Asynchronous vertical migrations of zooplankton in stratified lakes

Asynchronous vertical migrations of calanoid copepods Arctodiaptomus salinus were studied in two stratified lakes in the south of Siberia using the method of two-section enclosures. It was found that the presence of a pronounced thermocline and a depth maximum of phytoplankton (Lake Shira) contributes to the appearance of intensive individual migrations of copepods between areas of the epi- and hypolimnion.     

Predator-induced diet vertical migration in a planktonic copepod

Predator evasion is the most commonly hypothesized reason fordiel vertical migrations undertaken by a wide variety of planktonicorganisms in lakes and seas, yet direct evidence remains elusive.We tested the predation hypothesis by exposing enclosed populationsof a marine copepod Acartia hudsonica to caged or free-rangingindividuals of their natural predator, the planktivorous fishGasterosteus aculeatus. After little more than a week, adultcopepods changed their vertical distribution and migration behaviordepending on the presence or absence of predation. Only free-rangingfish induced vertical migration in the copepod population. Cagedfish had no effect, indicating that vertical migration was nota simple chemically mediated response of copepods to the predator.Rather, copepods seemed to react to the presence of predatorsby other means, perhaps visual or mechanical stimuli, and toexhibit a downward escape response which, because encounterswith visually orienting fish occur chiefly in the daytime, effectivelylimited the copepods' occurrence in the upper water column tothe night-time hours. Alternatively, because fish imposed heavymortality on copepods, it is possible that selective predationaltered the proportions of individuals with fixed, geneticallydetermined migration behaviors. We suggest experiments to distinguishthese alternatives.     

How are the vertical migrations of copepods controlled?

Using Calanus finmarchicus (Gunnerus) as a model organism, a hypothesis is suggested to explain the diel and seasonal vertical migrations of herbivorous copepods in boreal and polar waters. The hypothesis is based on the following assumptions. Hungry copepods are assumed to react to food smell by increased swimming. High lipid content is assumed to turn the copepods upside down. Light avoidance is assumed to operate solely while the copepods are satiated. The following three major peaks in downflux of phytoplankton remains are assumed to reach 1000 m depth or more: pre- and post-spring bloom peaks and the autumn increase. A minor “afternoon peak” in short-range downflux of phytoplankton is also assumed to exist. The assumptions are used to explain the following main traits in copepod migrations. The afternoon increase in downflux of phytoplankton material induces upward swimming of hungry copepods. If satiated, light avoidance brings them down again at dawn. The late stages of many species of copepods accumulate large amounts of lipids and if the above assumptions are valid, they will be turned upside down and swim down if activated. During midsummer, the downflux does not reach deep water and the copepods are assumed to spend some time in midwater until they moult. Copepods moulting from stage V into female adults use up to half of their lipids to produce eggs, which are more anteriorly located. This is assumed to turn their bodies back into an upright position and the copepods are assumed to swim up to the surface again when they smell sinking phytoplankton remains. Fat copepods are assumed to follow the downflux of phytoplankton material down to diapause depths, especially at the end of the spring bloom and in autumn. It is assumed that enough lipids are used up during the diapause to turn the copepods into head-up position again. The smell of fast-sinking fecal pellets containing prebloom phytoplankton is assumed to bring the copepods up from diapause again in late winter. The probable implications for the survival of cod larvae are discussed.     

Short-term changes in the mesozooplanktonic community in theSeine ROFI (Region of Freshwater Influence)(eastern English Channel)

Discrete-depth, hourly mesozooplankton samples were collectedover a 92 h period in May 1992 at an anchor station within theSeine Region of Freshwater Influence (ROFI) (English Channel).The mesozooplankton community defined as a euryhaline marineassemblage was dominated by the calanoid copepods Acartia spp.,Temora longicornis and Centropages hamatus, the cladoceran Evadnenordmanni and the appendicularian Oikopleura dioica. The semi-diurnaltidal current was the dominant factor in determining the short-termtemporal changes in the community in terms of density and speciescomposition so that zooplankton patches displayed oscillatingmotion in relation to tidal advection. Although a few species(e.g. Pleurobrachia pileus) exhibited higher densities aroundlow tide, maximum densities were observed for most species (e.g.T.longicornis and E.nordmanni) around high tide, according tosalinity variations. Diurnal changes were only reported forcyclopoid copepods (i.e. Halicyclops sp. and Cyclopina sp.)which wer$$$ mainly endobenthic during the day and moved intothe water column at night. Besides temporal changes in depth-averageddensities, most species exhibited vertical migrations at dieland/or tidal periods. Tidal vertical migrations were reportedonly for a few taxa and could be the result of passive mechanisms(e.g. vertical mixing) rather than of active behaviour. Dielvertical migrations were observed in most of the abundant taxa.While this migration pattern did not appear to be an adaptationto predator avoidance within the Seine ROFI, it could regulatehorizontal transport of organisms and promote their retention.The consequences of the short-term mesozooplankton fluctuationsfor sampling designs are discussed.     

Diel changes in vertical distribution of Oithona similis (Cyclopoida) and Oncaea curvata (Poecilostomatoida) under sea ice in mid-summer near Syowa Station, Antarctica

The diel vertical distributions of two small copepods, Oithona similis and Oncaea curvata, were investigated at 4-h intervals over a 24-h period under fast ice near Syowa Station during continuous daylight conditions in the Antarctic mid-summer, December 1993. Oithona similis and O. curvata exhibited small-scale diel vertical migrations during the study period, in a way opposite to what is expected, i.e., remaining mostly in the upper layer during the day and moving into deeper layers at night. The nighttime descent of both species coincided with the time of disappearance of a high algal concentration at the ice–water interface during the day and an increase of the algal concentration in the mid-water layer at night. This suggests the migration behavior of the copepods was responsible for the change of food availability. The daily grazing impact of these copepods was estimated to remove one-third of the algae daily released from ice during mid-summer at Syowa Station.     

On the Relations of Vertical Distribution,Diurnal Migration and Nutritional State of Herbivorous Zooplankton in the Northern North Sea during FLEX 1976

At a fixed station in the northern North Sea the occurrence of herbivorous and omnivorous zooplankton and the changes in its vertical distribution were studied over 2 1/2 months during the spring phytoplankton bloom in 1976. From a sporadical distribution of relative few plankton organisms in the mixed water column at the end of winter, in the middle of April a significant accumulation of organisms began to occur in the upper layers parallel to the development of stratification and the phytoplankton spring bloom. As exceptions, the populations of Microcalanus pusillus and Metridia lucens withdrew to below the thermocline. About ten days after the decline of the phytoplankton bloom, a few zooplankton species—especially wax ester producing copepods like Calanus finmarchicus, Pseudocalanus elongatus, Microcalanus pusillus, aad Metridia lucens—started diurnal vertical migrations. The movements of the species and stages occurred synchronously, leading in the same direction. Nevertheless, the amplitude of the migrations increased with each maturity stage reached. The vertical migrations, characterized by actual locations and velocities of the centers of gravity of the populations, are interpreted in terms of metabolism leading to fat storage, decomposition and reconstruction of lipids and proteins, in terms of a periodical change in the density of the organisms. Further data sets about the vertical distribution of organisms are cited to support the hypotheses stated.     

Control of Diapause by Acidic pH and Ammonium Accumulation in the Hemolymph of Antarctic Copepods

Life-cycles of polar herbivorous copepods are characterised by seasonal/ontogenetic vertical migrations and diapause to survive periods of food shortage during the long winter season. However, the triggers of vertical migration and diapause are still far from being understood. In this study, we test the hypothesis that acidic pH and the accumulation of ammonium (NH₄ ⁺) in the hemolymph contribute to the control of diapause in certain Antarctic copepod species. In a recent study, it was already hypothesized that the replacement of heavy ions by ammonium is necessary for diapausing copepods to achieve neutral buoyancy at overwintering depth. The current article extends the hypothesis of ammonium-aided buoyancy by highlighting recent findings of low pH values in the hemolymph of diapausing copepods with elevated ammonium concentrations. Since ammonia (NH₃) is toxic to most organisms, a low hemolymph pH is required to maintain ammonium in the less toxic ionized form (NH₄ ⁺). Recognizing that low pH values are a relevant factor reducing metabolic rate in other marine invertebrates, the low pH values found in overwintering copepods might not only be a precondition for ammonium accumulation, but in addition, it may insure metabolic depression throughout diapause.     

The structure and diurnal variations of the zooplankton of the Prince Edward Islands: Implications for the biomass build-up of higher trophic levels

Summary Short-term variations in the micro-, meso- and macrozooplankton communities at shelf and offshore stations in the vicinity of the Price Edward Islands were examined during April/May 1985 and 1986. Microzooplankton was dominated by copepod nauplii. Other holoplanktonic groups were represented in large numbers, while meroplanktonic larvae of benthic invertebrates were very scarce despite the large benthic population of the island shelf. Copepods dominated in terms of numerical abundance the meso- and macrozooplankton assemblage but cuphausiids were also very important in terms of biomass contribution. Dirunal vertical migrations were most pronounced for the euphausiids and the copepod Metridia gerlachei. The copepods Eucalanus longipes, Rhincalanus gigas and Microsetella sp. showed significant, but lesser, diurnal vertical migrations. A daylight decrease in zooplankton biomass was observed consistently on the island shelf, compared to the pattern found in deep-water. Data from daytime tows collected during April/May 1983 show that zooplankton biomass in the area increases with depth up to a stability level of about 50 mg/m³ (dry weight). It is suggested that an interaction between vertical migrations, surface Ekman drift and bottom topography results in zooplankton mass depletion by visual predators during daytime and replenishment during the night. The nocturnal advection of allochtonous zooplankton into the area may represent an input of food supply equivalent to as much as 2.2 times the local maximum phytoplankton production.     

Drifting meio- and macrobenthic invertebrates on tidal flats in Königshafen: A review

Many benthic species have been recorded to occur in the water column above the tidal flats in Königshafen. They were either passively suspended from the sediment or performed active migrations. Concerning both number of species and organisms, active migrations were more important. The causes for these migrations range from (1) individual escape from a sudden threat such as predator attack, over (2) group evasion of local subpopulations to avoid environmental deterioration, to (3) habitat changing of entire populations. The temporal scales involved range from seconds to seasons, and the spatial scales from cm to km. Such changes of distribution patterns have been demonstrated in juvenile molluscs and in meiofaunal copepods and plathelminths. Since migrations may interfere with many kinds of population studies in the field, new methods and concepts need to be developed to avoid and overcome pittalls. As a precaution, the migration potential of benthic species should be taken into account in any field study including population parameters.     

How plankton copepods avoid fish predation: From individual responses to variations of the life cycle

Analysis of original and published data on predator avoidance by marine and freshwater plankton copepods, a major diet of many young fishes, suggests that individual defense mechanisms play a minor role in copepod anti-predator behavior. Capture success by planktivorous fish depends largely on prey visibility and the ability of the prey to escape. Copepods have almost no chance to avoid relatively large fish when encountered, but they can be ranked according to their ability to escape from larvae and fries. In contrast to small pelagic fish, which are also under heavy predation pressure, copepods rely more on prevention of the threat of predation than on active attempts to escape. Seeking a refuge in habitats non-accessible to predators would be more effective for these small and rather slow animals. Retreat into such refuges is accomplished by vertical and horizontal migrations, either diel, seasonal or ontogenetic. A decrease in activity (feeding, metabolism, reproduction, movement), resulting in diapause in deep water layers in its most pronounced version, is the ultimate attempt by copepods to separate from their predators, both in time and in space.     

Fluorescence analysis of zooplankton gut contents and an investigation of diel feeding patterns

A method is described for measurement of the in situ feeding rate of planktonic herbivores. Freshly caught Zooplankton are washed with filtered sea water and frozen. The gut contents are later analysed using acetone extracts of the entire sample. Fluorescence peaks due to ingested chlorophyll are easy to quantify for sample sizes of 20–200 animals. This method obviates some of the problems, such as confinement and previous feeding history, associated with extrapolation from laboratory experiments to field conditions. The method is illustrated by an investigation of diel feeding patterns of marine copepods. Initial results indicate the complexity of these patterns, with either a single night peak or dawn and dusk peaks for adult copepods. Diel variations in feeding activity appear to occur in addition to, but closely interacting with, the diel vertical migrations of the animals.     

Shallow males, deep females: sex-biased differences in habitat distribution of the freshwater calanoid copepod Arctodiaptomus alpinus

In a small, circular high-altitude karst lake situated in the northeastern calcareous Alps of Austria the copepod Arctodiaptomus alpinus (Imhof) develops each spring from resting eggs after the lake basin fills in early spring. After mid-summer, late developmental stages and adults concentrate in a thin layer near the sediment throughout the lake basin. During daytime an average 86% and 71% of female and male copepods respectively resided near the bottom. Males exhibited stronger diel vertical migrations than females and part of the population concentrated in near-surface water at night, but the majority of copepods remained at the sediment during darkness. Macro-photographs and core samples revealed that the proportion of epibenthic male A. alpinus decreased constantly from >70% in 3 m depth to <20% below the 8-m isobath. The proportion of ovigerous females increased with depth, whereas non-ovigerous females were comparatively more abundant in shallow water. Concurrently the highest frequencies of copulating copepods were detected in areas of high male density. Above the 3-m isobath male A. alpinus formed dense swarms in scattered patches of the macrophyte Ranunculus eradicatus (>93% males; maximum abundance: 362 600 individuals m⁻²). Adults and larvae of the alpine newt Triturus alpestris are the top predators in this fishless alpine lake, and A. alpinus constitutes a major component of their diet. However, predator densities and different predation rates on male and female copepods were not sufficient to explain the observed horizontal distribution of A. alpinus . We argue that the optimization of successful sexual encounters in copepods is the ultimate driving force behind this segregation of both sexes of A. alpinus ('mate encounter hypothesis').     

Food-web manipulations influence grazer control of phytoplankton growth rates in Lake Michigan

Stocking piscivorous salmonids in Lake Michigan produced dramaticalterations in food-web structure, including higher numbersof large-bodied zooplankton (especially Daphnia pulicaria),lower summer chlorophyll concentrations and increased watertransparency. Experimental determinations of epilimnetic phytoplanktongrowth rates and of zooplankton grazing rates indicate thatherbivorous zooplankton controlled algal dynamics during thesummer of 1983 because grazers occupied the surface waters throughoutthe day. In 1985, however, both large- and small-bodied Daphniamade approximately equal contributions to total grazer biomass,and all grazers displayed pronounced diel vertical migrations,visiting epilimnetic waters only at night. This prohibited zooplanktonfrom controlling algal dynamics because grazing losses did notexceed phytoplankton growth rates. The changes in zooplanktoncommunity composition and behavior observed in summer 1985 probablyresulted from increased predation by visually orienting planktivorousfish, especially bloater chub (Coregonus hoyi). Effects of food-webmanipulations on phytoplankton dynamics were evident only duringJuly and August. During spring and early summer copepods dominateLake Michigan's zooplankton community. Owing to their smallbody size, copepods are less susceptible to fish predation andexhibit much lower filtering rates than Daphnia. Variabilityin zooplanktivorous fish abundance probably has little effecton phytoplankton dynamics during spring and early summer.     

The biology of mysids acclimatized in the reservoirs of the Volga River

Mysids were successfully introduced and aclimatized in the artificial reservoirs of the Volga River. They live there in the shallow sandy areas and their numbers amount to 300–500 ind./sq.m. They avoid direct sunlight, make slight vertical migrations according to the light intensity: during the morning hours their maximum is at the depth 0.2 m, at noon in 1.0–1.5 m. It has been proved that their biological features in the new localities do not differ from those in the original localities.     

Vertical migrations of herring,Clupea harengus,larvae in relation to light and prey distribution

Synopsis The influence of light and prey abundance on the vertical distribution of herring larvae was evaluated by three investigations made under calm weather conditions in the North Sea off the Scottish coast. The investigations took place at different time after hatching and the vertical distributions of three size groups of larvae (mean sizes 8,15 and 19 mm) were related to time of day and the vertical distribution of copepods. No migratory behaviour of copepods was observed but their vertical distribution differed between investigations. In the investigation on intermediate sized larvae, copepod density peaked at the pycnocline (40 m). Larvae concentrated at this depth at noon. At dawn and dusk larvae migrated towards the surface and the vertical distributions fluctuated semidielly. In the two other investigations, copepods were homogeneously distributed in the water column and after migration towards the surface at dawn larvae stayed in the upper water column during the day. The observations suggest that the daytime vertical distribution of larvae in calm weather is mainly determined by feeding conditions: the larvae move to depths were light is sufficient for feeding, and refinement within that zone is made according to a compromise between optimal light conditions for feeding and optimal prey densities.     

Rythmes d'activité natatoire de quelques copépodes hyponeustoniques (Anomalocera patersoni Templeton, Pontella mediterranea Claus, Labidocera wollastoni Lubbock) en fonction des conditions d'éclairement et de pression

Behaviour of some pontellid copepods (Anomalocera patersoni Templeton, Pontella mediterranea Claus, Labidocera wollastoni Lubbock) labelled with ¹³¹I was recorded under various experimental conditions. Some experiments were carried out in continuous darkness and others in regular light-dark. i.e., natural, conditions. When they are kept in darkness and when they are exposed to light, animals (males, females, and copepodite stages) exhibit a clearcut circadian and semi-circadian rhythm which persists as long as the experiments do. but sometimes there was some decrease in the activity after the first two or three days. Moreover, swimming activity of pontellids increases with increasing pressure (1 2 bars) and falls with decreasing pressure. Continuous pressure (2 bars) seems to attenuate endogenous periodicity and to enhance the exogenous rhythm of locomotory activity: hydrostatic pressure appears to synchronize rhythms. The comparison between, on the one hand, the rhythmic activity of pontellids exposed to various conditions of light and pressure and, on the other hand, the circadian fluctuations in abundance observed in the natural environment suggest that neustonic species display vertical migrations. The range and the duration of the upwards and downwards movements are probably less important than those of other pelagic copepods.     

Seasonal adaptations and the role of lipids in oceanic zooplankton

Oceanic zooplankton species exhibit quite diverse life history traits. A major driving force determining their life strategies is the seasonal variability in food supply, which is most pronounced in polar oceans where fluctuations in primary production are extreme. Seasonal adaptations are closely related to the trophic level of zooplankters, with strongest pressures occurring on herbivorous organisms. The dominant grazers, calanoid copepods and krill (Euphausiacea), have developed fascinating solutions for successful overwintering at higher latitudes. They usually exhibit a very efficient storage and utilization of energy reserves to reduce the effect of a highly seasonal primary production. The predominant larger Calanus species from the Arctic and Calanoides acutus from the Antarctic biosynthesize large amounts of high-energy wax esters with long-chain monounsaturated fatty acids and alcohols (20:1 and 22:1 isomers) as major components. They survive the dark season at depth in a stage of dormancy called diapause. In contrast, the Antarctic Calanus propinquus, a winter-active species, synthesizes primarily triacylglycerols, which are dominated by long-chain monounsaturated fatty acids with 22 carbon atoms (2 isomers) and yield even higher calorific contents. The omnivorous and carnivorous species, which are less subjected to seasonal food shortage, usually do not exhibit such an elaborate lipid biosynthesis. Herbivores usually do not utilize much of their enormous lipid reserves for overwintering, but channel this energy towards reproductive processes in late winter/early spring. Timing of reproduction is critical especially at high latitudes due to the short production period, and lipid reserves ensure early spawning independent of external resources. These energetic adaptations (dormancy, lipid storage) are supplemented by other life strategies such as extensive vertical migrations, change in the mode of life, and trophic flexibility.     

Orientation and navigation in coastal and estuarine zooplankton

The axiom that zooplankton species exhibit upwards migration behaviour at night is challenged by recent findings. Such behaviour is not universal, may vary during ontogeny, and is occasionally reversed. Moreover, in some estuarine and coastal zooplankton species vertical migration rhythms are of tidal, not diel, periodicity. There is evidence for several species that vertical migrations are endogenously controlled, occurring in constant conditions in the laboratory, suggesting that they have arisen under considerable selection pressure. They appear to play a significant role in orientation and navigation of coastal and estuarine zooplankton, predicated on the selective advantage of closure of life cycles. Vertical migrations between water masses moving in different directions at tidal, diel or longer timescales permit dispersal and recruitment, or retention, of planktonic larvae and adults in favourable ecological locations. Exogenous factors serving as cues for, or directly controlling, vertical migration rhythms include light, hydrostatic pressure, salinity, temperature, water movements and gravity responses, besides biological factors such as the timing of larval release, duration of larval life and predator/prey interactions. Behavioural characteristics should be quantified and factored into dispersal models which assume that zooplankton adults and larvae are advected as passive particles.     

Eat and run? The hunger/satiation hypothesis in vertical migration: history,evidence and consequences

The study of vertical migrations in aquatic organisms has a long and colourful history, much of it to do with the effects of changing sampling technology on our understanding of the phenomenon. However, the overwhelming majority of such studies carried out today still depend on detecting differences in vertical distribution profiles during some course of time, or acoustic echoes of migrating bands of organisms. These can not distinguish migratory activity of individual organisms, but can only assess net results of mass transfers of populations, which may integrate many individual migrations. This is an important distinction, for without knowing the actual movements of individuals it seems unlikely that we will be able to understand their causes, nor the effects of vertical migrations on the environment or on the migrators themselves. This review examines evidence for individual vertical movements gathered from 'tracers', mainly gut contents, and reviews the evidence for the hypothesis that such movements are in fact driven by hunger and satiation. The more recently appreciated vertical migrations of phytoplankters and their similarities in form and driving forces to those of zooplankton and nekton are also discussed. Finally, the role of vertical migrators in vertical fluxes of materials is discussed, along with the consequences of satiation-driven descent for such estimates.     

Spatial and seasonal variations in size, body volume and body proportion (prosome:urosome ratio) of the copepod Acartia tonsa in a semi-closed ecosystem (Berre lagoon, western Mediterranean)

Variations in size (prosome), body volume and proportion (prosome:urosome ratio) of female Acartia tonsa copepods were studied during three different seasons (June, October and November) in a network of 13 stations distributed throughout the Berre Lagoon, near Marseille. Strong morphological differences were found between the populations collected through the different seasonal surveys, but also between the different stations or groups of stations. They were related to the variations of environmental parameters (temperature, salinity, chlorophyll, particulate seston) according to the season and to the location of the stations (submitted to the marine influence in the south and to the intake of fresh water in the north of the lagoon). Considering all seasonal data, the size and body volume were inversely related to temperature. Body volume also showed a negative correlation with chlorophyll and carbon and a positive one with the C:N ratio of particles. The body proportion was positively correlated with temperature, chlorophyll and carbon. For each seasonal survey, the relationship between morphological features and environmental factors did not reach the significant level except in October when body proportion and volume were positively correlated to chlorophyll.Nevertheless, for each season, significant spatial changes in size or body proportion appeared in parts of the population of Acartia tonsa, in relation with local specific conditions of environmental factors, especially chlorophyll. These biometric differences suggest that individuals must develop in situ for at least the final period of larval growth, despite the dispersion effect caused by hydrodynamic movements. This stability in horizontal distribution may result from the diurnal vertical migrations of copepods between the surface and the bottom, two layers displaying currents of opposite directions. These results justify the use of somatic features (size and body proportion) to discriminate sets of individuals belonging to the same population. %