encyclopedia of inland waters || diel vertical migration

Diel Vertical MigrationL D Meester, Katholieke Universiteit Leuven, Leuven, Belgium

ã 2009 Elsevier Inc. All rights reserved.

Introduction

Diel vertical migration (DVM) is a conspicuous andwidespread behavior in planktonic organisms in bothinland waters and marine environments. The first sci-entific studies on this behavior date from more than100 years ago, and DVM has been observed in a widevariety of habitats worldwide. DVM has attracted alot of attention from researchers because of severalreasons: (1) it is a spectacular behavior, (2) its causeshave remained enigmatic for a long time, and (3) thebehavior has important ecological consequences.

DVM in its Various Forms

DVM is often associated with zooplankton, and zoo-plankton ecologists have indeed devoted most atten-tion to the phenomenon. However, it should bementioned that other organisms too have beenshown to perform DVM, such as fish, aquatic insects,and phytoplankton. Figures 1–3 provide illustrationsof DVM in zooplankton, chosen because they revealseveral key aspects of the behavior and its variation.Figure 1 illustrates the migration pattern of a calanoidcopepod in a subtropical lake. It shows the classicalpattern of a ‘standard’ migration: the animals residehigher in the water column during the night thanduring the day. During the day, they stay in deepwater layers, whereas at night, they distribute them-selves more evenly in the water column, and movetowards the surface water layers just after sunset andbefore dawn. The migration is over a long distance.Indeed, the animals move 30–40m twice every day,which is >40� 103 times their own body length (tohuman standards, this would translate into a travelingdistance of >60km a day). This illustrates why DVMis considered a spectacular behavior. Figure 1 alsoshows that the movement from the upper to the bot-tom water layers and vice versa is associated withdawn and dusk. Finally, the figure also illustratesanother very common feature: there is a clear tendencyfor more individuals to be caught during the nightthan during the day. This phenomenon is called the‘daytime deficit,’ and is often observed in studies onzooplankton populations showing extensive DVMpatterns. In the study system shown in Figure 1, itwas shown that a fraction of the copepod populationactually moves into the sediments during the day. Inmany systems, the daytime deficit is probably caused

by the fact that the zooplankton is either residingso close to the sediments during the day that itcannot be sampled by traditional sampling gear oris even moving into the surface layers of the sedi-ment. This is striking, as the sediment is a harsh envi-ronment for a zooplankton individual to reside induring part of the day. In many systems that show astrong oxycline, during the day the zooplankton residesat depths that are characterized by very low oxygenlevels. Figure 2 even shows a population of calanoidcopepods that resides part of the day in the anaerobicmonimolimnion of a meromictic lake. In this coastallake, there is no oxygen below 5m depth. Figure 2 alsoshows the pattern of changes in vertical distributionaround sunset, and illustrates that different life stagesmay differ in their daytime distribution and migrationpattern. Indeed, it is clear from this figure that theyoungest life stages tend to be distributed similarly totheir food during the day, whereas most (sub)adultanimals reside in the monimolimnion during thenight, and rapidly ascend to the mixolimnion aftersunset. Figure 3 illustrates strongly different DVMpat-terns among two congeneric species in the same habi-tat. This same study also reported seasonal changes inDVM, withDaphnia hyalinamigrating only verticallyduring the summer season. This pattern has beenreported for many deep lakes in temperate regions.Summarizing, DVM involves changes in depth distri-bution over a diel cycle, can take extreme forms andshows tremendous variation through time as well asamong life stages, species, and lakes. All the threeexamples illustrated by the figures represent the mostcommonly observed pattern of migration, with theanimals residing deeper in the water column duringthe day than during the night (‘standard migration’).However, it should be mentioned that many naturalzooplankton populations exhibit a clear-cut depth dis-tribution that does not change over a diel cycle, andthere are also reports on ‘reverse migration,’ wherepopulations reside higher in the water column duringthe day than during the night. This reverse pattern ofDVM is less common, but is nevertheless observed inseveral, often small-bodied species, such as rotifers.

In the face of this overwhelming diversity in DVMpatterns among species and populations, it is impor-tant to develop a broad perspective that allowsstructuring the observed variation. DVM can beconsidered a habitat selection behavior. More spe-cifically, it is a depth selection behavior that consistsof several elements that can vary among habitats,

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Figure 1 Diel vertical migration of adult females of the calanoid copepod Pseudodiaptomus hessei in Lake Sibaya (March 1972). The

broken line indicates a light intensity isocline (1 lux). X-axis: time of day; Y-axis: depth (m); thickness of bars indicates number of animals

(see scale bar). Reproduced from Hart RC (1976) The substrate bin – A new sampling device for studying diel vertical migratorymovements on to and off lake sediments. Freshwater Biology 6: 155–159, with permission from Blackwell Publishing.

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Figure 2 Vertical distribution of calanoid copepods in meromictic coastal Lake Nagada (Papua New Guinea); the lake has a

brackish mixolimnion and has no oxygen below 4.5m (cf. accumulation of hydrogen sulfide by anaerobic metabolism). (a) verticaldistribution of algae in amid-lake station, June 4, 1992. Filled circles: diatoms and flagellates; filled squares: cyanobacteria (Oscillatoria);

shaded area: bacterial plate. (b) vertical distribution of different ontogenetic stages (small nauplii: N1-3; larger nauplii: N4-6; young

copepodites: C1-3; larger copepodites: C4-6) of Acartia tonsa 1h before sunset (17.00; small empty symbols), at sunset (18.00; largerempty symbols) and 1h after sunset (19.00; filled symbols) at a mid lake station, May 30, 1992. Reproduced from De Meester L and

VyvermanW (1997) Diurnal residence of the larger stages of the calanoid copepod Acartia tonsa in the anoxic monimolimnion of a tropical

meromictic lake in New Guinea. Journal of Plankton Research 19: 425–434, with permission from Oxford University Press.

652 Zooplankton _ Diel Vertical Migration

species, and populations: the depth at which the ani-mals reside during the day, the depth at which theyreside during the night, and the timing and speed ofthe migration from one depth to the other. This viewhas proved productive in structuring the observedvariation in DVM patterns. For instance, it allowsthe viewing of the many observations of strong butconstant depth preference during day and night asa DVM phenotype that may be selected for if theoptimal depth is the same during the day and night,e.g., because it is only determined by the distributionof food. The ubiquity of standard migration wouldthen reflect the fact that under a wide variety ofcircumstances it is adaptive to stay deeper in thewater column during the day than during the night.It is revealing that DVM, viewed as a habitat selec-tion behavior, is essentially very similar to twilightactivity as reported for many small mammals, or themigration of aquatic macroinvertebrates in rivers tothe underside of stones during the day. Viewed as a

habitat selection behavior, DVM can also be seen asan alternative to diel horizontal migration (DHM).DHM has been observed in many zooplankton spe-cies in shallow lakes with a well-developed littoralzone. In a typical DHM migration, the zooplanktonresides between the macrophyte vegetation in thelittoral zone during the day, and moves into theopen water during the night. DHM shows many par-allels to DVM: it shows similar diel dynamics, withthe main movements being associated with dawn anddusk, and in both cases, the zooplankton moves torather marginal habitats during the day.

Causes of DVM

What causes zooplankton to migrate vertically in adiel cycle? It is important to make a distinctionbetween the proximate and ultimate factors leadingto DVM in nature.

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Figure 3 Vertical migration patterns of Daphnia galeata (plain)

and D. hyalina (shaded) during summer in Lake Constance. Theright panel shows depth profiles of temperature (solid line) and

particulate carbon (broken line) (particles <35mm, i.e., potential

food for Daphnia). Reproduced from Lampert W and Sommer U

(1997) Limnoecology: The Ecology of Lakes and Streams.New York, Oxford University Press; figure based on Stich HB and

Lampert W (1981) Predator evasion as an explanation of diurnal

vertical migration by zooplankton. Nature 293: 396–398, with

permission from Nature Publishing.

Zooplankton _ Diel Vertical Migration 653

Proximate Causes

Proximate causes are the stimuli from the immediateenvironment of the individual organism that triggerthe animal to move downwards or upwards. Thesestimuli–response processes relate to the physiology ofthe organism, and translate into a specific day- ornighttime depth as well as into a particular timingand speed of migration. It has been convincinglyshown that zooplankton DVM is a response to rela-tive changes in light intensity. Elegant laboratoryexperiments have revealed that a decrease in lightintensity that surpasses a specific threshold elicits anupwardmovement, whereas an increase in light inten-sity above a specific threshold elicits a downwardmovement. These responses have been called ‘second-ary phototaxis’ – in contrast to ‘primary phototaxis’that involves a response to a constant light intensity.Most detailed experiments have been carried out withthe water-fleaDaphnia, and have shown that changesin light intensity result in eye rotations, which thentranslate in a change in body orientation and anupward or downward swimming. These simple sec-ondary phototaxis responses may allow predicting theday and night time depth as well as the timing andspeed of migration. Moreover, several modifying fac-tors have been identified. For instance, the presence offish kairomones (which have not been chemicallyidentified yet, but characterized as nonvolatile, low-molecular-weight, lipophylic compounds of mediumpolarity and high thermal and pH stability) has been

shown to increase the sensitivity to changes in lightintensity, which translates into an increase in DVMamplitude in the presence of fish. A strong tempera-ture gradient may reduce the responsiveness to rela-tive changes in light intensity, and the same holds forhunger. The latter results in a less strong DVM behav-ior in the absence of food.

Ultimate Factors

To the question why the zooplankton engages inDVM, one can also answer with reference to the ulti-mate factors causing DVM, i.e., its adaptive signifi-cance. For a long time, the adaptive significance ofDVM has remained enigmatic, as researchers werepuzzled by the fact that the zooplankton remained indeep and cold water layers that are characterized bylow food concentrations during a large part of the dielcycle. Initially, many authors were convinced thatDVM was a side-product of the visual system. Othersbelieved that there were metabolic advantages asso-ciated with residing part of the time at a lower tem-perature. These hypotheses were, however, at oddswith model predictions and are unsatisfactory withrespect to the synchronized timing of DVM. The cur-rently most widely accepted explanation for DVM inzooplankton is that it acts as a predator-avoidancemechanism. The animals move into the deeper anddarker water layers during the day to avoid predationby fish. There are many lines of evidence pointingto the importance of predator-avoidance in DVM,most of them being related to the observation thatvariation in DVM can often be explained by variationin predator risk:

. First, the hypothesis is logical: fish need light todetect their prey efficiently, and by hiding in thedarker water layers, the zooplankton can reducemortality by visually hunting fishes. During thenight, the zooplankton moves to the upper waterlayers to feed on algae that remain in the epilimnionbecause they need sufficient light for photosynthe-sis. Thus, the hypothesis can explain both the day-time and nighttime depth distribution, as well as thetiming of the upward and downward migration.

. Overall, there is a tendency for larger zooplanktonspecies to migrate more or with more amplitudethan smaller species. For instance, large Daphniaspecies migrate to deeper water layers than smallercrustacean zooplankton, whereas rotifers often donot migrate at all.

. Smaller life stages often migrate with less ampli-tude or do not migrate at all (e.g., Figure 2).

. Individuals that are more conspicuous, such as egg-bearing females, oftenmigrate withmore amplitude.

Tuesday lake

Paul lake

Minnows

1985 1986 1987 1988

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Figure 4 Impact of fish on DVM: field data from Tuesday and

Paul Lake, two kettle lakes at the University of Notre Dame

Environmental Research Center. Shown are night (solid symbols)

and day (empty symbols) average depth of the Daphniaassemblage in the two lakes as determined from vertical profiles

taken during several sampling campaigns across four summers.

In 1985 and 1986, Tuesday lake was biomanipulated, resulting in

a strong reduction in fish predation pressure; in 1987, fish werereintroduced to the lake, resulting in quite high minnow densities.

DVM amplitude of the Daphnia assemblage is associated with

the presence of planktivores. Paul Lake was not biomanipulated,and harbored planktivorous fish during the whole period.

Reproduced from Dini ML and Carpenter SR (1991) The effect of

whole-lake fish community manipulations on Daphnia migratory

behavior. Limnology & Oceanography 36: 370–377, withpermission from American Society of Limnology and

Oceanography.

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Figure 5 The changes in day (left) and night (right) depth of

adult Daphnia hyalina x galeata hybrids in Lake Maarsseveen

during the course of the growing season (April 18 – September

141989). The dip in the bundle of lines indicates the period ofstrong DVM. This period corresponds with the massive

appearance of juvenile perch in the pelagial of the lake.

Reproduced from Ringelberg J, Flik BJG, Lindenaar D, and

Royackers K (1991) Diel vertical migration of Daphnia hyalina(sensu latiori) in Lake Maarsseveen: Part 1. Aspects of seasonal

and daily timing. Archiv fur Hydrobiologie 121: 129–145, with

permission from E. Schweizerbart’sche Verlagsbuchhandlung.


. It has been reported repeatedly that also within apopulation there is a tendency for a correlationbetween body size and daytime residence depthwithin a given lake.

. Among-population variation in DVM ampli-tude can sometimes be related to differences inpredation risk by fish (Figure 4). This is illustratedby the absence of migration in many fishless lakes.

. Many populations show seasonal variation inDVM amplitude that can be explained by seasonalvariation in predation risk by fish (highest in sum-mer when the young-of-the-year are roamingthrough the pelagial; e.g., Figure 5).

. Very direct evidence for the predator-avoidancehypothesis is provided by the many experimentalstudies that have shown that DVM behavior can beinduced by the presence of fish-specific chemicals(kairomones; Figure 6).

. Finally, several mathematical models have shownthat DVM behavior can indeed be adaptive as apredator-avoidance strategy, i.e., that the benefitsoutweigh the costs of staying at low food and tem-perature during part of the diel cycle.

The evidence for the predator-avoidance hypothesisis overwhelming. Yet, predator avoidance need not be

the sole adaptive value of DVM. Indeed, some casesof day depth distribution or DVM cannot be satisfac-torily explained by the predator avoidance hypo-thesis. For instance, zooplankton has been shownto migrate vertically in fishless alpine lakes, and insome studies the amplitude of the migration hasbeen shown to be unrelated to fish predation risk.Recently, evidence has accumulated that an impor-tant adaptive value of DVM may lie in the avoidanceof damage caused by UV-radiation, or more generally,by high light intensity. Especially in transparent high-altitude lakes, the presence of fish may not be theonly or even the main reason for DVM behavior todevelop. UV-avoidance can also explain the patternand timing of typical normal DVM behavior. More-over, there is an intrinsic interaction between UV- andpredator-avoidance, as animals have two mechanismsto avoid photodamage: they may accumulate protec-tive pigments or they may migrate vertically. In theabsence of predators, protective pigments may be themore beneficial option, but in the presence of preda-tors, pigmentation bears a high cost of increased riskof predation, and DVMmay be the better alternative.It is often observed in alpine lakes that copepods arepigmented whereas cladoceran zooplankton are notand migrate vertically. What drives this difference isinsufficiently understood, but it might be related tothe fact that most copepods are relatively small andbetter swimmers than water fleas. Alternatively, itmay be that copepods have better capacity to accu-mulate photoprotective pigments from their food.

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Figure 6 Induction of DVM by fish kairomone in a clonal population of Daphnia galeata x hyalina hybrids in an experiment using

the Plankton Towers at the Max Planck Institut for Limnology, Plon. The experiment was started without fish. Then, the upper 3m ofthe tank received water that was conditioned by one fish (Leucaspius delineatus, 5 cm body length). After four days of adaptation,

the vertical profile was recorded again at night and during the day. Subsequently, the number of fish was doubled, and the same

procedure was continued three more times (cf. up to 16 fish). Reproduced from Loose CJ (1993) Daphnia diel vertical migration

behavior: Response to vertebrate predator abundance. Archiv fur Hydrobiologie Beihefte Ergebnisse der Limnologie 39: 29–36,with permission from E. Schweizerbart’sche Verlagsbuchhandlung.


A third ultimate reason to exhibit a typical depthdistribution or DVM may be avoidance of competi-tion. This has been shown experimentally in rotifers,and is indeed likely in small zooplankton, which mayavoid depths at which larger bodied zooplanktonaccumulate. It should be noted, however, that thismechanism can only operate in a situation where thecompetitively superior species is restricted to its depthdistribution by additional factors. If competitivelydominant large bodied water fleas are forced to hidein the deeper water layers by predation risk or UVradiation, then thismay open awindow for coexistenceof smaller species that occupy the upper water layersduring the day. During the night, these smaller speciesmay avoid the upperwater layers to reduce interferencecompetition with the large bodied species. The result isa reverse migration. A reverse migration may also beassociated with the avoidance of invertebrate preda-tion. Indeed, small bodied species are more vulnerableto invertebrate predators, and as thesemay be forced tohide in deeper waters during the day because of preda-tion risk by fish, the small bodied species may increasetheir fitness by residing in the upper water layers dur-ing the day and then spread over the water columnduring the night, when the invertebrate predatorsmove up into the open water to search for prey.

Costs

Viewing DVM as a habitat selection behaviorprovides a flexible approach to the overwhelming

variation in DVM patterns, as the observed day andnight time depth distribution as well as timing andspeed of migration can be considered to be selectedbecause they provide a high benefit to cost ratio. Thebenefits of DVM are, as mentioned, often related toreduced mortality by predation, but also reduced dam-age from high light intensities, and in some cases,reduced damage by competition. The costs are largelycast in terms of reduced food intake, reduced foodquality, increased competition (when several com-peting taxa are migrating to the same refuge), andmetabolic costs associated with residing at a lowertemperature. In addition, there may be costs associatedwith residing in truly marginal habitats, such as near-anoxic conditions or being buried in sediments. Resid-ing at or near the sediments can also increase therisk of infection by increasing the likelihood of takingup infective parasite spores, as has been shown in thewater flea Daphnia. The costs associated with themovement from one depth layer to the other has,however, been shown to be relatively low or evennegligible. This is because the animals in generalmust actively swim to keep their position in the watercolumn anyway, coupled with the observation that thedescent is often passive. During the descent, the ani-mals become less active, and they become more activeduring the ascending phase. In simple wording: if awater flea shows its typical hop–sink–hop–sink swim-ming behavior to keep its vertical position, descendingis accomplished by hop–sink–sink–hop–sink–sink andascending by hop–hop–hop–sink behavior. One can

Clone 1 Clone 2 Clone 30


imagine that the energy lost during the ascendingphase matches more or less the energy gained duringthe descending phase.

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Figure 7 Vertical distribution of three Daphnia hyalina x galeata

hybrid clones isolated from the same population (Schohsee,Germany) in the Plankton Towers of the Max Planck Institute for

Limnology (Plon), showing genotypic differences in DVM

behavior. Top: average vertical distribution of adult females

during the day (open symbols) and night (solid symbols) in theabsence of fish chemicals or fish; bottom: daytime distribution in

the presence of fish chemicals (open symbols) and fish (solid

symbols). The dotted line indicates the thermocline. Theexperiment involved mixed populations of the three clones, and

individuals were identified using allozyme markers. Clone 3 has a

clearly different vertical distribution from clones 1 and 2.

Reproduced from De Meester L, Weider LJ, and Tollrian R (1995)Alternative antipredator defences and genetic polymorphism in a

pelagic predator–prey system. Nature 378: 483–485, with

permission from Nature Publishing.

Linking Proximate and Ultimate Factorsin DVM

The previous paragraph pictures a situation in whichthe most advantageous DVM behavior may stronglydiffer from lake to lake. The question then arises as tohow this is matched with the response to proximatestimuli. How do the physiological responses of theanimals lead to the right vertical distribution duringthe day and night and the right timing and speed of theascent and descent? There are two possible mechan-isms that are not mutually exclusive, and both aresupported by experimental evidence. On the onehand, part of the variation in DVM behavior can beaccomplished by phenotypic plasticity acting at thephysiological responses. It has, for instance, beenshown that predator kairomones and hunger influ-ence the sensitivity of zooplankton individuals torelative changes in light intensity, resulting in a modi-fication of DVM behavior. Populations may thusadjust their DVM behavior by showing the appropri-ate phenotypic plasticity in response to changes inenvironmental conditions. At the same time, it hasbeen shown that there is ample genetic variationfor DVM behavior, both through field studies as wellas through laboratory quantitative genetic analyses ofthe variation in response to a light gradient (Figure 7)and, to a lesser extent, changes in light intensity.More-over, it has been shown that there is also genetic varia-tion in phenotypic plasticity for DVM, at least in thewater fleaDaphnia. This sketches a picture of very highflexibility: populations can adjust their DVM behaviorby phenotypic plasticity of the individuals as well as bychanges in genetic composition with respect to DVM.It has been shown that populations show local geneticadaptation for DVM in relation to predation risk.Moreover, reconstruction of microevolution in DVMbehavior on laboratory populations hatched fromdormant egg banks that were isolated from a datedsediment core has shown that local populations cangenetically track changes in fish predation pressure.Finally, it has also been shown that animals isolatedfrom different depths during the day are often geneti-cally different and that seasonal changes in DVMbehavior may also have a genetic component.The picture that emerges is that DVM is an impor-

tant component of an antipredator strategy in manypopulations of zooplankton. It should be emphasizedthat DVM should not be seen in isolation from otherantipredator traits. It has, for instance, been shown

that there is often a relationship between size atmaturity and day-depth, with larger animals residingin deeper water layers than smaller ones. Differentgenotypes and species may thus differ in the antipre-dator strategy they employ. This is nicely illustratedin the differences in migration pattern of D. galeataand D. hyalina in Lake Constance shown in Figure 3.The strategy of D. galeata involves a high predationrisk associated with continuous residence at highertemperatures and under good food conditions. Thisresults in high birth and death rates. The strategyof D. hyalina is to reduce mortality rates and bearthe cost of having lower birth rates. The fact that bothspecies can co-occur in the lake illustrates thatboth strategies may have similar fitness. Part of thisrelationship is also observed at the genotypic level,with larger bodied genotypes showing a stronger


response to light gradients. Similarly, DVM behaviorand pigmentation are both photoprotection strate-gies, of which the relative importance may be stronglyinfluenced by predation risk. Both pigmentation andDVM have a cost, but the cost of pigmentation isstrongly dependent on predator risk.

Consequences of DVM

DVM has important consequences, as it stronglyinfluences the top–down impact in lake food webs.DVM reduces the direct impact of fish predation onzooplankton. A more subtle consequence of DVM isthat populations of large bodied zooplankton maysurvive longer in the lake, which thus extends thetime during which this food resource remains avail-able for the fish to feed on. In this sense, DVM can beexpected to strongly buffer predator–prey interac-tions between fish and zooplankton. With respect tothe zooplankton–algae interaction, DVM has beenshown to strongly affect the grazing impact of zoo-plankton on algae. Here again, however, this has twoaspects to it. On the one hand, grazing impact wouldindeed be higher in the absence of fish-induced DVM,because DVM reduces the time during which zoo-plankton resides in the epilimnion to feed on algae.On the other hand, DVM may allow large bodiedzooplankton to survive in lakes that harbor fish.Given that large bodied zooplankton are more effi-cient phytoplankton grazers than small bodied zoo-plankton, DVM may thus indirectly promote thegrazing impact of zooplankton on algae, as it allowslarge-bodied zooplankton to graze on phytoplanktonduring at least part of the day. In lakes in which thezooplankton exhibits a strong DVM, the algae areindeed fed on by large and efficiently grazing zoo-plankton during the night. The impacts of DVM onpopulation dynamics of zooplankton and algae andon predator–prey interactions are thus very pervasive.A nice illustration of the impact of DVM on popula-tion dynamics is given by the observation of a lunarcycle in zooplankton densities in Lake Kariba. Thepopulation densities of zooplankton species in thislake have been reported to show cyclic changes asso-ciated with increased predation success of fish onzooplankton during moonlit nights at full moon. Inthe tropics, sunset and moonrise occur quite fast, andat full moon, the zooplankton is trapped in the surfi-cial water layers by the suddenly rising full moon,providing a feast for the fish.An interesting avenue of thought in this context is

the impact of predator-induced plasticity in DVM onpredator–prey interactions and top–down control inlakes. Indeed, as DVM has strong impacts on

top–down control, it should be recognized that partof the impact of predators is actually mediated byinduced avoidance behavior rather than by directpredation itself.

Inverse Migrations

We focused on the DVM behavior of herbivorouszooplankton (mainly cladocerans, copepods, androtifers). Predatory zooplankton may, however, alsostrongly engage in DVM. The best examples are theextensive migrations carried out by phantom midgelarvae (Chaoborus) in fish-inhabited lakes. Chao-borus species that inhabit fishless habitats do notmigrate vertically, but in fish lakes, the animalsoften migrate to the sediments during the day andappear in the water column at night. This is believedto be a strong structuring force on the zooplankton inlakes. During the day, large-bodied zooplankton,including Chaoborus larvae, hide in the deep waterlayers as a refuge from fish predation. By doing so,they provide enemy-free space to small-bodied zoo-plankton. The small-bodied zooplankton may, how-ever, move out of the high food upper water layersduring the night, as these layers are then invaded byefficiently grazing large-bodied zooplankton andgape-limited invertebrate predators that hunt for thesmall-bodied zooplankton. The resulting pattern is astrong standard migration for crustacean zooplank-ton and Chaoborus and a reverse migration for smallzooplankton such as rotifers.

There is an interesting parallel in large, motile phy-toplankton. Algae often perform a ‘reverse’ DVM,residing deeper in the water column during the nightthan during the day. There is much less literature onDVM in algae, but here too, it is a profitableapproach to consider it a habitat selection behavior.There are two main reasons for DVM in algae. First,it may be a strategy to combine efficient photosynthe-sis during the day with a reduction of mortality bygrazing of zooplankton during the night, by spreadingout over the water column. Alternatively, DVM ofalgae may also be a strategy to increase nutrientuptake. By migrating to deeper and nutrient-richlayers during the night, the phytoplankton mayincrease nutrient availability for photosynthesis dur-ing the day, thus reducing the impact of nutrientdepletion in the epilimnion.

DVM in Fish

Fish have also been reported to migrate vertically.They may do so for two reasons. First, they may


follow their prey. Even though it is much less efficientto hunt in deeper water layers, it may still be moreprofitable to follow your food in suboptimal condi-tions than remaining in a habitat that is devoid of anyfood. It has been shown that fish may even venture forshort dives into the near-anoxic conditions to hunt forzooplankton that use these inhospitable layers as arefuge. This may actually explain why zooplanktonoften migrate deeper than the border zone of therefuge. A second reason why fish migrate may be toavoid their own predators. By avoiding the surfacewaters during the day, they may reduce their mortal-ity from fish-eating birds. Many fish populationshave been reported to either change their vertical orhorizontal distribution. In the latter case, they hide inthe littoral zone during the day and invade the pela-gial during the night. Given that studies often showthat both fish and zooplankton avoid the surficiallayers of the pelagial zone during the day, one maywonder why the zooplankton does not simply rein-vade the food-rich upper water layers. There areseveral explanations for this. First, it should beacknowledged that in zooplankton populations thatshow a very strong DVM behavior, intensive sam-pling of the superficial water layers often revealssome individuals residing there at very low densities.Secondly, it is easy to imagine that, if densities in thesuperficial layers would become higher, the fishwould also rapidly start exploring this food source.Given that hunting at high light intensities is so effi-cient, even short excursions of the fish to these waterlayers would decimate the zooplankton. So oneexpects densities of larger bodied zooplankton to bevery low in the epilimnion in such lakes, which is thepattern that is observed.

See also: Cladocera; Competition and Predation;Copepoda; Light, Biological Receptors; PhytoplanktonPopulation Dynamics: Concepts and PerformanceMeasurement; Regulators of Biotic Processes inStream and River Ecosystems; Role of Zooplankton inAquatic Ecosystems; Rotifera; Ultraviolet Light.

Further Reading

Bayly IAE (1986) Aspects of diel vertical migration in zooplankton,

and its enigma variations. In: De Deckker P and Willams WD(eds.) Limnology in Australia. Dordrecht: Junk.

Cousyn C, De Meester L, Colbourne JK, Brendonck L, Verschuren

D, and Volckaert F (2001) Rapid local adaptation of zooplank-

ton behavior to changes in predation pressure in absence ofneutral genetic changes. Proceedings of the National Academyof Sciences USA 98: 6256–6260.

De Meester L, Weider LJ, and Tollrian R (1995) Alternative anti-

predator defences and genetic polymorphism in a pelagicpredator–prey system. Nature 378: 483–485.

Gliwicz ZM (1986) A lunar cycle in zooplankton. Ecology 67:

882–897.

Haney JF (1988) Diel patterns of zooplankton behavior. Bulletin ofMarine Sciences 43: 583–603.

Lampert W and Sommer U (2007) Limnoecology: The Ecology ofLakes and Streams. 2nd edn. Oxford: Oxford University Press.

Ohman MD (1990) The demographic benefits of diel vertical mi-

gration by zooplankton. Ecological Monographs 60: 257–281.Ringelberg J (ed.) (1993) Diel Vertical Migration of Zooplankton,

Archiv fur Hydrobiologie – Advances in Limnology, vol. 39.Stuttgart: E. Schweizerbart’sche Verlagsbuchhandlung.

Ringelberg J (1999) The photobehaviour of Daphnia spp. As a

model to explain diel vertical migration in zooplankton.

Biological Reviews 74: 397–423.Stich HB and Lampert W (1981) Predator evasion as an explana-

tion of diurnal vertical migration by zooplankton. Nature 293:

396–398.Tollrian R and Harvell CD (eds.) (1999) The Ecology and Evolu-

tion of Inducible Defenses. Princeton: Princeton University Press.

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