Distribution and abundance of euphausiids and pelagic amphipods in Kongsfjorden,Isfjorden and Rijpfjorden (Svalbard) and changes in their relative importance as key prey in a warming marine ecosystem

Euphausiid (krill) and amphipod dynamics were studied during 2006–2011 by use of plankton nets in Kongsfjorden (79°N) and adjacent waters, also including limited sampling in Isfjorden (78°N) and Rijpfjorden (80°N). The objectives of the study were to assess how variations in physical characteristics across fjord systems affect the distribution and abundance of euphausiids and amphipods and the potential for these macrozooplankton species to reproduce in these waters. The abundances of euphausiids and amphipods were higher in Kongsfjorden than in Rijpfjorden and Isfjorden, and the highest abundances were observed at the innermost stations of Kongsfjorden, where Thysanoessa inermis and Themisto libellula dominated. The Atlantic species Thysanoessa longicaudata, Meganyctiphanes norvegica and Themisto abyssorum dominated at the outside Kongsfjorden. Inter-annual and seasonal variability in abundances of euphausiids and amphipods were evident. The presence of ripe euphausiids outside Kongsfjorden indicates that they may reproduce in these areas. Mature individuals of T. abyssorum were recorded mainly outside Kongsfjorden, whereas no mature or ripe T. libellula were present in both the inner and outer parts of this fjord. If the warming trend persists, as seen during the last decade, this would favour the Atlantic/boreal euphausiid species, while Arctic species, such as the amphipod T. libellula, may decline. Euphausiids and amphipods are major food of capelin (Mallotus villosus) and polar cod (Boreogadus saida), respectively, in this region, and changes in prey abundance will likely have an impact on the feeding dynamics of these important fish species.     

Temporal variations and pond age effect on plankton communities in semi-intensive freshwater marron (Cherax cainii,Austin and Ryan, 2002) earthen aquaculture ponds in Western Australia

The abundance and diversity of the plankton community represents the health of the aquatic ecosystem, and plays an important role in the growth of cultured animals under aquaculture conditions. The temporal variations of plankton abundance, taxonomic composition, diversity, evenness and species richness were studied in three old and three new semi-intensive marron (Cherax cainii, Austin and Ryan, 2002) ponds. Water parameters such as temperature, dissolved oxygen, pH, turbidity, TAN, nitrite, nitrate and reactive phosphate were recorded, and plankton samples were collected every two months, for one year of juvenile production cycle. A total of twenty-six phytoplankton and seven zooplankton genera were recorded. Chlorophyceae was the dominant class of phytoplankton throughout the year, followed by Trebouxiophyceae. Rotifera comprised 49.8% of the total zooplankton community (individuals L^?1), the largest proportion of any group. Temporal variations impacted the plankton abundance and community structure, and plankton abundance were more abundant during summer. The pond age did not influence the phytoplankton abundance, whereas zooplankton abundance was higher in older ponds.     

Distribution of tintinnid species from 42° N to 43° S through the Indian Ocean

Taking advantage of the transfer of the Italian R/V “Italica” from Italy to New Zealand for the Italian Research Expedition in the Antarctic, surface sampling of plankton was carried out from 42°N to 43°S from 14 November to 16 December 2001. Collaboration between several Italian research groups resulted in the Mediterranean, Indian and Pacific Ocean Transect (MIPOT), aimed to merge oceanographic measurements with remotely sensed data and to study latitudinal changes in plankton communities. We present preliminary results on the tintinnids (abundance and species composition) along the MIPOT1 transect. Tintinnid distributions and food vacuole contents are discussed in relation to chlorophyll concentrations and contribution of phytoplankton size classes. Generally, chlorophyll concentrations were well below 0.5 mg m^?3 and tintinnid numbers were only a few tens per liter. Eighty-six tintinnid species belonging to 27 genera were recorded. Highest diversity and number of tintinnid species occurred in the warm-water regions, from the Mediterranean Sea to the Tropic of Capricorn. Shannon diversity index ranged from 1.0 (Tasman Sea) up to 2.9 (tropical Indian Ocean). Each of the oceanic provinces showed a well defined tintinnid assemblage, a tintinnid ‘fingerprint’ characterizing each area. Data are compared to previous records for the different areas and the persistence in time of typical tintinnid species is discussed.     

First observation of krill spawning in the high Arctic Kongsfjorden, west Spitsbergen

In the past, two euphausiid species prevailed in the high Arctic Kongsfjorden, the arcto-boreal Thysanoessa inermis (Kroeyer, 1846) and Thysanoessa raschii (Sars. 1964). Both were considered expatriates from the Barents Sea or Norwegian Sea and non-reproductive due to low temperatures. The macro-zooplankton of the fjord has been studied as a component in an ecosystem context since 2006, including baseline investigation of distribution and functional performance of key species. In recent years, three additional krill species were regularly detected in the fjord and are the focus of an intensive long-term study. Of these species, Thysanoessa longicaudata (Kroeyer, 1846) and Meganyctiphanes norvegica (Sars, 1857) are typical for the boreal Atlantic whereas Nematoscelis megalops (Sars, 1883) has a broad distribution in temperate to subtropical provinces. Their occurrence in the Kongsfjorden clearly indicates increasing Atlantic influence. During the 2011 campaign, T. raschii was observed spawning in the field for the first time and showed development up to the naupliar stage in the laboratory. Should more evidence of reproduction be encountered in any of the five krill species in the Kongsfjorden in the future, it will be taken as an indication of a changing environment concerning temperature and food web composition.     

Near-bottom zooplankton aggregations in Kongsfjorden: implications for pelago–benthic coupling

Near-bottom zooplankton communities have rarely been studied despite numerous reports of high zooplankton concentrations, probably due to methodological constraints. In Kongsfjorden, Svalbard, the near-bottom layer was studied for the first time by combining daytime deployments of a remotely operated vehicle (ROV), the optical zooplankton sensor moored on-sight key species investigation (MOKI), and Tucker trawl sampling. ROV data from the fjord entrance and the inner fjord showed high near-bottom abundances of euphausiids with a mean concentration of 17.3 ± 3.5 n × 100 m^?3. With the MOKI system, we observed varying numbers of euphausiids, amphipods, chaetognaths, and copepods on the seafloor at six stations. Light-induced zooplankton swarms reached densities in the order of 90,000 (euphausiids), 120,000 (amphipods), and 470,000 ind m^?3 (chaetognaths), whereas older copepodids of Calanus hyperboreus and C. glacialis did not respond to light. They were abundant at the seafloor and 5 and 15 m above and showed maximum abundance of 65,000 ind m^?3. Tucker trawl data provided an overview of the seasonal vertical distribution of euphausiids. The most abundant species Thysanoessa inermis reached near-bottom concentrations of 270 ind m^?3. Regional distribution was neither related to depth nor to location in the fjord. The taxa observed were all part of the pelagic community. Our observations suggest the presence of near-bottom macrozooplankton also in other regions and challenge the current view of bentho–pelagic coupling. Neglecting this community may cause severe underestimates of the stock of pelagic zooplankton, especially predatory species, which link secondary production with higher trophic levels.     

Plankton richness in a eutrophic reservoir (Barra Bonita Reservoir,SP, Brazil)

Species richness of plankton was studied in a eutrophic reservoir (Barra Bonita Reservoir) of the Middle Tietê River, São Paulo State, Brazil, during the period 1985–1986. This reservoir is formed by two rivers: the Tietê and the Piracicaba (the main tributaries), of which the Tiête is more eutrophic, having conductivity and nutrient concentration values twice those of the Piracicaba. In addition, the reservoir is fed by 114 smaller tributaries. Monthly sampling was carried out at three stations representing different environmental conditions: St1 on the Tietê, S2 on the Piracicaba, and St3 at the confluence of the two rivers. For the phytoplankton community, the Piracicaba River (St2) proved the richest site, with a listed 72 species with abundance of Chlorophyta, while St1 and St3 registered 59 and 50 species, respectively, with abundance of Cyanophyta. For the zooplankton community a great difference was not registered in species number at the three stations but the species composition and dominant species of rotifers and copepods were quite different. The occurrence of Mesocyclops meridianus and Metacyclops mendocinus, specific for St2;Mesocyclops ogunnus and Notodiaptomus iheringi, specific for St1, and that of these two species plus Mesocyclops meridianusat St3 shows that the conditions combining at this station were favorable to Mesocyclops meridianus but not to Metacyclops mendocinus. Both for phytoplankton and zooplankton, high values found of species richness were compared to species richness of natural lakes, e.g., Dom Helvecio, a monomictic stable lake in eastern Brazil, and another fifteen lakes in the same region.In conclusion, this work shows that environmental gradients are strong selective factors that enhance plankton richness in eutrophic reservoirs exhibiting environmental instability. This fact could explain the presence of a high number of plankton species associated with a high number of individuals in Barra Bonita Reservoir, supporting the effects of the intermediate disturbance hypothesis.     

Pelagic nitrification and denitrification rates in an Arctic fjord during early spring

This study addresses factors governing nitrification and denitrification rates, along with the abundance of the bacterial groups likely involved in these activities, in Kongsfjorden, an Arctic fjord at Ny-Ålesund, Svalbard. The fjord was sampled three times during the month of March 2008 as day length and direct solar radiation increased. Although initially well mixed, cooler and more saline, the fjord became stratified, warmer and less saline during late March. The concentrations of NH₄ ⁺ (4.4?±?1.6 to 6?±?1.6 μM) and NO₂ ^? (1?±?0.3 to 1.2?±?0.4 μM) increased progressively with the decrease in NO₃ ^? (6.1?±?1.3 to 3.8?±?1.5 μM), reflecting the onset of primary productivity. Nitrification rates and the culturable population of nitrifiers decreased significantly from 1.6?±?0.9 to 0.4?±?0.1 ng at NH₄ ⁺-N l^?1 h^?1 and 5.1?±?0.3?×?10² to 29?±?14 cells l^?1, respectively. In contrast, denitrification rates increased (2.4?±?0.5 to 4.6?±?1.3 ng-at NO₃ ^?-N l^?1 h^?1), although the abundance of culturable denitrifiers did not vary significantly. A significant correlation of nitrifiers with NO₃ ^? during early March (p?<?0.01, r?=?0.51) indicated that nitrifiers may play an important role in regulating the NO₃ ^? pool and thereby in controlling the abundance of denitrifiers. However, the contribution of nitrification to the total NO₃ ^? pool decreased with time. Experimental simulations were also set up to understand the impact of change in duration of light and progressive increase in temperature on these processes. The application of 24 h light inhibited nitrification, suggesting that during peak Arctic summer the contribution of nitrification to the nitrate pool is minimal. It was also observed that a brief exposure to light (≤6 h) was enough to hamper nitrification rates. Experimental simulations suggested that a gradual increase in temperature in the fjord may enhance the magnitude of nitrification and denitrification in the fjord.     

Distribution patterns of polychaete fauna in an Arctic fjord (Hornsund, Spitsbergen)

Polychaetes are one of the most important groups of benthic organisms in marine ecosystems. They dominate on the Arctic shelf and play an important role in ecosystem functioning. This study focuses on the polychaete biodiversity and their distribution patterns in Hornsund, an open glacial fjord, in western Spitsbergen and provides important baseline data for future studies of temporal fluctuations in benthic fauna. The main aim of this study was to assess how the polychaete abundance, biomass, diversity, community structure, and function vary along the Hornsund fjord’s axis, in relation to the environmental factors. Eighty-eight polychaete taxa were identified; an average density was 457 ind. m^?2 ± 237.5 SD. Three assemblages were distinguished (INNER, MIDDLE, and OUTER) along the fjord axis, reflecting the intensity of glacial disturbance. A clear division between the highly disturbed inner part of the fjord (Brepollen) and the less impacted middle and outer parts was observed. Continuous gradients in abundance, biomass, and diversity were found with all those values diminishing toward the inner region of the fjord. The polychaete assemblages' indices were significantly correlated with bottom temperature, sediment characteristics (grain size), and distance to the glacier (longitude). No significant correlations were found with depth or total organic carbon content. Carnivore and motile surface deposit feeding polychaete species dominated in the areas close to the glaciers, while the OUTER community was dominated by carnivores and surface sessile and discretely motile species, and had more complex trophic structure, with multiple species representing different functional groups including carnivores, sessile, discretely motile, and motile surface deposit feeders and motile burrowers.     

Decadal change in macrobenthic soft-bottom community structure in a high Arctic fjord (Kongsfjorden, Svalbard)

Marine benthic macrofauna communities are considered a good indicator of subtle environmental long-term changes in an ecosystem. In 1997/1998 and 2006, soft-bottom fauna of an Arctic glacial fjord Kongsfjorden was extensively sampled and major communities were identified along the fjord axis, which were related to the diminishing influence of glacial activity. Spatial patterns in community structure and species diversity were significantly different in the central basin of Kongsfjorden between periods while there was no change in the inner part of the fjord. In 1997/98, three faunal associations were distinguished with significant differences in species richness and diversity (H′) while in 2006 only two faunal associations were identified and there were no differences any more between the two formerly distinct associations in the central fjord. The increased input of Atlantic water due to a stronger West Spitsbergen Current may be the reason for unification of previous clear faunal division. The faunal association in the inner, well separated glacial part of the fjord, characterized by strong glacier influence, was protected from Atlantic water inflow and, hence, the macrobenthic fauna essentially remained unaffected. Reduced abundance of species typical for glacial bays in the central part of the fjord in 2006 may result from the decreasing effect of Blomstrandbreen glacier, strong increase of input of Atlantic water into the fjord and increased temperature of West Spitsbergen Current. Higher values of POC in 2006 than in 1998 are likely the effect of increased primary production resulting from warmer water temperatures.     

Distribution and reproduction strategies of krill (Euphausiacea) on the Norwegian shelf

Meganyctiphanes norvegica and Thysanoessa inermis were the dominant krill species that were observed on the Norwegian shelf. T. inermis and M. norvegica were found in abundances of up to 14 and 22 no. m^?2, respectively, on the mid Norwegian shelf (Møre plateau, Halten, Sklinna, Træna banks) and up to the Lofoten area. The main distribution of the krill indicates that they are mostly restricted to the Atlantic waters of the shelf region in April and little krill was observed south of 62°N. The two dominant krill species appeared to have different strategies for the timing of spawning, thus avoiding direct overlap of spawning products. The majority of the T. inermis females were in the spawning condition in April. Meganyctiphanes norvegica appeared to spawn at least a month later. The sizes of the first time spawners of T. inermis were around 11–16 mm, and they were approximately 1 year old. The sizes of the second time spawners were around 17–22 mm. The spawning of krill on the Norwegian shelf coincides with the blooming of the spring phytoplankton in this region . Advection of eggs and larvae of krill, as well as other plankton, is important not only to the shelf and slope region, but also for the functioning of the wider Norwegian Sea ecosystem and the adjacent areas.     

Substratum as a structuring influence on assemblages of Arctic bryozoans

The nature of the substratum is a fundamental factor determining the types of organisms and communities found in many terrestrial and benthic habitats. The extent to which this is true in extreme environments was investigated using bryozoan assemblages as model organisms in an Arctic fjord (Kongsfjorden 79°N, 12°E) in summer 2001 using SCUBA. Twenty-seven substrate samples of 0.25 m² were taken at 10 m depth from the inner glacial basin to the mouth of the fjord. Multivariate analyses revealed four different bryozoan assemblages. The sea floor of the inner basin of Kongsfjorden near the glacial fronts was characterized by low diversity and dominance of the ctenostome species Alcyonidium disciforme Smitt. Highest richness and diversity occurred on rock substratum with mean size >10 cm², on which the most common species was the pioneer Harmeria scutulata Busk (abundance: 15%). On smaller rocks with mean size <2 cm², the runner-like pioneer species Electra arctica Borg comprised most individuals of the assemblage (98%). Yet another pioneer, Celleporella hyalina Linnaeus, was the most abundant species (49%) on substratum dominated by algae. Thus, in each habitat type, pioneers dominated but different species and to different extents. There was much variation in species composition and abundance within assemblages of heterogeneous habitats, and this study emphasizes the importance of microhabitats and physical conditions. Heterogeneity was evident at scales of <1 m.     

Diversity of anaerobic fungal populations in cattle revealed by selective enrichment culture using different carbon sources

We employed most probable numbers (MPNs) enumeration of enrichment cultures, combined with the use of a range of carbon sources (glucose, cellobiose, cellulose, xylan and wheat straw), to recover and identify morphologically different groups of anaerobic fungi (monocentric rhizoidal [Neocallimastix, Piromyces spp.], polycentric rhizoidal [Anaeromyces, Orpinomyces spp.], bulbous non-rhizoidal [Caecomyces, Cyllamyces spp.]) from rumen digesta, and fresh or frozen–thawed faeces of silage-fed cattle. Highest MPN counts (>10⁶ thallus forming units [TFU] g^?1 dry matter (DM)) were obtained using wheat straw but use of other carbon sources revealed large variation in the relative abundance of the morphotypes recovered in culture. Polycentric morphotypes were overall the most abundant fungi, comprising ca. 60 % of observations and recovered most frequently with xylan and wheat straw. Bulbous morphotypes showed a reciprocal pattern of occurrence, being most frequently observed on glucose, cellobiose and cellulose. Monocentric morphotypes were surprisingly the least abundant (<10 % overall), occurring mostly on glucose and wheat straw. Freezing of faeces (?20 °C/5 weeks) and thawing prior to enrichment culture reduced MPN counts by ca. 40 % from a mean of 1.8 × 10⁵ TFU g^?1 DM, but greater relative abundance of polycentric morphotypes in frozen–thawed faeces suggested differential survival in response to environmental stresses. PCR–RFLP demonstrated the simultaneous presence of seven ribotypes in one animal, but not all ribotypes could be associated with a particular genus.     

Distribution and ecology of tintinnids in the plankton of Lebanese coastal waters (eastern Mediterranean)

The tintinnids of Jounieh Bay in Lebanon were identified andtheir numerical abundance determined in horizontal and verticalnet samples from February 1979 to December 1980. Large fluctuationsin abundance and composition of the tintinnid fauna occurredduring the period of investigation. Two distinct peaks werenoted, a major one in May–June and a minor one in November–December.The first followed the main phytoplankton peak, suggesting apositive nutritional relationship between the two populations.The most abundant species were Tintinopsis beroidea and Eutintinnuslusus-undae. A few other species showed great abundance duringshorter periods. In all, 121 species were recorded during thisperiod, and of these 12 belong to the genus Tintinnopsis and11 to Eutintinnus.     

Population dynamics and production of the small copepod Oithona spp. in a subarctic fjord of West Greenland

The small cyclopoid copepod Oithona is widely occurring in polar areas; however, knowledge of its biology and ecology is very limited. Here, we investigate the population dynamics, vertical distribution, and reproductive characteristics of Oithona spp. from late winter to summer, in a subarctic fjord of West Greenland. During winter–early spring, the abundance of Oithona spp. was low (1.8 × 10³ ind. m^?2) and the population was mainly composed of late copepodites and adults, whereas in summer, abundance peaked and younger stages dominated (1.1 × 10⁶ ind. m^?2). In general, all stages of Oithona spp. remained in the upper 100 m, with nauplii exhibiting a shallower distribution. Although no general seasonal migration was found, a deeper distribution of the adult females in winter was observed. The mean clutch size of Oithona spp. varied from 16 to 30 eggs per female, peaking in summer. Egg production rates (EPR) were low in winter–early spring (0.13 ± 0.03 eggs female^?1 day^?1) and reached maximum values in summer (1.6 ± 0.45 eggs female^?1 day^?1). EPR of Oithona spp. showed a significantly positive relationship with both temperature and protozooplankton biomass, and the development of the population seemed to be appreciably affected by temperature. Oithona spp. remained active throughout the study, stressing the key importance of these small copepods in high-latitude ecosystems, especially in periods when larger copepods are not present in the surface layer.     

The distribution of picoplankton and nanoplankton in Kongsfjorden,Svalbard during late summer 2006

Abundance and biomass of pico- (<2 μm) and nanoplankton (2–20 μm) were investigated in relation to hydrography in Kongsfjorden, Svalbard (79°N, 12°E) during late summer 2006. Autotrophic and heterotrophic picoplankton abundance ranged from 0.1 × 10⁶ to 35.2 × 10⁶ cells L⁻¹ and from 0.4 × 10⁶ to 20.3 × 10⁶ cells L⁻¹, respectively. The highest number of bacteria in the entire water column was recorded at station 2 at 10 m (22.3 × 10⁸ cells L⁻¹); the lowest concentration was observed at station 1 (6.0 × 10⁸ cells L⁻¹). The abundance of autotrophic and heterotrophic nanoplankton varied from 0.4 × 10⁵ cells L⁻¹ to 46 × 10⁵ cells L⁻¹ and from 0.3 × 10⁶ to 9.1 × 10⁶ cells L⁻¹, respectively. Our results demonstrated that heterotrophic nanoflagellates and bacteria in Kongsfjorden microbial community were relatively important. The structure of plankton communities integrated with environmental variables could act as indicators of the variability of the inflow of Atlantic Water into Kongsfjorden.     

Behavioural differences in relation to pycnoclines during vertical migration of the euphausiids Meganyctiphanes norvegica (M. Sars) and Thysanoessa raschii (M. Sars)

An observation on differences in the vertical migration patternbetween Meganyctiphanes norvegica (M. Sars) and Thysanoessaraschii (M. Sars) caused by the presence of a thermocline isreported. Generally, M.norvegica did not migrate through a thermoclineat 50–60 m depth while T.raschii did. This observationis discussed in the context of how the combination of differencesin vertical migration and advection over sill depth may affectthe abundance of fjord stocks of these two euphausiid species.     

The significance of depth and substratum incline for the structure of a hard bottom sublittoral community in glacial Kongsfjorden (Svalbard, Arctic)—an underwater imagery approach

In order to reveal the structure of the sparsely known deeper sublittoral hard bottom communities of glacial Kongsfjorden, the macroepibenthos from six depth zones (30–200?m) was analysed. A total of 180 still images derived from 6-h video recorded at the Kongsfjordneset remotely operated vehicle station were assessed quantitatively. Overall 27 mainly suspension-feeding species/taxa were observed. Of these, two-thirds have an arcto-boreal distribution, while the remainder are cosmopolitan. The overall mean epibenthos abundance was 33 ind. m^?2 with maximum values at 150?m depth (97.9 ind. m^?2). The majority of the taxa inhabited the entire depth range. Encrusting red algae, an unidentified sponge and the sea anemone Urticina eques, characterized the assemblage of the shallow zone. The sea anemones Hormathia spp. were important below 30?m, the Serpulid polychaete Protula tubularia was characteristic for the community below 50?m and the demosponge Haliclona sp. was a key taxon between 100 and 200?m depth. Cluster analysis and non-metrical multidimensional scaling based on abundance data showed differences between the assemblages along the bathymetric gradient, but only in the shallower depths in relation to the substratum surface incline. As surface and tidal current impacts attenuate with increasing depth, there is a gradual trend from robust key species towards more fragile ones (i.e. P. tubularia), in line with the ‘Physical control hypothesis’.     

Sea ice microbial dynamics over an annual ice cycle in Prydz Bay, Antarctica

Microbial community dynamics within the fast sea ice of Prydz Bay (68°S?78°E) were investigated over an annual cycle at two sites (1 and 3?km offshore) between April and November 2008. There are few long-term sea ice studies, and few that cover the phase of winter darkness when autotrophic processes are curtailed. Mean chlorophyll a concentrations in the ice column ranged between 0.76 and 44.8?μg?L^?1 at the 1-km site (Site 1) and 3.11–144.6?μg?L^?1 at the 3-km site (Site 2). Highest chlorophyll a usually occurred at the base of the ice. Bacterial concentrations ranged between 0.30 and 2.08?×?10⁸?cells?L^?1, heterotrophic nanoflagellates (HNAN) between 0.21?×?10⁵ and 2.98?×?10⁵?cells?L^?1 and phototrophic nanoflagellates (PNAN) 0–1.06?×?10⁵?cells?L^?1. While HNAN occurred throughout the year, PNAN were largely absent in winter. Dinoflagellates were a conspicuous and occasionally an abundant element of the community (maximum 17,460?cells?L^?1), while ciliates were sparse. The bacterial community showed considerable morphological diversity with a dominance of filamentous forms. Bacterial production continued throughout the year ranging between 0 and 22.92?μg?C?L^?1?day^?1 throughout the ice column. Lowest rates occurred between late June and early August. The sea ice sustained an active and diverse microbial community through its annual extent. The data suggest that during winter darkness the microbial community is dominated by heterotrophic processes, sustained by a pool of dissolved organic carbon.     

Microplankton Patchiness in the Northwest Altantic Ocean

Plankton samples were collected in the Northwest Atlantic on the Grand Banks of Newfoundland in August 1976 and on Flemish Cap in March 1977. The samples were taken using a Hardy Continuous Plankton Recorder towed at a depth of 9.1 m. On the Grand Banks the following species were found: Ceratium arcticum (Ehrenberg), C. longipes (Ehrenberg), C. lineatum (Ehrenberg), C. macroceros (Ehrenberg), C. fusus (Ehrenberg). C. tripos (Müller) and Ceratium spp. Other microplankton found in the samples included Radiolaria, Globigerina humilis (Brady), and the tintinnid ciliates Tintinnopsis cylindrica (Daday) and Tintinnopsis spp. In the samples from Flemish Cap only a single species of diatom was found in small numbers. The paucity of individuals in these samples is most probably related to the season of the survey, winter. The population density of C. arcticum had a patchiness with a length scale on the order of 10⁴ m. The plankton tow on the Grand Banks was parallel to the north-south axis of the Labrador Current. This patchiness scale on the order of 10⁴ m may reflect a meandering of the Labrador Current.     

E-mail: J.Laybourn-Parry@nottingham.ac.ukAnnual plankton dynamics in an Antarctic saline lake

1. The plankton dynamics of Ace Lake, a saline, meromictic basin in the Vestfold Hills, eastern Antarctica was studied between December 1995 and February 1997. 2. The lake supported two distinct plankton communities; an aerobic microbial community in the upper oxygenated mixolimnion and an anaerobic microbial community in the lower anoxic monimolimnion. 3. Phytoplankton development was limited by nitrogen availability. Soluble reactive phosphorus was never limiting. Chlorophyll a concentrations in the mixolimnion ranged between 0.3 and 4.4 μg L^??1 during the study period and a deep chlorophyll maximum persisted throughout the year below the chemo/oxycline. 4. Bacterioplankton abundance showed considerable seasonal variation related to light and substrate availability. Autotrophic bacterial abundance ranged between 0.02 and 8.94 × 10⁸ L^??1 and heterotrophic bacterial abundance between 1.26 and 72.8 × 10⁸ L^??1 throughout the water column. 5. The mixolimnion phytoplankton was dominated by phytoflagellates, in particular Pyramimonas gelidicola. P. gelidicola remained active for most of the year by virtue of its mixotrophic behaviour. Photosynthetic dinoflagellates occurred during the austral summer, but the entire population encysted for the winter. 6. Two communities of heterotrophic flagellates were apparent; a community living in the upper monimolimnion and a community living in the aerobic mixolimnion. Both exhibited different seasonal dynamics. 7. The ciliate community was dominated by the autotroph Mesodinium rubrum. The abundance of M. rubrum peaked in summer. A proportion of the population encysted during winter. Only one other ciliate, Euplotes sp., occurred regularly. 8. Two species of Metazoa occurred in the mixolimnion; a calanoid copepod (Paralabidocera antarctica) and a rotifer (Notholca sp.). However, there was no evidence of grazing pressure on the microbial community. In common with most other Antarctic lakes, Ace Lake appears to be driven by ‘bottom-up’ forces.