A complete analysis of the macro-and microzooplankton of a warm monomictic lake indicates that Protozoa dominate the community numerically.

AbstructSwimming patterns, fecging behaviors, and ingestion, filtering, assimilation, and respiration rates were measured for Daphnia mugnu in food concentrations from 0 to 10" Chlamydomonas reinhnrdi cells * cm-", equivalent to O-20 mg C* liter-'. A Helling type 2 functional response curve or an Ivlev model without a threshold appear to be biologically meaningful descriptions of Daphnia feeding response to different food concentrations.Daphnia has no feeding threshold or reduced filtering activity at low concentrations such as are predicted by optimal foraging models. Above the incipient limiting concentration, constant ingestion rates and an over-collection of food result in increased rejection rates. These behaviors may cause high respiratory rates and reduced net assimilation rates at the highest food concentration. Daphnia does not orient to or alter swimming patterns in response to algal patches.

Daphnia parvula experiences a wide range of thermal (4°-30°C) and food conditions during its annual population cycle. We used cohort life table experiments to determine the synergistic effects of temperature and food concentration on its life history parameters. A 3×3 factorial design experiments was conducted with animals raised at all combinations of three naturally experienced temperatures (10°, 15° and 25° C) and food levels (0.02, 0.20 and 2.0 mg C/l). D. parvula showed an increase in survivorship with decreasing temperature at all food levels. Fecundity parameters (number of broods/female, brood size and net reproductive rate) increased with increasing food at the two lower temperatures but showed a mid-range food optimum at the highest temperature. Development rates and realized rates of increase (r) showed an increase with both increasing temperature and food such that they were maximum at the highest temperature-food level treatment. The life history parameters, average lifespan, age at first reproduction, brood duration time, brood size and number of young per reproductive female all showed significant interaction between temperature and food as was suggested by trends in R and r. Temperature had a reduced effect on fecundity, development rates and realized rates of increase at the low food level.Population birth rates of continuously reproducing zooplankton are typically calculated by the egg-ratio method (Edmondson 1960) and are based on thermally controlled development rates which neglect the effects of food limitation. Significant synergistic temperature-food effects on brood duration time and other life history parameters of Daphnia parvula suggest that food limitation and foodtemperature interaction should be considered when calculating field population birth rates. A comparison of realized rates of increase from this study with similar life table data for Daphnia parvula raised on natural food from Lake Oglethorpe indicate that synergistic effects are negligible in this eutrophic system due to abundant resources. However, synergistic effects are probably important in oligotrophic systems where resources are limited.

AbstructThe thermal demographic advantage of vertical migration was tested using life-table studies of Daphnia parvula cohorts fed on Chlamydomonas reinhurdi. At a high food level, lifespan and body size at death were relatively insensitive to temperature differences. Fecundity parameters (brood size, brood number, total number of young per reproductive female, and net reproduction rate) were highest at an optimal midrange temperature (15°C). However, age at onset of reproduction, instar at first reproduction and brood duration decreased with increasing temperature, resulting in an overall increase in the realized rate of increase (r) with increasing temperature.Realized rates of increase were higher in fluctuating temperatures than at comparable average constant temperatures but were always lower than those at the highest constant temperature in each range. A low food level reduced the effect of temperature on some growth and fitness parameters from those at a high food level, but T was still greater at the higher temperatures.Therefore, there is no apparent thermal demographic advantage to vertical migration at either a high or low food level within the natural range experienced by D. purvulu. Maximum fitness is achieved by remaining in the warmest surface waters at all times.

A cornerstone of evolutionary ecology is that population density affects adaptation: r and K selection is the obvious example. The reverse is also appreciated: adaptation impacts population density. Yet, empirically demonstrating a direct connection between population density and adaptation is challenging. Here, we address both evolution and ecology of population density in models of viral (bacteriophage) chemostats. Chemostats supply nutrients for host cell growth, and the hosts are prey for viral reproduction. Two different chemostat designs have profoundly different consequences for viral evolution. If host and virus are confined to the same chamber, as in a predator-prey system, viral regulation of hosts feeds back to maintain low viral density (measured as infections per cell). Viral adaptation impacts host density but has a small effect on equilibrium viral density. More interesting are chemostats that supply the viral population with hosts from a virus-free refuge. Here, a type of evolutionary succession operates: adaptation at low viral density leads to higher density, but high density then favors competitive ability. Experiments support these models with both phenotypic and molecular data. Parallels to these designs exist in many natural systems, so these experimental systems may yield insights to the evolution and regulation of natural populations.