Stages CIV Research Articles

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.

Diel patterns in stage-specific vertical migration of Calanus finmarchicus in habitats with midnight sun

Diel vertical migration (DVM) and feeding of Calanus finmarchicus were studied in relation to phytoplankton biomass and predator abundance along a latitudinal gradient in the Norwegian and Greenland Seas, including areas with midnight sun. Young stages (CI–CIII) were generally confined to the upper 30 m both day and night, but weak, significant DVM took place in Atlantic water masses. Older individuals were distributed progressively deeper with increasing stage. Stages CIV and males were non-migratory. Combining all stations, CV and females displayed significant DVM, but migrations were within a narrow depth range. Separating stations according to water mass, significant DVM was only observed within the Arctic domain. Older developmental stages consistently occurred in low numbers in the upper 10 m, both day and night. Contrary to expectations, the amplitude of the DVM tended to be largest in the northwestern part of the study area, representing habitats with midnight sun. Over the Norwegian Shelf and in the east of the Norwegian and Greenland Seas, herring and mackerel were regionally abundant in the upper 50 m, but their abundance decreased strongly towards the northwest. Calanus spp. were the most frequent prey items in their stomachs. The amplitudes of DVM were generally strongest in habitats with very low predator abundance. Neither concentrations of chlorophyll a nor its vertical distribution could account for the observed variations in DVM. Diel feeding rhythms (from gut fluorescence) were found throughout the ocean, except for areas with very low biomass of chlorophyll a. However, feeding patterns varied, and both enhanced and reduced nocturnal gut fluorescence were recorded. 2000 International Council for the Exploration of the Sea

Physiology and population structure of Calanus finmarchicus (Copepoda: Calanoida) during a Lagrangian tracer release experiment in the North Atlantic

A population of Calanus finmarchicus was followed in the open sea for a period of 10 days at the end of June 1996 using a tracer release method. Gut fluorescence, egg production, carbon content and stage specific abundance, together with phytoplankton concentration and composition, were measured during this period. Chlorophyll levels were less than 1 µg l-1 and the phytoplankton population was dominated by coccolithophorids. Gut fluorescence decreased during a period with strong winds, whereas egg production remained constant. Estimated phytoplankton ingestion was too low to cover egg production requirements. However, the microzooplankton concentration seemed to be high enough to complement phytoplankton ingestion, enabling egg production carbon require- ments to be covered. C/N ratios of adult females and stage CV were low, indicating low levels of lipid storage. The population was dominated by stage CIV, but shifted to a dominance of younger stages during the study. Mortality calculated from population structure indicates that higher death rates occurred at stage CV.

Field calibration of the Optical Plankton Counter with respect to Calanus finmarchicus

Zooplankton in the Faroe–Shetland Channel and north‐western North Sea were sampled concurrently with a profiling serial net system and an Optical Plankton Counter (OPC) during January and March 1995. Calanus finmarchicus, together with a small proportion of Calanus helgolandicus (< 3% overall), made up > 75% of the zooplankton in the region, with C. finmarchicus being predominant below 500 m depth. Sensitivity analysis of the OPC data was carried out to determine the particle size range providing the highest correlation between the concentrations of stage CIV–CV and stage CVI C. finmarchicus estimated from net catches, and particle concentrations from the OPC. Significant correlations were obtained for particle size ranges that corresponded closely with microscopic measurements of the dimensions of copepodites. The highest correlations were obtained for stages CIV–CV in January.According to the net sampling, the concentration of C. finmarchicus in January varied by at least four orders of magnitude over the survey region (< 0.05 to > 500 m−3), although only 4% of the samples contained > 100 m−3. When C. finmarchicus was present at concentrations of around 120 m−3, similar estimates of its abundance were obtained from the OPC data as from the net samples. At lower concentrations, the OPC increasingly overestimated the net catches, and the overestimate reached 40:1 at the detection limit of the nets (0.05 m−3). The average overestimate, taking into account the distribution of concentrations, was 2.5:1. This over‐estimate could not be wholly accounted for by the occurrence of other zooplankton species or potential errors associated with the OPC or net system, and the conclusion was that it arose mainly from the detection of detrital aggregates by the OPC. Thus, the OPC was effective at resolving the peak concentrations (> 100 m−3) of C. finmarchicus in the survey region, but was less effective at delineating the marginal areas of the distribution where C. finmarchicus was outnumbered by other similarly sized particles. Nevertheless, the results indicated that, with the application of calibration data such as described here, the OPC is a valuable tool for broad scale surveys of the spatial distribution of C. finmarchicus during the winter, when the animals are concentrated at depths > 500 m in areas such as the Faroe–Shetland Channel.

Selective feeding on natural phytoplankton by Calanus finmarchicus before, during, and after the 1997 spring bloom in the Norwegian Sea

Selective feeding by the calanoid copepod Calanus finmarchicus was investigated during a 3‐month study in spring 1997 at a permanent station in the Norwegian Sea (Sta. M, 66°N, 02°E). Phytoplankton biomass increased from 317 ng chlorophyll a (Chl a) liter−1 in the prebloom phase to 2,095 ng liter−1 during the bloom and declined after the bloom to 1,260 ng Chl a liter−1. In the prebloom phase, clearance rates of C. finmarchicus females were between 22 and 100 ml copepod−1 d−1, while during the bloom, they ranged from 75 to 92 ml copepod−1 d−1, with a decline in the postbloom phase (41 ml copepod−1 d−1). After the phytoplankton bloom, the C. finmarchicus population was dominated by copepodid stages CIV and CV, with clearance rates ranging from 7 to 23 ml copepod−1 d−1. Grazing rates of adult females on the phytoplankton standing stock were low in the prebloom phases (5–23 Chl a copepod−1 d−1, respectively), increased during the bloom from 82 to 219 ng Chl a copepod−1 d−1, and declined after the bloom (64 ng Chl a copepod−1 d−1 for adult females and 7–27 ng Chl a copepod−1 d−1 for copepodid stages CIV and CV). C. finmarchicus showed a selection for diatoms throughout the study period and for dinoflagellates, before and after the spring bloom, despite the low concentration of both groups in the pre‐ and postbloom phases. During the postbloom period, no differences were observed in the selective feeding behavior of the copepodid stages compared to the adults. The contribution of diatoms to the overall phytoplankton biomass was 8 and 14% in the pre‐ and postbloom periods, respectively, while dinoflagellates were ≦3%. Haptophytes (dominated by Phaeocystis pouchetii) and cryptophytes were ingested according to their abundance. Avoidance of cyanobacteria (Synechococcus spp.), pelagophytes, and “green algae” was observed throughout the study period.

Influence of food quantity and temperature on development and growth of the marine copepod Calanus chilensis from northern Chile

An experiment under laboratory conditions was conducted to test the hypothesis that development and growth of copepodite stages in Calanus chilensis are temperature-dependent and not subject to food shortage in the upwelling area of the Humboldt Current, northern Chile. Field data obtained from June 1994 to May 1995 in Bahia Mejillones (23°S) were used to define four combinations of temperature and food under which copepodites were reared from Stage CIII to adulthood. The high temperature was 18.1 °C and the low temperature 13.1 °C, whereas the high food level was in the range of 6.8 to 24.8 μg l−1 chlorophyll a and the low level 1.0 to 6.8 μg l−1 chlorophyll a. As food a mixture of three unknown species of phytoflagellates and the diatom Navicula cryptocephala was used. This phytoplankton was initially obtained from the same sampling sites as copepods and kept in f/2 media at stable levels and composition throughout the experiment. The development rate (1/t), estimated from the time (t) elapsing between Stage CIV and adult, was significantly affected by both temperature and food, although low-food effects were much more remarkable. Low-food conditions also significantly reduced body length and “structural” (lipid-discounted) body mass at adulthood, while temperature only affected body length. The weight-specific growth rate was also affected by food and temperature, but again food effects were much more drastic. The results indicate that C. chilensis is a highly sensitive species to lack of food, and is possibly subject to food shortage during its annual cycle in the coastal upwelling area of northern Chile. Food limitation may help explain the seasonal pattern of adult size reported by previous studies in the area and the lack of consistence between the number of generations predictable from a temperature-dependent model and that observed in the field during the annual cycle.

Winter ontogenetic migrations and the onset of gonad development in large dominant calanoid copepods in the Weddell Gyre (Antarctica)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 157:233-246 (1997) - doi:10.3354/meps157233 Winter ontogenetic migrations and the onset of gonad development in large dominant calanoid copepods in the Weddell Gyre (Antarctica) Vassily A. Spiridonov1,*, Ksenia N. Kosobokova2 1Zoological Museum of the M. V. Lomonosov Moscow University, Bolshaya Nikitskaya Str., 6, Moscow 103009, Russia 2P. P. Shirshov Institute of Oceanology, Nakhimov Ave., 36, Moscow 117218, Russia *E-mail: arthro@glasnet.ru Data on abundance, stage composition and vertical distribution, sex ratio, maturity states, and the inferred seasonal/ontogenetic migrations of Calanoides acutus, Calanus propinquus, and Rhincalanus gigas obtained during the Winter Weddell Gyre Study 1992 in early to mid-winter are presented. During this period copepodite Stages CIV and CV and adult females of C. acutus underwent an ontogenetic downward migration, which in CIV seemed to be delayed until early winter if moderate biomass of phytoplankton was sustained in the upper water layers. The descent of CV and CVI was close to completion already by June while CIV continued to sink down slowly during the winter. By mid-winter, sexual differentiation in CV, maturation of males, and the onset of female maturation were observed for C. acutus. Males of C. acutus maintained rather restricted vertical distribution centered in the core of the Warm Deep Water. C. propinquus showed no indication of regular ontogenetic migration, but did show some seasonal changes in the vertical distribution of particular stages with a strong geographical variability. The bulk of the overwintering C. propinquus population in the Weddell Gyre consisted of CIII and did not leave the Winter Water. The maturation of males and females in C. propinquus began in winter as well. R. gigas underwent a downward migration, which was only detected in those parts of the Weddell Gyre where some recruitment of that species took place, i.e. at the Weddell Front and in the Maud Rise area. Although sexual differentiation in CV was in progress in winter, the onset of maturation of both males and females in R. gigas appeared to be delayed until at least the end of winter. Copepods · Antarctica · Migrations · Maturation · Sex ratio Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 157. Publication date: October 16, 1997 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1997 Inter-Research.

Life cycle, abundance and transport ofCalanus finmarchicusin Faroese waters

Abstract Calanus finmarchicus is the dominant copepod in the open ocean around the Faroe Islands. Measurements from 1990-93 showed that C. finmarchicus in the Atlantic Water outside the Faroe Shelf has two annual generations.The overwintering individuals migrate towards the surface in March-April and their spawning usually peaks in May.The majority of the individuals of this generation reaches stage CV in mid-summer.Some of the overwintering individuals remain in the upper layer and a small number of them spawn again in late summer.To the north of the Iceland-Faroes Front, C. finmarchicus has only one generation the overwintering individuals of which migrate towards the surface later than in the Atlantic Water and have their peak spawning in June.The individuals of this generation reach stages CIV and CV in August-September, when they migrate downwards.The phytoplankton biomasses in spring were higher north of the Iceland-Faroes Front than south of it.Therefore the delay of maturation and spawning north o...

Mesoscale distribution and population dynamics of Rhincalanus gigas and Calanus simillimus in the Antarctic Polar Open Ocean and Polar Frontal Zone during summer

Two dominant Antarctic copepod species, Rhincalanus gigas and Calanus simillimus, were sampled at stations along a 420 km transect running from Maurice Ewing Bank (50°S, 42°W) towards South Georgia (54°S, 37.5°W) during early January 1994 and again 1 mo later. The transect spanned the Polar Frontal Zone (PFZ), crossing the Polar Front (PF) into Antarctic surface waters. In January, diatom blooms were present immediately south of the PF and at open ocean stations further south where unusually high total pigment levels up to 800 mg m-2 were recorded. In early January, north of the PF, R. gigas had not yet commenced spawning although recruitment had begun to the south. Early stages of C. simillimus copepodites were, however, present at all stations sampled. South of the PF both gonad maturity and lipid levels in R. gigas showed a clear positive relationship with the elevated pigment concentrations. C. simillimus females, on the other hand, were ripe at the majority of stations sampled regardless of pigment levels. A month later postbloom conditions were present over the greater part of the transect and the summer generation of both species was present at all stations. A proportion of the summer generation of C. simillimus had already reached adulthood and their gonads were maturing. The populations of both species were older in PFZ waters, and mean population age showed a strong correlation with water temperature. Prosome length of stages CIV, CV and CVI-female R. gigas was negatively correlated with sea surface temperature. This was not the case for C. simillimus females, suggesting that they had been recruited earlier in the season under different environmental conditions. Lipid analysis indicated that R. gigas predominantly stored wax esters, in contrast to C. simillimus where the dominant lipid class was triacylgycerol. These observations coupled with literature data on feeding and seasonal vertical distribution seem to indicate fundamently different life history patterns.

Seasonal variations in distribution and population structure of Microcalanus pygmaeus and Ctenocalanus citer (Copepoda: Calanoida) in the eastern Weddell Sea, Antarctica

The abundance, vertical distribution and population structure of two important small calanoid copepod species, Microcalanus pygmaeus (G. O. Sars) and Ctenocalanus citer Heron and Bowman, were studied in the eastern Weddell Sea in summer (January/February 1985), in late winter/early spring (October/November 1986) and in autumn (April/May 1992). The population of Microcalanus pygmaeus consisted mainly of copepodite stages CII and CIII in late winter/early spring and were concentrated between 500 and 200 m depth. In summer, stage CIV was the modal stage and the bulk of the population had ascended above 300 m. In autumn the population structure was bimodal with CI and CV dominating. Most of the population was concentrated between 300 and 200 m. In all investigation periods M. pygmaeus had their maximal concentrations in the thermo-pycnocline. The developmental stages CIII to CV of Ctenocalanus citer formed the bulk of the population in late winter/early spring. In October all developmental stages had their main distribution between 500 and 200 m, except females, which were concentrated in the upper 50 m. In November most of the population occurred between 200 and 50 m. The summer population was concentrated in the upper 50 m, and numbers increased dramatically as the new cohort hatched. Copepodite stages CII and CIII dominated the population at the end of January, while CIV dominated 2 wk later. In autumn, CV was the modal stage. The majority of the population was concentrated in the upper 100 m, but there was an increase in abundance below 300 m compared to summer. Age structure changed with depth with a younger surface population and an older one in deeper water layers. The seasonal change in number of M. pygmaeus is much smaller than that of C. citer; the summer:winter:autumn ratio of the former being about one, whereas the winter:summer/autumn of the latter was about nine. Early copepodite stages and adults of M. pygmaeus occurred throughout all investigation periods. The large proportion of early copepodite stages in April and in mid-October suggests autumn and early to midwinter breeding. Apparently, M. pygmaeus may reproduce and grow year-round or perhaps has a 2-yr life-cycle. In contrast, the dramatic increase in abundance of early copepodite stages of C. citer in summer suggests springtime reproduction.

Comparative demography and population dynamics of two coexisting copepods in a Venezuelan floodplain lake

I used an inverse matrix projection model to estimate vital rates (growth, survival, and reproduction) for two coexisting copepod species in a Venezuelan floodplain lake as a first step in identifying the ultimate environmental factors that determine distribution and abundance of a species. Zooplankton were sampled four times a week from June through December 1984, and the resulting abundance data were used together with the estimation technique to extract time series of vital rates over a 6‐month period. There were marked temporal differences in population maxima of each species. Abundance of Diaptomus negrensis fluctuated with changes in recruitment and mortality of stages NV, NVI, CIII, and CIV. Peak abundance of D. negrensis coincided with a peak in the abundance of the invertebrate predator Chaoborus sp. In contrast, population size of Oithona amazonica was unrelated to recruitment but directly related to survival of the youngest copepodite stages, which was poor until October when abundance of Chaoborus declined. These demographic analyses target the stages most susceptible to environmental (selection) pressures, providing a basis for understanding the evolution of life‐history strategies, and suggest the hypothesis (as yet untested) that prolonged coexistence is facilitated between species whose abundance is determined by different demographic (and thus environmental) factors.

Diel vertical migration and feeding of copepods at an oceanic site near South Georgia

Twelve Longhurst Hardy Plankton Recorder (LHPR) profiles were taken over a 16 h period in January 1990, in order to study feeding of four copepod species at an Antarctic oceanic site near South Georgia. Vertical distributions of their life stages, as well as those of dominant competitors and predators, are described in relation to the feeding cycles of Calanoides acutus CV, Calanus simillimus CV, Calanus propinquus CV and Rhincalanus gigas CIII, CV and CVI♀. Comparisons with vertical ring-net catches, which were used for concomitant gutevacuation experiments, demonstrated the suitability of the LHPR for these fine-scale studies. Planktonic predators, with the exception of the diel migrant Themisto gaudichaudii, resided deeper than the herbivores. During the day and around midnight, when feeding rates were low, species and stages reached their maximum vertical separation. At these times, new generation copepodites of the four species lived progressively deeper and the overwintered generation (i.e., R. gigas Stages CIV, CV, CVI) were progressively shallower. During the afternoon or evening (depending on species), all stages older than CII, as well as Euphausia frigida and T. gaudichaudii, migrated upwards, to amass in the surface mixed layer. Feeding was restricted to darkness, although R. gigas commenced several hours before dusk. In detail their migration and feeding differed widely, with combinations of unimodal and apparent bimodal cycles. As a whole, the results suggest that (1) feeding could occur during sinking as well as during upward migrations, (2) upward migrations were not always associated with feeding increases, and (3) individuals appeared to descend after filling their guts.

Predatory feeding behavior of an Antarctic marine copepod, Euchaeta antarctica

During the austral summer, fall and winter, feeding rates of Euchaeta antarctica were measured in the laboratory. Measurements were taken over 24 hours in the dark on a mixing device at the ambient temperature using lively prey and predators in good condition with intact first antennae. Under these conditions, I found that no feeding occurred during winter so the following characterizes summertime feeding behavior. Adult females of Euchaeta antarctica are exclusive carnivores, exhibiting highest feeding rates on prey having a prosome length of 1200 ?m. The size of these preferred prey was 65% the length of the second basipodal segment of the maxilliped, the main appendage used in prey capture by E. antarctica. These prey, juvenile stages of Metridia gerlachei, also were the most abundant prey available in the plankton. The mean saturation ingestion rate of the adult female on their preferred prey of 18.7 Melridia/female/d is 8.8% its body weight, of which 17.3% is respired. A three-week starvation period caused less than a 1.5 fold increase in the feeding rate. Maximum volume effectively searched by this predatory copepod was 2.4 liters/day. Fecal pellets were produced at a constant rate of 0.697 pellets/hour; this rate was not influenced by the level of gut fullness. In order to produce one fecal pellet, E. antarctica must ingest 1.63 prey of the preferred size. Assuming that the rate of egestion is equal to the rate of ingestion, prey were ingested at a rate of 1.14/h. Many of the field-caught copepods evacuated 3-5 pellets which converts to 5-8 copepod prey ingested. This in situ meal more than adequately covers their daily respiratory costs. Smaller predators. CIV and CV E. antarctica, exhibited higher rates on smaller prey while males did not feed at all. The younger and potentially faster growing stage CIV consumed a higher proportion of their body weight per day than did the older mature predators; they were able to meet their metabolic costs on large and small prey. Older stages may not be able to obtain enough food to meet their metabolic needs during the late summer when prey availability is limited; they cease feeding and go into the overwintering state.

Gonad maturation as an indication of seasonal cycles for several species of small copepods in the Barents Sea

In a remote oceanic area like the Barents Sea, it is often difficult to follow the seasonal development of copepod populations in detail. Information on the gonad maturation stage of older juveniles and adults of a species will reveal the immediate state of reproduction and the expected development of juveniles into reproductively active adults. Winter “resting stages” in juveniles can also be recognised. Zooplankton were caught during Pro Mare cruises in early March, May, July/August, mid-September and mid-October. Abundance and composition of developmental stages of small copepods were determined for several stations from each cruise. Samples consisting of Stages CIV to CV1 of Pseudocalanus acuspes (Giesbrecht 1881), P. minutus (Kroyer), Microcalanuspusillus (Sars), and M. pygmaeus Sars were stained with carmine and analysed with respect to gonad maturation stage, length, width, and area of the prosome and the area of the gonad and the oil sac. Image analyses were performed from photographs or drawings of copepods using a digitising pad. With additional information on abundance and stage composition, and by comparing the present data set with information on Pseudocalanus spp. from Balsfjorden, northern Norway, seasonal cycles for the species could be inferred.

Comparison of Catch Rates among Small and Large Bongo Sampler for Calanus finmarchicus Copepodite Stages

Comparison of catch rates between small (20-cm, 0.165-mm-mesh nets) and large (61-cm, 0.333- and 0.505-mm-mesh nets) plankton samplers was carried out for Calanus finmarchicus copepodite stages sampled simultaneously during six different times. It is concluded that C. finmarchicus stages CI and CII were undersampled in 0.333-mm-mesh samples, while stages CIV–CVI were undersampled in 0.165-mm-mesh samples. Stage CIII catch rates were not different for these two samplers. Comparison of catch rates between 0.165- and 0.505-mm-mesh samples demonstrated that stages CI–CIII were undersampled in the 0.505-mm-mesh samples, stages CV and CVI were undersampled in the 0.165-mm-mesh samples, and there was no difference in catch rates for stage CIV. There was evidence of substantial variability in results among cruises for most stages. This is attributed to inherent variability associated with plankton sampling due to their patchy distribution. Due to the large variation in catch rates between these different samplers, it is recommended that the appropriate sized sampler be used to collect different stages of C. finmarchicus copepodites rather than attempting to correct for catch rate differences among different sized samplers.

Life cycles ofCalanoides acutus, Calanus simillimus andRhincalanus gigas (Copepoda: Calanoida) within the Scotia Sea

The life cycles and distribution of three dominant copepods,Calanoides acutus, Calanus simillimus andRhincalanus gigas were studied from the “Discovery” collections in the Scotia Sea earlier this century.C. simillimus is a sub-Antarctic species which mates in the top 250 m mainly in spring. The rapid development of the summer generation may allow a second mating period and a smaller second generation to appear in late summer.C. simillimus remains in the surface layers for a longer period thanCalanoides acutus orR. gigas, and its depth distribution is bimodal throughout the winter.R. gigas is most abundant in sub-Antarctic waters to the north of the Polar Front. It mates within the top 750 m later in spring, and development seems less synchronised than that of the other two species, with egg laying and the growth season being more protracted. Stages CIII and CIV are reached by the first autumn and further development resumes very early the following spring. It is not clear whether the majority then spawn or whether a further year may be needed to complete the life cycle. The predominantly Antarctic species,C. acutus mates below 750 m in middle to late winter and the summer generation develops rapidly to either CIV or CV. Its lifespan seems typically 1 yr, but some of the CVs which fail to moult and spawn in winter survive into their second summer, and their subsequent fate is uncertain. The cold-water speciesCalanus propinquus is comparatively rare in the Scotia Sea and aspects of its distribution and life cycle are briefly described for comparison. Regional variations in the timing of these events were apparent forC. simillimus and possiblyCalanoides acutus, but were not seen inR. gigas. Their geographic and vertical separation, together with their asynchronous life cycles support the concept of habitat-partitioning of these dominant herbivores.

Gonad maturation as an indication of seasonal cycles for several species of small copepods in the Barents Sea

In a remote oceanic area like the Barents Sea, it is often difficult to follow the seasonal development of copepod populations in detail. Information on the gonad maturation stage of older juveniles and adults of a species will reveal the immediate state of reproduction and the expected development of juveniles into reproductively active adults. Winter “resting stages” in juveniles can also be recognised. Zooplankton were caught during Pro Mare cruises in early March, May, July/August, mid-September and mid-October. Abundance and composition of developmental stages of small copepods were determined for several stations from each cruise. Samples consisting of Stages CIV to CV1 of Pseudocalanus acuspes (Giesbrecht 1881), P. minutus (Kröyer), Microcalanuspusillus (Sars), and M. pygmaeus Sars were stained with carmine and analysed with respect to gonad maturation stage, length, width, and area of the prosome and the area of the gonad and the oil sac. Image analyses were performed from photographs or drawings of copepods using a digitising pad. With additional information on abundance and stage composition, and by comparing the present data set with information on Pseudocalanus spp. from Balsfjorden, northern Norway, seasonal cycles for the species could be inferred.

Dynamics of Calanoides acutus (Copepoda: Calanoida) in Antarctic coastal waters

Observations of the population dynamics, distribution, abundance, feeding activity and egg production of Calanoides acutus were made in the period from mid-December 1986 to late March 1987. Spawning is presumed to have begun as early as October, since the population reached the copepodite I stage of development by early December. Egg production, which was not related to ambien chlorophyll concentrations in the range 4.4–18.5 μgI −1, ceased by mid-January. Feeding intensity of adult females, indicated by gut pigment content, was correlated with surface chlorophyll concentration. No evidence was found for diel periodicity in feeding. Younger copepodite stages (CI-CIII) were concentrated in the 0–100 m depth stratum, whereas later stage copepodites tended to be equally distributed throughout the upper 200 m. No evidence was found for diel vertical migration. All copepodite stages in the Gerlache Strait tended to be concentrated in the upper 100 m. Abundance of C. acutus was consistently greater in Gerlache Strait waters than elsewhere in the study area. The population attained overwintering copepodite stages CIV and CV by March. Downward ontogenetic migration of the overwintering stages occurred in the Drake Passage by late March, but was delayed in the relatively food-rich waters of the Gerlache Strait.

Seasonal cycles in population abundances and egg production rates in the planktonic copepods Centropages typicus and Acartia clausi

Laboratory studies on the seasonal cycling of egg production rates for Centropages typicus and Acartia clausi did not reflect seasonal changes in densities for these species at sea. Maximum egg production rates occurred from autumn to spring when population abundances at sea were low. In late spring, an increment in population numbers was followed by a drastic decline in egg deposition which continued into summer. The percentage loss due to mortality, calculated using egg data and total number of copepodid stages CIII and CIV recorded at sea ˜2 weeks later, indicated greatest mortality (80–99%) from eggs to copepodids during periods of high breeding intensity. These results suggest that high and low density phases in population numbers may be the outcome of variations in reproductive potential and survival rates of eggs and immature stages rather than reproductive potential per se .

Large-scale and long-term variations in the zooplankton community of the Gullmar fjord, Sweden, in relation to advective processes

Since 1978, long-term variations in zooplankton biomass have been studied in relation to the hydrography of the Gullmar fjord, west coast of Sweden. The effects of water exchange processes on the zooplankton community were given special attention. During each autumn extremely variable numbers of Calanusspp. (Stages CIV to V) were transported to the fjord by inflows of Skagerrak surface water Once inside the fjord, Calanus spp. descended into the deep water, but were more or less completely washed out again by the annual renewal of deep water during winter or spring. An anomaly in the zooplankton biomass which was observed in 1982-84 was caused by a more than 10-fold increase in the abundance of Calanus. It was concluded that advective processes were the major factor regulating zooplankton biomass in the fjord.