Resource depression in Daphnia galeata, Daphnia cucullata, and their interspecific hybrid: life history consequences

Abstract. In this study, the effect of the food concentration on several life history parameters of Daphnia galeata, Daphnia cucullata and the hybrid between these two species was examined. The food concentration had a significant effect on both growth and reproduction of all three taxa. Mortality was highly species specific, and to a lesser extent dependent on the food concentration. Changes in life history parameters resulted in changes in the intrinsic rate of population increase (r). The food dependence of the different species was found to be different. Daphnia cucullata had a relatively constant and low r. The r values for the hybrid varied most, with low values compared with the parental species when grown at low food levels and relatively high values at higher food concentrations. Our results suggest that when food concentrations are high the hybrid will have a selective advantage over both parental species, but that at lower food levels D.galeata will have an advantage over both D.cucullata and the hybrid

Long-term dynamics of small-bodied and large-bodied cladocerans in a shallow reservoir exposed to eutrophication with special attention for Chydorus sphaericus

Eutrophication in Tjeukemeer involved a gradual increase in chlorophyll concentrations from ca. 30 mg m(-3) in 1968-69 to 125 mg m(-3) in 1976. From 1976 onwards, chlorophyll concentrations remained at a high level fluctuating between 100-225 mg m(-3). Hillbricht-Illkowska (1977) hypothesized that small-bodied species will become increasingly abundant and dominant over large-bodied species with increasing eutrophication. We tested this hypothesis using observations from life history experiments on Chydorus sphaericus, combined with data from 25 years of field observations on the population dynamics of cladocerans in Tjeukemeer. In life history experiments with C. sphaericus, the fitness measure r in treatments with natural lake seston and laboratory cultured green algae was significantly higher on lake seston from Tjeukemeer, containing a high proportion of detritus. This suggests that detrital particles are good quality food for C. sphaericus. Field observations during the period 1968-1976 showed that all three categories of cladocerans: C. sphaericus, 'other' small-bodied cladocerans (predominantly Bosmina spp.) and large-bodied cladocerans (predominantly Daphnia galeata), increased in biomass with increasing chlorophyll concentration. However, of these three cladoceran categories only C. sphaericus showed a distinct and significant increase whereas the other two only showed a marginally significant increase. During the period 1977-1992, both 'other' small-bodied cladocerans and C. sphaericus significantly decreased in biomass with increasing chlorophyll concentration, whereas the biomass of the large-bodied cladocerans significantly increased with increasing chlorophyll content. These observations are not in agreement with the hypothesis that small-bodied zooplankton become increasingly abundant with increasing eutrophication. We suggest that the observed trends are partially caused by a food effect, and partially caused by predation pressure. Daphnia shows a better response to the increase in detritus and filaments of Cyanobacteria than small-bodied cladocerans, but is more vulnerable to fish predation. Densities of 0+ zooplanktivorous fish show strong annual fluctuations in Tjeukemeer, and because of hydrological conditions, 0+ fish abundance in this lake is probably negatively related to chlorophyll content. [KEYWORDS: Chydorus; body size; zooplankton community; eutrophication; detrital food web; Cyanobacteria Green-alga scenedesmus; filamentous cyanobacterium; daphnia-hyalina; body size; lake; zooplankton; tjeukemeer; abundance; growth; fish

Decrease in Daphnia egg viability at elevated pH

The effect of high pH on the reproduction of two Daphnia galeata clones was experimentally investigated in the laboratory. We observed that the mortality of juveniles and adults did not increase with increasing pH in the range pH 9.0- 10.5, which agrees with what is generally reported in the literature for cladoceran zooplankton and suggests that the threshold value for mortality is between pH 10.5 and 11.5. However, both egg mortality and the proportion of stillborn neonates increased at pH 10.0 and above, and the two clones differed in their sensitivities to pH. Consequently, pH already affects population growth rate markedly from pH 10.0 onward. Because pH values greater than or equal to 10.0 are common during spring and summer in many eutrophic and hypertrophic lakes due to intense photosynthetic activity, we expect that high pH has a much larger effect on the population structure of Daphnia and the community composition of microcrustacean zooplankton in such water bodies than was assumed previously. [KEYWORDS: Zooplankton; growth; limitation; balance; magna; lake; food; fish; size

Food effects on life history traits and seasonal dynamics of Ceriodaphnia pulchella

1. This paper describes the effects of differences in food quantity and quality on selected life history traits of Ceriodaphnia pulchella. Animals were fed with four concentrations of two green algae, Scenedesmus obliquus and Chlamydomonas globosa, given separately as well as in a 1:1 mixture. 2. In most cases growth, reproduction and condition of the animals and consequently the rate of population increase, r, increased with food concentration. Life history parameters of animals raised on S. obliquus were lowest, whereas no significant differences were found between animals fed with C. globosa and with the mixture of the two algal species. 3. In contrast to the large effects of food quantity and quality found in the laboratory, an analysis of the population dynamics of C. pulchella in Lake Tjeukemeer showed that the effect of changes in food conditions during the growing, season of this species was limited, although overall food levels seemed to be poor throughout the year. 4. Changes in the birth rate of the population are mainly governed by changes in temperature, and possibly by changes in the densities of the invertebrate predator Leptodora kindtii. [KEYWORDS: Leptodora-kindti; growth-rates; birth-rate; daphnia;quality; phytoplankton; cladoceran; bosmina; lake; diet