Life cycle of Boeckella poppei Mrazek and Branchinecta gaini Daday (King George Island, South Shetlands)

In Lake Wujka, a shallow, polymictic Antarctic lake situated at 15 m from the seashore, several yearly cohorts occur of the copepod Boeckella poppei and one of the fairy shrimp Branchinecta gaini. There is circumstantial evidence that the two species compete for food, and perhaps adult fairy shrimp feed on the nauplii of the copepod. Both species are positively influenced by a measure of salinity. However, when autumn storms massively sweep seawater into the lake, all fairy shrimp are wiped out; no hatching occurs until next spring. In B. poppei, some nauplii and copepodites survive or hatch after the salt flows out of the lake. This is an advantage to the copepod that may balance its coexistence with the shrimp. Its cycle is, however, aborted by the freezing of the lake. In contrast to many other Antarctic lakes, the life cycle of the crustaceans is therefore controlled by salinity rather than freezing.     

Revised estimates for the biomass and production of zooplankton in Lake George, Uganda

The relationship of total body length to formalin dry weight for individuals of Thermo-cyclops hyalinus and Mesocyclops leuckarti, from the tropical Lake George, Uganda, is given and shows that previous estimates of individual biomass and, therefore, of zooplankton standing crop and production based on them, were too high. The annual mean standing crop of total Crustacea, in 1969–70 was 828 mg (dw) m^?2 and for T. hyalinus, the dominant species, 559 mg (dw) m^?2 Biomass tended to be higher in the mid-lake area than inshore. The annual mean production of T. hyalinus is now estimated to be 44 mg (dw) m^?2 day.     

Predaceous feeding habits of Limnocalanus macrurus

Limnocalanus macrurus, a large, glacial-relict copepod, hasbeen assumed an omnivore or a herbivore; predaceous habits ofthe species are unknown. The predaceous feeding habits of Limnocalanusfrom Lake Michigan were studied in the laboratory using naturalprey. Predation rates were highest on copepod nauplii. Copepoditesof Diaptomus spp. and Cyclops spp. were preyed upon at lowerrates. Limnocalanus preyed selectively upon nauplii <300µm. Small cyclopoid copepodites (<–750 µm)were also selected over large copepodites. Experiments usingtwo prey types showed that nauplii were selected over all copepodites,and that no selectivity existed for either diaptomid or cyclopoidcopepodites. Predaceous feeding habits began in the fourth copepoditestage of Limnocalanus. Predaceous feeding rates of Limnocalanuschanged seasonally being highest in late spring and autumn andlowest in summer and early winter. Since Limnocalanus also feedson net-phytoplankton, predation rate changes may be relatedto changes in the relative abundance of large phytoplanktonand naupliar prey in nature. Limnocalanus predation may be animportant factor in structuring the zooplankton community. Present address: Great Lakes Research Division, University Michigan,Ann Arbor, MI 48109, USA     

Relationship Between N:P Ratio and Growth Rate During the Life Cycle of Calanoid Copepods: An in situ Measurement

The mean nitrogen and phosphorus contents of the calanoid copepodMixodiaptomus laciniatus Lilljeborg were measured to the stagelevel throughout the ice-free period of a high mountain lake.Our results indicate large intraspecific variations in calanoidelemental composition. While mean N as dry weight increasedfrom 3.0 ± 1.3 in nauplii to 6.0 ± 2.1 in copepodites,mean P content showed the opposite trend, varying intraspecificallyfrom 0.98 ± 0.26% in nauplii, to 0.87 ± 0.21%in copepodites and 0.51 ± 0.16% in adults. Thus, themean N:P ratio increased ontogenetically from 3.3 in naupliito 13.3 in copepodites and 24.6 in adults. Two ontogenetic parameters,the growth rate and body size, were associated with zooplanktonstoichiometry. Among all 11 copepod stages, growth rate waspositively related to %P and negatively related to %N and N:Pratio. A two-part analysis of these relationships, before andafter metamorphosis, showed that the nauplii growth rate explainednearly all the variance in naupliar P content. A high P contentin nauplii may reflect a high content of RNA, translating intorapid growth rates. Overall, these results tend to support thehypothesis linking specific growth rate with P content for copepods,but these results also suggest that the validity of this hypothesisis robustness when differences in the life history of copepodsas a consequence of metamorphosis are accounted for. We suggestthat the intra-stage variation in P content is associated withpeaks of intensive metabolic activity during the process ofmolting in copepods, and we emphasize the importance of newempirical evidence to examine this hypothesis further.     

Seasonal vertical distribution and diel migration of zooplankton in a temperate stratified lake

The investigation of the vertical distribution of the zooplankton community in the temperate Lake Trichonis during four seasons in 2005, showed the existence of vertical segregation among species, ontogenetic stages and sexes within and between the major groups. In each season, the two or three more abundant rotifer species distributed at separate depth layers, while this feature was maintained during the entire 24 h period, since no diel vertical migrations (DVM) were performed. In contrast, the crustacean community, comprised mainly by the calanoid copepod Eudiaptomus drieschi and the cladoceran Diaphanosoma orghidani, showed various patterns of DVM, being more pronounced in spring and summer. Females of E. drieschi distributed deeper than males, while the copepod nauplii were found mainly in the surface layer in all four seasons. Temperature was the most important abiotic factor affecting directly and indirectly the vertical distribution and migration of various species. During stratification, the metalimnion was the most productive layer in Lake Trichonis, having maximum values of dissolved oxygen and low transparency due to high concentration of organic matter and phytoplankton. The DVM patterns of the crustaceans indicate that the metalimnion acts probably as a daylight refuge against predation by Atherina boyeri, which is the dominant planktivorous fish in the lake.     

Zur Biologie vonParamphiascella fulvofasciata (Copepoda,Harpacticoida)

The benthic harpacticoid copepodParamphiascella fulvofasciata Rosenfield & Coull was collected from holdfasts ofLaminaria hyperborea from a subtidal area of Helgoland (North Sea). All developmental stages ofP. fulvofasciata are raptorial feeders. The feeding of the nauplii is advanced by a marginal setule-crest of the labrum which prevents food-particles from being swept away. The oral appendages of the copepodites circumscribe a frustal space ventral to the mouth which facilitates uptake of food-particles. The nauplii are not able to swim and perform stalking movements with their antennal endopodites. Good swimming ability as well as digging-in-behaviour and negative phototaxis of the copepodites indicate epi- as well as inbenthic mode of life. Several life-cycle characters are described. Precopula lasts ca. one day. The mean egg-number is 27, and mean egg-diameter is 87 × 75 µm. The number of nauplii per egg (double)-sac amounts to 25–30. Developmental time at 19°C is 6–9 days (nauplii) and 20–24 days (copepodites). The whole developmental period lasts 28 days. The maximal lifespan in the laboratory is 193 days. Sex-ratio is almost balanced. Females produce egg-sacs more than 3.5 times during their life period. Seasonal effects on reproductive activity have not been detected in laboratory cultures.     

Zooplankton community changes in Lake Kinneret (Israel) during 1969–1985

Long-term records (1969–1985) of zooplankton density in Lake Kinneret indicated significant reduction of biomass (Copepods, Cladocera, Rotifera) and production (Copepods, Cladocera). Nauplius and adult copepod densities decreased but those of copepodites did not change. Mesocyclops was suppressed more than the smaller Thermocyclops and males of both genera became more abundant relative to the larger females. Ratios of small/large Cladocera densities became higher. Numbers of total cladocerans were stable, and therefore reduction of Cladocera grazing capacity is assumed. The abundant Keratella spp. were reduced. It is likely that intensification of fish visual-attack-predation pressure shifted the size-class structure towards smaller adult copepods and cladocerans. Reduction of Keratella spp. and copepod nauplii was probably affected by increasing pressure of fish filter-feeders. Data on fish food consumption, feeding behaviour and fisheries management suggested their direct impact on long-term changes of zooplankton in Lake Kinneret.     

Diurnal residence of the larger stages of the calanoid copepod Acartia tonsa in the anoxic monimolimnion of a tropical meromictic lake in New Guinea

We studied diel vertical migration of the calanoid copepod Acartiatonsa in a coastal meromiclic lake (Lake Nagada, Papua New Guinea).During the day, the animals migrate several metres below theoxycline to just above or into the sediments, and remain inanoxic water. Vertical migration patterns differ between ontogeneticstages; adults and larger copepodites do not occur in appreciablenumbers in the water samples during the day (deepest sampletaken at {small tilde}0.5 m above the sediment). Around sunset,the larger copepodites gradually appear in the water column,reaching the near-surface food layer (0–2 m) only wellafter sunset. Our results suggest that the evening ascent ofthe copepods in this tropical lake is related to relative changesin light intensity, and that there are age- and sex-relateddifferences in the responses to these stimuli.     

Suppression of microzooplankton by zebra mussels: importance of mussel size

1. The zebra mussel (Dreissena polymorpha) is amongst the most recent species to invade the Great Lakes. We explored the suppressive capabilities of mussels 6–22-mm in size on Lake St Clair microzooplankton (< 240)μm) in laboratory experiments. 2. Absolute suppression of rotifers and Dreissena veliger larvae was proportional to mussel shell length for individuals larger than 10 mm; larger zooplankton, mainly copepod nauplii and Cladocera, were not affected. Mussel clearance rates on rotifers generally exceeded those on veligers, although rates for both increased with increasing mussel size. Rotifer-based clearance rates of large (22 mm) mussels approached published values for phytoplankton food. 3. Most zooplankton taxa, particularly rotifers, declined significantly in western Lake Erie during the late 1980s concomitant with the establishment and population growth of zebra mussels in the basin. Densities of some taxa subsequently increased, although rotifers and copepod nauplii densities remained suppressed through 1993. Available evidence indicates that direct suppression by Dreissena coupled with food limitation provides the most parsimonious explanation for these patterns.     

Evolutionary shifts in copepod acid tolerance in an acid-recovering lake indicated by resurrected resting eggs

We employed zooplankton resting eggs to track population-level shifts in acid tolerance over the last century in a boreal shield lake recovering from acidification. Shifts in mean and variance of ecological tolerances of species that occupy ecosystems recovering from anthropogenic stress are important to consider because of their potential to influence eco-evolutionary dynamics at community and ecosystem levels. In a laboratory experiment, we compared juvenile survival of Leptodiaptomus minutus copepods hatched from resting eggs from three time periods (80- to 100-year- old: pre-industrial, 20- to 50-year- old: lake acidification, and 8- to 10-year- old: recovery of lake-water back to pH ≥ 6.0) under several pH treatments. Mitochondrial DNA was used to confirm species identity and to test for population bottlenecks as a possible mechanism to explain our results. We expected that nauplii hatched from eggs deposited prior to industrialization (lake-water pH ≈ 6.0) and from the period of pH recovery (lake-water pH ≥ 6.0) would have lower mean and more variable survival at acidic pH compared to nauplii hatched from the period of peak lake acidification (lake-water pH ≈ 4.7). Our results, which are likely a combination of both genetic and environmental effects, suggest support for this hypothesis. Nauplii hatched from eggs deposited during the period of acidification in George Lake had reduced variation in pH tolerance compared to the recovery period. This was likely driven by strong selection rather than genetic drift because we found no evidence of a population bottleneck. However, we could not detect differences in the variance of naupliar survival between pre-industrial and acidic time periods. Trends in mean naupliar survival from different time periods matched findings from other field-based studies that detected a relationship between lake acidification history and acid tolerance in L. minutus.     

Mean individual dry weight and length-weight regressions of some zooplankton of Lake Kariba

Individual dry weights of various crustacean zooplankton in Lake Kariba were determined. All the zooplankters were collected from pelagic waters. Regression equations relating dry weight to body length were computed for those species or groups in which size classes could be separated: Bosmina longirostris, Ceriodaphnia cornuta, and copepod copepodites and adults. The mean individual dry weights obtained for most of these zooplankters are similar to those reported in other studies.     

The Zooplankton community of Croker Passage,Antarctic Peninsula

Summary Zooplankton species composition, abundance and vertical distribution were investigated in the upper 1000 m of Croker Passage, Antarctic Peninsula during the austral fall (March–April, 1983). 106 species were identified, many being mesopelagic and reported previously from the Southern Ocean. The most numerous species (>1000/100 m³) were the copepodsMetridia gerlachei, Microcalanus pygmaeus, Oncaea antarctica andOncaea curvata. Oncaea curvata alone constituted half the zooplakton population. Zooplankton biomass was dominated by three copepod species,Metridia gerlachei, Calanoides aculus andEuchaeta antarctica,which comprised 74% of the biomass. Size analysis revealed most of the zooplankton numbers were in the >1 mm fraction. The biomass distribution was polymodal with major maxima in the >1 mm and the 4–4.9 mm size classe. The >1 mm peak, exclusive of protozoans, was primarily copepod nauplii and copepodites ofOncaea, Metridia andMicrocalanus. The 4–4.9 mm peak was mostlyCalanoides acutus andMetridia gerlachei.All of the principal species had broad vertical distributions both day and night. There was some suggestion of diel vertial migration byMetridia gerlachei andEuchaeta antarctica, with segments of their populations migrating into the upper 100 m and 200 m, respectively, at night. Most of the dominant and subdominant species were concentrated below 200 m,with only the subdominantOithona similis having its maximum in the epipelagic zone. The occurrence of zooplankton at winter depths appears to have been earlier in Croker Passage in 1983 than has been generally reported for waters south of the Polar Front.Total standing stock of net-caught zooplankton (>15 mm) in the upper 1000 m was estimated at 3.1 gDW/m², which is somewhat higher than values reported for the West Wind Drift and for open ocean areas of temperate to tropical latitudes.Euphausia superba (17–52 mm) dominated the pelagic biomass, exceeding zooplankton standing stock under a square meter of ocean by a factor of 15. This is in contrast to lower latitudes where zooplankton biomass is usually greater than macrozooplankton-micronekton.     

Diel vertical migration by males,females, copepodids and nauplii in a limnetic population of diaptomus (Copepoda)

Diel vertical migration (DVM) by the sexes and life history stages of the copepod Diaptomus novamexicanus (Herrick) was investigated during two summers in a subalpine lake (Castle Lake, California, USA). Migratory behavior was quantified repeatedly by 1300 and 21–2200 hr sampling in the epilimnion. Population density and vertical distribution were estimated routinely at 1300 hr and at 2200 hr on selected dates. Males had greater night to day biomass differentials and ratios than females. The sexes showed comparable DVM participation (% of total biomass migrating) and amplitudes of migration (distance of ascent). Copepodids displayed higher participation, night/day ratios and amplitudes than younger copepods, but adult participation differed only from that of nauplii. Active participation in high amplitude ascents thus increased with age and was greatest in adult males. Analysis suggests that such relative behaviors probably occur in most limnetic Diaptomus, and that the magnitude of ascent into the epilimnion is associated with daytime vertical distribution.     

Appendage activity recordings of the sub-Antarctic copepod Drepanopus pectinatus in relation to its feeding behaviour

Summary Movements of the cephalic appendages of nauplii (stages 3 and 4), copepodites (stage 4) and adult female Drepanopus pectinatus were investigated using a computerized micro-impedance unit. Direct measurements and visual observations of the behaviour of restrained copepods were made in the presence of filtered seawater, naturally occurring phytoplankton of different sizes, prefiltered extracts of phytoplankton and seston. The normal appendage movements recorded in filtered seawater were similar for adult females and copepodites. Traces for nauplii were different compared with those of the two other developmental stages. Appendage movements were modified when adult females were offered particles of different sizes and extracts of phytoplankton, showing significant changes in the frequency of the limb beats in relation to each stimulus. Impedance traces distinguished between activity of copepod appendages related to feeding and to swimming such as: flicking, stroking, jerking, combing, handling and rejecting particles. Prolonged recordings of restrained females in association with visual observations suggested that the activity of the mandibular palp was related to gut fullness and peristaltic movements, and to fecal pellet formation and evacuation. These results indicated that D. pectinatus is an intermittent beater responding to the physical and chemical characteristics of food.     

How important is bacterial carbon to planktonic grazers in a turbid, subtropical lake?

This study examined the relative contributions of bacterialand phytoplankton production to the pelagic carbon flow of LakeOkeechobee, a large and shallow subtropical lake. Due to thepredominance of cyanobacteria in this lake, we hypothesizedthat bacterial carbon flow would be larger than phytoplanktoncarbon flow to grazers. Using epifluorescent and light microscopyand radiotracer techniques, we measured the carbon biomass ofplanktonic functional groups and carbon flow between these groups.The functional groups that we used in this study included: picophytoplankton,autotrophic nanoflagellates (ANAN), microphytoplankton, bacteria,heterotrophic nanoflagellates (HNAN), ciliates, microzooplankton(rotifers and copepod nauplii) and macrozooplankton (cladocerans,copepodites and adult copepods). Microphytoplankton dominatedthe carbon biomass of all plankton, whereas the calanoid copepod,Diaptomus, dominated the carbon biomass of the grazers. Phytoplanktoncarbon flow often was higher than bacterial carbon flow to grazers;however, bacterial carbon constituted a large percentage ofthe total carbon flow to grazers (33.7 ± 22.4%). Bacterialcarbon provided roughly one quarter of the carbon flow to macrozooplankton(27.1 ± 25.4%), whereas it provided half of the carbonflow to microzooplankton (57.4 ± 20.3%) and to protozoans(47.2 ± 25.8%). These results suggest that microbialpathways play an important role in the energetics of subtropicallake plankton communities. Although microbial loop pathwaysare important in many systems, direct bacterial carbon flowto macrozooplankton also may be important in copepod- and cyanobacteria-dominatedlakes.     

Demographic characteristics of the copepod <Emphasis Type="Italic">Acanthocyclops americanus</Emphasis> (Sars, 1863) (Copepoda: Cyclopoida) fed mixed algal (<Emphasis Type="Italic">Scenedesmus acutus</Emphasis>)-rotifer (<Emphasis Type="Italic">Brachionus havanaensis</Emphasis>) diet

The predatory copepod Acanthocyclops americanus is commonly found in shallow ponds and lakes in Mexico. We tested the survivorship and reproduction-related variables of this copepod, isolated from Lake Huetzalin (Xochimilco, Mexico City), on four mixed diets comprising two algal concentrations (0.8 × 10⁶ and 1.6 × 10⁶ cells ml⁻¹ of Scenedesmus acutus) and two rotifer densities (1 and 8 ind. ml⁻¹ of Brachionus havanaensis). Survivorship patterns of naupliar and adult stages of A. americanus were evaluated at 24 ± 1°C. The demography experiments were initiated with five females and five males (1:1 ratio, 5th copepodites) into each of the 16 test [= 4 diet combinations × 4 replicates (cohorts)] jars containing 50 ml medium and with one of the chosen algal-rotifer diets. Data on the survival of naupliar and copepodites (up to V) showed a high death rate (80%) in A. americanus by the time nauplii reached the adult stage. The duration of developmental time for nauplii and the copepodite V (C-V) was about 28 days by which time A. americanus became adults. Food combination had a significant effect on both survival and duration of the naupliar and C-V but not on the development time in C-I, C-II, C-III, and C-IV. Naupliar stages showed highest life expectancy at low food levels (rotifers or alga). Adult male and female copepods showed better survival under higher availability of rotifers in the medium, while higher algal density resulted in steep mortality during the first days. Average adult lifespan and life expectancy of A. americanus varied from 19 to 23 days, depending on the diet combination. Age-specific reproductive output (m _x ) of female A. americanus showed low reproduction (<10 offspring per female on any given day) when raised on low rotifer density, regardless of the algal concentration. On the other hand, enhanced offspring production (average of about 50 nauplii per female) was recorded when the rotifer density was higher (i.e., 8 ind. ml⁻¹). Gross and net reproductive rates varied from 48 to 438 and 27 to 318 offspring per female, respectively. In general, higher availability of rotifers in the medium resulted in higher (8–10 times as compared to those in low rotifer density) gross and net reproductive rates. There was no significant difference in the generation time (13–14 days) in relation to the diet combination. The rate of population increase (r) (range: 0.29–0.69 per day) increased with greater abundance of B. havanaensis in the medium. Thus, our study showed that inclusion of animal protein in the diet is necessary for high population growth rates of A. americanus.     

Ontogeny of growth, respiration and feeding rate of the freshwater calanoid copepod Eudiaptomus graciloides

The calanoid copepod, Eudiaplomus graciloides, was reared fromegg to adult on uni-algal diets (0.1. 0.5 and 2.5 mg dry wt1^–1) using the green alga, Chlamydomonas reinhardtii,as food, or on a mixed diet consisting of Lake Esrom water filteredthrough a plankton net with pore size 45 µm and supplementedwith C. reinhardtii (2.5 mg dry wt 1^–1). On the mixeddiet at 21.0°C growth in body dry wt (W, µg dry wt)was exponential, and the growth constants were 0.21 day^–1in the early to mid juvenile stage (N1 - C4) and 0.11 day^–1in the late juvenile to early adult stage (C4-A). At 14.5°Cthe corresponding growth rate constants were 0.10 and 0.08 day^–1.Similar growth rates were found at uni-algal concentrationsof 0.5 and 2.5 mg dry wt I^–1, and it was argued that thethreshold concentration for growth in Eudiaptomus was closeto 0.1 mg dry wt I^–1. The clearance (C, ml h^–1)of copepodites was measured on the uni-algal diets. The constantsof the regression (C = aW^b) were: a = 0.125, b = 0.858 (2000C. reinhardtii ml^–1), a = 0.068, b = 0.849 (10 000), a= 0.028, b = 0.875 (50 000). Ingestion rates were calculatedfrom the clearances and the average algal concentrations. Atthe three food levels the average daily rations were 30, 67and 125% of body dry wt. The respiration rate (R, nl O₂ h^–1)was measured in individuals reared on the mixed diet. The constantsof the regression (R = aW^b) were: a = 4.82, b = 1.07 (nauplii,14.5°C), a = 4.17, b = 0.904 (copepodites and adults, 14.5°C),a = 6.87, b = 0.757 (copepodites and adults, 21.0°C). Nosignificant difference in the respiration rate of copepoditesreared on uni-algal diets and the mixed diet could be demonstrated.Energy budgets were calculated. The assimilation efficiencyand the gross growth efficiency of copepodites decreased markedlywith increasing food concentration, the net growth efficiencyvaried from an average of 0.44 at the lowest algal concentrationto 0.60 on the mixed diet. The results are discussed in relationto previous findings with both freshwater and marine copepods.     

Genetic effects of habitat restoration in the Laurentian Great Lakes: an assessment of lake sturgeon origin and genetic diversity

Lake sturgeon (Acipenser fulvescens) have experienced significant habitat loss, resulting in reduced population sizes. Three artificial reefs were built in the Huron‐Erie corridor in the Great Lakes to replace lost spawning habitat. Genetic data were collected to determine the source and numbers of adult lake sturgeon spawning on the reefs and to determine if the founder effect resulted in reduced genetic diversity. DNA was extracted from larval tail clips and 12 microsatellite loci were amplified. Larval genotypes were then compared to 22 previously studied spawning lake sturgeon populations in the Great Lakes to determine the source of the parental population. The effective number of breeders (N_b) was calculated for each reef cohort. The larval genotypes were then compared to the source population to determine if there were any losses in genetic diversity that are indicative of the founder effect. The St. Clair and Detroit River adult populations were found to be the source parental population for the larvae collected on all three artificial reefs. There were large numbers of contributing adults relative to the number of sampled larvae. There was no significant difference between levels of genetic diversity in the source population and larval samples from the artificial reefs; however, there is some evidence for a genetic bottleneck in the reef populations likely due to the founder effect. Habitat restoration in the Huron‐Erie corridor is likely resulting in increased habitat for the large lake sturgeon population in the system and in maintenance of the population's genetic diversity.     

Spatial overlap of a predatory copepod,Acanthocyclops robustus,and its prey in a shallow eutrophic lake

Spatial overlap between Acanthocyclops robustus, with special emphasis on the adult females, and other zooplankton in one basin of a shallow (approximate depth of 2 m) eutrophic lake was studied.Horizontal distribution patterns were analysed on two dates. On both dates, most taxa examined showed large-scale patchiness between the three sections of the lake basin (approximate length of 1.2 km). Similarly, most taxa, with the important exception of the adult female Acanthocyclops robustus, were significantly patchily distributed on the small-scale (i.e. within sections). However, the intensity of such patchiness was, in general, relatively low. There was no consistent evidence of aggregation by the adult females or copepodites and adult males (the latter two were considered together) of the predator in such small-scale prey patches.Diurnal vertical distribution patterns were studied on two 24–25 hour periods. The first period was characterized by calm weather. Adult female, and perhaps male, Acanthocyclops robustus, Chydorus sphaericus, Bosmina Coregoni, Keratella cochlearis, Asplanchna species, Polyarthra vulgaris and Pompholyx sulcata seemed to show diurnal migration patterns, while seven other taxa showed consistent preferences for particular depths. Only copepod nauplii and Daphnia species were approximately evenly distributed. Negative correlations were found between the vertical distributions of the adult female predator and seven of the seventeen potential prey recognized.The first half of the second period was characterised by strong winds which abated during the second half. Most zooplankton taxa showed inconsistent heterogeneous vertical distributions or were homogeneously distributed with vertical heterogeneity developing towards the end of the period. Only Bosmina longirostris and Daphnia species seemed to show vertical migration patterns. Thus, no consistent vertical segregation between predator and prey was detected.     

Temporal changes in zooplankton composition in a hypersaline,Antarctic lake subject to periodic seawater incursions

Colonisation of Lake Fletcher, a hypersaline, meromictic lake in the Vestfold Hills, Antarctica, by the calanoid copepod Drepanopus bispinosus, the cyclopoid copepod Oncea curvata and an undescribed cydippid ctenophore is discussed. In 1978, salinity directly under the ice was 66‰ and repeated net hauls found no zooplankton. In 1983, adults of D. bispinosus were found, and in 1984, a reproductively active population of this species. Surface water salinity in 1984 was 56‰. During winter 1986, surface salinity was 54‰ and three zooplankton species (D. bispinosus, O. curvata and an undescribed cydippid ctenophore) had established populations in the lake. In 1986/87, high tides caused nearby Taynaya Bay to flood into the lake, and three further species (the calanoid, Paralabidocera antarctica, and two harpacticoids, Harpacticus furcatus and Idomene sp.) were found in the lake. It appears that periodic flooding after 1978 caused a salinity decrease in the lake from 66 to 54‰, and this enabled some invertebrate species to maintain year-round populations, whereas others require marine incursions to re-establish summer only populations.