Size-specific swimming behavior of Daphnia pulex

Body length affects several aspects of the behavior of quietlyswimming Daphnia pulex Swimming and sinking rates were measuredat 0.033 s intervals during the ‘hops’ characteristicof Daphnia swimming behavior Larger animals swim faster, covermore distance, and produce more powerful swimming strokes. LargerDaphnia also sink faster, but the sinking rate scales as lengthto the 0 58 power, far lower than the power of 2 00 predictedby Stokes Law considerations. The number of hops s^–1 wasindependent of body size, although a theoretical analysis predictshopping rate (antennal beat frequency) should increase as bodylength squared. Turning behavior, measured as the ratio of displacementto total distance, during 5 s, is also independent of body sizeIndependence of several parameters of body motion and body sizeimplies that factors other than simple mechanics affect Daphniaswimming behavior     

Cladoceran filtration rate-body length relations: model improvements developed for a Microcystis-dominated hypertrophic reservoir

Filtering rates were measured for zooplankton species in Situon single-celled Chlorella and on four Microcystis colony sizefractions (5–20, 20–40, 40–60 and 60–100µm) in a hypertrophic reservoir. Natural-log-transformedfiltration rates of five cladoceran species, one copepod andone rotifer were included in an all-food-particle, all-speciesmultiple regression model which explained 43% of the variancein filtration rate as a function of animal body length. An additional14% and 7.6% of the variance was attributable to food type andzooplankton species respectively, with temperature accountingfor <4% of the variance. Restricting the filtration ratemodel to cladocerans alone explained 51% of the variance asa function of animal length, 16% as a function of food type,7.5% as a function of species and only 0.2% as a function oftemperature. In linear filtration rate models for each foodtype, cladoceran body length explained 70% of the variance whenfeeding on Chlorella and between 57 and 67% of the varianceon the four Microcystis colony fractions. Models describingcladoceran filtration rates on Chlorella and the 5–20µm Microcystis colony fraction were significantly differentfrom the three models on larger colonies due to cladoceran responsesto increasing food particle size. Accordingly, a combined modelfor Microcystis colonies >20 µm was developed. Inclusionof food quality factors such as cyanophyte colony size seemsjustified in models aimed at estimating clearance rates, resourceutilization and phytoplankton grazing losses in plankton orecosystem studies when applied to eutrophic or hypertrophiclakes where large cyanophyte particles are abundant.     

DYNAMICS OF NERVE CELLS : II. THE TEMPERATURE COEFFICIENTS OF CARBON DIOXIDE PRODUCTION BY THE HEART GANGLION OF LIMULUS POLYPHEMUS.

1. It is possible to determine by the colorimetric method the rate of production of carbon dioxide by the cardiac ganglion of Limulus. 2. Carbon dioxide formation in the cardiac ganglion was found to run parallel to the rate of heart beat for different temperatures. 3. The conclusion seems justified that the rate of cardiac rhythm of Limulus depends upon a chemical reaction in the nerve cells of the cardiac ganglion and that this reaction is associated with the production of carbon dioxide since the rate of beat and the rate of CO₂ production are similarly affected by changes in temperature.     

Dehydration markedly impairs cardiovascular function in hyperthermic endurance athletes during exercise

González-Alonso, José, RicardoMora-Rodríguez, Paul R. Below, and Edward F. Coyle.Dehydration markedly impairs cardiovascular function inhyperthermic endurance athletes during exercise. J. Appl. Physiol. 82(4): 1229-1236, 1997.Weidentified the cardiovascular stress encountered by superimposingdehydration on hyperthermia during exercise in the heat and themechanisms contributing to the dehydration-mediated stroke volume (SV)reduction. Fifteen endurance-trained cyclists [maximalO₂ consumption(O_{2 max}) = 4.5 l/min] exercised in the heat for 100-120 min and either became dehydrated by 4% body weight or remained euhydrated by drinkingfluids. Measurements were made after they continued exercise at 71%O_{2 max} for 30 minwhile 1) euhydrated with anesophageal temperature (T_es) of38.1-38.3°C (control); 2)euhydrated and hyperthermic (39.3°C);3) dehydrated and hyperthermic withskin temperature (T_sk) of34°C; 4) dehydrated withT_es of 38.1°C and T_sk of 21°C; and5) condition4 followed by restored blood volume. Compared withcontrol, hyperthermia (1°C T_esincrease) and dehydration (4% body weight loss) each separatelylowered SV 7-8% (11 ± 3 ml/beat;P < 0.05) and increased heart ratesufficiently to prevent significant declines in cardiac output.However, when dehydration was superimposed on hyperthermia, thereductions in SV were significantly (P < 0.05) greater (26 ± 3 ml/beat), and cardiac output declined 13% (2.8 ± 0.3 l/min). Furthermore, mean arterialpressure declined 5 ± 2%, and systemic vascular resistanceincreased 10 ± 3% (both P < 0.05). When hyperthermia wasprevented, all of the decline in SV with dehydration was due to reducedblood volume (~200 ml). These results demonstrate that thesuperimposition of dehydration on hyperthermia during exercise in theheat causes an inability to maintain cardiac output and blood pressurethat makes the dehydrated athlete less able to cope with hyperthermia.

     

Rates of Progress towards Flowering and Podding in Bambara Groundnut (Vigna subterranea) as a Function of Temperature and Photoperiod

The influence of temperature and photoperiod on phenologicaldevelopment of three bambara groundnut (Vigna subterranea) selectionsfrom Botswana, Zimbabwe and Mali was investigated in a semi-controlledenvironment experiment with factorial combinations of threeconstant temperatures (20.9, 23.4 and 26.2 °C) and fourconstant photoperiods (10.0, 12.5, 13.5 and 16.0 h d^-1). Inall three selections, the onset of flowering was influencedby temperature but not by photoperiod, while the onset of pod-growth(‘podding’) of all three selections was influencedby both factors. The influence of temperature and photoperiodwas quantified by means of photothermal models, linking developmentrates to temperature and photoperiod with linear equations.The rate of progress from sowing to flowering of the three selectionscould be described very well (r²>95%) as a function of temperature;the rate of progress from flowering to podding was describedreasonably well as a function of both temperature and photoperiodby a combination of one to three response planes (r²for thedifferent selections ranging from 63 to 90%). Model testingwith independent data sets showed good agreement between observedand predicted times to flowering and podding. Vigna subterranea; Voandzeia subterranea; bambara groundnut; phenology; development; flowering; podding; photoperiod; temperature; modelling     

Development and regulation of heart rate in embryos and hatchlings of gulls (Larus schistisagus and Larus crassirostris) in relation to growth

We compared the developmental patterns of mean heart rate in Larus crassirostris and L. schistisagus embryos and chicks with other avian species of different hatchling developmental modes. We proposed that, since mean heart rate is inversely related to fresh egg mass in all birds, larger species reached a higher fraction of their hatchling mean heart rate by the end of the early phase of incubation and that heart rate contributions to supplying the increasing metabolic demands during mid and late incubation phases were less important than in smaller avian species. Mean heart rate was essentially independent of age throughout the mid-incubation phase (33% of normalised incubation until pipping), but increased with time during early (L. schistisagus only investigated) and late-incubation phases in both species. The O₂ pulse of L. schistisagus embryos and chicks increased linearly with yolk-free body mass (log-log) after the early-phase of incubation until shortly before pipping, but was independent of body mass in the periods before and after. We conclude that a high heart rate in this first period is probably more important for increasing the circulation of nutrients to the embryo at a stage when extra-embryonic circulation to the yolk sac is limited by the size of the growing area vaculosa. Furthermore, large increases in mean heart rate during the late-incubation phase are probably important for increasing the cardiac output to hatching levels with onset of endothermy. However, mean heart rate is stable over the mid-incubation while O₂ pulse increases, suggesting that increases in stroke volume and other circulatory adjustments may be entirely responsible for the largest increases in O₂ transport during incubation of large avian species. Accepted: 18 May 2000     

Tone-entropy analysis on cardiac recovery after dynamic exercise

Oida, Eiichi, Toshio Moritani, and Yukio Yamori.Tone-entropy analysis on cardiac recovery after dynamic exercise. J. Appl. Physiol. 82(6):1794-1801, 1997.Autonomic controls on heart rate variabilityhave been investigated; however, sympathovagal interactive modulationsremain unexplored. The purpose of this study is to presenta new method, tone-entropy analysis (T-E analysis) of heart periodfluctuations, and to make clear an intensive cooperation of autonomicnetworks in heart recovery. On the basis of evidence obtained in animalexperiments, we hypothesized that heart periods are lengthened orshortened beat to beat by assumed physiological mediators: acceleratorand inhibitor. Their operations were evaluated through anormalized successive variation of the period, that is, the percentageindex (PI). The process was described through PI distributions by usingtwo indexes, tone and entropy, standard values of which were obtainedthrough pharmacological autonomic blockade experiment. T-E analysis wasapplied to heart recovery (70 min) after dynamic exercise by 12 femaleathletes. Interactive autonomic modulations were expressed by a curvedpath in tone-entropy space. Results suggested that heart rate decayproceeds not by withdrawal of one pathway but by increasing activity ofboth pathways as vagosympathetic balance inclines slightly butsignificantly to the vagus division in the course of recovery. Theprocess was examined through Fourier spectral analysis as well.

     

Population biomass, feeding, respiration and growth rates, and carbon budget of the scyphomedusa Aurelia aurita in the Inland Sea of Japan

We investigated the seasonal occurrence, wet : dry : carbon: nitrogen weight ratios, population biomass, gastric pouchcontents, and rates of feeding, growth and respiration of thescyphomedusa Aurelia aurita in the central part of the InlandSea of Japan. Aurelia aurita medusae began to appear in January/Februaryas ephyrae, reached annual maximum body size in July/August,and disappeared, presumably due to death, by November. Initialslow growth in early spring was followed by a period of exponentialgrowth (mean growth rate: 0.069 d^–1) between April andJuly. In the Ondo Strait, which is characterized by strong tidalmixing, the A. aurita population (mean carbon biomass: 66.0mg C m^–3) overwhelmingly dominated the zooplankton-communitybiomass (mean biomass of micro- and mesozooplankton: 23.7 mgC m^–3) between May and early August The gastric contentanalysis revealed that A. aurita ate almost all micro- and mesozooplankters,of which small copepods were most important. On the basis ofdigestion time for small copepods (60 min) and their abundancein the gastric pouch of field-collected A. aurita, we determinedthe weight specific feeding rates and clearance rates. The formerincreases linearly with increasing copepod abundance, but thelatter was relatively constant irrespective of the food supply.We also measured the respiration rates of A. aurita and expressedthem as functions of body weight and temperature. These physio-ecologicalparameters enabled us to construct the carbon budget of theA. aurita population typical of early summer in the Ondo Strait.Predicted population-feeding rate (6.07 mg C m^–3 d^–1)was higher than the population-food requirement for both metabolismand growth (4.55 mg C m^–3 d^–1), indicating thatfood supply was sufficient to sustain the observed growth rate.This feeding rate was equivalent to 26% of micro- and mesozooplanktonbiomass, a significant impact on zooplankton.     

EFFECT OF BODY SIZE ON THE RESPIRATION OF CERITHIDEA (CERITHIDEOPSILLA) CINGULATA (GMELIN, 1790) AND CERITHIUM CORALIUM KIENER, 1841

The effect of body size on the oxygen consumption and metabolicrate of Cerithidea (Cerithideopsilla) cingulata (Gmelin, 1790)andCerithium coralium Kiener, 1841 was studied at a constant temperatureof 25°C. An exponential relationship has been observed inboth animals. Oxygen consumption showed a positive linear correlationwith a ‘b’ value of 0.6518 in C. cingulata and 0.7667in C. coralium. A negative linear correlation was obtained forthe metabolic rate with a (b-1) value of –0.3482 in C.cingulata and –0.2333 in C. coralium.     

Regulation of cardiac rhythm in hibernating mammals

The dramatic fall in heart rate exhibited by mammals entering hibernation begins before there is any noticeable fall in body temperature. The initial, progressive decrease in heart rate is the result of a cyclic parasympathetic activation that induces skipped beats and regular asystoles as well as slows the even heart beat. As body temperature subsequently falls, the parasympathetic influence is progressively withdrawn and periods of parasympathetic and sympathetic dominance alternate and give rise to regular periods of arrhythmia (tachycardia followed by bradycardia), and occasional long asystoles or periods of highly irregular cardiac activity. Superimposed on this is a vagally-mediated, respiratory sinus arrhythmia that is accentuated in species that breathe episodically. These events give way to a uniform heart rate in deep hibernation at low temperatures where both parasympathetic and sympathetic tone appear absent. The complete absence of tone is not a function of reduced temperature but is reflective of the state of deep, steady state hibernation. The elevation in heart rate that accompanies the onset of arousal is the result of dramatic increases in sympathetic activation that precede any increases in body temperature. As body temperature then rises, sympathetic influence is slowly withdrawn. Arrhythmias are also common during natural arousals or shifts from lower to warmer hibernation temperatures as periods of parasympathetic and sympathetic dominance again alternate en route to re-establishing a steady state in euthermia. The mechanism behind, and the biological significance of, cardiac changes mediated through orchestrated arrhythmias remain unknown.     

Effects of temperature and body size on the clearance rate of Oikopleura dioica

The effects of temperature and body size on the clearance rateof the appendicularian Oikopleura dioica were investigated.The clearance rate increased with the trunk length of the animalsaccording to a power function with exponent 3.2 ± 0.17(range 2.9–3.8). The intercepts of the three regressionswere significantly different with the highest rates at 20°C,corresponding to a Q₁₀ value of 1.78 (10–20°C).     

A Canopy Photosynthesis Model for the Dynamics of Size Structure and Self-thinning in Plant Populations

A dynamic model for growth and mortality of individual plantsin a stand was developed, based on the process of canopy photosynthesis,and assuming an allometric relationship between plant heightand weight, i.e. allocation growth pattern of plant height andstem diameter. Functions G(t, x), for the mean growth rate ofindividuals of size x at time t, and M(t,x), for the mortalityrate of individuals of size x at time t, were developed fromthis model and used in simulations. The dynamics of size structurewere simulated, combining the continuity equation model, a simpleversion of the diffusion model, with these functions. Simulationsreproduced several well-documented phenomena: (1) size variabilityin terms of coefficient of variation and skewness of plant weightincreases at first with stand development and then stabilisesor decreases with an onset of intensive self-thinning; (2) duringthe course of self-thinning, there is a power relationship betweendensity and biomass per unit ground area, irrespective of theinitial density and of the allocation-growth pattern in termsof the allometric parameter relating plant height and weight.The following were further shown by simulation: (a) competitionbetween individuals in a crowded stand is never completely one-sidedbut always asymmetrically two-sided, even though competitionis only for light; (b) plants of ‘height-growth’type exhibit a greater asymmetry in competition than plantsof ‘diameter-growth’ type, (c) the effect of competitionon the growth of individuals in a crowded stand converges toa stationary state, even when the stand structure still changesgreatly. All of these theoretical results can explain recentempirical results obtained from several natural plant communities.Finally, a new, general functional form for G(t, x) in a crowdedstand is proposed based on these theoretical results, insteadof a priori or empirical growth and competition functions. Canopy photosynthesis, competition mode, continuity equation, self-thinning, simulation, size distribution     

Size-based variation in antipredator behavior within a snake (Agkistrodon piscivorus) population

Variation in an animal's response to a predator likely reflectsthe complex interaction of factors that influence predationrisk. Due to their high degree of behavioral variation and simplifiedbauplan, snakes offer a unique model for investigating the influenceof sex and body size on antipredator behavior. We examined variationin antipredator behavior within a cottonmouth (Agkistrodon piscivorusleucostoma) population. Behavioral responses to human-inducedpredation risk were compared across a continuous scale of bodysize. Defensive responses significantly declined with increasingbody size. After controlling for body size, no differences betweenthe sexes were detected. Although this study suggests that variationin antipredator behavior is, in part, related to body size,some studies on snakes have not found this relationship. Likewise,some studies have demonstrated differences between sexes. Suchdisparate patterns of variation indicate a need for future comparativestudies examining the complex interaction of factors that mayinfluence predator–prey relationships.     

Pulse pressure response to the strain of the Valsalva maneuver in humans with preserved systolic function

Arterial pulsepressure response during the strain phase of the Valsalva maneuver hasbeen proposed as a clinical tool for the diagnosis of left heartfailure, whereas responses of subjects with preserved systolic functionhave been poorly documented. We studied the relationship between theaortic pulse amplitude ratio (i.e., minimum/maximum pulse pressure)during the strain phase of the Valsalva maneuver and cardiachemodynamics at baseline in 20 adults (42 ± 14 yr) undergoingroutine right and left heart catheterization. They were normal subjects(n = 5) and patients withvarious forms of cardiac diseases(n = 15), and all had a leftventricular ejection fraction 40%. High-fidelity pressures wererecorded in the right atrium and the left ventricle at baseline and atthe aortic root throughout the Valsalva maneuver. Aortic pulseamplitude ratio 1) did not correlatewith baseline left ventricular end-diastolic pressure, cardiac index(thermodilution), or left ventricular ejection fraction(cineangiography) and 2) waspositively related to total arterial compliance (area method) (r = 0.59) and to basal mean rightatrial pressure (r = 0.57) (eachP < 0.01). Aortic pulse pressureresponses to the strain were not related to heart rate responses duringthe maneuver. In subjects with preserved systolic function, the aorticpulse amplitude ratio during the strain phase of the Valsalva maneuver relates to baseline total arterial compliance and right heart fillingpressures but not to left ventricular function.

     

Immediate exercise hyperemia in humans is contraction intensity dependent: evidence for rapid vasodilation.

We tested the hypothesis that rapid vasodilation proportional to contraction intensity contributes to the immediate (first cardiac cycle after initial contraction) exercise hyperemia. Ten healthy subjects performed single 1-s isometric forearm contractions at 5, 10, 15, 20, 30, 50, and 70% maximal voluntary contraction intensity (MVC) in arm above heart (AH) and below heart (BH) positions. Forearm blood flow (FBF; brachial artery mean blood velocity, Doppler ultrasound), mean arterial pressure (arterial tonometry), and heart rate (electrocardiogram) were measured beat by beat. Venous emptying (measured with a forearm strain gauge) was already maximized at 5% MVC, indicating that increases in contraction intensity did not further empty the forearm veins. Immediate increases in FBF were linearly proportional to contraction intensity from 5 to 70% MVC in AH (slope = 4.4 +/- 0.5%DeltaFBF/%MVC). In BH, the immediate increase in FBF demonstrated a curvilinear relationship with increasing contraction intensity and was greater than AH at 15, 20, 30, and 50% MVC (P < 0.05). Peak changes in FBF were greater in BH vs. AH from 10 to 50% MVC, even when venous refilling was complete (P < 0.05). These data support the existence of a rapid-acting vasodilatory mechanism(s) at the onset of human forearm exercise.     

Cardiac development in crayfish: ontogeny of cardiac physiology and aerobic metabolism in the red swamp crayfish Procambarus Clarkii

The cardiovascular system performs key physiological functions even as it develops and grows. The ontogeny of cardiac physiology was studied throughout embryonic and larval development in the red swamp crayfish Procambarus clarkii using videomicroscopic dimensional analysis. The heart begins to contract by day 13 of development (at 25°C, 20 kPa O₂). Prior to eclosion, heart rate (ƒH) decreases significantly. Previous data suggests that the decrease in cardiac parameters prior to hatching may be due to an oxygen limitation of the embryo. Throughout development, metabolizing mass and embryonic oxygen consumption primarily increased while egg surface area remains constant. The limited area for gas exchange of the egg membrane, in combination with the increasing oxygen demand of the embryo could result in an inadequate diffusive supply of oxygen to developing tissues. To determine if the decrease in cardiac function was the result of an internal hypoxia experienced during late embryonic development, early and late stage embryos were exposed to hyperoxic water (PO₂ =40 kPa O₂). The ƒH in late stage embryos increased significantly over control values when exposed to hyperoxic water suggesting that the suppression in cardiac function observed in late stage embryos is likely due to a limited oxygen supply.     

Body, eye, and chorioallantoic vessel growth are not dependent on cardiac output level in day 3-4 chicken embryos

Normal aerobic metabolic rates persist in the early chicken embryo after elimination of cardiac output, but the dependence of tissue growth and differentiation on blood flow is unknown in these early stages. We partially ligated (25-50% occlusion) the ventricular outflow tract of Hamburger-Hamilton stage (HH) 16-18 embryos, producing a wide range of cardiac output. For the next approximately 48 h (to HH 24), we measured heart rate (HR), stroke volume (SV), and cardiac output (CO), as well as these growth indicators: eye diameter, chorioallantoic vessel density, and body mass. Acutely, HR declined with partial ligation (from 108 to 98 beats/min). Paradoxically, SV and CO decreased sharply in most embryos but increased in others, collectively producing the desired large variation (up to 25-fold) in CO and permitting assessment of tissue growth over a very large range of blood perfusion. Eye diameter doubled (from 0.6 to 1.2 mm) with development from HH 16 to HH 24, but within a developmental cohort there was no significant correlation between eye diameter and CO over a 25-fold range of CO. Similarly, chorioallantoic membrane vessel index was independent of CO over the CO range at all stages. Finally, body mass increase during development was not significantly affected by partial conal truncal ligation. Collectively, these data suggest that normal eye and vessel growth and body mass accumulation occur independent of their rate of blood perfusion, supporting the hypothesis of prosynchronotropy-that the heart begins to beat and generate blood flow in advance of the actual need for convective blood flow to tissues.     

Phospholamban overexpression in transgenic rabbits

There has been considerable interest in pursuing phospholamban as a putative therapeutic target for overcoming depressed calcium handling in human heart failure. Studies predominantly done in mice have shown that phospholamban is a key regulator of sarcoplasmic reticulum calcium cycling and cardiac function. However, mice differ significantly from humans in how they regulate calcium, whereas rabbits better recapitulate human cardiac function and calcium handling. To investigate phospholamban’s role in the rabbit heart, transgenic rabbits that overexpressed wild-type phospholamban in the ventricular cardiomyocytes and slow-twitch skeletal muscles were generated. Rabbits expressing high levels of phospholamban were not viable due to severe skeletal muscle wasting, the onset of cardiac pathology and early death. A viable transgenic line exhibited a 30% increase in PLN protein levels in the heart. These animals showed isolated foci of cardiac pathology, but cardiac function as well as the response to β-adrenergic stimulation were normal. SR-calcium uptake measurements showed that the transgenic hearts had the expected reduced affinity for calcium. The data show that phospholamban-overexpressing transgenic rabbits differ markedly in phenotype from analogous transgenic mice in that rabbits are quite sensitive to alterations in phospholamban levels. Exceeding a relatively narrow window of phospholamban expression results in significant morbidity and early death.     

Ontogeny of the Immune System in Amphibians

SYNOPSIS. Experiments with amphibians have revealed that tissuesallografted in embryonic and early larval life may later succumbto a host immunological attack. In Xenopus, the ontogeny ofthis alloimmune response is correlated with the lymphoid maturationof the thymus. This article describes experiments primarilydesigned to ascertain if such a correlation exists in Rana pipiens. Initially, a histological study of the differentiation of thelymphomyeloid complex of the leopard frog was undertaken. At18–21°C lymphoid histogenesis is well under way inthe thymus and is beginning in many other organs during thethird week of larval life. Extensive growth and differentiationof these organs follow. Observations are also presented on thestructure, function and development of the lymphoid and Iymphomyeloidorgans from late larval life throughmetamorphosis to adulthood. Experiments were then performed to determine the onset of thealloimmune response to embryonically transplanted neural foldmaterial. At 18–21°C incompatibility phenomena, albeitslight, are first detected in these grafts as early as 17 daysafter fertilization, i.e. 15 days after transplantation. Thus,in the leopard frog, the alloimmune response develops soon afterlymphoid maturation of the thymus. At later stages of development,a more vigorous response is witnessed, concomitant with a rapidphase of lymphoid organ differentiation.     

Is the midsummer decline of Daphnia really induced by age-0 fish predation? Comparison of fish consumption and MDaphnia mortality and life history parameters in a biomanipulated reservoir

We analysed the temporal pattern of a Daphnia galeata populationand the development of the age-0 Ash community in a long-termbiomanipulated lake (Bautzen reservoir, Germany) during springand early summer of two successive years. In Bautzen reservoir,the age-0 Rsh-Daphnia interaction is a key process within thefood web due to the low abundance of adult zooplanktivorousfish. Daily consumption of daphnids by age-0 fish, as estimatedby a bioenergetics model, was compared to daily mortality ratesof daphnids. In addition, we estimated life history parametersof Daphnia that may indicate predation impact by fish. A midsummerdecline of daphnids occurred only in July of 1996, whereas in1995 the Daphnia biomass remained >4 mg wet weight 1^–1for the entire summer. The percentage of total Daphnia mortalitydue to fish predation before the onset of the midsummer declinewas –2% day^–1. Temporal patterns of individual size,clutch size and size at maturity also indicated that the 1996midsummer decline of daphnids was not the exclusive consequenceof age-0 fish predation. Instead, low reproductive capacityof daphnids also contributed significantly to the decline. Consequently,year-to-year variation of the Daphnia dynamics may be determinedby a fine-tuned ‘timing’ between the period of reducedfecundity and the time of the strongest predation impact byage-0 fish.