Reproductive cycles of three dominant Calanus species in Disko Bay, West Greenland

This study describes the annual reproductive cycles of the three dominant Calanus species, C. finmarchicus, C. glacialis and C. hyperboreus, in Disko Bay (West Greenland) in relation to seasonal phytoplankton development. Relative abundance of females, copepodite stage V (CV) and males, and the developmental stage of the female gonad were examined from plankton samples collected at weekly to monthly intervals from May 1996 to June 1997 with a WP2 net or a pump. During spring and summer, egg production rates were determined. Females of all three species were present year round. Maximum relative abundance was reached by C. hyperboreus females at the beginning of February, by C. glacialis in mid-February, and by C. finmarchicus in April. All three species reproduced successfully in Disko Bay. Their reproductive cycles were considerably different with respect to the timing of final gonad maturation and spawning, and hence in their relation to seasonal phytoplankton development. In all three species, early gonad development took place during winter, before living food became plentiful, suggesting that these processes were largely food independent. Final gonad maturation and spawning in C. finmarchicus was related to the phytoplankton concentration, reflecting that final gonad maturation processes are food dependent in this species. C. glacialis females matured and spawned prior to the spring bloom. Our results indicate that first internal lipid stores and later ice alga grazing supplied final gonad maturation and egg production. Maximum egg production rates of C. glacialis were found in spring and summer, when the chlorophyll a concentration was high. Mature female C. hyperboreus were found from February until mid-April, when the chlorophyll a concentration was still low. In this species, reproductive activity was decoupled from phytoplankton development, and final maturation processes and spawning were solely fuelled by internal energy stores.     

Large-scale oceanic distribution and population structure of Calanus finmarchicus, in relation to physical environment, food and predators

The investigation was carried out from 62°N to 73°N and from 14°E to 11°W in the Norwegian Sea during 19 June-12 July 1997. Regional differences in the phase of the seasonal development of the plankton community were evident, most pronounced across the Arctic front. In the Coastal and eastern Atlantic domains, post-bloom conditions prevailed, characterised by low chlorophyll a (chl a) levels and a phytoplankton assemblage dominated by coccolithophorids and small flagellates. During the study period, egg production rates of Calanus finmarchicus were low (<10 eggs female^-1 day^-1), older copepodite stages dominated, and the seasonal descent to deeper waters had started. In the Arctic domain, bloom conditions were evident by high chl a levels and a high abundance of large diatoms. Egg production rates were higher (a maximum of 29 eggs female^-1 day^-1), but the dominance of stages CI-CIII indicated that considerable spawning had already occurred prior to the spring bloom. The seasonal descent had barely started. Both invertebrate and fish predators were most abundant in the Coastal and eastern Atlantic domains, with abundance strongly decreasing north-westwards. No tight relationship between total abundance of invertebrate or fish predators and that of C. finmarchicus was apparent. However, a weak, but significant, relationship between abundance of young stages of chaetognaths and Euchaeta spp. versus young stages of C. finmarchicus was found, indicating that these invertebrate predators develop parallel to the development of the new cohort of C. finmarchicus. In early summer, C. finmarchicus had reached overwintering stages, and had started to accumulate in deeper waters in areas with the highest abundance of horizontally migratory planktivorous fish.     

Mesozooplankton community structure, abundance and biomass in the central Arctic Ocean

During the "International Arctic Ocean Expedition 1991" (20 August-21 September 1991) mesozooplankton was sampled at six stations in the Nansen, Amundsen and Makarov Basins of the central Arctic Ocean from 1,500 m depth to the surface by multiple opening/closing net hauls. Total mesozooplankton abundance decreased from 268 ind. m^-3 in the surface layer (0-50 m) to <25 ind. m^-3 below 200 m depth. The small copepods Oithona similis and Microcalanus pygmaeus, as well as copepod nauplii, were most abundant close to the surface, while Oncaea borealis and Spinocalanus spp. frequently occurred at greater depth. Mesozooplankton dry mass (DM) integrated over the upper 1,500 m of the water column was surprisingly stable throughout the investigation area and measured 2.0ǂ.3 g DM m^-2. Dry mass in the upper 50 m measured 20.9 mg m^-3 and was dominated by Calanus hyperboreus (57.4%) and C. glacialis (21.1%). C. finmarchicus was very abundant only in the Nansen Basin. Below 200 m the calanoid copepods Metridia longa, Microcalanus pygmaeus and Pareuchaeta spp., the decapod Hymenodora glacialis and chaetognaths of the genus Eukrohnia were the principal contributors to biomass values of <1 mg DM m^-3. Hence, vertical changes in abundance, biomass and species composition were much more pronounced than regional differences between the basins. Three different mesozooplankton communities were differentiated according to their faunistic composition and are discussed in context with the major water masses: Polar Surface Water, Atlantic Layer and Arctic Deep Water.     

Egg production and feeding by copepods prior to the spring bloom of phytoplankton in Fram Strait,Greenland Sea

Reproductively activeCalanus hyperboreus (Krøyer) andC. glacialis Jaschnov were captured in the upper 100 m of Fram Strait (77° to 79°N) in late winter 1987. There was no evidence of a phytoplankton bloom; chlorophylla concentrations were uniformly low (<0.1 mg m^–3), and nitrate concentrations were uniformly high (>11.3 mg-at m^–3). Gut-fullness measurements indicated that females were ingesting very little. The maturation state of gonads of bothC. hyperboreus andC. glacialis indicated that 75% of females were in a ripe condition consistent with observed egg laying. The lipid content of females laying eggs was reduced in both species compared to that of females not laying eggs. InC. hyperboreus the reduction was 39% and inC. glacialis it was 44%. All the evidence suggests that bothC. hyperboreus andC. glacialis were laying eggs in late winter by using lipids stored previously; they were not relying on ambient concentrations of phytoplankton. The daily rate of egg laying byC. glacialis using lipids in late winter exceeded the rate reported for summer when ambient food supplies have been shown to be necessary. It is suggested that individuals, spawned well in advance of the spring bloom of phytoplankton, may comprise a major portion of the annual recruitment to the entire population ofC. glacialis in this area, and that their life cycle can be completed within 1 yr. NeitherMetridia longa (Lubbock) norC. finmarchicus (Gunnerus) laid eggs during this study.     

Evaluation of fatty acids as biomarkers for a natural plankton community. A field study of a spring bloom and a post-bloom period off West Greenland

An investigation of the fatty acid composition of a natural arctic plankton community was carried out over two fishing banks located between 63°N and 65°N off the West Greenland coast. Samples for fatty acid analyses, species determination and biomass assessments of the plankton community were taken at the depth of fluorescence maximum. High biomass and diatom dominance during the spring bloom and low biomass and flagellate dominance in the post-bloom period were reflected by the fatty acid profiles. The total amount of fatty acid ranged from 55 to 132 µg l^-1 during the spring bloom and from 1 to 5 µg l^-1 during the post bloom. Analysis of the fatty acids showed that when the plankton was dominated by diatoms of the genera Thalassiosira and Chaetoceros, the proportions of C16:1(n-7) and C20:5(n-3) were correspondingly high. C18s, and particularly C18:1(n-9), were more abundant when the plankton was dominated by small autotrophic flagellates, primarily haptophytes. We found a good positive correlation between the common diatom marker, C16:1(n-7)/C16:0, and the biomass percentage of diatoms (r=0.742, P<0.001), as well as between the biomass percentage of flagellates and total C18 fatty acids (r=0.739, P<0.001). This supports the use of these specific fatty acids and fatty acid ratios as general biomarkers of the plankton community. However, the fatty acids are not specific enough to sufficiently characterise the composition of the plankton community, and microscopical support is needed to verify observed trends.     

Population structure of two Antarctic ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, in winter sea ice

The spatial distribution and population structure of two dominant ice-associated copepods, Drescheriella glacialis and Paralabidocera antarctica, were studied during winter at nine locations in east Antarctic fast ice. These species accounted for at least 90% of the total metazoan abundance at each location. Abundances were high, reaching 175 individuals l^-1 (190,000 m^-2) for D. glacialis and 660 l^-1 (901,000 m^-2) for P. antarctica. These abundances were probably partly supported by the high biomass of ice-algae (Pearson correlation coefficient, r=0.75), as indicated by chlorophyll-a concentrations (1.7-10.1 µg l^-1). The population structures of each species suggested very different life-history strategies. All developmental stages of D. glacialis were isolated from the ice cores, including females with egg sacs, supporting the hypothesis that this species reproduces in the sea ice during winter. This strategy might assist D. glacialis in leading a continually colonising existence, whereby it responds opportunistically to the availability of favourable habitat patches. The populations of P. antarctica were composed primarily of nauplii (>99%), consistent with past observations of a synchronised life cycle for this species. The strong coupling of the developmental cycle of P. antarctica to the growth and decay of sea ice suggests that local extinctions might occur in areas where ice break-out is unpredictable.     

Calanus marshallae,a new species of calanoid copepod closely allied to the sibling species C. finmarchicus and C. glacialis

Species of the copepod genus Calanus frequently dominate the marine zooplankton in boreal and arctic waters. Up to now there have been no operational means of identifying several species closely related to Calanus finmarchicus. Reanalysis of these taxa, using material from plankton samples collected throughout Northern Hemisphere polar and coreal waters, shows that there are 3 sibling species which have been previously combined under the names C. finmarchicus and C. glacialis. Several new taxonomic characters permit unequivocal identification of C. finmarchicus, C. glacialis, and a new species, C. marshallae. Claims that C. finmarchicus and C. glacialis are subspecies are refuted; there is no evidence that the two species continuously intergrade either where they co-occur or where they are allopatric, nor is there evidence that the two species hybridize. C. finmarchicus is basically restricted to the North Atlantic Ocean and C. marshallae to the North Pacific Ocean and Bering Sea; C. glacialis is primarily an Arctic species, but its geographical distribution slightly overlaps those of the other two species. Taxa closely related to C. finmarchicus and C. helgolandicus probably represent two separate, but closely linked evolutionary lineage; species of these two lineages are placed in one of two species groups, the finmarchicus group and the helgolandicus group.     

Multiplexed species-specific PCR protocol to discriminate four N. Atlantic Calanus species, with an mtCOI gene tree for ten Calanus species

Accurate species identification is the cornerstone of any ecological study - yet this fundamental step is not always possible for marine zooplankton. Routine species' identification, especially of juvenile andlarval stages, is difficult for Calanus species (Copepoda; Calanoida) in the N. Atlantic Ocean, where two or three species may co-occur. A rapid, simple, and inexpensive molecularly based protocol to identify individual copepods of any life stage has beendeveloped. This protocol will routinely identify four Calanus species in the N. Atlantic and will help to accurately understand the role of each species in coastal and open ocean ecosystems. The DNA sequence of a 633 bp region of the mitochondrial cytochrome oxidase I (mtCOI) was determined for ten Calanus species: C. australis (Brodsky, 1959), C. chilensis (Brodsky, 1959), C. finmarchicus (Gunnerus, 1770), C. glacialis (Jaschnov, 1955), C. helgolandicus (Claus, 1863), C. hyperboreus (Krøyer, 1838), C. marshallae (Frost, 1974), C. pacificus (Brodsky, 1948), C. simillimus (Giesbrecht, 1902), and C. sinicus (Brodsky, 1965). MtCOI sequences were used to design species-specific oligonucleotide primers for C. finmarchicus, C. glacialis, C. helgolandicus, and C. hyperboreus and to optimize a competitive, multiplexed, species-specific PCR (SS-PCR) protocol to discriminate the four species. This corrects and improves a previously published protocol for three Calanus species (Bucklin et al. 1999: Hydrobiologia 401:239-254), unambiguously identifying individual copepods and copepodites from diverse geographic regions of the four species' ranges. In order to further examine the pattern of mtCOI evolution within Calanus (an important consideration for molecular systematic characters), consensus mtCOI sequences were used to reconstruct phylogenetic relationships among the ten species; the mtCOI gene tree agreed with morphological and molecular (based on mt16S rRNA) phylogenies, except that the affiliation of C. sinicus could not be resolved.     

Under-ice amphipods in the Greenland Sea and Fram Strait (Arctic): environmental controls and seasonal patterns below the pack ice

During three "Polarstern" cruises to the ice-covered Greenland Sea (spring 1997, summer 1994, autumn 1995) studies on the under-ice habitat (morphology, hydrography, ice-algal biomass) and on the macrofaunal, autochthonous under-ice amphipods (species diversity, abundance) were carried out in order to describe environmental controls and seasonal patterns in this community. In spring, the ice underside was rather smooth and whitish, while in summer melting structures and sloughed-off ice-algal threads were observed, in autumn detritus clumps accumulated in depressions at the ice underside. Only in summer, a thin layer of warm (up to -0.6°C) and less saline (as low as S=6.3) water was found at the ice-water interface above Polar Water. Integrated ice-algal biomass was highest during autumn (2.6 mg chl a m^-2) and lowest during summer (1.2 mg chl a m^-2). Four species of under-ice amphipods occurred in spring and summer (Apherusa glacialis, Onisimus glacialis, O. nanseni, Gammarus wilkitzkii), but only the last species was observed at the ice underside in autumn. A. glacialis and G. wilkitzkii were equally abundant in spring; A. glacialis dominated in summer. The highest total abundance of amphipods occurred during summer (31.9 ind. m^-2), compared to lower abundances in spring and autumn (5.3 and 1.1 ind. m^-2, respectively). A factor analysis revealed seasonal patterns in the data set, which mainly influenced A. glacialis, and species-specific relations between several environmental factors and the distribution of under-ice amphipods. Abundance of A. glacialis was closely related to the under-ice hydrography and ice-algal biomass, whereas the other amphipod species were more influenced by the under-ice morphology. It is therefore stated that the observed thinning of the Arctic sea ice and the resulting increased meltwater input and change in morphology of floes will have a profoundly adverse effect on the under-ice amphipods.     

Molting rates and cohort development of Calanus finmarchicus and C. glacialis in the sea off southwest Nova Scotia

Species of Calanus were sampled at stations off southwest Nova Scotia during an 8-d cruise in June, 1982. A simple model using shipboard measurement of molting rates of predominant copepodite stages accurately predicted the changes in cohort structure that occurred at stations along a transect at the beginning and end of the cruise. The combined durations of C. finmarchicus (Gunnerus) stages CIII and CIV, calculated from the inverses of molting rates corrected for presence of diapausing C. glacialis Jaschnov, corresponded well with development times based on laboratory rearing experiments. We conclude that our molting rate measurements accurately reflected processes in nature. This methodology is useful for analyzing the dynamics and production of copepod populations.     

Molecular systematics of six Calanus and three Metridia species (Calanoida: Copepoda)

The discrimination of species of the copepod genus, Calanus (Copepoda; Calanoida), is problematical-especially in regions of sympatry. Although the species of Calanus exhibit exceptional morphological similarity, they are quite distinct in genetic character. The DNA base sequences of the mitochondrial large subunit (16S) ribosomal RNA (rRNA) gene unambiguously discriminated C. finmarchicus (Gunnerus 1765), C. glacialis (Jaschnov 1955), C. marshallae (Frost 1974), C. helgolandicus (Claus 1863), C. pacificus (Brodsky 1948), C. sinicus (Brodsky 1965), and C. hyperboreus (Kroyer 1838). Sequence differences among Calanus species for this gene portion range from 7.3% (between C. glacialis and C. marshallae) to 23.9% (between C. glacialis and C. sinicus). Differences among conspecific individuals were approximately 1 to 2%. [These sequence data were determined between April and November 1993; the sequenced domain is similar to that published previously in Bucklin et al. (1992) but are derived from analysis of additional individuals.] Statistical analysis of the sequence data using a variety of tree-building algorithms separated the taxa into one group of species corresponding to the C. finmarchicus group (C. finmarchicus, C. marshallae, and C. glacialis) and another ungrouped set of species corresponding to the C. helgolandicus group (C. helgolandicus, C. pacificus, and C. sinicus). The C. helgolandicus group may be older than the C. finmarchicus group, making the tree topology less reliable in this area. Calanus hyperboreus was an outlier; Nannocalanus minor (Claus 1863) was the outgroup. Similar analysis of Metridia species confirmed that M. lucens (Boeck 1864) and M. pacifica (Brodsky 1948) are distinct species; M. longa (Lubbock 1854) was still more divergent. These sequence data will allow the design of simple, molecular tools for taxonomic identifications. Diagnostic characters, assayed by rapid molecular protocols, will enable biological oceanographers to answer important questions about the distribution and abundance of all life stages (as well as patterns of reproduction) of morphologically similar species, such as those of Calanus.     

Diet composition and quality for <Emphasis Type="Italic">Calanus finmarchicus</Emphasis> egg production and hatching success off south-west Iceland

Egg production and hatching success of the copepod Calanus finmarchicus was measured during spring and summer in the waters south-west of Iceland. Egg-production rates varied greatly, both temporally and spatially, with highest average rates found at a station with low chlorophyll-a concentrations (0.4 mg m^-3). Excluding this high production rate from statistical analysis, the remaining egg-production rates were found to be positively correlated with phytoplankton biomass, as well as with parameters representing healthy phytoplankton condition, food quality and diatom-type fatty acids. Hatching success of eggs was negatively correlated with some saturated and monounsaturated fatty acids related to phytoplankton senescence.     

Pools of chlorophyll and live planktonic diatoms in aphotic marine sediments

Chlorophyll a and numbers of live pelagic diatoms were recorded from sediment depth profiles at 11 stations in the oligotrophic Øresund, Denmark, in late-June. Extraction efficiency of chlorophyll a analysed fluorometrically did not differ significantly between paired samples of frozen-thawed and fresh sediment. The depth profiles of chlorophyll a could be explained by a diagenetic model involving two different chlorophyll pools: one reactive pool declining exponentially with core depth, and one non-reactive pool, of about 1 µg Chl ml^-1 wet sediment, being constant with depth. The number of live diatoms, quantified by the dilution-extinction method, and expressed in terms of most probable number (MPN), declined from an average of about 300,000 g^-1 in the surface sediment to zero values at a depth of 13 cm. The number of live cells was significantly correlated with the sediment chlorophyll a, and the profiles of live cells as well as reactive chlorophyll followed the same exponential decline with core depth, suggesting that the main source of chlorophyll in the sediment was live pelagic diatoms. Taxonomic composition of diatoms in the sediment, dominated by the pelagic genera Chaetoceros, Thalassiosira and Skeletonema, matched the species composition in the water column 3 months earlier during the spring bloom. Regular recordings of the phytoplankton community in the water column showed that only these specific bloom species could be the source of the sediment content of diatoms and chlorophyll a. Further, the ratios between live cells and chlorophyll a were similar in the sediment and in the spring bloom. A conservative estimate of depth-integrated pools of diatoms in the sediment suggested that about 44% of the total phytoplankton biomass during the spring bloom was still present as live cells in the sediment after 3 months. This indicates that the spring bloom input to the sediment is not degraded immediately by the benthic fauna.     

Growth,under laboratory conditions,of turbot,Scophthalmus maximus,from the Ría de Vigo (north-west Spain)

Egg production rates of Calanus glacialis (Jaschnov) were measured in the laboratory (in 1985) and in the field (in 1983–1984). In the laboratory, daily egg production was studied over one month at alternating feeding and fasting conditions. Spawning ceased after 3 d of starvation and started as soon as females were reintroduced to food. Egg production increased stepwise at 3-d intervals. Females survived more than nine months in captivity. In the field, egg production was measured during PREMIZEX 1983 and MIZEX 1984 at 5 and 16 stations, respectively. High egg production was found in polynyas on the East Greenland Shelf, where melt water induced stratification which supported a spring bloom. Highest egg production was converted into 6.1% body carbon female^-1 d^-1. Under thick pack-ice no eggs were spawned. Spawning was induced in females from a station with low food abundance by feeding them on board ship. These results from both experiments and field studies show that egg production in C. glacialis is closely related to food availability. Thus, C. glacialis exhibits a reproductive behavior similar to that of C. finmarchicus, but not C. hyperboreus, the other two dominant species in this region.     

Significance of food type for growth of ephyrae Aurelia aurita (Scyphozoa)

We studied growth of newly released Aurelia aurita ephyra larvae fed five different food types, including a large-sized copepod, a phytoflagellate, and suspended POM (particulate organic matter) made from bivalve meat. Experiments were run at saturated food concentration in two different temperatures over 10 days. The effect of small differences in temperature was inconsistent and interacted with the effect of food type, which, in turn, was highly significant. A low average growth rate (4-9% day^-1) was shown when feeding on the large-sized copepod Calanus finmarchicus (80 µg AFDW individual^-1), in spite of an extremely high daily ration of up to 1500% of body AFDW. When feeding on the cryptophyte Rhodomonas baltica (ca. 8 µm cell diameter), the ephyrae showed an average growth rate over the 10 day experiment of 7-11%, but with a considerably higher growth rate during the first days. Suspended POM generated an average growth rate of 7-9% day^-1, whereas fresh bivalve meat, manually placed into the stomach of the ephyra, gave an average growth rate of 12-14% day^-1. Artemia nauplii (ca. 3 µg AFDW individual^-1), used as a general reference, resulted in higher growth rates than any of the other food types (17-31% day^-1). We conclude that A. aurita ephyrae can capture and feed on phytoplankton, large copepods, and POM; that phytoplankton might be of nutritive significance early in development; and that the high quantity of large-sized copepods ingested is inefficiently converted to growth during early development. POM is a potential food source because of the ability of the ephyrae to encounter and ingest it, although concentration, size distribution, and nutritional composition of natural POM probably constrain its effect on growth.     

In situ feeding rates and grazing impact of zooplankton in a South African temporarily open estuary

Recent studies in temporarily open estuaries of South Africa have shown that phytoplankton biomass is at times low, when compared to the high standing stock of the grazers. In situ grazing rates of the dominant zooplankton species were estimated at the Mpenjati Estuary once during the winter closed phase, in August 1999, and once during the summer open phase, in February 2000. The study aimed at determining what proportion of the energetic demands of the dominant grazers of the estuary is met by the available phytoplankton. Results show that the gut of all species exhibited higher pigment concentrations during the night than during the day, both in winter and summer. Gut pigment contents ranged from 0.27 to 5.38 ng pigm individual^-1 in the mysid Gastrosaccus brevifissura, from 0.16 to 1.63 ng pigm individual^-1 in the copepod Pseudodiaptomus hessei, from 0.12 to 0.45 ng pigm individual^-1 in the copepod Acartia natalensis, and from 0.8 to 5.44 ng pigm individual^-1 in the caridean Palaemon sp. [where pigm is the sum of chlorophyll-a (chl-a) and phaeopigments]. During the winter closed phase, gut evacuation rates for G. brevifissura, P. hessei, and A. natalensis were 0.62, 0.42, and 0.46 h^-1, respectively. In summer, gut evacuation rates were 0.68, 0.48, and 0.46 h^-1 for G. brevifissura, P. hessei, and Palaemon sp., respectively. The rate of gut pigment destruction for G. brevifissura was 99.6% of the total ingested, one of the highest values ever recorded for any crustacean. A gut pigment destruction of 79.0% was measured for Palaemon sp., 95.7% for P. hessei, and 93.8% for A. natalensis. During winter the total grazing impact of the dominant zooplankton species ranged from 5.05 to 22.7 mg chl-a m^-2 day^-1 and accounted for 34-69% of the available chl-a in the water column. During summer, the grazing impact ranged between 0.45 and 0.65 mg chl-a m^-2 day^-1, accounting for 17-41% of the available chl-a in the water column. This shows that the dominant zooplankton species of the Mpenjati have a very high grazing impact on algal cells. At times this may exceed 100% of the available phytoplankton production, suggesting that the zooplankton community may often resort to other food sources to meet all its energetic demands.     

Simple molecular method to distinguish the identity of Calanus species (Copepoda: Calanoida) at any developmental stage

Diagnostic morphological characteristics of copepods of the genus Calanus are restricted largely to minor variations in secondary sex characteristics. This presents a persistent problem in the identification of individuals to species level, especially for immature stages. We have developed a simple molecular technique to distinguish between the North Atlantic Calanus species (C. helgolandicus, C. finmarchicus, C. glacialis and C. hyperboreus) at any life stage. Using the polymerase chain-reaction (PCR), the mitochodrial large subunit (16S) ribosomal RNA (rRNA) gene was amplified from individual copepods preserved in ethanol. Subsequent digestion of the amplified products with the restriction enzymes DdeI and VspI, followed by electrophoretic separation in 2% agarose (Metaphor, FMC Ltd), produced a characteristic pattern for each species. The versatility of the method is demonstrated by the unambiguous identification to species of any life stage, from egg to adult, and of individual body parts. Received: 11 May 1998 / Accepted: 5 August 1998     

Under-ice feeding and diel migration by the planktonic copepodsCalanus glacialis andPseudocalanus minutus in relation to the ice algal production cycle in southeastern Hudson Bay,Canada

The marine planktonic copepodsCalanus glacialis Jaschnov andPseudocalanus minutus (Kroyer) typically dominate the copepod biomass in spring under the ice in southeastern Hudson Bay, Canada. Females of both species exhibited significant diel feeding cycles, as measured by gut pigment content, throughout a bloom of ice algae at the ice-water interface in 1986. Periods of grazing correlated well with a nighttime vertical migration by females to within 0.2 m of the ice-water interface, suggesting that feeding took place at or just below the thermohaline boundary between seawater and the interfacial layer containing the ice algae. Seasonal melting of the ice bottom in mid-May resulted in freshening of the surface layer and release of the ice algae into the water column. FemaleC. glacialis andP. minutus responded by ceasing migration to the interface. Gut pigment content, and by reasonable assumption, feeding activity in the water column, increased substantially immediately after this event. In mid-May, the water column phytoplankton consisted of flagellates, sedimenting ice algal cells, and diatoms (Navicula pelagica andChaetoceros sp.) previously found at the interface and then growing in the water column. We conclude that algae growing at the ice-water interface, and sedimenting or actively growing algae derived from this interfacial layer, are a regular and principal source of nutrition for these pelagic copepods during and immediately after the ice algal bloom.     

Seasonal changes in feeding, gonad development and lipid stores in Calanusfinmarchicus and C. hyperboreus from Malangen, northern Norway

 Seasonal dynamics of feeding activity, oil sac volume, gonad development, sex ratio and spawning periods in the two sibling species Calanusfinmarchicus and C. hyperboreus, the key zooplankton copepod organisms throughout the northern Atlantic waters, were studied simultaneously in Malangen, northern Norway, during 1992. We were also tracing differences in surface- and deep-dwelling components of these populations in terms of oil sac volume and gonad development during the time period when the G₁ is preparing for a subsequent generation (G₂) or hibernation. The main difference in the life cycle strategies of these species is the earlier maturation and spawning of C. hyperboreus. No feeding activity in either of the two species was found in February, but both commenced feeding in March, prior to the spring phytoplankton peak. The larger copepod, C. hyperboreus, had a more intensive energy deposition than C. finmarchicus. The period of active feeding was much shorter for the former species, only from March through July in copepodite stages CIV and CV, and even less in females – March and April. Basically, a similar pattern of seasonal changes in gonad length and lipids was observed in the two species. In June, oil sacs in the surface- and deep-dwelling specimens were about equal, during the rest of the year, lipids in the deep CVs exceeded those in the surface. We propose that as copepods accumulated sufficient lipid reserves, they started to descend, while others, containing less fat, stayed in the upper layers feeding. The mean length of the gonads in the surface-dwelling copepods was consistently less than in their deep counterparts from October to February, so that gonad development at the expense of accumulated reserves during resting stage was confirmed. C. finmarchicus males were found in considerable numbers only in February and March, and were only occasionally found in the upper layers (0–100 m), while adult male C. hyperboreus were present from October to March, but were never found in the surface layers. The differences in life cycle timing among the two species are discussed in relation to tradeoffs with regard to foraging strategies, generation numbers, bioenergetics and predator avoidance. Received: 31 March 1999 / Accepted: 23 November 1999     

Geographical range and taxonomic divergence in North Atlantic Calanus (C. helgolandicus,C. finmarchicus and C. glacialis)

The known distribution of Calanus helgolandicus in the North Atlantic Drift is difficult to explain in the absence of a reproductively active population inhabiting continental waters off eastern North America. New evidece indicates that this population, overlooked in the past, does in fact exist. The species has been found in a study of zooplankton samples from a number of MARMAP (Marine Resource Monitoring, Assessment, and Prediction Program) cruises surveying ichthyoplankton between Cape Hatteras and the New York Bight. Sexual activity in these stocks of C. helgolandicus was indicated by the ripeness of ovaries, the frequency of males, the presence of sperm in the females' seminal receptacles and the appearance of a female bearing a Calanus spermatophore. The new records provide a likely origin for the presence of the species in the vicinity of the Labrador Grand Banks as well as further east in the North Atlantic Drift. C. finmarchicus, similarly sexually active, was found to be sympatric with C. helgolandicus in the MARMAP collections which were taken from a region contiguous with the southernmost, known distribution, of C. glacialis. The distribution of integumental organs (i.e., pore signature patterns) was examined in the three species to determine whether they would be taxonomically useful. Strikingly different patterns were found on the female urosome. Pore signature differences between the polar species C. glacialis and the temperate C. helgolandicus proved to be as pronounced as those between the boreal C. finmarchicus and its two neighboring species. The successively overlapping ranges and the distinctive differences in pore signature patterns suggest that divergence from the generic pattern of integumental organ distribution has been a product of selection against hybridizing among the three species. If this is in fact the case, the reproductive range of C. helgolandicus has overlapped with those of C. finmarchicus and C. glacialis for appreciable periods in the history of the three species.