Seasonal Vertical Migration and Aestivation of <Emphasis Type="Italic">Rhyacodrilus hiemalis</Emphasis> (Tubificidae,Clitellata) in the Sediment of Lake Biwa,Japan

The vertical distribution of the tubificid worm Rhyacodrilus hiemalis Ohtaka, the numerically dominant species of oligochaete in the littoral of Lake Biwa, was studied with special reference to seasonal vertical migration in the lake sediment. Monthly collections of lake sediment cores were made using PVC tubes. Core sections of sample sediments ranged from 76 to 117 cm. The vertical distribution of the worms showed no diurnal variation; therefore diel vertical migration was not evident. Seasonal downward migration started in April, and upward migration started in October. From December to March, almost all worms remained in the near-surface sediment layer (surface to 30 cm deep), while from July to September almost all worms remained deeper than 30 cm. However, few individuals migrated deeper than 90 cm. No discontinuous layers were found in grain size composition, water content, loss on ignition, particulate carbon, nitrogen or phosphorus. In deep sediment there was no free oxygen, as evidenced by negative ORP values. For 4 months in summer, R. hiemalis aestivated, probably utilizing anaerobic respiration. It appeared that R. hiemalis moved deeper in the sediment in response to sediment temperature, because sediment temperatures in the deep layers seemed to converge at around 20–21 °C in the summer months. The life history traits of seasonal vertical migration and summer aestivation perhaps arose as an adaptation to the climatic conditions accompanying the geographical origin of R. hiemalis, and they also serve to minimize predation risk during summer when most invertebrate predatory fishes are active.     

Diel vertical migration and gut pigment rhythm of Paracalanus parvus,P. crassirostris,Acartia erythraea and Eucalanus subcrassus (Copepoda,Calanoida) in Tolo Harbour,Hong Kong

Diel vertical migration and gut pigment rhythm of calanoid copepods were studied in the highly eutrophic waters of Tolo Harbour, Hong Kong in summer between July 30 and August l, 1991 and in winter between January 24 and 25, 1992. Paracalanus parvus and P. crassirostris showed neither diel vertical migration nor diel gut pigment rhythm during the summer study. Acartia erythraea and Eucalanus subcrassus exhibited prominent diel vertical migration and diel gut pigment rhythm. Diel changes in gut pigment content of surface dwelling individuals and increase of gut pigment content before the onset of nocturnal upward migration suggest that gut pigment rhythm in E. subcrassus was not the result of animals moving in and out of a food-rich surface layer. During the winter study, the harbour was affected by a bloom of Noctiluca scintillans. P. parvus and P. crassirostris resided in deeper waters presumably to avoid the dense populations of N. scintillans at the surface. P. crassirostris remained non-migratory during the winter study. There was a nocturnal increase in the number of P. parvus in the surface waters, although no diel change in the mean depth of the population was observed. Both Paracalanus species showed diel gut pigment rhythm. The presence of diel gut pigment rhythm in the non-migratory P. crassirostris during the winter study suggested the presence of an independent feeding rhythm.     

Salinity tolerance of the gemmules of Eunapius fragilis (Leidy) and the inhibition of germination by various salts

We report results of a field test of the predator avoidance hypothesis as an explanation of the adaptive significance of diel vertical migration in zooplankton. We determined the vertical distribution and diel migration of the planktonic copepod Acartia hudsonica, concurrently with the abundance of pelagic fish, transparency and thermal stratification of the water column, on six cruises over a one year period in a temperate marine lagoon (Jakles Lagoon, San Juan Island, Washington, USA). Striking seasonal variability was observed in all biological and environmental variables. Linear regressions of the strength of diel vertical migration in A. hudsonica on these environmental variables resulted in only one statistically significant relationship, that between copepod diel vertical migration and predator abundance. These results, together with those of previous studies, point to diel vertical migration as a widespread behavioral response of planktonic prey to the presence of their predators.     

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.     

Daphnia size structure,vertical migration,and phosphorus redistribution

The timing and magnitude of diel migration in two daphnid assemblages were determined from a series of vertical profiles of daphnid size distribution. Animals were collected concurrently for gut fullness determination. Only large daphnids (> 1.4 mm) migrated, but these animals could account for substantial vertical and diel differences in phosphorus excretion rate. Gut fullness measurements and time courses of diel vertical migration suggested that large Daphnia can cause a net downward flux of phosphorus during summer in thermally stratified lakes.     

Day‐Night Vertical Distribution and Feeding Patterns of Fourth Instar ofChaoborus Larvae in a Neotropical Reservoir (Socuy Reservoir,Venezuela)

The day‐night vertical distribution, diel feeding activity and diet of fourth instar of Chaoborus larvae were analyzed in lacustrine zone of a neotropical reservoir which shows seasonally contrasting hypolimnetic oxygen conditions. Larvae stayed in sediment and water bottom during day and ascended to surface during night. Results indicate that feeding activity is limited mainly to the plankton population. Phytoplankton, rotifers or remains of Chaoborus larvae were not found in crops. With the exception of ostracods, all crustacean prey available in the zooplankton occurred in the guts. Ceriodaphnia cornuta and Moina micrura were the most frequent food items (about 75% of occurrence frequency) and were positively selected. The remainder crustacean zooplankton taxa were negatively selected by larvae. The most intense feeding activity in larvae occurred near midnight and sunrise, in dates when the hypolimnion was anoxic. When oxygen was available on the bottom, a higher and not changing diel feeding activity was detected. Our results indicate that vertical migration may promote a spatial separation between larvae and zooplankton, and feeding activity of larvae occurred only when both overlapped. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diel and seasonal vertical distribution of meiobenthic copepods in muddy sediments of a eutrophic lagoon (fish ponds of Arcachon Bay)

The vertical distribution of meiobenthic copepods was investigated within muddy sediments of a eutrophic lagoon (fish ponds of Arcachon Bay, France). The aim of the study was to determine if in muddy sediments, as previously established in sandy sediments, meiobenthic copepods migrate vertically according to the seasons or diel periods. Two experimental approaches were used, viz: a three-season comparison was made of the diel vertical distribution of the harpacticoid Canuella perplexa T. & A. Scott (1893) and secondly the depth distribution of a meiobenthic copepod assemblage was followed for a 24 h period, in shallow water subtidal locations. The harpacticoid C. perplexa vertically migrated through the top three centimeters of the sediment, showing diel and seasonal variations in depth distribution. The differential vertical distributions shown by the dominant meiobenthic populations suggest that emergence into the water column may mainly concern surface dwelling copepods. The physical and biological factors affecting seasonal and diel changes in the copepod assemblage of the fish ponds are discussed.     

Life history characteristics and distribution of Bythotrephes longimanus Leydig (Crustacea,Onychopoda) in the Biesbosch reservoirs

The diel and horizontal distributions of Bythotrephes longimanus Leydig, Leptodora kindti (Focke) and Anomopoda (cladocerans), as well as several life history parameters of each developmental stage of B. longimanus were studied in the non-stratified Biesbosch reservoirs. Anomopoda avoided the surface layer during the day only in the pelagic zone. In the surface layer near the shore, however, they were very abundant. Because densities in the pelagic zone during the day were very low, and high during the night, the population undoubtedly moved from the littoral zone or from the sediments to the pelagic. Accumulation of zooplankton at the littoral site is either the result of random dispersal or wind-induced movements. Bythotrephes' diel distribution pattern is possibly related to that of their prey, the anomopods. Leptodora mostly exhibited a normal diel vertical migration with a small amplitude. Wind most likely influenced the horizontal distribution of the zooplankton. Bythotrephes body length increased with higher temperatures, whereas spine length was constant throughout the year. Bythotrephes can already be fertile in the first developmental stage, indicating that a rapid adaptation to a favourable environment is possible. The number of parthenogenetically produced eggs per ovigerous female was higher at the start of the growing season and constant throughout the rest of the year. Relatively few resting eggs and males were found in autumn.     

Diel vertical migration thresholds of Karenia brevis (Dinophyceae)

Light and nutrient availability change throughout dinoflagellate diel vertical migration (DVM) and/or with sub-population location in the water column along the west Florida shelf. Typically, the vertical depth of the shelf is greater than the distance a sub-population can vertically migrate during a diel cycle, limiting the ability of a sub-population to photosynthetically fix carbon toward the surface and access nutrients sub-surface. This project investigated changes of Karenia brevis (C.C. Davis) G. Hansen et Moestrup intracellular carbon, nitrogen, internal nitrate (iNO₃), free amino acid (FAA), and total lipid concentrations in high-light, nitrate-replete (960 μmol quanta m⁻² s⁻¹, 80 μM NO₃), and high-light, nitrate-reduced (960 μmol quanta m⁻² s⁻¹, <5 μM NO₃) mesocosms. The nitrate-reduced mesocosm had a slowed cell division rate when compared to the nitrate-replete mesocosm. Minimum intracellular carbon, nitrogen, iNO₃, FAA, and total lipid concentrations during the largest surface sub-population aggregations led to the conclusion that daughter cells resulting from cell division received unequal shares of the parental resources and that this inequality influenced migration behavior. Nutrient reduced daughter cells were more strongly influenced by light and phototaxis for carbon production than their replete same cell division sister cells during vertical migration thus rapidly increasing the fulfillment of constituents through photosynthesis. Vertical migration was consistent with an optimization scheme based on threshold limits through utilization or formation of photosynthate. We propose a simplified conceptual model describing how K. brevis is transported along the benthos of the west Florida shelf from off-shore to on-shore. Dynamic carbon thresholds are also suggested for future DVM modeling efforts on K. brevis populations transported between nitrogen replete and nitrogen reduced environmental conditions.     

Density and depth variations of Daphnia multilocus genotypes during a summer period in Lake Maarsseveen

The genotype composition of a Daphnia population complex during a summer period in Lake Maarsseveen (The Netherlands) was determined by allozyme analysis. The depth distribution, diel vertical migration and several parameters of the total population were measured. Young-of-the-year (0⁺) perch (Perca fluviatilis) were caught and species and allozyme types of Daphnia in the perch gut were also analysed. During May 1997, the densities of D. hyalina, D. galeata, the back-cross D. g × h — hyalina and the multilocus allozyme genotypes of the hybrid D. g × h decreased, except one multilocus genotype (MMMF). Total population size decreased and the ratio of females with eggs to those without eggs decreased as well. Food limitation during this clear-water phase in the lake is considered responsible. All genotypes, except MMMF, gradually descended in the water column. This drift is thought to be a reaction to the abundantly present 0⁺ perch or to the kairomones of this fish, although predation on the daphnids was still absent. In June, diel vertical migration started, except again part of the MMMF subpopulation. The other part migrated over a short distance compared with the other taxa and allozyme types. Within two weeks, the upper 5 m of the epilimnion was devoid of Daphnia, and guts of perch were predominantly filled with MMMF. The daphnids in the gut and the lake did not differ in allozyme type composition. By the end of July, population density had increased again. The size and composition of the Daphnia population complex continuously changed during the study period, as did the depth distribution of the components. Different genotypes within the population complex seem to have developed different strategies to cope with starvation and predation and the state at a particular moment can be understood only if past and present factors are considered.     

Vertical, lateral and longitudinal movement of zooplankton in a large river

1. The spatial distribution and movement patterns of zooplankton in large rivers are little known compared with those in lake environments. We conducted a series of studies in the Ohio River (U.S.A.) during the low flow period to assess diel vertical (DVM), longitudinal and lateral movement of crustacean zooplankton. 2. The dominant large zooplankter, the copepod Eurytemora affinis, showed a consistent vertical migration pattern of daytime ascent and night‐time descent during all sampling periods – the reverse of the most common migratory pattern of zooplankton in lakes. The cladoceran Bosmina migrated in a similar way in two of the three sampling periods. Surveys taken longitudinally in the river showed similar trends for both taxa. 3. During the lateral surveys, E. affinis was significantly more abundant in the shallow littoral zone during the night than in the daytime. The combination of vertical and lateral movement patterns along with the diel distribution of zooplanktivorous fish suggest that these movements are a predator‐avoidance mechanism. 4. Sampling programmes in large rivers should consider that larger zooplankton such as E. affinis may not be randomly distributed in the river channel and behaviours such as diel vertical migration may be just as evident in river habitats as in lakes.     

Diurnal vertical migration of Daphnia hyalina Leydig, 1860 (Crustacea: Cladocera) in Lake Bled (Slovenia) in relation to temperature and predation

Diurnal vertical migration (DVM) of Daphnia hyalina in Lake Bled was most intense during summer stratification. The extent of DVM varied with the size of the animal and its reproductive state. Migration distances were shortest in immature specimens and longest in ovigerous females. During daytime, ovigerous females stayed deeper in the water column than females without ova or immatures. The daytime temperature of water at the median depth of the ovigerous females did not exceed 10 °C, even in the warmest season. At night they migrated upward to an environment which was warmer by as much as 9 °C.Laboratory observations indicate that specimen's size and water temperature determine the velocity of passive sinking, such that morning descent of the different groups of Daphnia can be explained by passive sinking alone.Our hypothesis is that the distribution of different groups of D. hyalina in Lake Bled is influenced by two types of predators: fish (Perca fluviatilis L. and Rutilus rutilus (L.)) and larvae of Chaoborus flavicans (Meig.), the latter appearing in the epilimnion during the night. Fish predation has a key-role at the beginning of thermal stratification. Supposing that in spring the gene pool of Daphnia consists of a mix of different genotypes, distributed at different depths during the day, fish predation combined with a presence of fish chemicals favored genotypes with a lower day-depth during the spring/summer period.     

Migrations of haemoglobin-rich Daphnia longispina in a small,steeply stratified,humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.     

Contribution ofAzospirillum brasilense to the nitrogen needs of grain sorghum (Sorghum bicolor (L) Moench) in humid sub-tropics

Summary Field experiments were conducted to assess the contribution ofAzospirillum brasilense to the N needs of grain sorghumcv. CSH — 5 during monsoon (June–October) seasons of 1978 and 1979.A. brasilense contributed to the N uptake by crop in the range from 5.8 to 19.6 kg N/ha. However, the contribution ofA. brasilense to the N needs of sorghum was more when sorghum was manured with farmyard manure at the rate of 10 tons/ha. Publication of G.B.P.U.A.T. Expt. Station, Pantnagar — 263145, India.     

Vertical distribution of epipelagic siphonophores at the confluence between Benguela waters and the Angola Current over 48 hours

The diel distribution of epipelagic siphonophores at a station off northern Namibia (18°00 S 10°30 E) was studied. This area is characterized by the mixing of surface waters of the Angola Current with the waters of the northernmost Benguela region. During the sampling period, the continuous flow of the Angola Current gave rise to a marked thermocline at ca 30 m depth. In order to study the diel vertical distribution patterns of epipelagic species under these hydrographic conditions, narrow depth horizons in the top 200 m of the water column were sampled repeatedly over a 48 h period. Thirty-four species were collected. Sphaeronectes gracilis and Chelophyes appendiculata predominated. Increasing numbers of species and individuals were observed on the second day of sampling, particularly in the layers above the thermocline. Only a few species showed an upward nocturnal migration that crossed the boundary between 2 water masses. The differences in the depth distributions of the various species were related to 3 distinct phenomena: the existence of 2 separate siphonophore populations associated with each of the 2 water masses; vertical migration by the most abundant species, which were able to cross the thermocline; and differences in patch size for the most epipelagic species.     

Diel vertical migration ofEudiaptomus gracilis during a short summer period

Several aspects of a diel vertical migration (DVM) of adultEudiaptomus gracilis in Lake Maarsseveen (The Netherlands) are described. The period of DVM lasted from the end of May until the middle of August. On May 21, 1989, the population was found divided into a deep dwelling part and a part in the upper five meter. Large shoals of juvenile perch were observed in the open water for the first time. On June 7, the whole population was down below 10 m and concentrated in a zone of high chlorphyll-a concentrations. One week later, a regular DVM was performed. The amplitude of this migration gradually decreased towards the end of the migration period. The ascent in the evening and the descent in the morning took place after sunset and before sunrise, respectively. The movements coincided with high relative changes in light intensity. Population size increased rapidly during the period of DVM but decreased again before the end of this period.     

Diel patterns of feeding and vertical migration in daphnids and diaptomids during the clear water phase in Lake Geneva (France)

In lacustrine environments, little attention has been paid to small-scale interactions between zooplankton diel vertical migration (DVM) and feeding rhythms. Moreover, most of the information on in situ diel feeding and migratory rhythms is based on low sampling frequencies. The kinetics and the degree of coupling of these processes are thus only roughly known. Here, we present a study conducted on a diel cycle in Lake Geneva to establish the temporal and spatial relationships between DVM and grazing activity of the dominant planktonic crustaceans. Our methodological approach is based on reliable and frequent (every 30 minutes) sampling, and on gut fullness analysis. We test the hypothesis of temporal and spatial segregation in DVM and feeding activity of sympatric taxa to counteract resource competition. We also evaluate the variation in DVM and feeding activity between taxa, size and sexes. In Lake Geneva, the Daphnia complex of different species and size (D. hyalina × galeata) and the diaptomid (Eudiaptomus gracilis) have distinct DVM and diel feeding patterns which lead to temporal and spatial segregation. Differences arise from the amplitude and kinetics of DVM and diel feeding rhythms. A strong day/night contrast in depth distribution and feeding activity was observed for the large daphnids while the small daphnids and the diaptomids had lower amplitudes of DVM and weaker diel changes in feeding activity. Large Daphnia exhibited a bimodal feeding pattern coupled with dynamic interchange of individuals between the epi- and hypolimnetic layers at dusk and dawn. In contrast, little coupling between DVM and feeding patterns was found for the diaptomid. These distinct behaviours can be viewed as specific adaptive strategies developed by calanids and daphnids to limit interspecific competition and to compromise between avoidance of starvation in deep waters and avoidance of visual predators in surface layers. Our study supports the hypothesis of exogenous control of Daphnia DVM by the relative change in light intensity at dusk and dawn, but also suggests that small Daphnia (not large ones), are controlled by absolute light variations when this major stimulus is lacking. Our results also support the hypothesis that selective predation by fish is responsible for the observed differences in DVM and diel feeding patterns of sized-daphnids and diaptomids. Other factors explaining the coupling of DVM and feeding patterns are hunger, vertical temperature gradient and for daphnids, size. Thus, ecological plasticity in crustacean DVM and feeding patterns results from the interactive effect of multiple abiotic and biotic driving forces. Finally, our study also shows that large Daphnia have a marked contribution to the acceleration of downward nutrient fluxes in Lake Geneva, via their diurnal rhythm in feeding and vertical migration. Ecological implications of the study for lake management and sampling design of zooplankton grazing studies are also presented.

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Résumé Dans les écosystèmes lacustres, les interactions à fine échelle temporelle entre les patrons diurnes de migration verticale et de broutage du zooplancton sont peu étudiées. En outre, jusq'à présent, les études ont généralement été réalisées selon des chroniques temporelles assez lâches. La cinétique et le degré d'interaction entre les patrons journaliers de migration verticale et de broutage sont donc encore mal connus. La présente étude, conduite au Lac Léman (Lac de Genéve) au cours d'un cycle nycthéméral, tente de préciser les liaisons spatiales et temporelles existant entre les migrations journalières et la consommation de phytoplancton chez les taxons de Crustacés les mieux représentés. Notre approche méthodologique repose sur une maille temporelle d'échantillonnage fine et sur l'analyse de la fluorescence du contenu stomacal. Nous testons l'hypothèse d'une ségrégation spatio-temporelle visant à réduire la compétition entre les taxons sympatriques et reposant sur des différences entre les patrons journaliers respectifs de migration et d'alimentation. Nous évaluons pour ces rhythmes d'activité les différences entre les espèces, les classes de tailles et les sexes. Au lac Léman, le complexe de différentes espèces et tailles de daphnies (Daphnia hyalina × galeata) et le diaptomide (Eudiaptomus gracilis) présentent des patrons journaliers de migration et d'alimentation distincts, assurant une ségrégation spatio-temporelle. Les différences proviennent de variations dans la cinétique et l'amplitude des migrations et dans les niveaux d'alimentation. Les grandes daphnies affichent un fort contraste jour/nuit dans leur répartition verticale et leur état de réplétion, tandis que les petites daphnies et les diaptomides présentent une faible amplitude de migration et de variations circadiennes de réplétion. Les grandes daphnies ont un rythme alimentaire bimodal couplé avec un relais dynamique des organismes entre l'épilimnion et l'hypolimnion au crépuscule et à l'aube. Chez le diaptomide, les interactions sont au contraire faibles entre les patrons de migration verticale et de réplétion. Ces différents comportements peuvent être perçus comme des stratégies adaptives spécifiques développées par les daphnies et les diaptomides pour limiter la compétition interspécifique et aboutir à un compromis satisfaisant entre l'évitement de la famine en eaux profondes et de la prédation par les poissons dans les eaux superficielles. Notre étude conforte l'hypothèse d'un contrôle exogène de la migration verticale de Daphnia par les changements relatifs de la lumière au crépuscule et à l'aube. En l'absence de ce stimulus, la répartition verticale des petites daphnies semble par contre contrôlée par les variations absolues de lumière. Nos observations confortent également l'hypothèse que la prédation sélective par les poissons est responsable des différences observées dans les patrons de migration des grandes daphnies et ceux des petites daphnies et des diaptomides. Les autres facteurs pouvant influencer les patrons de migration et d'alimentation des crustacés du Lac Léman sont la famine, le gradient thermique vertical et, chez les daphnies, la taille. En définitive, la plasticité écologique des patrons journaliers de migration et d'alimentation résulte des effets interactifs de daphnies ont un rôle très important dans le transfert des nutriments dans les couches profondes durant l'été, via leurs migrations verticales et les variations circadiennes d'activité alimentaire. Les implications écologiques pour l'aménagement lacustre et la planification des études portant sur le broutage du zooplancton sont aussi présentées.

     

Contrasting migration behaviour of Daphnia pulicaria and D. galeata x hyalina, in avoidance of predation by 0+perch (Perca fluviatilis)

In spring and early summer, a small population of the large-bodied Daphnia pulicaria coexists with a much larger population of the medium-sized hybrid Daphnia galeata × hyalina in the epilimnion of Lake Maarsseveen (The Netherlands). When large shoals of juvenile perch (Perca fluviatilis) appear in the open water, both species start to migrate vertically. Since D. pulicaria has a larger body-size than D. galeata × hyalina, and is therefore competitive dominant over the hybrid, it is unlikely that both species interact via their common food resource, but they react both to fish predation. However, since they differ in size, and therefore in vulnerability for fish predation, both species adopt different strategies. The smaller bodied, and less vulnerable D. galeata × hyalina exhibits diel vertical migration ascending to the surface at dusk, and staying there during the night. In this way, benefiting from the higher temperatures of the surface layers. In contrast, the large-bodied, and more vulnerable D. pulicaria selects the deep cold hypolimnion water layers as refuge against fish predation. In this way it benefits from a safe habitat, free from fish predators, but on the other hand suffers from low water temperatures, which decrease its fitness. It is likely that the relatively higher temperature in the upper water layers serves as a proximate factor for the downward migration of D. pulicaria.     

The importance of worm identity and life history in determining the vertical distribution of tubificids (Oligochaeta) in a riverine mud

The vertical distribution of Tubifex tubifex, Limnodrilus hoffmeisteri and L. udekemianus, in a riverine mud was studied over a year. Distribution was dependent upon species identity, season and locality. The size structure of the population, and hence species life history, was of fundamental importance in determining temporal patterns of vertical distribution. A positive correlation between worm length and depth of penetration was clearly demonstrated for all three species. However, the mid-stream populations of the three species were closer to the mud surface during the summer and autumn than were the near-bank populations, although the size structures of the populations were not significantly different. Thus, another factor, possibly the distribution of food, also influenced the vertical distribution of the worms. Cocoon distribution was similar to that of the adult worms. Cocoons of the larger species, L. udekemianus, occurred deeper in the mud than those of T. tubifex. A comparison between the vertical distributions of tubificids in cores sectioned in situ, on the stream bank, and 30 mins later, showed there to be no significant change in distribution with time.     

Depth distribution of Daphnia in response to a deep-water algal maximum: the effect of body size and temperature gradient

1. In the absence of fish predation, Daphnia exploiting a deep‐water algal maximum are faced with a trade‐off. They can either dwell in the epilimnion where development in the warm water is fast, but food shortage causes low egg production, or in the hypolimnion, where food availability is high but development is slow because of low temperatures. 2. We tested the hypotheses that (i) depth distributions of various ontogenetic stages (size classes and egg‐bearing females) differ because daphnids react to light with size‐specific diel vertical migration (DVM) even in the absence of fish (residual predator avoidance hypothesis) and (ii) differently sized daphnids select different depths because the relative importance of temperature and food varies for ontogenetic stages (physiological hypothesis). We used large indoor mesocosms (Plankton Towers) to test these hypotheses experimentally. 3. Temperature was the strongest factor governing the distribution, with larger proportions of the population dwelling in the food‐rich hypolimnion if the temperature gradient was shallow. There were small but significant differences between ontogenetic stages during the day, but not at night. This suggested the existence of a ‘residual’ effect of light on depth distribution in the absence of a fish cue. 4. Although large individuals exhibited greater amplitude of DVM, the physiological hypothesis had to be rejected. A stage‐specific physiological effect is unlikely to be directly triggered by light, hence vertical movement of the individuals should not be synchronised. Rather, being forced into deeper layers by the residual light response during the day, large and egg‐bearing females experience a lower average temperature during day than juveniles. They probably compensate for this by spending longer time periods in warm waters at night.