Phytoplankton in the Weddell Sea,Antarctica: composition,abundance and distribution in water-column assemblages of the marginal ice-edge zone during austral autumn

At the compacted, north-south line of the ice edge, phytoplankton were sampled during early austral autumn of 1986 in the northwestern Weddell Sea. Cells from discrete water bottle samples from 12 stations on two east-west transects were counted to gain quantitative information on the composition, abundance, distribution, and condition of the phytoplankton in water-column assemblages. Over 70 species were found. The highest numbers of total cells (integrated through the top 150 m) were found in open water, well-separated from and to the east of the ice edge on the southern transect, with 6.01×10¹⁰ cells m^-2. The relative abundance of diatoms was low at ice-convered stations (< 35% of the total phytoplankton in preserved samples) and high at open-water stations (> 80%); however, the relative abundance of the prymnesiophyte Phaeocystis sp. was high at ice-covered stations (> 60%) and low at open-water stations (< 16%), with lower absolute abundances than during a previous austral-spring phytoplankton increase. In the open ocean, the dominants were the pennate diatoms Fragilariopsis cylindrus, Pseudonitzschia prolongatoides, F. curta, and a small form of the centric diatom Chaetoceros dichaeta in chains. Although the three pennate diatoms were frequently dominant in number, they represented less biomass than C. dichaeta in open waters. Mean phytoplankton abundance was low (0.2×10⁶ cells l^-1) but, overall, the diatom cell density (0.14×10⁶ cells l^-1) was similar to that found previously during a northward transect from ice-covered to ice-free water at the Weddell-Scotia Sea ice edge (spring 1983). The phytoplankton spatial patterns in the two autumn transects differed, with the more southerly transect exhibiting a higher abundance of diatoms and dinoflagellates. The ratio of full to empty diatoms was higher on the southern transect, indicating a healthy population, while lower ratios of full/empty frustules on the northern transect suggested a generally declining population. However, Phaeocystis sp. was more abundant on the northern transect.     

Interannual variability in the spring phytoplankton bloom in Auke Bay,Alaska

Data on phytoplankton primary production, biomass, and species composition were collected during a 5 yr (1985–1989) study of Auke Bay, Alaska. The data were used to examine the interannual differences in the timing, duration, and magnitude of the spring phytoplankton blooms during each year and to relate these differences to interannual variations in weather patterns. Within any given year, a pre-bloom phase was characterized by low available light, low rates of primary production, low biomass, and predominantly small (<10µm) diatoms. During the primary bloom, integrated production rates rose to 4 to 4.5 g C m^–2 d^–1, and integrated biomass levels reached 415 to 972 mg chlorophyll m^–2. Primary blooms were usually dominated by large diatoms (Thalassiosira spp.), and in a single year (1989) byChaetoceros spp. The primary blooms terminated upon nutrient depletion in the euphotic zone. Secondary blooms, triggered by nutrient resupply from below, occurred sporadically after the primary bloom and accounted for 4 to 31% of total spring production. The date of initiation and the duration of the primary bloom varied little from year to year (standard deviation 3 and 5 d, respectively). Seasonal production rates and biomass levels varied interannually by a factor of 2 to 3. In contrast, intra-annual variations of more than an order of magnitude, especially in biomass, occurred over periods as short as 10 d. These large variations over short time periods indicate the importance of synchronous timing between spring blooms and the production of larval fish and shellfish, which depend on an appropriate and adequate food supply for growth and survival. Parameters describing primary production (e.g. peak daily production, mean daily production, and total production during the primary bloom and the entire season) exhibited little interannual variation (coefficient of variation, CV = 10 to 19%), but a large degree of intra-annual variation (CV = 77 to 116%). Similarly, interannual variations in biomass (peak chlorophyll, mean chlorophyll) were also lower (CV = 20 to 33%) than intra-annual variations (CV = 85 to 120%).     

Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day⁻¹) and microzooplankton grazing (range: 0.00–2.10 day⁻¹) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations also varied widely (0.19–13.65 μg l⁻¹). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l⁻¹. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska, and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries in the transfer of primary production to higher trophic levels in these coastal marine food webs. Received: 12 November 1999 / Accepted: 4 October 2000     

Spatial and temporal variability of phytoplankton biomass and taxonomic composition around Elephant Island,Antarctica, during the summers of 1990–1993

During the austral summers of 1990–1993, phytoplankton studies were conducted in the vicinity of Elephant Island, Antarctica, to investigate the spatial and temporal variability of phytoplankton biomass and taxonomic composition. There was much intraannual variability, with a trend for increasing biomass from January–February (Leg I) to February–March (Leg II), except in the 1993 studies. There was also a change in phytoplankton composition between the two legs. During 1990–1991 the increase was due mostly to diatoms, during 1992 mostly to an increase of flagellates; during 1993 there was a decrease in total biomass between the two legs, with diatoms decreasing, so that dinoflagellates, which increased slightly in numbers, dominated the biomass during the second leg. There was also much inter-annual variability, with the summers of 1990–1991 having greater biomass and higher proportions of microplanktonic diatoms than that of 1992–1993, which had a higher proportion of flagellates. Cluster analyses revealed the presence of four major phytoplankton assemblages, with varying geographical distributions. The northwestern portion of the grid (Drake Passage waters), was characterized by nanoplanktonic diatoms during 1990–1991 and 1993, but by nanoplanktonic flagellates during 1992. The central area (Drake-Bransfield confluence) was characterized by microplanktonic diatoms in 1990–1991, but by cryptophytes or flagellates in 1992–1993. The south and southeastern portion of the area (Bransfield Strait waters) was characterized mainly by either cryptophytes or other flagellates during all 4 yr. The spatial and temporal variability of phytoplankton could not be ascribed specifically to the geographical extent of the different water masses found in the study area, but appears to be due to changing growth conditions in the upper water column as influenced by physical mixing and meteorological conditions, as well as to effects of differential grazing.     

Natural diets of vertically migrating zooplankton in the Sargasso Sea

The feeding preferences of three common diel vertically migrating zooplankton were investigated from December 1999 to October 2000 at the U.S. JGOFS Bermuda Atlantic Time-Series Study (BATS) station in the Sargasso Sea. Gut content analysis of the copepods Pleuromamma xiphias (Giesbrecht) and Euchirella messinensis (Claus) and of the euphausiid Thysanopoda aequalis (Hansen) indicated that all three species fed on a wide variety of phytoplankton, zooplankton, and detrital material. Diet changes generally reflected seasonal trends in phytoplankton community structure. However, species-specific feeding preferences and differences in feeding selectivity among the three species were evident, and in general agreement with feeding habits predicted from the analysis of mouthpart morphology. The euphausiid T. aequalis fed equally on more different food types compared to both copepod species. The copepod P. xiphias consumed a diverse assemblage of phytoplankton from late winter through the summer (78-93% of gut items, by number, were phytoplankton) and based its diet more strongly on carnivorous feeding in autumn and early winter (31% and 61% of gut items were phytoplankton, respectively). E. messinensis showed the greatest feeding specialization, with a strong preference for pennate diatoms in winter and spring and for coccolithophorids during late summer and fall (constituting 67-93% of gut items by number). All three species consumed diatoms more than other phytoplankton taxa, even though diatoms form only a small fraction of the phytoplankton biomass in the Sargasso Sea. Although the majority of gut items identified were phytoplankton cells, the relative biomass contribution of these small cells may be lower than that of zooplankton and detritus. Zooplankton on which the three species primarily preyed were protozoans and crustaceans, but also included other metazoans such as chaetognaths and cnidarians. Marine snow was also an important component of the diet in all three species, with typically >50% and rarely <20% of the gut content being olive-green debris. Marine snow from larvacean houses was found in the guts of all three species, while E. messinenis appeared to selectively consume marine snow aggregates enriched with bicapitate Nitzschia spp. Large cyanobacteria (>4 µm in diameter) found in guts were also likely consumed with marine snow. The species-specific differences in the diets of these three migrating species suggest that an individual species approach is important in determining how feeding habits affect the structure of pelagic food webs and carbon cycling in the sea. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0815-8.     

Comparison of chlorophyll a and carotenoid pigments as predictors of phytoplankton biomass

Chlorophyll a concentration was compared with carotenoid concentration as a predictor of seasonal changes in phytoplankton biomass within Bedford Basin, Nova Scotia, Canada (1976–1977). For all seasons, predictions of biomass from different measures of chlorophyll a were poor and were not improved when chlorophyll a was measured accurately by chromatography. Chlorophyll a and a carotenoid (fucoxanthin) were highly correlated and equally good predictors of total biomass, but neither was related to changes in peridinin concentration. Correlations between specific carotenoids and diatom or dinoflagellate biomass indicate that carotenoids may be useful to describe changes in biomass composition. For all pigments measured, predictions of biomass were hampered when large dinoflagellate cells were present, which biased estimates of total cell volume. Regardless of species composition or cell density, dinoflagellate biomass contributed on the average 68% of the total cell volume measured each day compared with only 14% for diatoms and 17% for flagellates, the most abundant taxa.     

Spatial variability in lipid composition of calanoid copepods from Fram Strait,the Arctic

The calanoid copepods Calanus hyperboreus and C. finmarchicus were investigated in view of their lipid and wax ester content and their fatty acid and alcohol composition. Analyses were performed in females and copepodid stages V and IV from the Fram Strait region between Greenland and Spitsbergen in 1984. This region offers different food conditions like diatom blooms in the North East Water Polynya, food shortage in areas with very close ice cover, high phytoplankton biomass in the marginal ice zone and lower biomass in the open Atlantic water. Lipids contained generally more than 70% wax esters. Highest levels were found in C. hyperboreus with more than 90%. This percentage was not very variable, in spite of large differences in dry weight and lipid content. Copepods with particularly high weight and lipid content were found in the North East Water Polynya. The lightest individuals were found under the pack ice. Lipid proportions per unit dry weight were higher in C. hyperboreus than in C. finmarchicus, whose lowest values were found in the open Atlantic water. Spatial variability in fatty acid composition was much higher than in alcohol composition. The principle alcohols, 20:1 and 22:1, generally accounting for more than 80% of total alcohols. In the North East Water Polynya, the predominant monounsaturated fatty acid was 16:1, while under the ice 20:1 and 22:1 dominated. In the marginal ice zone and in the open water, the 18:4 acid reached percentages up to 30% of total fatty acids. These changes were related to the different food conditions. C. hyperboreus appears to be best adapted to the cold water and unfavourable conditions of polar regions because of its high lipid and wax ester store with long-chain wax esters of high calorific value.     

Size structure of phytoplankton biomass and photosynthesis in subtropical Hawaiian waters

In a subtropical Hawaiian ecosystem, phytoplanton size structure analyses (November–December, 1980) showed that ultraplankton (>3μm), nanoplankton (>20μm) and netplankton (>20μm) accounted for ca. 80, 98, and 2% of total chlorophyll standing stock, respectively, on the basis of chlorophyll. Similar trends were evident, for other biomass indices (e.g. cell numbers, total cell volume, ATP, particulate organic carbon, particulate organic nitrogen). The ultraplankton fraction consisted primarily of small flagellates (1 to 3 μm diam) and coccoid cells (?1 μm diam); the 3 to 20 μm fraction was represented by dinoflagellates, coccolithophores, diatoms, and chrysophytes; and the netplankton fraction consisted principally of dinoflagellates and centric diatoms. Community photosynthesis had a size distribution similar to that of biomass. Sinking rates for the 3 μm, 3 to 20 μm, and >20 μm fractions averaged 0.0, 0.09, and 0.29m d^?1, respectively. The absence of measurable sinking rates for the ultraplankton, together with the relative abundance of biomass in this fraction, result in very small phytoplankton losses due to sinking in such subtropical surface waters.     

Annual population development and production by small copepods in Disko Bay,western Greenland

The population of small copepod species (approximately <1 mm) were investigated during an annual cycle in Disko Bay, western Greenland. The small species considered were Acartia longiremis, Pseudocalanus spp., Oithona spp., Oncaea spp., Microsetella spp., and Microcalanus spp. Most of the small species were present in the surface waters year round and numerically dominated the community, and in biomass from late summer and throughout winter. Oithona spp. was numerically the main contributor, while Pseudocalanus spp. dominated in terms of biomass. In the uppermost 50 m, maximum abundance, biomass and secondary production were observed in late September after the phytoplankton production practically had terminated and the winter initiated. The free spawning Acartia longiremis showed a strong seasonal fluctuation in biomass and egg production, in contrast to the egg carrying species Pseudocalanus spp. and Oithona spp. These had a long spawning season and maintained a more stable biomass year round. Secondary production was estimated by three different ways: (1) based on the obtained specific egg production rates, (2) a temperature dependent equation, and (3) a multilinear regression taking temperature, body weight and chlorophyll into consideration. The contribution of the small species was insignificant when compared to the large Calanus species during the spring- and post-bloom. However, during late summer and winter, where Calanus had left the upper water strata for hibernation, the small species played a crucial role in the pelagic carbon cycling.     

Natural diet of larval Penaeus merguiensis (Decapoda: Penaeidae) and its effect on survival

The relationship between Penaeus merguiensis protozoea larvae and their phytoplankton diet was examined using seasonal plankton surveys and in situ rearing experiments. Larval abundance, phytoplankton community structure, and chlorophyll a concentration in Albatross Bay, Gulf of Carpentaria, were monitored monthly for 2 yr. Larval abundance peaked in November (spring) and March (autumn), at which times diatoms were the most abundant group in net samples of phytoplankton and in the guts of larvae. During November 1989 and March 1990, larvae were reared in nylon mesh enclosures positioned throughout the water column at three depths: 0 to 3 m, 3 to 6 m and 6 to 9 m. Overall, larval survival and gut fullness were both higher in November than in March. In both months, larval survival was lower at the surface than at other depths. This correlated with lower chlorophyll a concentrations, but lower total cell densities were not detected. During the in situ experiments, diatoms were the most abundant phytoplankton group in the water column and in the guts of larvae and, therefore, appeared to be the principal diet of larvae. Pigment analysis demonstrated that while gut contents generally reflected the composition of the phytoplankton community, the larvae were not feeding exclusively on diatoms. They also ingested green algae and possibly seagrass detritus. The in situ experiments demonstrated that the predominantly diatom flora in Albatross Bay can provide a nutritionally adequate environment for prawn larvae even at seasonally low levels. It is unlikely, therefore, that starvation is a major cause of mortality of P. merguiensis larvae during either of the biannual peaks in their abundance in Albatross Bay, Gulf of Carpentaria.     

Spring phytoplankton in the Subarctic North Pacific Ocean

The report includes quantitative and qualitative data on the phytoplankton from the First Canadian Transpacific Oceanographic Cruise from March to May 1969. Nanoplanktonic species (<20 m) were numerically dominant and, together with nertic diatoms, made up the larger portion of the biomass at all stations outside the Western Subarctic and Alaskan Gyres, where Denticulopsis seminae predominated. The nanoplankton consisted mainly of haptophycean, dinophycean and cryptophycean flagellates. Tropical oceanic dinoflagellates and diatoms were abundant in the net samples from Station 17 in the warm Kuroshio current but absent from Stations 24–27 in the Western Subarctic Gyre, Stations 2, 37–39 in the Alaskan Gyre, in which cold water oceanic species predominated, and Stations 31 and 33, south of the Aleutians. Subsurface maxima were common on the outward bound (westerly) leg in March.     

Phytoplankton growth and microzooplankton grazing rates in a restricted Mediterranean lagoon (Bizerte Lagoon,Tunisia)

Phytoplankton growth and microzooplankton grazing were investigated in the restricted Bizerte Lagoon in 2002 and 2004. The 2002 study, carried out at one station from January to October, showed significant seasonal variations in phytoplankton dynamics. High growth rates (0.9–1.04 day⁻¹), chlorophyll a (Chl a) concentrations (6.6–6.8 μg l⁻¹) and carbon biomass (392–398 μg C l⁻¹) were recorded in summer (July), when several chain-forming diatoms had intensively proliferated and dominated the carbon biomass (74%). In 2004, four stations were studied during July, a period also characterized by the high proliferation of several diatoms that made up 70% of the algal carbon biomass. In 2004, growth rates (0.34–0.45 day⁻¹) and biomass of algae (2.9–5.4 μg Chl a l⁻¹ and 209–260 μg C l⁻¹) were low, which may be related to the lower nutrient concentrations recorded in 2004. Microzooplankton >5 μm were mainly composed of heterotrophic dinoflagellates and ciliates. Microzooplankton biomass peaked during summer (2002 320–329, 2004 246–361 μg C l⁻¹), in response to the enhanced phytoplankton biomass and production. The grazer biomass was dominated by ciliates (71–76%) in July 2002 and by heterotrophic dinoflagellates (52–67%) in July 2004. Throughout the year and at different stations, microzooplankton grazed actively on phytoplankton, removing 26–58% of the Chl a and 57-84% of the primary production. In 2002, the highest grazing impact was observed on the large algae (>10 μm) during the period of diatom dominance. These results have a significant implication for carbon export to depth. Indeed, the recycling of most of the diatom production by the microbial food web in the upper water column would reduce the flux of material to the seafloor. This should be considered when modeling the carbon cycling in coastal environments and under conditions of diatom dominance. During both studies, ciliates had higher growth rates (0.5–1.5 day⁻¹) and a higher carbon demand (165–470 μg C l⁻¹ day⁻¹) than dinoflagellates (0.1–0.5 day⁻¹, 33–290 μg C l⁻¹ day⁻¹). Moreover, when grazer biomass was dominated by ciliates (in July 2002), herbivory accounted for 71–80% of the C ingested by microzooplankton while it accounted only for 14–23% when dinoflagellates dominated the grazer biomass (in July 2004). These results suggest that, in contrast to findings from open coastal waters, ciliate species of the restricted Bizerte Lagoon were more vigorous grazers of the large algae (diatoms) than were dinoflagellates.     

Carbon assimilation and lipid production in phytoplankton in northern Norwegian fjords

Carbon assimilation and lipid production were studied in phytoplankton in Balsfjorden and Ullsfjorden, northern Norway, during the exponential growth phase of the spring bloom in 1983 (6–7 April). In Balsfjorden, phytoplankton biomass was constant with depth and equivalent to 1.5 g chlorophyll a 1^-1. Phytoplankton biomass in Ullsfjorden varied with depth, with a maximum of ca. 7 g chlorophyll a 1^-1 occurring at 5 to 10 m. Particulate carbon-14 assimilation was about 18 mg C per m^-2 h^-1 in Balsfjorden and about 39 mg C per m^-2 h^-1 in Ullsfjorden over the depth range 4 to 8 m. In Balsfjorden, the percentage of total fixed carbon recovered as total lipid was 14.7 and 20.4% at 4 and 8 m depth, respectively. In Ullsfjorden, the corresponding values were 8.8 and 28.1% at 4 and 8 m, respectively. The percentages of total fixed carbon present as fatty acids were 1.1 and 1.6% at 4 and 8 m, respectively, in Balsfjorden, and 0.8 and 6.4% at 4 and 8 m in Ullsfjorden. The majority of the radioactivity in lipid at both locations and at both depths was present as polar lipid, with small percentages present in triacylglycerols and very small percentages present in free fatty acids. On average, about 18% of the total carbon-14 incorporated into phytoplankton over a 6 to 7 h mid-day period was recovered as total lipid and its percentage tended to increase with depth. The relatively low percentage of incorporated carbon-14 present as fatty acids in total lipid implies that most of the radioactivity is present in glyceryl and/or glucosyl moieties and that measurement of total radioactivity in total lipid does not necessarily give an accurate estimation of lipogenesis in phytoplankton. Fatty acid analyses of total phytoplankton in Balsfjorden and Ullsfjorden in 1983 and of a surface slick at the end of a bloom of Phaeocystis pouchetii in Balsfjorden in May 1980 showed an abundance (more than 40% of the total) of (n-3) polyunsaturates in all cases. C-18 polyunsaturates, especially 18:4 and 18:5, were very abundant (about 30% of the total) in the P. pouchetii surface slick in Balsfjorden in 1980. Both P. pouchetii biomass and C-18 polyunsaturates were more abundant in Ullsfjorden than in Balsfjorden (1983). Lipids from the P. pouchetii surface slick were deficient in C-16 polyunsaturates and relatively deficient in C-20 polyunsaturates, but both these classes of fatty acids were abundant in Balsfjorden and Ullsfjorden in 1983. The phytoplankton in both locations in 1983 was dominated by P. pouchetii and diatoms; Chaetoceros socialis was especially abundant in Balsfjorden. The results are discussed in terms of the fatty acids present in herbivorous zooplankton in northern Norwegian fjords.     

Continuous plankton records: Changes in the composition and abundance of the phytoplankton of the north-eastern Atlantic ocean and North Sea, 1958–1974

In most areas of the north-eastern Atlantic Ocean, diatoms have declined drastically in abundance in the last decade. Additionally, in areas to the north of 59°N Ceratium species and an index of total phytoplankton have also declined. South of 59°N the phytoplankton index has increased, diatoms have declined and Ceratium species have remained at a constant level of abundance. A possible explanation of the increase in the phytoplankton index at a time when the diatoms were declinig south of 59°N is the development of unidentified phytoplankton organisms such as microflagellates. As many of the variables influencing phytoplankton standing crop are governed in turn by the prevailing weather, the phytoplankton changes may well be a consequence of the general deterioration, since 1940, of North Atlantic weather. Changes in phytoplankton which may be attributed to an amelioration of climate since 1971 are evident as yet only in the southern North Sea.     

Microplankton of the Gulf of Elat: Aspects of seasonal and bathymetric distribution

The components of 4 major groups of microplankton were identified and their numerical abundance determined in net samples collected at depth intervals down to 600 m at a permanent station off the H. Steinitz Marine Biological Laboratory, Elat. The samples analyzed were collected once a fortnight over a period of 1 year beginning in June 1974. The groups studied were the Cyanophyta, Bacillariophyta and Pyrrhophyta of the phytoplankton and the Tintinnina of the microzooplankton. The pattern of vertical distribution of the phytoplankton as a whole showed a general decrease in cell numbers with increasing depth. The blue-green algae, consisting mainly of Trichodesmium sp. trichomes, were confined primarily to the upper 100 m. The diatoms were unevenly distributed, with one species, a minute centric diatom, Thalassiosira subtilis, associated with a massive bloom during March 1975 between 300 and 400 m. The peridinians, the group with the largest number of species, included forms which were evenly distributed throughout the whole water column and forms limited either to the upper or deeper water strata in accordance with their light intensity preferences. The mass occurrence of newly-emerged dinoflagellate cysts of Pyrophacus horologicum, a weakly-armoured dinoflagellate, in the 200 to 300 m depth interval during April 1975, suggests that reproductive processes in dinoflagellates may also be light-controlled. The tintinnids, like the phytoplankton groups, were most abundant in the upper 100 m with a gradual decrease in numbers of individuals, though not in species, in the deeper water strata. The overall yearly pattern of microplankton distribution indicates 3 peaks: late fall and early summer peaks consisting primarily of blue-green algae and one in early spring consisting of several species of diatoms and peridinians and of species of tintinnids which thrive in the same niches as the phytoplankton. Both phytoplankton and tintinnid production was lowest during the summer months.     

Preponderance of a few diatom species among the highly diverse microphytoplankton assemblages in the Bay of Bengal

The microphytoplankton assemblages were studied from water samples collected at eight discrete depths in the top 120 m at five central (open ocean) and four western (shelf/slope region) locations in the Bay of Bengal. The Bay is a low-productive warm pool regime with poor inorganic nutrient inputs to its intensely stratified surface layer despite the very large riverine influx. In addition, the prolonged cloud cover has an adverse effect on the top 25–40 m, on primary production, chlorophyll concentration and phytoplankton assemblages. Microphytoplankton were the most abundant in the northern area of the Bay. A total of 153 phytoplankton species were identified during this study. The most abundant species (at least 1,800 individual cells belonging to a given species or ≥2% of the total counts of identified specimens) during this study were: Thalassiothrix longissima, Thalassiothrix fauenfeldii, Rhizosolenia styliformis, Nitzschia angularis, Thalassionema nitzschioides, Coscinodiscus radiatus, Chaetoceros eibenii, Skeletonema costatum, Coscinodiscus concinnus and Chaetoceros coarctatus. Similarly, there were 20 moderately abundant (≥0.5 but <2%) species. Thirty-three of the least abundant species (<0.5%) occurred at one station. Diatom species, T. fauenfeldii and T. longissima were ubiquitous in the study area. The exclusive occurrence of S. costatum in the northernmost stations suggests that it proliferates only in the low-salinity regions with adequate silica from the land inputs. Abundance of pennate diatoms was higher in the open Bay compared to that of centric diatoms in the more productive northern locations and the western Bay. There appears to be a basic difference between near shore flora and offshore flora. This first analysis of phytoplankton assemblages from the offshore Bay suggests that while there is predominance of only a few species, the Bay harbors very diverse diatom communities that seem to be syntrophic, non-competitive and co-habiting in the generally low nutrient, stratified surface waters.     

Vertical fine structure of the biomass and composition of algal communities in Arctic pack ice

The biomass and composition of algal communities in sea ice were studied during two summer expeditions to the central Arctic Ocean and the Greenland Sea. Based on algal pigment determination and cell counts, high biomass accumulations were found at the surface, in the interior and in the bottom layer of the ice floes. Pennate diatoms dominated in the bottom layer, while phototrophic flagellates and cysts of unknown origin were the most abundant taxa in the upper parts. The lowermost 20 to 40 cm contained between 4 and 62% of the entire algal biomass. Consequently, ice biological studies, which deal only with the bottom few centimetres of the ice floes, will underestimate algal biomass and production by factors of up to 25. Differences between the results of this study and published data from coastal locations point towards different biological regimes in Arctic sea ice. The algal biomass in coastal ice is about two orders of magnitude higher and composed mainly of diatoms, probably supported by nutrient influx from the water column. In the pack ice of the central Arctic, nutrient supply is probably reduced, and flagellates contribute substantially to total algal biomass. However, methodological problems might partially be responsible for the observed differences. Received: 12 June 1998 / Accepted: 11 December 1998     

Distribution and structure of heterotrophic protist communities in the northeast equatorial Pacific Ocean

The distribution and structure of heterotrophic protist communities and size-fractionated chlorophyll a were studied during the Korea Deep Ocean Study 98 (KODOS 98) research expedition (July 1998) in the northeast equatorial Pacific Ocean (5–11°N). Areas of convergence and divergence formed at the boundaries of the South Equatorial Current (SEC), North Equatorial Current (NEC), and North Equatorial Counter Current (NECC) during the expedition. Water column physicochemical characteristics significantly influenced the size structure of heterotrophic protist communities. Intense vertical mixing and high nutrient and chlorophyll a concentrations characterized SEC and NECC areas, which were affected by converging and diverging water masses, respectively. Nanophytoplankton dominated in SEC and NECC areas; both areas also had relatively high heterotrophic protist biomasses (average 743 µg C m^–2). NEC areas were characterized by a stratified vertical structure, low nutrient and chlorophyll a concentrations, and picophytoplankton dominance. The heterotrophic protist biomass in NEC areas averaged 414 µg C m^–2; nanoprotists (<20 µm) dominated the community. The nanoprotist biomass comprised 49–54% of the total heterotrophic protist biomass in SEC/NECC areas and 67–72% in NEC areas. The biomass of heterotrophic protists was higher in SEC/NECC areas than in NEC areas, but the relative importance of nanoprotists was greater in NEC areas than in SEC/NECC areas. Heterotrophic dinoflagellates were dominant components of the <20 µm and >20 µm size classes in both water columns. The biomass of heterotrophic protists significantly correlated with the net-, nano-, and picophytoplankton biomass in SEC/NECC areas and with the nano- and picophytoplankton biomass in NEC areas. Heterotrophic protists and phytoplankton also showed strong positive correlation in the study area. The size structure of the phytoplankton biomass coincided with that of heterotrophic protists; the heterotrophic protist biomass positively correlated with the protists prey source. These relationships suggest that the community structure of heterotrophic protists and the microbial food web depended on size classes within the phytoplankton biomass. Microzooplankton grazing and phytoplankton growth rates were higher in SEC/NECC areas than in NEC areas. In contrast, the potential primary production grazed by microzooplankton was relatively high in NEC areas (127.3%) compared with SEC/NECC areas (94.6%). Our results indicate that the relative importance and size structure of heterotrophic protists might vary according to two distinct water column structures.Communicated by T. Ikeda, Hakodate     

Subsurface chlorophyll maximum in the tropical and subtropical western Pacific Ocean: Vertical profiles of phytoplankton biomass and its relationship with chlorophylla and particulate organic carbon

Vertical distribution of phytoplankton biomass in terms of carbon content (PC) and its relationship with chlorophylla and particulate organic carbon (POC) were examined together with phytoplankton growth rates in the tropical and subtropical western Pacific in 1979, where a prominent subsurface chlorophyll maximum (SCM) developed between 65 and 150 m. Fluorescence microscopy combined with image analysis was used for measurement of cell volume which was converted to PC. The SCM coincided consistently with subsurface maximum of PC, and the SCM primarily reflected in situ accumulation of phytoplankton biomass. The PC:chlorophylla ratio decreased with depth; the ratio was 1.8 times, on average, higher in populations at the SCM compared to those near the surface. This increase in relative cellular chlorophylla along with depth accentuated the magnitude of the SCM. The PC:POC ratio was substantially lower near the surface, 0.17 on average, and increased sharply around the SCM, with a mean value of 0.53. Thus suspended particles around SCM were richer in phytoplankton than those in the upper layers. A major part of PC was contributed by autotrophic eukaryotes both near the surface and at the SCM, and prokaryotic picoplankton comprised a relatively small proportion (6.3 to 14.9%) of PC. The high phytoplankton biomass around the SCM was suggested to be ascribed to in-situ growth of phytoplankton.Please address all correspondence and requests for reprints to Dr Furuya at his present address: Institute of Bioresources, Miè University, Kamihama, Tsu 514, Japan     

Coccolithophores: a major phytoplankton component in mature upwelled waters off the Cape Peninsula,South Africa in March, 1983

Coccolithophores, together with other phytoplankton, were sampled at ten stations along a transect following the general orientation of the Cape Peninsula upwelling plume in March, 1983. Thirteen coccolithophore species were identified with Emiliania huxleyi dominating the assemblages. Coccolithophores and other microflagellates were more abundant than diatoms along the transect except at one station in mature upwelled water. Very high populations (2 340×10³ cells l^-1) of coccolithophores were found in mature upwelled water adjacent to the oceanic front. This count is one of the highest counts ever recorded in an upwelling area. The anomalous warm event prior to sampling may have had a residual effect on seeding, contributing towards the predominance of small-celled phytoplankton over chain-forming diatoms.