Distribution of chlorophyll a and Gymnodinium catenatum associated with coastal upwelling plumes off central Portugal

The composition and the distribution patterns of phytoplankton were studied during the early stages of an upwelling event in Lisbon Bay, in September 1999. The distribution of chlorophyll a and phytoplankton assemblages was asymmetric about the upwelling centre, with the highest abundance of chlorophyll a observed at the inner (coastal) side of the plume. The diatom Cylindrotheca closterium dominated in the upwelling core and the chain forming diatom Proboscia alata dominated at the outer side of the feature within mature oceanic waters. Chain forming dinoflagellates such as Alexandrium affine and the toxin producer Gymnodinium catenatum were most numerous at the inshore side of the upwelling front. These patterns were compared with the distribution of G. catenatum observed in the same area in 1985 and 1994, under conditions of fully developed upwelling when chlorophyll a and G. catenatum maxima were observed extending southwards on the inshore side of an upwelling plume. Different hydrodynamic conditions at each side of upwelling plumes associated with pronounced Capes are evoked to explain the asymmetry on the phytoplankton patterns.     

Plankton distribution across a slope current-induced front in the southern Bay of Biscay

Relatively warm (12.50–12.75°C) and high-salinity[<35.640 practical salinity units (PSU)] water flowing eastwardwas detected at the shelf-break during a cruise carried Outin the southern Bay of Biscay in Spring 1987. The slope currentinduced the formation of a convergent front separating well-mixedoceanic waters from haline-stratified coastal waters. Very highconcentrations of dissolved oxygen (295 µmol kg^–1)and chlorophyll a(>4.5 mg m^–3) were found at the outeredge of the frontal boundary. Small autotrophic flagellatesdominated the phytoplankton community. Primary production peakedat the boundary region. Estimated phytoplankton growth ratesindicated that active growth was taking place, with lower turnovertimes integrated over the water column at the frontal station(2.5–5 days) than at coastal (1.5–2.8 days) or oceanic(1.5–3.5 days) stations. The lowest doubling times (1–2days) were calculated for surface frontal populations. Accumulationof zooplankton was also observed associated with the convergentphysical structure, although this relationship was less markedthan for phytoplankton. Copepods, mainly Paracalanus parvus,Acartia clausi and Oithona helgolandica, formed the bulk ofthe mesozooplankton biomass. Compatibility between the sizeof phytoplankton cells and copepod size spectra indicate highfood availability for these animals, particularly in the vicinityof the front. The distribution of fish eggs and fish larvaewas also coupled with the slope current-induced front. Sardinelarvae were more abundant at the coastal side of the front,whereas larval stages of blue whiting reached the highest densitiesat off-shelf stations. Larvae of lamellibranch molluscs andbryozoa were restricted to nearshore waters, as the frontalboundary prevented larval dispersion to the open ocean. Theresults presented in this paper suggest that the Iberian slopecurrent and its associated shelf-break frontal structure werecrucial in controlling phytoplankton primary production, activityof grazers, distribution of larvae of fishes and benthic invertebrates,and ultimately in determining the structure of the pelagic foodweb in the southern Bay of Biscay during the seasonal periodof vertical mixing.     

THE EFFECTS OF IRON AND COPPER AVAILABILITY ON THE COPPER STOICHIOMETRY OF MARINE PHYTOPLANKTON1

We studied the interactive effects of iron (Fe) and copper (Cu) availability on the growth rates, Cu quotas, and steady‐state Cu‐uptake rates (ρ_ssCu) of 12 phytoplankton (from four classes and two marine environments). A mixed‐effect statistical model indicated that low Fe significantly decreased phytoplankton growth rates. In contrast, lowering Cu levels only decreased the growth rates of the oceanic phytoplankton. Under Fe/Cu sufficiency, the Cu quotas ranged from 0.36 to 3.8 μmol Cu · mol^?1 C. Copper levels in the growth medium had a significant positive effect on the Cu quotas, and this effect was dependent on the algal class. Under Fe/Cu sufficiency, the highest average Cu quotas were observed for the Bacillariophyceae, followed by the Cyanophyceae, Prymnesiophyceae, and lastly the Dinophyceae. Similar taxonomic trends were observed for the ρ_ssCu. Although the Cu:C ratios were not significantly higher in oceanic strains, there are five independent lines of evidence supporting a more important role of Cu in the physiology of the oceanic phytoplankton. The mixed‐effect model indicated a significant Cu effect on the growth rates and ρ_ssCu of the oceanic strains, but not the coastal strains. In addition, lowering the Cu concentration in the media decreased the Cu quotas and ρ_ssCu of the oceanic strains to a greater extent (5.5‐ and 5.4‐fold, respectively) than those of the coastals (3.8‐ and 4.7‐fold, respectively). Iron limitation only had a significant effect on the Cu quotas of the oceanic strains, and this effect was dependent on Cu level and taxonomic class. Our results highlight a complex physiological interaction between Fe and Cu in marine phytoplankton.     

Phytoplankton community structure derived from HPLC analysis of pigments in the Faroe-Shetland Channel during summer 1999: the distribution of taxonomic groups in relation to physical/chemical conditions in the photic zone

A study of the phytoplankton community in the Faroe-ShetlandChannel was conducted in July 1999. Samples were collected atvarious depths in the photic zone along three transects (thenorthern entrance, the center and the southern entrance). Exceptfor a few easterly stations where nitrate and silicate werebelow 1 µM, all nutrients (phosphate, silicate, ammonium,nitrite and nitrate) were non-limiting for phytoplankton growth.HPLC pigment analysis revealed a pronounced (>50%) dominanceof Prymnesiophyceae at all stations. Their pigment ratio ofdiatoxanthin + diadinoxanthin/Chl a (DDX/Chl a) indicated thatthe phytoplankton community was controlled by light. Primaryproduction in the delayed spring bloom varied from 1.2 to 1.8g C m^–2 day^–1 along the northern transect. Alongthe other two transects, primary production ranged from 1.6to 3.8 g C m^–2 day^–1. Associated with the characteristicsindicating the establishment of a bloom, the relative contributionof diatoms and Prymnesiophyceae increased, whereas that of Prasinophyceae,Cryptophyceae, Chrysophyceae and Cyanobacteriaceae decreased.With respect to their vertical distribution, Cyanobacteriaceae,Chrysophyceae and Dinophyceae tended to have a higher abundance,relative to other taxonomic groups, in the surface layers. Therelative abundance of diatoms and Chlorophyceae increased withdepth. The DDX/Chl a ratio of the Prymnesiophyceae decreasedwith depth, indicating that vertical mixing in the upper 30m of the photic zone occurred less frequently than the timespan of physiological acclimation of cellular pigment composition.     

Mesozooplankton Distribution and Copepod Grazing in the Subtropical Atlantic Near the Azores: Influence of Mesoscale Structures

Mesozooplankton distribution and copepod grazing were investigatedin the Subtropical Atlantic Ocean near the Azores during theAZORES I (August) and II (April–May) cruises. Mesozooplanktonbiomass and abundance remained low throughout the region, butsignificant increases were found related to the presence ofthe Azores Front. The Azores Front also exhibited maximum valuesof copepod community ingestion, reaching 250 mg C m^–2ingested daily. This increase in ingestion was related to increasesin copepod abundance, but not in copepod gut contents. No relationshipwas found between gut contents, or ingestion, and phytoplanktonbiomass or production. Daily cycles were found in copepod gutcontents, being higher during the night, but not in copepodabundance. Multi-dimensional scaling analysis revealed differencesin copepod taxonomic composition between both sides of the front.During spring, daily copepod ingestion represents an averageof 6% of the integrated chlorophyll (Chl) a concentration and22% of the primary production. These percentages increase to15% of Chl and 61% of production if we only consider large (>2µm) phytoplankton. No clear influence of the cycloniceddy LETICIA was found in mesozooplankton biomass or grazing.A significant effect of the Great Meteor Tablemount was foundin copepod abundance and grazing, with higher values locatedwest of the mount.     

Formation and retention mechanisms of phytoplankton peak abundance in the Kuroshio front

Observations on phytoplankton and environmental factors weremade across the Kuroshio front. Phytoplankton standing stockwas high just north of the temperature front, and the widthof the peak abundance was <10 km. By analyses of speciescomposition and water masses, the entrainment of coastal waterand/or the Oyashio water was considered to be an important factorfor the formation of the phytoplankton peak abundance. The comparativelyhigh contribution of the microplankton fraction >10 µmin the peak abundance also supported the importance of entrainmentprocess. The incubation experiments of natural phytoplanktonassemblages with nutrient addition showed that the microplanktonfraction has higher growth potential at the front than in theneighboring waters. Dome-like risings of isopleths of temperatureand nutrients were demonstrated in the cross sectional observationof the front Possible upwelling accompanied with nutrient supplyseemed to contribute to the retention of the phvtoptankton peakabundance at the front.     

Factors influencing the short-term variation in phytoplankton composition and biomass in coral reef waters

The short-term temporal dynamics of phytoplankton composition was compared among coral reef waters, the adjacent ocean and polluted harbour water from July until October along the south-western coast of Curaçao, southern Caribbean. Temporal variations in phytoplankton pigment 'fingerprints' (zeaxanthin, chlorophyll b, 19'-hexanoyloxyfucoxanthin, fucoxanthin, 19'-butanoyloxyfucoxanthin, chlorophyll c₂ and c₃ relative to chlorophyll a) in the ocean were also observed in waters overlying the reef. However, with respect to specific pigments and algal-size distribution, the algal composition in reef waters was usually slightly different from that in the oceanic water. Phytoplankton biomass (chlorophyll a) was either higher or lower than in the oceanic water. The relative amount of fucoxanthin and peridinin was usually higher, and the relative and absolute amount of zeaxanthin was significantly lower than in oceanic water. Zeaxanthin-containing Synechococci were significantly reduced in reef water. Average algal cell size increased from the open water to the reef and the harbour entrance. Large centric diatoms (>20 m Ø) were better represented in reef than in oceanic water. In reef-overlying waters, the nitrate and nitrite concentrations were higher than in oceanic water. In front of the town, anthropogenic eutrophication (sewage discharge and ground water seepage) resulted in higher NH₄, NO₃ and PO₄ concentrations than at other reef stations. This concurred with significantly enhanced phytoplankton biomass (chlorophyll a), chlorophyll c₂ and peridinin amounts at Town Reef compared with the other reef stations. Polluted harbour water usually showed the highest phytoplankton biomass of all stations, dominated by diatoms and dinoflagellates. Conditions in reef waters and harbour water promoted the occurrence and the relative abundance of diatoms and dinoflagellates. Harbour water did not influence the phytoplankton composition and biomass at reef stations situated >5 km away from the harbour entrance. We conclude that phytoplankton undergoes a shift in algal composition during transit over the reef. The dominant processes appear to be selective removal of zeaxanthin-containing Synechococcus (by the reef benthos) and (relative) increase in diatoms and dinoflagellates. The difference in the phytoplankton composition between reef and oceanic waters tends to increase with decreasing dilution of reef water with ocean water.     

Seasonal and fine-scale spatial variations in egg production and triacylglycerol content of the copepod Acartia tonsa in a river-dominated estuary and its coastal plume

We measured egg production rates of the estuarine calanoid copepodAcartia tonsa in Mobile Bay, an estuary in the northern Gulfof Mexico. Two stations were sampled approximately monthly,one at the mouth of the bay and the other just beyond the mouthin the salinity front between bay and coastal waters. Over thewhole year, temperature was the most important environmentalvariable controlling egg production. Rates increased with temperatureup to 30°C and 140 eggs female^–1 day^–1. We foundno evidence of food limitation. There was no correlation betweenegg production and phytoplankton abundance, nor increased eggproduction in response to supplements of phytoplankton addedto natural food, suggesting that non-phytoplankton food wasimportant in the diet. At the highest egg production rates,the amount of the storage lipid triacylglycerol (TAG) in adultfemales was greatly diminished, to <50 ng female^–1.This suggests that lipids in the diet can be very tightly coupledto egg production. Both egg production and TAG content of femalesshowed significant variability on spatial scales of 3–15km, especially in relation to the salinity front separat ingwater outwelling from the bay from open coastal water. For organismsthat are using copepods and their eggs as food, this variabilitywould result in a heterogeneous food environment, both in termsof the abundance of food and its nutritional content.     

Benthic food web structure in the southeastern Chukchi Sea: an assessment using ��13C and ��15N analyses

The benthos of the southeastern Chukchi Sea shelf is typified by high faunal abundance and biomass resulting from settlement of a large proportion of seasonal phytoplankton under highly nutritious offshore Bering Shelf Anadyr Water (BSAW). In contrast, inshore Alaska Coastal Water (ACW) is much less productive. Yet the Chukchi Bight and Kotzebue Sound, located under ACW in the southeastern Chukchi Sea, contain a substantial faunal abundance and biomass of invertebrates, fishes and marine mammals. We examined food web structure to gain an understanding of how a relatively rich benthic fauna with a high biomass can be supported under ACW with a supposedly low flux of carbon to the benthos. We measured stable isotope (δ¹³C and δ¹⁵N) values of selected organisms (from zooplankton to fishes) as markers of food sources and trophic position to compare fauna on the shelf under BSAW with that in the Chukchi Bight and Kotzebue Sound under ACW. Relative isotope position of organisms in all three regions was similar, even though some pelagic species within the Sound were depleted in δ¹³C compared to the other regions. We attribute the depletion to the influence of terrestrially derived carbon. We suggest that the hydrodynamics along an oceanic front between the Chukchi Shelf and the Chukchi Bight support the advection of nutrient-rich POC into the Bight and Sound as additional food sources to local production. We conclude that local conditions and multiple POC sources in the Bight and Sound support the substantial population of benthic invertebrates and the fishes, seabirds, and marine mammals that feed on them.     

Phytoplankton communities and biomass size structure (fractionated chlorophyll “a”), along trophic gradients of the Basque coast (northern Spain)

Eutrophication is a major threat to coastal ecosystems. Within Europe, the Water Framework Directive (WFD) has established the need of developing methods of assessment. Bulk chlorophyll “a” is used world-wide as an indicator of eutrophication. However, the size structure of the phytoplankton communities has not been investigated, in detail, in relation to eutrophication pressures. This study investigates the suitability of fractionated chlorophyll “a” (<10 and >10 μm), as an indicator of eutrophication. Along the Basque coast (northern Spain) several water bodies, ranging from offshore waters to the heads of the estuaries, were surveyed during 2008. Physico-chemical conditions and chlorophyll “a” showed a distinct spatial gradient. Trophic richness increased towards the middle and inner parts of the estuaries, where nutrient inputs, from natural or anthropogenic sources, together with the residence time of the water are usually higher. In summer, phytoplankton biomass and abundance decreased, generally, in offshore and coastal waters; in estuaries, they tended to increase. The chlorophyll distribution within the two size fractions was coherent with the phytoplankton taxonomic composition. In summer, the relative abundance of small-sized cells (diatoms and non-siliceous taxa) increased. The relationships between size-fractionated chlorophyll and physico-chemical variables differed, when comparing the offshore and coastal waters, with the estuaries. In the offshore and coastal waters, a strong seasonality was observed; in estuaries, an important spatial component was found. Fractionated chlorophyll provided complementary useful information on anthropogenic pressures. However, more research is necessary to utilise this variable as a tool for ecological status assessment, within the WFD.     

Warming alters the size spectrum and shifts the distribution of biomass in freshwater ecosystems

Organism size is one of the key determinants of community structure, and its relationship with abundance can describe how biomass is partitioned among the biota within an ecosystem. An outdoor freshwater mesocosm experiment was used to determine how warming of～4 °C would affect the size, biomass and taxonomic structure of planktonic communities. Warming increased the steepness of the community size spectrum by increasing the prevalence of small organisms, primarily within the phytoplankton assemblage and it also reduced the mean and maximum size of phytoplankton by approximately one order of magnitude. The observed shifts in phytoplankton size structure were reflected in changes in phytoplankton community composition, though zooplankton taxonomic composition was unaffected by warming. Furthermore, warming reduced community biomass and total phytoplankton biomass, although zooplankton biomass was unaffected. This resulted in an increase in the zooplankton to phytoplankton biomass ratio in the warmed mesocosms, which could be explained by faster turnover within the phytoplankton assemblages. Overall, warming shifted the distribution of phytoplankton size towards smaller individuals with rapid turnover and low standing biomass, resulting in a reorganization of the biomass structure of the food webs. These results indicate future environmental warming may have profound effects on the structure and functioning of aquatic communities and ecosystems.     

Bacterial and phytoplankton responses to nutrient amendments in a boreal lake differ according to season and to taxonomic resolution

Nutrient limitation and resource competition in bacterial and phytoplankton communities may appear different when considering different levels of taxonomic resolution. Nutrient amendment experiments conducted in a boreal lake on three occasions during one open water season revealed complex responses in overall bacterioplankton and phytoplankton abundance and biovolume. In general, bacteria were dominant in spring, while phytoplankton was clearly the predominant group in autumn. Seasonal differences in the community composition of bacteria and phytoplankton were mainly related to changes in observed taxa, while the differences across nutrient treatments within an experiment were due to changes in relative contributions of certain higher- and lower-level phylogenetic groups. Of the main bacterioplankton phyla, only Actinobacteria had a treatment response that was visible even at the phylum level throughout the season. With increasing resolution (from 75 to 99% sequence similarity) major responses to nutrient amendments appeared using 454 pyrosequencing data of 16S rRNA amplicons. This further revealed that OTUs (defined by 97% sequence similarity) annotated to the same highly resolved freshwater groups appeared to occur during different seasons and were showing treatment-dependent differentiation, indicating that OTUs within these groups were not ecologically coherent. Similarly, phytoplankton species from the same genera responded differently to nutrient amendments even though biovolumes of the majority of taxa increased when both nitrogen and phosphorus were added simultaneously. The bacterioplankton and phytoplankton community compositions showed concurrent trajectories that could be seen in synchronous succession patterns over the season. Overall, our data revealed that the response of both communities to nutrient changes was highly dependent on season and that contradictory results may be obtained when using different taxonomic resolutions.     

Monterey Bay phytoplankton I. Seasonal cycles of phytoplankton assemblages

Netplankton collections from shallow-water stations in MontereyBay, California were examined for 1976 and 1977. The predominatingphytoplankton forms were neritic diatom species. Recurrent speciesgroups were identified using Fager's regroup analysis and theirdistributions related to hydrographic seasons. Neritic groupswere relatively more abundant during the Davidson Current andupweiling periods; however, many of the species were presentthroughout much of the year. The appearance of oceanic groupsand species was seasonal and associated with advective events. One neritic species group was of overwhelming numerical importancebut within the group conspicuous changes in relative abundanceof phytoplankton populations occurred seasonally among speciesfrom this group. This group appears to be composed of residentspecies and may be locally retained as vegetative cells or benthicresting stages. Species cycles in Monterey Bay may be regardedas largely the result of suc-cessional changes or cycles ofautochthonous species, while species introduced by adjectiveprocesses appear to be of minor importance. The study has suggested that the presence of species in a ‘seedpopulation’, either as vegetative cells or benthic restingstages, may be important as the growth rate of the species inunderstanding and predicting composition and relative abundancein coastal phytoplankton assemblages.     

Scaling-up from nutrient physiology to the size-structure of phytoplankton communities

In many community assemblages, the abundance of organisms isa power-law function of organism size. In phytoplankton communities,changes in size structure associated with increases in resourceavailability and total biomass have often been interpreted asa release from grazer control. A metapopulation-like approachis used to scale up from the individual physiological responsesto environmental conditions to community size structure assumingthe community taxonomic composition reflects the species pool.We show that the size scaling of cellular nutrient requirementsand growth can cause (1) the power-law relationship betweencell size and abundance, (2) dominance of small phytoplanktoncells under oligotrophic conditions, and (3) relative increasein abundance of larger phytoplankton cells under eutrophic conditions.If physiological differences associated with the taxonomic compositionof different community size fractions are considered, then themodel can replicate detailed field observations such as theabsence of small, slow-growing Prochlorococcus spp. and therelative dominance of large diatom species in nutrient-rich,upwelling regions of the ocean. This paper was presented in a session on "Size Structure ofPlankton Communities", at the ASLO Summer International Meeting,held in Santiago de Compostela, Spain, between 19 and 24 June,and coordinated by Xabier Irigoien, Roger Harris and Angel Lopez-Urrutia.     

PHYTOPLANKTON COMMUNITY STRUCTURE: TEMPORAL VARIABILITY IN A TROPICAL UPWELLING ECOSYSTEM

The Cariaco Basin (Southeastern Caribbean Sea) is the largest anoxic basin of oceanic character. Its surface waters are affected by coastal upwelling during boreal winter and spring. Historical information on the phytoplankton communities in the basin is scarce. In November 1995, an oceanographic time-series station was established at 10.5°N - 64.66°W. In this study changes in the structure of the phytoplankton community in the upper 100-m layer were studied. Monthly samples to determine phytoplankton abundance and pigment composition (HPLC) were collected from November 1995 to January 1999; water temperature was used as proxy for upwelling. Surface waters reached temperatures ≤24° C during the upwelling season, and surpassed 26° C during the rest of the year. A total of 300 species were found. Generally, the highest number of cells (>500 cells ml⁻¹) were measured from January to April every year. Diatoms were the dominant group in terms of abundance, species composition (168 species) and pigments (Chl c and fucoxanthin) during this period. The abundance and marker pigment concentration (peridinin, but- and hex-fucoxanthin) for other groups support this observation. During the rest of the year diatom and total abundance decreased markedly (<100 cells ml⁻¹). Small organisms (<5 μm) became dominant. The maximum concentration values of zeaxanthin and But- and Hex-Fuco in this period indicated the presence of cyanobacteria and prymnesiophytes. The sparse and relatively low concentration of Chl b , lutein and prasinoxanthin indicated that chlorophytes and its allies are a minor floral component.     

Phytoplankton of the Middle Paraná River During an Anomalous Hydrological Period: A Morphological and Functional Approach

The Paraná River mainstem was sampled monthly (November 1999–December 2000) in order to analyse phytoplankton structure and dynamics during a period of extreme low water discharge. Physiognomic groups, size fractions, taxonomic groups, species and Reynolds et al. [2002, Journal of Plankton Research, 24: 417–428] functional groups were studied in relation to environmental variables. The phytoplankton comprised an unusual development of small unicellular flagellates (1–20 mm), whereas large size algae such as the commonly recorded Aulacoseira were present with very low abundances. Groups Y (Cryptomonas spp.), X2 (Chlamydomonas spp., Rhodomonas minuta followed by a silicean chrysophyte) and D (Skeletonema cf. subsalsum and small centrics in minor proportion) dominated the phytoplankton community. Groups X2 and Y dominated at drought phase (November–December 1999). During the coupling of increasing waters and high-suspended solids (February–May 2000) minimum abundance was exhibited. The highest densities were registered during mid-waters (Y, D, X2) when a constant river-floodplain feedback occurred (September–December). The connectivity between the main channel and its alluvial valley, and their interaction resulting from the regular alternation of hydrological fluctuations, play a steering role in structuring the potamoplankton of large rivers. The multivariate RDA showed that water discharge and transparency highly explained a significant proportion of the functional groups variation. The functional classification is proposed as a useful approach in reconstructing the phytoplankton community. The differences in structure from the typically described potamoplankton for the Paraná were maintained throughout the study period. The perseverance of the community changes that occurred during the drought depends, in part, on the persistence and magnitude of this anomalous phase.     

Inter-annual variation in summer zooplankton community structure in Prydz Bay, Antarctica, from 1999 to 2006

Inter-annual variations in zooplankton community structure in Prydz Bay were investigated using multivariate analysis based on samples collected with a 330-μm mesh, 0.5-m² Norpac net during the austral summer from 1999 to 2006. Two distinct communities, an oceanic and a neritic community, were consistently identified in all surveys. Oceanic communities had higher diversity and were indicated by species such as Haloptilus ocellatus, Heterorhabdus austrinus, Thysanoessa macrura, Rhincalanus gigas, Scolecithricella minor and Oikopleura sp.. Neritic communities were indicated by Euphausia crystallorophias and Stephos longipes and were characterized by fewer but more abundant species. In 1999 and 2006, a transitional community was also distinguished near the continental shelf edge, where ice coverage was more extensive than either the oceanic or neritic regions. Significant inter-annual variations in community structure (mainly involving species abundance rather than species composition) were found in both oceanic and neritic communities, being more obvious in the latter. The timing and amplitude of sea ice retreat (polynya appearance), and its effect on food availability, had strong influences on zooplankton community structure. In oceanic communities during years with earlier ice retreat, the extra time available for phytoplankton blooms to accumulate resulted in a higher proportion of large copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei) (especially the younger copepodites) in the zooplankton assemblage. In neritic communities, zooplankton such as the ice krill E. crystallorophias, and large copepods (C. acutus, C. propinquus, M. gerlachei), also showed higher abundance and earlier developmental stages in years with larger polynya. On the other hand, in years with later ice retreat, smaller polynya, and less time for phytoplankton blooms to form, the abundance of large copepods was lower and older age classes were more common.     

Structure and grazing impact of the mesozooplankton community during late summer 1994 near South Georgia, Antarctica

Mesozooplankton abundance, community structure and grazing impact were determined during late austral summer (February/March) 1994 at eight oceanic stations near South Georgia using samples collected with a Bongo and WP-2 nets in the upper 200-m and 100-m layer, respectively. The zooplankton abundance was generally dominated by copepodite stages C3–C5 of six copepod species: Rhincalanus gigas, Calanus simillimus, Calanoides acutus, Metridia spp., Clausocalanus laticeps and Ctenocalanus vanus. Most copepods had large lipid sacs. All copepods accounted for 41–98% of total zooplankton abundance. Juvenile euphausiids were the second most important component contributing between 1 and 20% of total abundance. Pteropods, mainly Limacina inflata, were important members of the pelagic community at two sites, accounting for 44 and 53% of total abundance. Average mesozooplankton biomass in the upper 200 m was 8.0 g dry weight m⁻², ranging from 4.3 to 11.5 g dry weight m⁻². With the exception of Calanussimillimus, gut pigment contents and feeding activity of copepod species were low, suggesting that some species, after having stored large lipid reserves, had probably started undergoing developmental arrest. Daily mesozooplankton grazing impact, measured using in situ gut fluorescence techniques and in vitro incubations, varied widely from <1 to 8% (mean 3.5%) of phytoplankton standing stock, and from 5 to 102% (mean 36%) of primary production. The highest grazing impact was found northeast of the island co-incident with the lowest phytoplankton biomass and primary production levels. Received: 30 October 1996 / Accepted: 23 February 1997     

Increased frequency of dividing cells of a phototrophic species of Cryptophyceae at a frontal structure off the Antarctic Peninsula

The hypothesis that enhanced phytoplankton growth explainedthe increase in phytc-plankton biomass observed at a frontalstructure off the Antarctic Peninsula during the summer of 1993was examined by analysing the phytoplankton cells undergoingmitosis. The frequency of dividing cells (FDC) of an unidentifiedphototrophic species of Cryptophyceae, which dominated phytoplanktonbiomass at the front, varied between 1.2 and 31.6%, with higherpercentages associated with the frontal structure. FDC valuesincreased as phytoplankton biomass increased, suggesting thatactive growth, rather than passive accumulation, was responsiblefor the enhanced phytoplankton biomass observed. The low temperatures(mean ± SE = 1.11 ± 0.52°C) that characterizedthe Antarctic waters sampled imposed an upper limit to the maximumFDC reached by the Cryptophyceae, but the relationship betweenFDC and temperature suggests a clear response of the maximalgrowth rate of this species to small changes in temperature.The stabilization of the water column, resulting in higher lightavailability and heating of the surface water in this frontalarea, appeared to promote the growth of the phototrophic speciesof Cryptophyceae and emphasizes the importance of mesoscaleprocesses as determinants of phytoplankton growth dynamics.