for this SF compared to the smaller SFs. To the north of the HCZ, primary production was approximately equal between the three SFs. Data from a transect across the HCZ showed the phytoplankton to be dominated by larger diatoms, although a maximum in Phaeocystis was present immediately to the north of the frontal boundary. A succession in the phytoplankton taxa was inferred northwards from the ice-edge to the frontal boundary. A steady-state modelling approach indicated a loss rate of the phytoplankton from the HCZ by mesoscale activity of 0.071 day−. Extrapolation of the data obtained during the survey gave an estimate of primary production within the HCZ associated with the Southern Polar Front between longitudes 135-75°W in the Bellingshausen Sea, that is over an approximate distance of 2470 km, of 50 × 103 tonnes C day−1.

Size-fractionated primary productivity measurements were done in the northeast Atlantic from May to July 1989 as part of the Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment. Samples were taken on the 20°W meridian at 47, 52, 56 and 60°N, with most intensive sampling at 47 and 60°N. At the time of the first sampling at 47°N, the bloom was over but, at the other stations, substantial phytoplankton biomass and productivity were measured. The northward progression in maximum phytoplankton biomass was associated with increasing water stability and resulted in a decline in the concentration of nitrate and silicate in the surface mixed layer. The maxima in phytoplankton biomass and productivity were associated with phytoplankton cells that were larger than 5 μm diameter, but the immediate pre-bloom conditions at 60°N were dominated by small nanoplankton ( 50% of the daily productivity of 500–800 mgC m−2 day−1. In the post-bloom conditions at 47°N, nanophytoplankton again became increasingly important. Picophytoplankton (