Patterns and processes of population dynamics with fluctuating habitat size: a case study of a marine copepod inhabiting tide pools

The logistic model is a fundamental population model often used as the basis for analyzing wildlife population dynamics. In the classic logistic model, however, population dynamics may be difficult to characterize if habitat size is temporally variable because population density can vary at a constant abundance, which results in variable strength of density‐dependent feedback for a given population size. To incorporate habitat size variability, we developed a general population model in which changes in population abundance, density, and habitat size are taken into account. From this model, we deduced several predictions for patterns and processes of population dynamics: 1) patterns of fluctuation in population abundance and density can diverge, with respect of their correlation and relative variability; and 2) along with density dependence, habitat size fluctuation can affect population growth with a time lag because changes in habitat size result in changes in population density. In order to test these predictions, we applied our model to population dynamics data of 36 populations of Tigriopus japonicus, a marine copepod inhabiting tide pools of variable sizes caused by weather processes. As expected, we found a significant difference in the fluctuation patterns of population abundance and density of T. japonicus populations with respect to the correlation between abundance and density and their relative variability, which correlates positively with the variability of habitat size. In addition, we found direct and lagged‐indirect effects of weather processes on population growth, which were associated with density dependence and impose regulatory forces on local and regional population dynamics. These results illustrate how changes in habitat size can have an impact on patterns and processes of wildlife population dynamics. We suggest that without knowledge of habitat size fluctuation, measures of population size and its variability as well as inferences about the processes of population dynamics may be misleading.     

Vertical distribution and seasonal numerical abundance of the Calanidae in oceanic waters to the south-west of the British Isles

The vertical distribution and seasonal abundance of the copepodite and adult stages of Calanus finmarchicus, C. helgolandicus, C. tenuicornis, Neocalanus gracilis, Nannocalanus minor and Calanoides carinatus from a series of Longhurst Hardy Plankton Recorder hauls taken in the oceanic waters, off the continental shelf, to the south-west of the British Isles are described. The sampling area was selected because the geographical distributions of the major Calanidae copepods of the north-east Atlantic Ocean are shown to overlap in this region. It marks the southern boundary of the distribution of C. finmarchicus, the central area of C. helgolandicus and the approximate northern limit of distribution of C. tenuicornis, N. gracilis, N. minor and C. carinatus. These four southern species occasionally penetrate further north (60° N) in the open ocean but do not breed at these northern latitudes. In autumn and winter, when C. finmarchicus and C. helgolandicus were overwintering below 400 m primarily as Stage V copepodites, N. gracilis occurred in the upper 200 m of the water column in a breeding condition; all copepodite stages were present. This copepod reproduced throughout the year in this regions while C. tenuicornis was observed to breed primarily in spring and summer. The geographical and vertical distributions of the Calanidae are related to the observed seasonal temperatures of the North Atlantic and the breeding strategies of species are compared.     

Fecundity limitation of Calanus helgolandicus, by the parasite Ellobiopsis sp.

The effect of the parasite Ellobiopsis sp., on the fecundityof Calanus helgolandicus and Calanoides carinatus in the Bayof Biscay, was investigated in May 2003. An average of 6.8%of C. helgolandicus females were infected with Ellobiopsis sp.,whereas none of the C. carinatus were found to be infected.An objective method of estimating gonad development was appliedto quantitatively measure the effect of the parasitism on thereproduction of the copepod. Parasitism by Ellobiopsis sp. hasthe potential to reduce the fecundity of C. helgolandicus females.     

Hypoxia tolerance in the copepod Calanoides carinatus and the effect of an intermediate oxygen minimum layer on copepod vertical distribution in the northern Benguela Current upwelling system and the Angola-Benguela Front

There is a growing concern that hypoxic and anoxic areas in the sea spread in extent and intensity, posing a severe risk to marine ecosystems and fisheries. Hypoxia may affect fish stocks directly or via detrimental effects on important prey species, such as zooplankton. A unique feature of the northern Benguela Current upwelling region and Angola-Benguela frontal system is a pronounced intermediate oxygen minimum layer (IOML) at 60-500 m depth with oxygen concentrations ≤ 1.4 mg O₂l^− 1 (minimum < 0.7 mg O₂l^− 1). Field studies during February-March 2002 demonstrated that the abundance of calanoid copepods and the biomass of mesozooplankton in general were severely reduced within the IOML. The dominant copepod Calanoides carinatus showed a bimodal vertical distribution with parts of the population either comprising all developmental stages concentrated in the surface layer (0-60 m), or copepodids C5 diapausing below 400 m depth apparently avoiding the IOML. Accordingly, abundances of other calanoid copepods were higher at the surface and below 300 m than in the centre of the IOML. The scarcity of planktonic life within the IOML raises the question whether this layer represents an effective barrier for zooplankton vertical migrations. Especially in C. carinatus, ontogenetic vertical migration plays a key role in the retention of the population within the productive upwelling region and for the rapid re-colonisation of plumes of newly upwelled water. To address this issue, the hypoxia tolerance of C. carinatus was determined in a series of laboratory-based, closed-bottle experiments in January 2005. Copepods were kept in gas-tight bottles and the decreasing oxygen concentrations were monitored to establish their minimum oxygen demands. Although copepodids survived apparently unharmed at surprisingly low oxygen concentrations of ca. 1.5 mg O₂l^− 1, they could not tolerate oxygen levels < 1.1 mg O₂l^− 1, implying that the core of the IOML, where O₂ concentrations are below this threshold, is uninhabitable for C. carinatus. In contrast, the IOML may represent a refuge from competition and predation for other copepod species specifically adapted to hypoxic environments.     

Spatio-Temporal Variability of Copepod Abundance along the 20°S Monitoring Transect in the Northern Benguela Upwelling System from 2005 to 2011

Long-term data sets are essential to understand climate-induced variability in marine ecosystems. This study provides the first comprehensive analysis of longer-term temporal and spatial variations in zooplankton abundance and copepod community structure in the northern Benguela upwelling system from 2005 to 2011. Samples were collected from the upper 200 m along a transect at 20°S perpendicular to the coast of Namibia to 70 nm offshore. Based on seasonal and interannual trends in surface temperature and salinity, three distinct time periods were discernible with stronger upwelling in spring and extensive warm-water intrusions in late summer, thus, high temperature amplitudes, in the years 2005/06 and 2010/11, and less intensive upwelling followed by weaker warm-water intrusions from 2008/09 to 2009/10. Zooplankton abundance reflected these changes with higher numbers in 2005/06 and 2010/11. In contrast, zooplankton density was lower in 2008/09 and 2009/10, when temperature gradients from spring to late summer were less pronounced. Spatially, copepod abundance tended to be highest between 30 and 60 nautical miles off the coast, coinciding with the shelf break and continental slope. The dominant larger calanoid copepods were Calanoides carinatus, Metridia lucens and Nannocalanus minor. On all three scales studied, i.e. spatially from the coast to offshore waters as well as temporally, both seasonally and interannually, maximum zooplankton abundance was not coupled to the coldest temperature regime, and hence strongest upwelling intensity. Pronounced temperature amplitudes, and therefore strong gradients within a year, were apparently important and resulted in higher zooplankton abundance.     

The occurrence of Calanidae species in waters off Argentina

As food of planktivorous fish and likely good predictors of natural perturbations, members of the family Calanidae are recognised to be key species in ecosystems worldwide. The distribution and seasonal relative abundance of the Calanidae species occurring in the Argentine Sea were reviewed from published and unpublished data collected over the last three decades. Species are also figured in order to elucidate any possible taxonomic uncertainty. Calanoides cf. carinatus, Calanus australis and Calanus simillimus are the most abundant calanids in the region. The former two species typically inhabit inner and middle shelf waters decreasing offshore, while Calanus simillimus is distributed in the middle and outer shelf, its abundance increasing towards the shelf-break. The southern limit of the distribution of Calanoides cf. carinatus appears to be 46° S. Calanus australis is the most common large copepod in coastal and inner shelf waters off southern Patagonia. Neocalanus tonsus and Calanoides patagoniensis are a much rarer species. The latter is recorded in the southwestern Atlantic, for the first time, immediately east of Magallanes Strait and the Beagle Channel. The taxonomic status and worldwide biogeographic distribution of the region's calanids are briefly described and the patterns identified off Argentina are discussed in relation to the major hydrographic characteristics.     

Distribution of six major copepod species around South Georgia during an austral winter

Summary A net sampling survey was conducted around the island of South Georgia during July/August 1983. This study compares the age structure and vertical and horizontal distributions of the dominant copepods Calanoides acutus, Calanus simillimus, C. propinquus, Rhincalanus gigas, Metridia lucens and M. gerlachei. The chief physical and biological factors affecting the distributions of these species are assessed and the results are compared with those from a similar survey around the island carried out in early summer (1981/1982). The survey grid lay within the Polar Front during the winter survey, and horizontal changes in copepod abundance corresponded well to the temperature gradient across the front. This pattern was interrupted by the South Georgia shelf where the seasonal migrants (Calanoides acutus, Rhincalanus gigas and Calanus simillimus) occurred in high abundance. The concentration of these migrants over the shelf relative to the oceanic surface layer was attributed to the shelf having prevented their seasonal migration. Within the oceanic area the copepods occupied differing depths, with Calanoides acutus and Metridia gerlachei living deeper than Calanus simillimus, C. propinquus and M. lucens. The populations also tended to live deeper in the warmer (NW) portion of the oceanic survey area. In contrast to the summer survey the age structure of each species showed little variation throughout the survey area. This was attributed mainly to the decreased rates of copepod growth and metabolism in winter.     

Abundance,population structure and vertical distribution of dominant calanoid copepods on the eastern Weddell Sea shelf during a spring phytoplankton bloom

The main emphasis of this study was to analyse the short-term development of abundance, population structure and vertical distribution of the dominant calanoid copepods during a phytoplankton bloom in the coastal area of the eastern Weddell Sea in December 2003. Microcalanus pygmaeus was by far the most abundant calanoid species. Metridia gerlachei, Ctenocalanus citer, Calanoides acutus, Calanus propinquus and the ice-associated Stephos longipes were also present in considerable proportions. The observed changes in the population characteristics and parameters of these species are described in detail and discussed in the context of the spring phytoplankton bloom. A conspicuous event occurring during the final stage of the study was the development of a strong storm. While the results suggest that this storm did not have any considerable influence on the populations of all other investigated copepod species, it very likely caused pronounced changes in the S. longipes population present in the water column. Before the storm, S. longipes was found primarily in the upper 100 m of the water column, and its population was dominated by adults (mean proportion = 41%) and the copepodite stage I (mean proportion = 30%). After the storm, the abundance increased considerably, and the copepodite stage I contributed by far the largest proportion (53%) of the total population indicating that the early copepodite stages probably had been released from the sea ice into the under ice water layer due to ice break-up and ice melt processes caused by the storm.     

Effects of a rapidly receding ice edge on the abundance,age structure and feeding of three dominant calanoid copepods in the Weddell Sea,Antarctica

Open-water, marginal-ice and in-ice zones were sampled in the Weddell Sea during November and December, 1993 in an effort to examine the influence of the early spring bloom on the diet and population structure of the three biomass dominant copepods: Metridia gerlachei, Calanus propinquus, and Calanoides acutus. The abundance of all three species in the upper 200 m was highest at stations in the open water, but individually, each species displayed a unique trend. M. gerlachei, which showed the least variability, was significantly more abundant in open water than in the marginal-ice zone. The abundance of Calanus propinquus was higher in open water than in the marginal-ice zone or in the ice. Calanoides acutus displayed the highest variability, with significant differences between all three ice-cover zones. Diet analysis revealed no significant differences in the number of food items within each ice-cover zone and diatoms were the most numerous item identified in the guts of all three species. However, M. gerlachei and Calanus propinquus also contained metazoan material, while Calanoides acutus did not. There were dramatic differences in the age composition of the species between the zones. Early copepodite stages of all three species predominated at the ice edge and in open water. Numbers of M. gerlachei adult females were roughly equivalent in all three zones while Calanoides acutus and Calanus propinquus adult females composed a higher fraction of the total population within the ice. These results compare well with life-history data compiled by other authors and reinforce the importance of the ice edge to bloom-dependent Antarctic zooplankton. Accepted: 5 April 1999     

Seasonal succession of Copepoda in the Bahía Blanca estuary

The abundance and species composition of Copepoda with respect to other mesozooplanktonic groups were studied at the harbour of Ingeniero White in the inner zone of the Bahfa Blanca estuary, between July 1990 and August 1991. Maximal copepod abundances of 4.7 × 10 m^–3 and 4.9 × 10 m^–3 were observed in January 1991 and May 1991, respectively. Minimal abundances of 6 m^–3 were found in June 1990. Acartia tonsa was present throughout the year with high dominance in summer-autumn (December to May). Eurytemora affinis was present from July to October 1990 (first pulse) and from July to September 1991 (second pulse). Euterpina acutifrons was most abundant during spring 1990, whereas Paracalanus parvus was most abundant during winter-spring (July–October) 1990 and April–May 1991. The rest of the copepods were observed during winter and spring 1990 and July–August 1991. Calanoides carinatus and Labidocera fluviatilis, both species from the outer estuarine waters, were only found on two sampling dates. The proportion of meroplanktonic forms was high in certain months of the annual period considered. Differences between the copepod seasonal succession studied here and those observed during several years in the 1980's are discussed.     

Effect of food composition on egg production and hatching success rate of two copepod species (Calanoides carinatus and Rhincalanus nasutus) in the Benguela upwelling system

We have analysed the daily egg production (EPR) and hatchingsuccess rates of the calanoid copepods Calanoides carinatusand Rhincalanus nasutus as a function of nano- and microplanktonconcentration and composition in the northern Benguela upwellingsystem off Namibia. Food concentration explained 55% (R. nasutus)to 62% (C. carinatus) of the EPR variability. We found no relationbetween the residuals of the food concentration–EPR regressionand the percentage of the different taxonomic components ofthe nano- and microplankton. Nor was there a relation with theproportion of the diatom Skeletonema costatum that dominatedthe major blooms or with the number of nano- and microplanktonspecies. We conclude that food quality differences could notbe attributed to the relative composition of microplanktonicparticles of the different groups (i.e. taxonomic composition).     

Gut contents and diel feeding rhythm in dominant copepods in the ice-covered Weddell Sea,March 1992

Zooplankton was sampled at a diurnal station during the drift of the Ice Station Weddell-1 in the western Weddell Sea in March 1992. Gut contents of Metridia gerlachei, Calanoides acutus, Calanus propinquus and Rhincalanus gigas were studied. Calanoides acutus was trophically inactive, but 18%, 33% and 45% of Calanus propinquus, R. gigas and M. gerlachei, respectively, contained food in their guts. Diel changes in gut levels and percentage of individuals with full guts with maximum during the dark period were observed in M. gerlachei copepodite stage V and females. The possible reasons for observed differences in foraging patterns of the four copepod species are discussed.     

Population dynamics of two small pelagic fish in the central-south area off Chile: delayed density-dependence and biological interaction

It is widely believed that environmental variability is the main cause for fluctuations in commercially exploited small pelagic fish populations around the world. Nevertheless, density-dependent factors also can drive population dynamics. In this paper, we analyzed thirteen years of a relative abundance index of two clupeoids fish populations coexisting in the central-south area off Chile, namely the common sardine, Strangomera bentincki, and anchovy, Engraulis ringens. We applied the classical diagnostic tools of time series analysis to the observed time-series. Also, the realized per capita population growth rate was studied with the aim of detecting the feedback structure that is characterizing the population dynamics of the two species. The analysis suggests that population fluctuations of the two species have an important density-dependent component, displaying first-order (direct density-dependent) and second-order (delayed density-dependent) simultaneously. The density-dependent component explained 70.5 and 55.6 % of the realized per capita population growth rate of common sardine and anchovy, respectively. The deterministic skeleton model showed an asymptotic convergence to equilibrium density. In presence of a stochastic environment, fluctuations were reproduced for the species showing a component of fluctuation with a period of 4 year. The intrinsic dynamics of each species is typical of interacting species resulting from trophic interactions. It is postulated that the second-order dynamics of S. bentincki and E. ringens in central-south Chile, may be the result from interactions with a specialist predator (the fishing fleet), interacting with exogenous environmental factors.     

Zooplankton distribution in the coastal upwelling system along the Banc d'Arguin,Mauritania

In the framework of the Dutch Mauritania Expedition 1988, zooplankton was sampled in the beginning of the spring upwelling season at 44 stations along the Banc d'Arguin, Mauritania. Distribution patterns of oceanic and neritic copepod species and especially of the upwelling indicator Calanoides carinatus (Kröyer) were analysed in search for direct proof of circulation of ocean upwelling water over the inshore banks. The vertical temperature distribution at the stations pointed to upwelling in two different parts of the shelf area. According to the copepod data the two upwellings were of a different nature. A restricted area at the shelf edge near Cap Timiris appeared to represent a relatively stable coastal upwelling situation. Upwelled water present near Cap Blanc was found to have traveled quite recently over a considerable part of the shelf. In the central area between the two upwellings, Banc-water extended from the shelf into the ocean. The possible impact of the observed circulation of ocean water on productivity of the shelf benthic system is discussed.     

Limits to genetic bottlenecks and founder events imposed by the Allee effect

The Allee effect can result in a negative population growth rate at low population density. Consequently, populations below a minimum (critical) density are unlikely to persist. A lower limit on population size should constrain the loss of genetic variability due to genetic drift during population bottlenecks or founder events. We explored this phenomenon by modeling changes in genetic variability and differentiation during simulated bottlenecks of the alpine copepod, Hesperodiaptomus shoshone. Lake surveys, whole-lake re-introduction experiments and model calculations all indicate that H. shoshone should be unlikely to establish or persist at densities less than 0.5–5 individuals m⁻³. We estimated the corresponding range in minimum effective population size using the distribution of habitat (lake) sizes in nature and used these values to model the expected heterozygosity, allelic richness and genetic differentiation resulting from population bottlenecks. We found that during realistic bottlenecks or founder events, >90% of H. shoshone populations in the Sierra Nevada may be resistant to significant changes in heterozygosity or genetic distance, and 70–75% of populations may lose <10% of allelic richness. We suggest that ecological constraints on minimum population size be considered when using genetic markers to estimate historical population dynamics.     

Temporal and spatial variation of dominant pelagic Copepoda (Crustacea) in the Weddell Sea (Southern Ocean) 1929 to 1993

The abundances of four dominant Antarctic copepod species, Metridia gerlachei, Rhincalanus gigas, Calanoides acutus and Calanus propinquus, were examined in the Southern Ocean in a combination of a literature review, analysis of museum samples and field sampling. The data were analysed for spatial and temporal variations. The data included in the analysis were from the Weddell Sea area in the summertime at periods 1929–1939 and 1989–1993. The results are discussed in the light of environmental changes and their hypothesised and observed consequences in the Southern Ocean: global temperature change, ozone deficiency and cascading trophic interactions. Combining all these hypothetical effects our null hypothesis was that there were no consistent long-term changes in the abundance of dominant pelagic Copepoda. The null hypothesis was rejected, since several taxons did show statistically significant long-term changes in abundance. The changes were not uniform however. The numbers of adults and juveniles of Calanus propinquus increased significantly between the periods studied. Adult stages of Calanoides acutus were the only taxon decreasing in abundance, in concert with the cascading trophic interactions theory. Latitudinally, only Metridia gerlachei showed a significant increase from north to south. Longitudinally, the abundances of Calanus propinquus juveniles and both adults and juveniles of Rhincalanus gigas increased from west to east. There were no significant variations between day and night samples. Interannual changes were statistically significant in juvenile stages of all the species and in adults of Calanus propinquus. We conclude that no uniform and consistent abundance changes could be observed in the pelagic Copepoda of the Weddell Sea that could be connected to major environmental changes, expected to affect the whole planktonic ecosystem of the Southern Ocean. Significant changes in some of the species studied show that the pelagic ecosystem is not in a steady state, but in addition to interannual changes, there are also major fluctuations extending over decades. Received: 5 December 1996 / Accepted: 24 March 1997     

Aspects of the study of the life cycles of Antarctic copepods

Different approaches to the study of life cycle strategies of Antarctic copepods are described in an attempt to shed new light on our present knowledge. To date, most studies were carried out on abundance, horizontal and vertical distribution and stage composition during different seasons and in various regions. Hence, the seasonal pictures had to be compiled from different years and sampling regions. The physiological method includes measurements on e.g. egg production, feeding, respiration and excretion rates, C:N and O:N ratios, lipid and protein contents. However, both physiological and biochemical data are still rare. Results of field observations are given in this paper for investigations conducted within the last 15 years in the eastern Weddell Sea, while data of physiological parameters are based on a broader geographical region. In the eastern Weddell Sea, eight copepod species account for about 95% of copepod abundance and for more than 80% of copepod biomass. Within the calanoids, the small species Microcalanus pygmaeus dominates by numbers with 66%, while the large species Calanoides acutus and Calanus propinquus comprise together 52% of the biomass. Species abundance is lowest in winter and highest in summer/autumn, however, seasonal changes in the abundance of M. pygmaeus are small and this species occurs in similar quantities throughout the year. All copepod species show a distinct seasonal vertical distribution pattern and they occur in upper water layers in summer, in contrast to the other seasons. However, the depth layers of maximum concentration differ between species. The ontogenetic vertical migration is most pronounced in C. acutus and relatively weak in C. propinquus. The age structure also shows seasonal differences with the youngest population observed in summer for C. acutus, C. propinquus, Ctenocalanus citer or autumn for Metridia gerlachei, whereas the M. pygmaeus population is oldest during summer. The youngest copepodite stage and the males are not always present in C. acutus and C. propinquus. In contrast, all developmental stages and both sexes occur throughout the year in M. gerlachei, M. pygmaeus and C. citer. Gonad maturation in the dominant calanoid species proceeds well before the onset of phytoplankton production in the eastern Weddell Sea. However, the highest portion of females with ripe gonads and hence highest egg production rates coincide with the productive period in spring and summer. In autumn, ovaries of the three larger species C. acutus, C. propinquus and M. gerlacheiare all spent. In contrast, the percentage of ripe females of the two smaller species, C. citer and M. pygmaeus, stays high in autumn. Egg production rates are highly variable within one region and species. Many copepods accumulate large depots of lipid, mainly wax esters. In contrast, five species (C. propinquus, C. simillimus, Euchirella rostromagna, Stephos longipes and Paralabidocera antarctica) almost exclusively synthesise triacylglycerols and not wax esters. The lipid content exhibits distinct seasonal patterns, and is highest in autumn. A seasonal difference is also obvious in metabolic activities with lowest rates during the dark season. The adaptation to the pronounced seasonality in the Southern Ocean differs greatly between copepod species, and most Antarctic copepods stay active during the dark season. Calanoides acutus seems to be the only true diapause species. Calculations of summer developmental rates and winter mortality rates of the large species C. acutus and C. propinquus suggest that both species have a 1-year life cycle with few females overwintering and probably spawning a second time. In contrast, a 2-year life cycle is more likely in R. gigas. However, life cycle durations of all species studied are still uncertain and regional differences are very probable.     

Detection of zooplankton items in the stomach and gut content of larval krill, Euphausia superba, using a molecular approach

The usefulness of a molecular approach based on polymerase chain reaction (PCR) was investigated to identify and quantify the feeding of larval krill on zooplankton organisms in the Lazarev Sea during winter in 2006. Different primers and probes of dominant copepod species (Oithona sp., Ctenocalanus citer, copepodid stages of Metridia gerlachei and Calanoides acutus), co-occurring with larval krill under sea ice during winter, were developed for quantitative PCR (qPCR) and their species specificity was tested on target and non-target species. The qPCR results showed that larval krill were exclusively feeding on Oithona sp. This result was confirmed by microscopic analysis of stomach and gut contents of larvae from the same stations.     

The occurrence of Rhincalanus gigas,Calanoides acutus,and Calanus propinquus (Copepoda: Calanoida) in late May in the area of the Antarctic Peninsula

Summary The Antarctic copepod species Rhincalanus gigas, Calanoides acutus and Calanus propinquus were studied in the area of the Antarctic Peninsula in May 1986. Research was focussed on vertical distribution and stage composition of the populations. Rhincalanus gigas occurred in greatest densities in the upper layers of the water column, and copepodite stages CI and CII and nauplii dominated the population. Gut content analyses suggest that R. gigas was actively feeding. Copepodite stage CV dominated the Calanoides acutus population. At two deep basin stations (water depth>1000 m) the C. acutus population occurred below 500 m, whereas at shallower stations the majority was found above 300 m. Most specimens had empty guts. Calanus propinquus occurred in low densities, mainly in the upper water layers, and copepodite stage CV dominated. Most individuals of stage V had food in their guts. Our results suggest that C. acutus had ceased feeding and was overwintering in a resting stage (diapause), while C. propinquus and R. gigas were still active, the latter species having finished a late autumn spawning.     

Distribution of six major copepod species around South Georgia in early summer

Summary An intensive net sampling survey was conducted around the island of South Georgia during November/December 1981. The distribution and copepodite stage structure of the dominant copepods Calanoides acutus, Calanus simillimus, C. propinquus, Rhincalanus gigas, Metridia lucens and Metridia gerlachei were compared. The herbivorous species had completed their spring vertical migration and their summer generations were developing during the survey. At every station, Calanoides acutus was noticeably more advanced than Rhincalanus gigas in its reproductive cycle. The species were also more advanced in their development in the SE oceanic part of the survey area than in the NW. However, copepod development rates are rapid at this time of year, and this apparent regional difference may be due mainly to temporal variation during the four weeks of the survey. A truly regional variation in timing of reproduction was found when development was compared between shelf and oceanic waters. Spawning of Rhincalanus gigas and particularly of Calanoides acutus was later over the shelf. The age structure of the epi-mesopelagic metridinids also differed between shelf and oceanic waters, but no age differences were found for either Calanus simillimus or C. propinquus, both of which live and spawn higher in the water column than the other species. The presence of the South Georgia shelf also limited the overall abundance of the two deeper living metridinids. However, the length of time between spawning (and production of large numbers of early copepodids) and sampling had the largest influence on observed species abundance within the survey area.