Population structure,growth and fecundity of perch (Perca fluviatilis L.) in an acidified river system in Southern Norway

Population structure, density and reproductive potential of perch (Perca fluviatilis L.) from 4 acidified lakes in a river system in Southern Norway were investigated. The upper 3 lakes were most affected by acidification and the number of perch caught per unit effort indicate three-fold increase in density from the upper Gjerstadvann to the lower Brøbørvann. Low density of perch in the upper lakes is explained by; 1) Abnormal mortality occurring in episodes due to acidification, probably caused by aluminium toxicity at pH = 5.2–5.4. 2). Acid water (pH ≤ 5.0) during spawning and development of eggs and larvae, resulting in recruitment failure. Juvenile mortality of perch may depend on the density of the parental stock. In the 3 upper lakes, the density of adult perch probably was too low to produce significant juvenile mortality, and in these lakes the recruitment probably depended more on the May water quality. Improved water quality increases egg hatching and survival and also benefits zooplankton production, the main food for perch during the first summer. Low population density has reduced competition for food and thus improved growth of perch in the 3 upper lakes compared to the lower lake. The perch in Brøbørvann mature at higher age and have lower individual fecundity than perch in Gjerstadvann, an effect of both slower growth and lower length specific fecundity in Brøbørvann. In Gjerstadvann, the perch therefore have a higher reproductive potential relative to the stock density, and may, when the water quality is good enough, give rise to strong year-classes and more frequent year-class fluctuations than perch in the less acid Brøbørvann.     

Small fish use the hypoxic pelagic zone as a refuge from predators

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Diel vertical migrations of bathypelagic perch fry

The behaviour of young‐of‐the‐year (YOY) perch Perca fluviatilis as a dominant species in the assemblage of fry in the pelagic of Slapy Reservoir (Czech Republic), was studied during late May and mid‐June 2002 using acoustic methods and complementary net catches. During the day, perch fry were present simultaneously in littoral, epipelagic and bathypelagic habitats. Bathypelagic perch fry, forming a scattering layer, migrated vertically each day between the epilimnion and hypolimnion, with an amplitude of 11·0 m in May and 12·5 m in June. At dusk, the migratory bathypelagic fry mixed in the epilimnion with non‐migrating epipelagic fry and spent the night close to the thermocline (abundance maximum at 3–4 m in May, 0–2 m in June). In June, shoaling behaviour by some of the bathypelagic perch fry was also observed: the shoaling fry remained higher in the water column than the non‐shoaling fry. Both depths of the scattering layer and the depths of the fry shoals were strongly controlled by the light intensity. The contribution of the bathypelagic part of the population to the total numbers of pelagic perch fry decreased from 28·1% in May to 4·7% in June, while the density of all pelagic perch fry increased (c. 96 000 individuals ha^?1 in May and 142 000 individuals ha^?1 in June). In May, the bathypelagic (average total length, L_T, 11·9 mm) and epipelagic (average L_T 14·6 mm) perch fry differed in size while, in June, the epipelagic fry were divided into two distinct size groups. The more abundant group, of small epipelagic perch fry (average L_T 14·6 mm), was similar in size to the bathypelagic fry (average L_T 14·6 mm) while the less abundant group, of larger epipelagic fry (average L_T 34·4 mm), was similar in size to littoral perch fry (average L_T 35·0 mm). The results suggest that in perch fry three different survival strategies with different risks can be used in the same locality, time and year.     

Recruitment variation in a stream galaxiid fish: multiple influences on fry dynamics in a heterogeneous environment

1. The relative importance of density‐dependent and density‐independent processes in explaining fluctuations in natural populations has been widely debated. In particular, the importance of larval supply and whether it may control the type of regulatory processes a population experiences has proved contentious. 2. Using surveys and field experiments conducted in streams in Canterbury, New Zealand, we investigated how variation in the survival of non‐migratory Galaxias vulgaris fry was affected by density‐dependent and density‐independent processes and how this variation influenced recruitment dynamics. 3. Fry populations with high settlement densities experienced a 70–80% reduction in population size from density‐related mortality during the first fourteen days after peak settlement but thereafter the influence of density‐dependent processes on fry was weak. The impact of environmental conditions on fry populations was dependent on fry size and the magnitude of the perturbation, such that flooding effects on fry survival were most severe when fry were small. 4. In streams not affected by flooding, the size and density of introduced trout (Salmo trutta and Oncorhynchus mykiss) were the most significant factors determining the abundance of eventual recruits. A field experiment manipulating brown trout access to fry populations revealed that trout as small as 110 mm may be capable of greatly reducing and possibly preventing galaxiid recruitment. 5. Overall, the results indicated density‐dependent population regulation was only possible at sites with high native fish densities because trout were likely to be suppressing the number of potential recruits at sites with low native fish numbers. Whilst density‐dependent processes had a strong effect on fry survival following the period of peak fry abundance, density‐independent processes associated with flow and predatory trout influences on fry survival largely determined recruitment variability among galaxiid populations. Focusing conservation efforts on improving habitat to increase fry retention and reducing the impacts of trout on galaxiids would ensure more native fish populations reached their potential abundance.     

Priority effects among young‐of‐the‐year fish: reduced growth of bluegill sunfish (Lepomis macrochirus) caused by yellow perch (Perca flavescens)?

1. When available, Daphnia spp. are often preferred by age‐0 yellow perch and bluegill sunfish because of energetic profitability. We hypothesised that predation by age‐0 yellow perch could lead to a midsummer decline (MSD) of Daphnia spp. and that priority effects may favour yellow perch because they hatch before bluegill, allowing them to capitalise on Daphnia spp. prior to bluegill emergence. 2. Data were collected from 2004 to 2010 in Pelican Lake, Nebraska, U.S.A. The lake experienced a prolonged MSD in all but 1 year (2005), generally occurring within the first 2 weeks of June except in 2008 and 2010 when it occurred at the end of June. MSD timing is not solely related to seasonal patterns of age‐0 yellow perch consumption. Nevertheless, when Daphnia spp. biomass was low during 2004 and 2006–2010 (<4 mg wet weight L^?1), predation by age‐0 yellow perch seems to have suppressed Daphnia spp. biomass (i.e. <1.0 mg wet weight L^?1). The exception was 2005 when age‐0 yellow perch were absent. 3. Growth of age‐0 bluegill was significantly faster in 2005, when Daphnia spp. were available in greater densities (>4 mg wet weight L^?1) compared with the other years (<0.2 mg wet weight L^?1). 4. We conclude that age‐0 yellow perch are capable of reducing Daphnia biomass prior to the arrival of age‐0 bluegill, ultimately slowing bluegill growth. Thus, priority effects favour age‐0 yellow perch when competing with age‐0 bluegill for Daphnia. However, these effects may be minimised if there is a shorter time between hatching of the two species, higher Daphnia spp. densities or lower age‐0 yellow perch densities.     

Increased growth and recruitment of piscivorous perch, Perca fluviatilis, during a transient phase of expanding submerged vegetation in a shallow lake

1. In this study, we examine how a 7‐year period of expanding submerged stonewort (Chara spp.) vegetation during a shift from turbid to clear water in a shallow lake influenced individual growth and population size structure of perch (Perca fluviatilis). We expected that a shift from phytoplankton to macrophyte dominance and clear water would improve feeding conditions for perch during a critical benthivorous ontogenetic stage, and enhance the recruitment of piscivorous perch. 2. Growth analysis based on opercula showed that growth during the second year of life was significantly higher in years with abundant vegetation than in years with turbid water and sparse vegetation. Growth was not affected during the first, third and fourth year of life. Stable isotope analyses on opercula from 2‐year‐old perch showed that the increase in growth coincided with a change in carbon source in the diet. Stable nitrogen ratio did not change, indicating that the increased growth was not an effect of any change in trophic position. 3. Following the expansion of submerged vegetation, perch size range and abundance of piscivorous perch increased in central, unvegetated areas of the lake. In stands of stoneworts, however, mainly benthivorous perch were caught, and size range did not change with time. 4. Our findings provide empirical support for the notion that establishment of submerged vegetation may lead to increased recruitment of piscivorous perch, because of improved competitive conditions for perch during the benthivorous stage. This is likely to constitute a benthic‐pelagic feedback coupling, in which submerged vegetation and clear water promote the recruitment of piscivorous perch, which, in turn, may increase water clarity through top‐down effects in the pelagic.     

The effects of nymphaeid (Nuphar lutea) density and predation by perch (Perca fluviatilis) on the zooplankton communities in a shallow lake

1. The effects of addition of juvenile perch (Perca fluviatilis) on the microcrustacean and rotifer communities associated with nymphaeid beds were studied, at three different plant densities [high (normal), medium (reduced by a half) and low (reduced to a third of normal)], in eighteen 2 m × 1 m enclosures in a shallow lake. 2. At the low and medium densities of lilies, Daphnia densities were high in the absence of perch but very low in the presence of perch. They increased, even in the presence of perch, to high densities (comparable with those in the absence of perch) at the highest plant density. Body sizes of Daphnia hyalina were consistent with high predation by perch at low and medium plant densities but reduced predation at high plant densities. Patterns of chlorophyll a concentration, in the presence of perch, inversely reflected those of D. hyalina density. 3. At naturally high densities but not at reduced densities, the plants appeared to act as refuges against predation for the Daphnia. Reductions in oxygen concentrations in the plant beds were not responsible for the refuge effect, nor could there be avoidance of the beds by the fish. The mechanism of the refuge effect must therefore lie in frustration of the process of capture of the Daphnia by the fish. 4. Numbers of other small Crustacea and rotifers were mostly unaffected by fish predation. Numbers of Asplanchna sp., Chydorus sphaericus and copepodites were higher in the presence of fish and, although there was no main effect of fish on numbers of Diaphanosoma brachyurum, there was a plant–fish interaction, with this species being less abundant in the presence of fish at low plant densities but more abundant in the presence of fish at medium and high plant densities. Main effects of plants were few, with only Asplanchna sp. and Keratella sp. decreasing in numbers with increasing plant density. Most taxa changed in numbers with time but interaction effects between time and plants, and fish and plants, were few.     

Effect of vertebrate predation on the spatio-temporal distribution of cladocerans in a temperature eutrophic lake

We analysed the spatio-temporal distribution of zooplankton along a profile of 10 stations from the shore to the pelagic zone from April to September 1988, the period when the larvae and juveniles Rutilus rutilus, the most abundant species in the Lake, are in the littoral zone. The digestive tracts of the young roach were analysed. They fed essentially on rotifers and on cladocerans. For comparison, zooplankton was also analysed at one littoral area without fish fry. There was an increase of cladoceran density from the vegetated nearshore zone to the offshore zone. Considering the density of Bosmina longirostris, Daphnia longispina, Chydorus sphaericus and Ceriodaphnia quadrangula, we observed a different distribution pattern in the course of the year. In the nearshore zone, the relative abundance of small species, Bosmina and Chydorus, was much higher than that of the larger Daphnia. From April to September, predation pressure mainly affected the smallest species: in contrast to the inshore station without fish fry, the density of Bosmina decreased in May in the littoral with fish. Chydorus was concentrated in the littoral between February and April, then grew into the pelagic zone, where predation pressure obviously was low during the warm season. The number of Daphnia, which was eaten by the fish fry at any time, remained low in the nearshore zone, which suggests that the presence of fish may cause Daphnia to avoid this zone. Ceriodaphnia which was not affected by this predation, was scarce in the nearshore zone during mid-summer. The low density of the cladocerans in the nearshore zone is likely associated with vertebrate predation by roach fry and juveniles, the result of such a process being either a depletion in density of the prey, or an avoidance behaviour.     

Biomanipulation by ecotone management in a lowland reservoir

The comparative studies on fry of dominant fish species in the lowland Sulejow Reservoir have demonstrated the importance of an unstable ecotonal shoreline zone in whole system dynamics, which affects water quality. Ecotone zone utilization by fish determines their reproductive success and consequently pressure of fry on a zooplankton community. Density of perch, which dominates in the fry community of the Reservoir, influences the utilization of ecotonal resources by dominant cyprinid fry-roach. A reverse relationship between perch and roach fry densities has been demonstrated, which in turn, affects utilization of zooplankton by roach fry. At stations of highest perch density roach reduces feeding on zooplankton and eats more algae and insects. The knowledge of competitive relations between perch and cyprinid fry has been an important aspect of application of the biomanipulation concept in the reservoir ecosystem, to improve fish yield and water quality. Hypothetical models of energy flow through the reservoir's ecosystem with poor and well developed flooded riparian ecotonal zones are established.     

Diet niche relationships among predator and prey fish species in their early life stages in Lake Võrtsjärv (Estonia)

In Lake Võrtsjärv pikeperch was observed not to shift to piscivory in their first autumn of life, although juvenile stages of a variety of fish species were abundant in the lake. It was hypothesized that the diets of predator and prey fish fry overlap and that coarse fish species are important food competitors for juvenile piscivores and thus, pikeperch and perch fry do not shift to piscivory during their first growing season. To discover the possible linkages in this pattern, in 2009 the feeding relationships of pikeperch, perch, ruffe and roach fry were analysed. The stomach content analyses showed that in the summer period, Mesocyclops leuckarti was the most frequent prey for perch and ruffe, pikeperch consumed Leptodora kindti in large quantities, and roach ate mostly plant material. Towards autumn, M. leuckarti was the most abundant prey for all percids. However, average stomach content weight and the number of prey items eaten by ruffe were considerably higher than for other fish fry. Since the feeding opportunities of fish fry are considered poor in the examined lake, the prey has the potential to restrict the recruitment to piscivory of their predators, as prey fish seem to have better abilities to persist in this ecosystem. Furthermore, supposed competition in the juvenile stage may result in a reduced top‐down effect on coarse fish.     

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.     

Life history of Daphnia galeata in a hypertrophic reservoir and consequences of non-consumptive mortality for the initiation of a midsummer decline

SUMMARY 1. Field and laboratory investigations were combined in a 2‐year study on the initiation of a midsummer decline of Daphnia galeata Sars in a hypertrophic reservoir. Quantitative field samples were taken twice a week, and, adult and juvenile mortality rates were calculated. Patterns of reproduction and survival of daphnids born during spring and early summer under fluctuating food conditions were determined in life‐table experiments. 2. The abundance of Daphnia increased strongly in early May and declined in June 1998 (midsummer decline). In 1999, Daphnia density increased only slowly in spring and remained constantly high throughout the summer. 3. Food conditions (concentrations of POC_{<30 μm}) for daphnids deteriorated in both years in response to increasing Daphnia densities, resulting in a clear‐water phase of about 4 weeks. When Daphnia abundance declined in 1998, POC_{<30 μm} concentrations increased greatly, whereas in 1999 food conditions improved only slightly and Secchi depth remained high. 4. Survival of daphnids in life‐table experiments decreased greatly after food became rare and was strongly reduced in those animals born during the clear‐water phase compared with those born later. In addition, age at first reproduction was retarded during the clear‐water phase, resulting in very low population growth rates. Survivorship patterns in life‐table experiments suggest a strong impact of non‐consumptive mortality on Daphnia population dynamics. 5. Field data of mortality point to differences in mortality patterns between years, probably resulting from different predation impacts of juvenile fish. In both years, however, adult mortality contributed substantially to overall mortality at the end of the clear‐water phase. As bottom‐up effects on D. galeata were very similar in both years, the significance of non‐consumptive mortality on the initiation of midsummer declines appears to depend largely on recruitment patterns before the clear‐water phase. A high impact can be expected when Daphnia populations are dominated by a peak cohort of nearly identical age during the clear‐water phase.     

Feeding behaviour determining differential capture success of evasive prey in underyearling European perch (<Emphasis Type="Italic">Perca fluviatilis</Emphasis> L.) and roach (<Emphasis Type="Italic">Rutilus rutilus</Emphasis> (L.))

The effect of feeding behaviour on the prey capture efficiency of young-of-the-year European perch and roach was investigated in laboratory experiments using planktonic crustaceans possessing different escape abilities—Daphnia sp. and Cyclops sp. Two sets of experiments were performed. In the first set, the feeding efficiency and behaviour of 270 fish individuals were determined by stomach content analyses and video record evaluations. In the second set of experiments, analysis of attack-effort, which was evaluated as attack-distance and repeated strikes, was undertaken. Except for situations in which Daphnia was offered at high densities, the feeding efficiency of perch was significantly higher compared to roach in all other combinations of prey types and densities. Roach consumed significantly less prey compared to perch when feeding exclusively on the evasive Cyclops and when it was offered in a 1:1 ratio mixture with Daphnia. The mean swimming speed was similar in both fish species, but behavioural differences were evident during prey search and capture. Perch swam through the aquaria in short and fast movements that were interrupted by many stops. Roach exhibited rather continuous swimming that was punctuated by slowdowns instead of stops. The perch attacks were very intensive and repeated strikes occurred, particularly when feeding on evasive Cyclops. On the other hand, roach revealed strong schooling behaviour restricting the fish during inspection of the experimental aquaria. The distinct differences in feeding efficiency between perch and roach were demonstrated to be closely related to differences in their feeding behaviour. Discontinuous searching for prey, vigorous attacks, occurrence of repeated strikes and the absence of schooling increased perch prey capture efficiency, particularly when foraging on evasive copepods.     

Food and habitat partitioning among juveniles of three fish species in the pelagial of a mesotrophic lake

An analysis of the diet of 0⁺ perch, smelt and roach caught at night of the pelagial of a mesotrophic lake showed that their food was composed mainly of herbivorous and predatory cladocerans, copepods and Chaoborus larvae during summer, and of herbivorous cladocerans and copepods during October. An analysis of habitat use by juveniles revealed separation among the species: roach occurred in the upper, perch and smelt in the deeper water layers. Food and habitat were alternatively partitioned among the juveniles. In early summer the lowest food overlap between perch and smelt coincided with high habitat overlap. In October high food overlap between perch and smelt corresponded with low habitat overlap. Smelt and roach fry used common food resources throughout the season, but they were segregated in habitat.     

The role of ecotones as feeding grounds for fish fry in a Bohemian water supply reservoir

The food composition of young-of-the-year (YOY) roach, common bream and perch, the dominant fish species in ímov reservoir, was investigated during 1989 and 1990. The littoral fry assemblages were dominated by roach and perch, while bream larvae migrated into deeper water early in their development. The YOY roach fed opportunistically. Up to 15 mm length they remained close to the shoreline, preferring periphyton, Chydoridae and larval Chironomidae. Perch larvae between 10–15 mm long gradually became demersal but their diet continued to consist of the planktonic Cyclops spp. and Diaptomus spp. Resource partitioning was a characteristic feature of the littoral fry assemblages. The only significant overlap in diet occurred with Polyphemus pediculus, which was positively selected by both roach and perch fry.     

The consequences of a drastic fish stock reduction in the large and shallow Lake Wolderwijd,The Netherlands. Can we understand what happened?

In 1990 an experiment started in the large and shallow lake Wolderwijd (2700 ha, mean depth 1.5 m) to improve the water quality. About 75% of the fish stock was removed (425 000 kg fish). The fish was mainly composed of bream and roach. In May 600000 young pikes (3–4 cm) were introduced.In May 1991 the water became very clear (Secchi depth 1.8 m) during a spring bloom of large Daphnia. Then the grazing by zooplankton was eight times higher than the primary production of algae and the total suspended matter concentration became very low. Compared to the situation before the fish reduction, the grazing had increased only slightly, while the primary production had decreased significantly in early spring. The fish stock reduction might have contributed to the reduction in primary production by a reduced internal nutrient load. The clear water period lasted six weeks. Daphnia disappeared in July due to food limitation, the algal biomass increased and the Secchi depth became 50 cm. Daphnia did not recover during summer, due to predation that was not caused by 0 + fish but by the mysid shrimp Neomysis integer. Neomysis could develop abundantly, because of the reduced biomass of the predator perch. The production of young fish had been low because of the cold spring weather. The cold weather was probably also responsible for the slow increase in density of macrophytes. After 1991, perch probably can control Neomysis. Due to lack of spawning places and shelter for 0 + pike, pike was probably not able to control the production of 0 + fish. In a lake of this scale, it will not be easy to get more than 50% coverage of macrophytes, which seems necessary to keep the algal biomass low by nutrient competition. Therefore, we expect also in the future a decrease in transparency in the summer. Locally, especially near Characeae, the water might stay clear.     

Trophic dynamics in a whole lake experiment: size-structured interactions and recruitment variation

Horizontal and vertical heterogeneity as a result of size‐structured processes are important factors influencing indirect effects in food webs. In a whole‐lake experiment covering 5 years, we added the intermediate consumer roach (Rutilus rutilus) to two out of four lakes previously inhabited by the omnivorous top predator perch (Perca fluviatilis). We focused our study on the direct consumption effect of roach presence on zooplankton (and indirectly phytoplankton) versus the indirect effect of roach on zooplankton (and phytoplankton) mediated via effects on perch reproductive performance. The patterns in zooplankton and phytoplankton abundances were examined in relation to population density of roach and perch including young‐of‐the‐year (YOY) perch in the light of non‐equilibrium dynamics. The presence of roach resulted in changed seasonal dynamics of zooplankton with generally lower biomasses in May–June and higher biomasses in July–August in roach lakes compared to control lakes. Roach presence affected perch recruitment negatively and densities of YOY perch were on average higher in control lakes than in treatment lakes. In years when perch recruitment did not differ between lakes as a result of experimental addition of perch eggs, total zooplankton biomass was lower in treatment lakes than in control lakes. Phytoplankton biomass showed a tendency to increase in roach lakes compared to control lakes. Within treatment variation in response variables was related to differences in lake morphometry in treatment lakes. Analyses of the trophic dynamics of each lake separately showed strong cascading effects of both roach and YOY perch abundance on zooplankton and phytoplankton dynamics. Consideration of the long transients in the dynamics of top predators (fish) in aquatic systems that are related to their long life span involving ontogenetic niche shifts is essential for making relevant interpretations of experimental perturbations. This conclusion is further reinforced by the circumstance that the intrinsic dynamics of fish populations may in many cases involve high amplitude dynamics with long time lags.     

Recruitment patterns of Daphnia: a key for understanding midsummer declines?

The population dynamics of D. galeata was studied during the period May–July of 5 successive years in Bautzen Reservoir (Germany). Sampling intervals of 3–4 days and knowledge of the egg–age distribution made it possible to calculate recruitment of the population for each sampling interval. In every year, high Daphnia densities and high Secchi depth readings (clear-water stage) were recorded in spring. In two of the 5 years, Daphnia abundance remained on a high level throughout summer, whereas in the remaining 3 years a midsummer decline of Daphnia was observed. Birth rates declined while the abundance increased in spring in every year but the timing of this decline in relation to abundance and the onset of the clear-water stage revealed no obvious pattern, distinguishing between years with or without a midsummer decline. However, cumulative recruitment did reveal a clear pattern: in years without a midsummer decline, cumulative recruitment increased slowly, but continually, starting from a low level in early May. By contrast, in years with a midsummer decline, recruitment increased strongly during one week in May, starting from a relatively high density level (>30 ind. l^–1). This quick increase was followed by extended periods with low recruitment. Consequently, in these years the population was dominated by a strong peak-cohort of about the same age during the clear-water stage. If all members of this cohort, while approaching their maximum life-span, die off during a short time interval, this may have a pronounced effect on the initiation of a midsummer decline. From an extensive literature search, evidence is provided that the findings of this study represent a demographic mechanism which is of general importance for the initiation of a midsummer decline in temperate waters.     

Foraging behavior by <Emphasis Type="Italic">Daphnia</Emphasis> in stoichiometric gradients of food quality

Mismatches in the elemental composition of herbivores and their resources can impact herbivore growth and reproduction. In aquatic systems, the ratio of elements, such as C, P, and N, is used to characterize the food quality of algal prey. For example, large increases in the C:P ratio of edible algae can decrease rates of growth and reproduction in Daphnia. Current theory emphasizes that Daphnia utilize only assimilation and respiration processes to maintain an optimal elemental composition, yet studies of terrestrial herbivores implicate behavioral processes in coping with local variation in food quality. We tested the ability of juvenile and adult Daphnia to locate regions of high-quality food within a spatial gradient of algal prey differing in C:P ratio, while holding food density constant over space. Both juveniles and adults demonstrated similar behavior by quickly locating (i.e., <10 min) the region of high food quality. Foraging paths were centred on regions of high food quality and these differed significantly from paths of individuals exposed to a homogeneous environment of both food density and food quality. Ingestion rate experiments on algal prey of differing stoichiometric ratio show that individuals can adjust their intake rate over fast behavioral time-scales, and we use these data to examine how individuals choose foraging locations when presented with a spatial gradient that trades off food quality and food quantity. Daphnia reared under low food quality conditions chose to forage in regions of high food quality even though they could attain the same C ingestion rate elsewhere along a spatial gradient. We argue that these aspects of foraging behavior by Daphnia have important implications for how these herbivores manage their elemental composition and our understanding of the dynamics of these herbivore–plant systems in lakes and ponds where spatial variation in food quality is present. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Restoring lakes by using artificial plant beds: habitat selection of zooplankton in a clear and a turbid shallow lake

1. Return of large‐bodied zooplankton populations is of key importance for creating a shift from a turbid to a clear‐water state in shallow lakes after a nutrient loading reduction. In temperate lakes, recovery is promoted by submerged macrophytes which function as a daytime refuge for large zooplankton. However, recovery of macrophytes is often delayed and use of artificial plant beds (APB) has been suggested as a tool to enhance zooplankton refuges, thereby reinforcing the shift to a clear‐water state and, eventually, colonisation of natural plants. 2. To further evaluate the potential of APB in lake restoration, we followed the day–night habitat choices of zooplankton throughout summer in a clear and a turbid lake. Observations were made in the pelagic and littoral zones and in APB in the littoral representing three different plant densities (coverage 0%, 40% and 80%). 3. In the clear lake, the zooplankton (primarily Daphnia) were mainly found in the pelagic area in spring, but from mid‐May they were particularly abundant in the APB and almost exclusively so in mid‐June and July, where they appeared in extremely high densities during day (up to 2600 ind. L⁻¹). During night Daphnia densities were overall more equally distributed between the five habitats. Ceriodaphnia was proportionally more abundant in the APB during most of the season. Cyclopoids were more abundant in the high APB during day but were equally distributed between the five habitats during night. 4. In the turbid lake, however, no clear aggregation was observed in the APB for either of the pelagic genera (Daphnia and Bosmina). This may reflect a higher refuge effect in the open water due to the higher turbidity, reduced ability to orient to plant beds and a significantly higher fish density (mainly of roach, Rutilus rutilus, and perch, Perca fluviatilis) in the plant beds than in the clear lake. Chydorus was found in much higher proportions among the plants, while cyclopoids, particularly the pelagic Cyclops vicinus, dominated in the pelagic during day and in the pelagic and high density plants during night. 5. Our results suggest that water clarity is decisive for the habitat choice of large‐bodied zooplankton and that introduction of APB as a restoration measure to enhance zooplankton survival is only a useful tool when water clarity increases following loading reduction. Our results indicate that dense APB will be the most efficient.