Cladoceran body size and vulnerability to copepod predation

We examined species-related and age-related mortality caused by a predaceous copepod (Acanthocyclops robustus) in five closely related cladocerans representing a wide spectrum of body size. Animals of all five species from clonal cultures were exposed for 1, 2, or 4 d in medium from a copepod-infested enclosure that either did or did not contain natural densities of Acanthocyclops adults and copepodites (40-50 copepods liter-1). In each of the five species, the 1-d-old neonates were most, and adults least, vulnerable to the copepods. Small-bodied Daphnia cucullata and Ceriodaphnia reticulata remained vulnerable until maturation, but the susceptibility of large Daphnia magna drastically decreased in 3- and 5-d-old animals, and 7-d-old adults were immune to copepod predation

Egg predation by copepods in Daphnia brood cavities

Both field observations and enclosure experiments show that juvenile copepods enter Daphnia brood cavities to feed on Daphnia eggs and embryos. The ability to perform such in vivo exploitation is reported for both cyclopoid and calanoid copepods. Copepodites of Acanthocyclops robustus (G.O. Sars) were found to eat eggs in brood cavities of D. magna, D. pulex and D. pulicaria in experimental enclosures rich in algae. Copepodites of Eudiaptomus gracilus (G.O. Sars) were found in brood cavities of D. hyalina in a mesotrophic lake. The copepods' intrusions into brood cavities caused dramatic declines in the clutch size of infested Daphnia, and this predation effect could easily be confused with the effect of severe food limitation

Clutch-size variability in Daphnia: body-size related effects of egg predation by cyclopoid copepods

Cyclopoid copepods had unexpected effects on four species of Daphnia in an enclosure study. Small copepodites entered the brood pouch of large Daphnia and preyed on eggs. The resulting low clutch sizes could erroneously be ascribed to food limitation, but a clear difference between copepodite and food-limited populations could be seen when clutch sizes were plotted vs. body lengths. Food limitation resulted in consistently lower egg numbers for all individuals; egg predation resulted in large variability of clutch sizes because individuals were randomly affected by the copepods. Only small copepodites could enter brood pouches, and only large daphniids were affected. Daphniids below a body size of 2.25 mm were immune to egg predation regardless of species or size at first reproduction. This unusual mechanism of size-selective invertebrate predation (affecting large cladocerans) may explain unexpected results found in field studie

Underestimation of primary production as indicated by measurements with size-fractionated phytoplankton in Lake Maarsseveen I (The Netherlands)

Primary productivity of four size classes of phytoplankton (<150μm, <50μm, <20μm and <5μm) was measured from March through October 1986 in Lake Maarsseveen I with an incubator technique. The mean column production was approximately 400 mg C.m−2.day−1, with a range of values between 150 and 750 mg C.m−2.day−1. The mean contribution of the size fractions <50μm, <20μm and <5μm to the size fraction<150μm was 80%, 60% and 35%, respectively. During their appearance the grazing impact of small herbivorous zooplankton,e.g. rotifers, can give an underestimation of the size fraction <150μm. An indication of this phenomenon is given