Marine cladocerans in Toyama Bay, southern Japan Sea: seasonal occurrence and day-night vertical distributions

Marine cladocerans in the zooplankton samples collected vertically(0–500 m) with Norpac nets at 2–4 week intervalsfrom February 1990 through January 1991 at a station in ToyamaBay, southern Japan Sea, were examined. From mid-March throughearly December, cladoceran populations exhibited five distinctpeaks. in which seven species occurred and showed a species-specific pattern of seasonal distribution. In mid-July. Peniliaavirostris exhibited a prominent peak in population density.Three species of Evadne, E.nordmanni, E.spinifera and E.tergestina,showed a clear ecological separation in time. The first twospecies had a peak density in April and July. respectively,but the last species demonstrated three distinct peaks betweenJune and September. Two congeneric species of the genus Podon,P.leuckarti and P.schmackeri, also did not co-exist in time.Simultaneous horizontal hauls with MTD nets made in June andSeptember 1986 revealed a strikingly marked surface distributionof all species. Evadne nordmanni occurred in June and demonstrateda clear ‘reverse’ did vertical migration, residingin the 30 m depth during the night time and ascending up tothe uppermost water column of 0–10 m during the daytime.Among the four species which occurred in September, E.spinifera,E.tergestina and P.schmackeri also showed a reverse migrationbetween the 30 m depth during the night and the very surfaceduring the day. A different pattern was observed in P.avirostrzs,which was distributed at a depth of 30 m during both the dayand the night with no sign of diel vertical migration. Amongthe more prevalent parthenogenetic individuals, some gamogeneticforms were found to exist in most species occurring in ToyamaBay.     

A change in the pattern of vertical migration of Chaoborus flavicans after the introduction of trout

The pattern of diel vertical migration exhibited by Chaoborusflavicans in Lake Lenore, Washington has changed over the pastdecade. Formerly Chaoborus larvae and pupae were present inthe water column during the day. Since 1982, a strong patternof diel vertical migration has been evident in this population.Third and fourth instar larvae and pupae reside in the sedimentsduring the day and then ascend to the surface waters at night.The change in migratory activity of C. flavicans was coincidentwith the introduction of cutthroat trout (Salmo clarki henshawi)into Lake Lenore in 1979. Predation by trout on fourth instarlarvae and pupae of C. flavicans was probably responsible forthe observed changes in migratory behavior.     

Vertical distributions, diel and ontogenetic vertical migrations and net avoidance of leptocephali of Anguilla and other common species in the Sargasso Sea

Day- and night-time vertical distributions and their ontogeneticchanges in Anguilla leptocephali and other common species ofleptocephali were determined and compared during five cruisesin the Sargasso Sea using an opening - closing 2-m ring netto sample discrete depth strata between 0 m and 350 m deep.No difference in vertical distribution was ever found betweenAnguilla rostrata (American eel) and A. anguilla (European eel).Anguilla leptocephali <5 mm long did not exhibit a diel verticalmigration, as they were distributed between 50 m and 300 m bothby day and by night. The vertical distribution of these smalllepto-cephali is probably roughly representative of the depthdistribution of adult spawning. Anguilla     

The effect of ultraviolet radiation on the vertical distribution and mortality of estuarine zooplankton

The effect of UV-B radiation on the vertical distribution ofthree calanoid copepod species (Tortanus dextrilobatus, Acartiuraspp. and Acanthacartia spp.) and three larval stages of Pacificherring, Clupea pallasi (1-, 7-, and 14-day-old larvae) wasinvestigated. A series of 2 m high columnar tanks equipped withinfra-red light and video-microscopy was used to investigatethe vertical distribution of zooplankton in the presence andabsence of UV-B radiation. In the presence of UV-B radiation,T.dextrilobatus and 1-day-old C.pallasi resided about 50 cmdeeper than in the absence of UV-B, while Acartiura spp. andAcanthacartia spp. showed no (or only minimal) change in verticaldistribution. Mortality experiments were also conducted outdoorsin which each copepod species was exposed to full or reducednatural radiation levels. Only T.dextrilobatus showed an increasein mortality when exposed to full radiation. Our results showedthat T.dextrilobatus and 1-day-old C.pallasi larvae were sensitiveto UV radiation (UVR), and to reduce or eliminate UV-inducedstress, they avoided the surface of the water column when UV-Bradiation was present. Copepod species were chosen to span arange of pigmentation: T.dextrilobatus (heavily pigmented),Acartiura spp. (moderately pigmented) and Acanthacartia spp.(not pigmented). The pigmentation did not appear to play a rolein UVR tolerance of the copepods, but may be a factor determiningUV tolerance of C.pallasi.     

Chemical, mechanical and visual cues in the vertical migration behavior of the marine planktonic copepod Acartia hudsonica

Recent experimental evidence in both marine and freshwater systemsindicates that predators can induce vertical migration behaviorin individual zooplankters, yet the specific cues by which zooplankterssense their predators appear to vary. In situ manipulation experimentswere carried out with enclosed populations of the marine planktoniccopepod Acartia hudsonica to re-examine the potential role ofchemical cues in the behavior of A.hudsonica, and to test explicitlyfor the role of mechanical or visual stimuli in triggering verticalmigration behavior in this species. Adult female copepods wereinduced to vertically migrate (descend) when exposed to fishmimics during the day, but no such response occurred when thecopepods were exposed to Fish mimics during the night. Moreover,copepods exhibited no changes in vertical distribution whenexposed to water which, having recently held a natural predator(the threespine stickleback, Gasterosteus aculeatus), was presumedto be laden with predator-produced chemical exudates. Predator-mediatedmechanical or visual cues, or a hierarchy of both, are responsiblefor eliciting vertical migration behavior in adult female A.hudsonica.These results, together with those of other investigations demonstratingthe inducing role of chemical exudates, indicate that the stimulieliciting vertical migration in zooplankton can be expectedto vary between species.     

Zooplankton-phytoplankton interactions in a eutrophic lake

Enclosure experiments were made in a cyanobacteria dominatedlake (Lake Rotongaio) to assess the impact of zooplankton (>150µm) grazing on algal growth rates and determine the effectof diel and vertical changes in zooplankton grazing intensityand nutrient (NH₄-N) regeneration upon abundance of phytoplankton.The filamentous cyanobacterium Anabaena minutissima var. attenuataand diatom Cyclotella meneghiniana showed a negative linearchange in abundance with a gradient in zooplankton grazing intensity.Phytoflagellates were not grazed and showed a positive linearchange in abundance with increasing zooplankton biomass. Theseeffects, as well as shortening of filament length of Anabaena,were caused by raptorial feeding by the alanoid copepod Boeckellapropinqua which dominated the zooplankton. Phytoplankton growthwas not stimulated by addition of nutrients, suggesting nutrientregeneration was not important. Diel and vertical changes infeeding and NH₄-N regeneration rates were measured in Marchand June 1988. Diel differences were more pronounced in Marchwhen the water column was stratified. Specific feeding rateswere more important than vertical changes in zooplankton biomassin determining community grazing rates in March, but in Junewhen the water column was mixed, vertical distribution of zooplanktonbiomass was important. Zooplankton grazing was an importantloss process for phytoplankton in the lower part of the epilimnionin Lake Rotongaio.     

Ontogenetic shift in geotaxis for walleye pollock,Theragra chalcogramma free embryos and larvae: potential role in controlling vertical distribution

Synopsis The behavioral capability of walleye pollock,Theragra chalcogramma free embryos and larvae to control vertical distribution was assessed by examining buoyancy during resting and swimming orientation and activity as they developed in complete darkness from hatching to first feeding readiness (1 to 7 d post hatching at 6° C). Free embryos exhibited positive geotaxis 1 d post hatching, actively swimming through a density gradient to remain in the lower water column. Activity increased with free embryo development and by 7 d post hatching, feeding-ready larvae reversed their vertical orientation, now exhibiting negative geotaxis as they migrated to the upper water column. The results indicate that even at the earliest developmental stages, walleye pollock possess the capability to control vertical distribution. Laboratory results are compared with patterns of vertical distribution observed in the sea.     

Feeding, excretion and egg production by individuals and populations of the marine, planktonic copepods, Acartia spp. and Centropages furcatus

Diel variations in vertical distribution, gut pigment content,ammonium excretion and egg production were investigated foradult females of Acartia erythraea and A.pacifica in the verticallymixed Inland Sea of Japan and Centropages furcatus in the stratified,neritic Gulf of Mexico. Gut pigment content and egg productionrate were maximal at night and ammonium excretion was maximalduring the daytime. Neither A.erythraea nor A.pacifica adultfemales showed an apparent diel migration, but the former werehighly concentrated in the surface layer during the afternoon.In contrast, C.furcatus adult females showed a clear diel migration,residing immediately above the bottom during the daytime andbeing concentrated between 10 and 25 m depth during the nighttime.Individual-based data on gut content and excretion and egg productionrates were combined with vertical-distribution data to calculatepopulation values. In the Inland Sea of Japan, the resultantpattern for Acartia spp. reflected the diel variation in physiologicalrates and even distribution of adult females, except for theafternoon, surface aggregation of A.erythraea. In the Gulf ofMexico, the pattern for C.furcatus reflected largely the dielvariation in each rate process and the heterogeneous distributionof adult females in the water column. Elevated nocturnal feedingactivity of these copepods may be due to an endogenous rhythm.The daytime maximum in ammonium excretion and night-time maximumin egg production rate indicated approximate half-day and daytime lags, respectively, after the intake of food until itsconversion into dissolved excreta and released eggs.     

State-dependent vertical distribution of the carnivore copepod Pareuchaeta norvegica

We explored the impact of body condition on vertical distributionof the carnivore copepod Pareuchaeta norvegica. Individualssampled in the upper part of the water column had lower dryweight and slightly shorter prosome than individuals sampleddeeper. Non-ovigerous females occurred higher in the water columnthan ovigerous females. Our results suggest that ovigerous femalesand individuals with large energy reserves to a large extentprioritize predator avoidance in deep water versus feeding inthe upper part of the water column compared to non-ovigerousfemales and individuals with low energy reserves. Adult femalescollected closest to the surface were less pigmented than thosefound deeper. This, however, proved to be a quickly adjustabletrait depending on prevailing light conditions.     

Interrelations between the vertical distribution of Daphnia and chlorophyll a in two large limnetic enclosures

This paper summarises the major factors influencing the daytimevertical distribution of Daphnia hyalina var. laciutris (Sars)in two large experimental enclosures (Lund Tubes). In both tubes,the depth at which the Daphnia aggregated during the day wasclosely related to sub-surface ir-radiance. On a few occasionsaggregations of Daphnia were found in ‘dark’ waterassociated with a deep chlorophyll maximum. On many occasions,however, the animals' light response per se ensured their aggregationat depths of maximum phytoplankton abundance. The most importantfactor influencing the dispersion of animals in the water columnwas the steepness of the light gradient. In turbid water verticalaggregations were well defined, whereas in clear water the animalswere widely dispersed around their depth of maximum abundance.Daphnia also tended to disperse in the water column when theirpopulation density was high or when food was scarce. A simplemodel based on surface irradiance, water turbidity and populationdensity explained the basic pattern of vertical distributionthroughout the season. The implications of these findings arediscussed in relation to the effects of zooplankton grazingand speculations on the adaptive significance of depth regulation.     

The life history and vertical distribution of the chaetognath, Sagitta elegans, in Dabob Bay, Washington

Seasonal changes in the population structure and patterns ofvertical distribution and migration of the dominant chaetognath,Sagitta elegans, in a fjord off Puget Sound were determinedwith samples collected at approximately semi-monthly intervalsfrom November 1972 to November 1973. S. elegans bred continuouslyfrom March through August, with major recruitment in June andJuly. Seasonal changes in the vertical distribution of the populationand in the proportion of the population that vertically migratedreflected the stage of maturity and size of the individuals.Small chaetognaths were non-migratory and distributed in thetop 100 m. Stage II individuals were dispersed throughout thewater column; some did migration was evident. Breeding stageswere restricted to a layer between SO and 100 m during the dayand migrated to the surface layer at night, enhancing the probabilityof successful reproduction.     

Evaluation of genes involved in Norway lobster (<Emphasis Type="Italic">Nephrops norvegicus</Emphasis>) female sexual maturation using transcriptomic analysis

In marine ecosystems, the most significant migration observed in terms of biomass distribution is the one connected with the vertical movements in the water column. In the present study, the vertical profiles of the mesopelagic shrimps Gennadas elegans, Eusergestes arcticus, Sergia robusta, and the epipelagic Parasergestes vigilax in the Balearic Sea (western Mediterranean), during the stratified (summer) and non-stratified (autumn) hydrographic conditions, were investigated through their ontogeny, from the larval to adult stages. The mesopelagic adults were observed to move down to the deeper layers during the night more than during the daylight hours. Most larvae aggregated within the limits of the upper water column. The P. vigilax larvae were collected only during the stratified period. The first two larval stages vertical distribution indicates that the mesopelagic crustacean spawning could occur at greater depths. During the non-stratified period, the larvae of the mesopelagic species tended to remain at about 500 m depth at night, rising towards the upper layers at sunrise. Vertical patterns are discussed, as strategies associated with predator–prey trade-offs. To our knowledge, the present study is the first such attempt to jointly analyze the vertical migrations of the developmental stages of the pelagic shrimps in the Mediterranean Sea.     

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.     

Vertical and temporal distribution of two copepod species, Cyclops scutifer and Diaptomus pribilofensis, in 24 h arctic daylight

Factors structuring zooplankton communities in areas with 24h of sunlight are not well understood. In stratified temperatelakes with fish, zooplankton generally undergo a diel verticalmigration (DVM) to avoid predation. Zooplankton abundance mayalso be linked to the quality and quantity of food. To explorefactors that determine vertical structure of zooplankton whereDVM does not occur, we obtained time-series data on the verticaldistribution of the two dominate copepods, Cyclops scutiferand Diaptomus pribilofensis, in Toolik Lake, Alaska, a sitewithin the Arctic Long Term Ecological Research area. Thesetwo species compose 80–90% of pelagic mesozooplanktoncommunity density. Temperature plays a large role in the verticaldistribution of these two species. During the summers of 2002and 2003, C. scutifer underwent a seasonal vertical migrationfrom the epilimnion to the hypolimnion as water temperaturesincreased. Diaptomus pribilofensis exhibited a strong preferencefor warmer water and were not present in the water column untilepilimnetic temperatures reached 9°C. During both yearsof this study, C. scutifer and D. pribilofensis segregated verticallyin the water column along thermal gradients when the two speciesoverlapped temporally.     

An <Emphasis Type="Italic">in situ</Emphasis> method for the investigation of vertical distributions of zooplankton in lakes: test of a two-compartment enclosure

Two-section enclosures were designed for the investigation of the effect of various physicochemical and biological factors on vertical distribution of zooplankton in situ. The framework of the enclosure was a cylindrical polyethylene column without any partitions inside, in which the isolation of animals in different sections after in situ exposure was achieved by pinching the flexible central part of the column. Enclosures were tested at the brackish stratified meromictic Lake Shira (Russia, Khakasia). The absence of fish and carnivorous zooplankton in the lake suggests that the vertical distribution of zooplankton is mainly determined by physicochemical gradients in the water column. Experiments and field observations demonstrated that all age and size groups of Arctodiaptomus salinus and Brachionus plicatilis strongly avoided surface layers during the daylight. The escape of zooplankton from the anoxic hypolimnion was less active. Statistically significant avoidance was observed only for copepodites C4–C5 and females of A. salinus. The relatively simple construction of the columns and easy handling during the experiment were the factors that favoured the use of this device to perform in situ basic tests of the effect of different factors on the vertical distribution of zooplankton.     

Seasonal variations in the diel vertical distribution of phytoplankton and zooplankton in a shallow pond

The seasonal succession of phytoplankton diversity, and the variations in the diel vertical distribution of phyto‐ and zooplankton were investigated in a small shallow pond (1.7 m water depth) in 2003. It was inferred that the water tended to stratify weakly in the daytime from February to June. In February and April, the green alga Golenkinia radiata Chodat dominated the phytoplankton assemblage. The cell density of G. radiata greatly decreased in April, when rotifers increased near the bottom. The vertical mixing was attenuated in June, large populations of the euglenoids (Lepocinclis salina Fritsch, Phacus acuminatus Stokes, Trachelomonas hispida (Perty) Stein et Deflandre) developed, and the cyanobacterium Aphanizomenon flos‐aquae var. klebahnii Elenk. appeared at low density. Euglenoids and A. flos‐aquae were mostly distributed in the bottom layer. In late September, when the water was mixed throughout the day, euglenoids and A. flos‐aquae were distributed evenly throughout the water column. The zooplankton (cyclopoid copepods and rotifers) densities in September were the lowest throughout the year. The vertical mixing increased in November, and the phytoplankton community was composed of A. flos‐aquae, P. acuminatus, T. hispida and the green alga Ankistrodesmus falcatus (Corda) Ralfs. In November, at the final stage of water bloom of A. flos‐aquae, its population density decreased with depth. The two euglenoids exhibited similar cell distributions at 0.8 m and 1.6 m during 1–3 November. A. falcatus was distributed evenly throughout the water column; however, when the vertical mixing lessened, the cells at the surface started to sink. Copepod nauplii and rotifers appeared at high densities in November. Seasonal variation in the phytoplankton community structure in the pond seemed to be related to the vertical mixing of the water. In addition, zooplankton, especially rotifers, might play an important role in initiating a spring clear‐water phase and in the bloom collapse of A. flos‐aquae.     

Observations on the vertical distribution of zooplankton, including post-larval teleosts, off Plymouth in the presence of a thermocline and a chlorophyll-dense layer

During 1973 – 1977, when Sagitta elegans was returningto dominance over S. setosa in Plymouth waters, there was often,in summer in stratified conditions, a community characterizedby S. setosa in the warm water above the thermocline, and atypical S. elegans community in the colder layer below the thermocline.This vertical separation persisted at night even though somespecies from the S. elegans community migrated up into the warmlayer. Such vertical migration was not stopped by a temperaturediscontinuity of from 3° to 6°C, but fewer individuals,or a lesser proportion of species, took part at the higher temperaturedifference. Only a small part of the zooplankton, comprisingsmall individuals of Limacina retroversa, the appendicularians,and, possibly, post-larvae of Arnoglossus laterna, showed signsof aggregation at the thermocline or the chlorophyll maximumclose to it.     

Predator-limited population growth of the copepod Pseudocalanus sp.

The impact of predators on population growth of Pseudocalanussp. was investigated in Dabob Bay, Washington. Mortality ofPseudocalanus sp. was determined from stage-specific survivorship,from seasonal changes in mortality rates of adult males andfemales and from incidence of injuries to adult copepods. Theprincipal predators of adult Pseudocalanus were identified asthe predatory copepod Euchaeta elongata, the omnivorous euphausiidEuphausia pacifica and the chaetognath Sagirta elegans. Predatorattack rates - and prey mortality rates - are highly density-dependentand thus sensitive to prey dispersion in the water column, particularlyto layering in the vertical plane. Predation rates by the threeprincipal predators exceeded 100% of the recruitment rate toadult Pseudocalanus sp. beginning in early summer, thus restrictingpopulation growth. Planktivorous fish predation (by adult three-spinestickleback, Gasterosteus aculeatus, and juvenile chum salmon,Oncorhynchus keta) on Pseudocalanus sp. adults was estimatedto be two orders of magnitude lower than consumption rates bypredatory zooplankton, at a deep water station in July. Analysisof seasonal changes in prey ingested by Sagitta elegans revealedthat Pseudocalanus sp. was the major prey item of S. elegansin April (61.0% of prey) and in June (67.0% of prey), thereafterdeclining seasonally in importance. Predation by S. elegansvaried seasonally with changes in chaetognath stage structure,vertical distribution and diapause, not size structure alone.Although chaetognath recruitment and population growth appearto be directly coupled to the abundance of Pseudocalanus sp.,predation by S. elegans has little reciprocal impact on Pseudocalanussp. population growth; hence asymmetries may occur in the interaction of planktonic prey and predators.     

Vertical migration and habitat partitioning of six euphausiid species in the northern Benguela upwelling system

The vertical distribution of the adults of six species of euphausiids,Thysanöessa gregaria, Nematoscelis megalops, Euphausiaamericana, E.gibboides, E.hanseni, Stylocheiron longicorne,as well as the larval stages of E.hanseni, were studied duringa 48 h cycle at a fixed station in the northern Benguela upwellingsystem. All the species, except T.gregaria and S.longicorne,proved to perform diel vertical migration, but both migratoryand non-migratory species appeared to be segregated in spaceduring night-time hours, regardless of potential prey (phytoplanktonand copepods). It is suggested that water column structure andhydrographic discontinuities caused by a warm, depth-localizedintrusion, as well as the reproductive strategies of particularspecies, are responsible for this pattern. Eggs and larval stagesof E.hanseni were concentrated near the surface, and the effectof the short-term pulses of the intrusion on their abundanceis discussed.     

Diel reproduction and vertical distributions of the marine cladocerans, Evadne tergestina and Penilia avirostris, in contrasting coastal environments

Evadne tergestina and Penilia avirostris were sampled duringseveral diel cycles in the well-mixed, turbid Inland Sea ofJapan and the more stratified, less turbid Gulf of Mexico shelf.Parthenogenetic female Evadne contained embryos with pigmentedeyes only at night, and apparently released neonates near dawnor in the morning. There was no significant diel vertical migrationin the Gulf of Mexico, and a ‘reverse’ migrationin the Inland Sea. Female Penilia could contain mature embryosat any time, but were most likely to do so at night. Peniliadid not migrate dielly, and in the Gulf of Mexico was concentratedin a nepheloid layer near the bottom.