Size selectivity spectra of 28 planktonic predators from 18 studies in the literature are compared. The linear size ratio between predators and their optimal prey is 1 : 1 for a dinoflagellate, 3 : 1 for other flagellates, 8 : 1 for ciliates, 18 : 1 for rotifers and copepods, and ∼50: 1 for cladocerans and meroplankton larvae. These size ratios seem consistent within groups, and their validity is supported by quantitative information from the literature. However, a difference between filter feeders and raptorial‐interception feeders, preferring relatively smaller and larger prey respectively, is evident across the taxonomic groups. A classification of planktonic predators into functional groups is therefore crucial for the construction of models of pelagic food webs.

In order to study the size dependency of grazing and growth rates in zooplankton, data wcrc collected from laboratory studies in the literature, covering both limnic and marinc organisms. Data were obtained from about 60 species of nano-, micro-, and mesozooplankton, representing flagellates, ciliates, rotifers, meroplankton larvae, copepods, and cladocerans. Estimates of maximum ingestion and clearance were extracted from functional responses (ingestion rates as a function of food density) cstablishcd from laboratory experiments. Maximum specific rates were expressed as a function of predator body volume. Maximum specific clearance and ingestion rates decreased with predator volume within each group of zooplankton, with a-common exponent (scaling factor) of -0.23 (SE = 20.12) in accordance with previous findings. However, significant differences wcrc found between groups. In particular, among the protists, ciliates display maximum ingestion, growth, and clearance rates that exceed those of dinoflagellates by a factor of 2-4. Among the metazooplankton, Calanoid copepods have maximum clearance rates that cxcccd those of filter-feeding cladocerans and meroplankton larvae by a factor of 10. Because of these differenccs between the groups, the entire set of observations could not be fitted by an overall regression.

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During spring (May-June) 1988 an extensive subsurface bloom of the haptophycean flagellate Chrysochromulina polylepis developed in Scandinavian waters (the KattegatSkagerrak area). Here we report on the vertical distribution of bacteria, heterotrophic flagellates, ciliates and copepods at a permanent station in the Southern Kattegat during the bloom. At the height of the bloom, the density of C. polylepis reached 60 to 70 X 10' cells 1-' in the pycnocline. At this time no potential grazers were present in the subsurface bloom and bacterial production was extremely low. Field and laboratory experiments showed that C. polylepis inhibited the activity of planktonic bacteria, ciliates and copepods. During the decay of the bloom, algae were colonized by bacteria and the pycnocline was subsequently re-lnvaded by heterotrophic flagellates, small aloricate ciliates and copepods. Two weeks after the height of the bloom, the normal pelagic food web structure was re-established.

Image analysis was applied to epifluorescense microscopy of acridine orange-stained plankton samples. A program was developed for discrimination and binary segmentation of digitized video images, taken by an ultrasensitive video camera mounted on the microscope. Cell volumes were estimated from area and perimeter of the objects in the binary image. The program was tested on fluorescent latex beads of known diameters. Biovolumes measured by image analysis were compared with directly determined carbon biomasses in batch cultures of estuarine and freshwater bacterioplankton. This calibration revealed an empirical conversion factor from biovolume to biomass of 0.35 pg of C ,um-3 (+ 0.03 95% confidence limit). The deviation of this value from the normally used conversion factors of 0.086 to 0.121 pg of C ,um-3 is discussed. The described system was capable of measuring 250 cells within 10 min, providing estimates of cell number, mean cell volume, and biovolume with a precision of 5%.