Grazing experiments were conducted at different seasons with the large Calanus finmarchicus, C. glacialis and C. hyperboreus, and the small Acartia longiremis in Disko Bay, West Greenland and Young Sound, NE Greenland. Female copepods incubated in 200 µm screened natural water preferred large protists. Thus, particularly during the post-bloom period, the relatively large heterotrophic protists (ciliates and heterotrophic dinoflagellates) contributed substantially to the trophic coupling between protists and copepods. However, low grazing by C. glacialis and C. hyperboreus in mid-June suggests that large parts of the populations of these species had terminated feeding at this time, prior to overwintering. Clearance increased with ciliate and dinoflagellate size above 10 µm equivalent spherical diameter (ESD), equal to the size of the smallest heterotrophic protists. At a size of 30 to 40 µm ESD maximum clearance was observed. Grazing on Phaeocystis single cells of 5 µm by C. finmarchicus showed a lower size-limit for capture of this species < 5 µm which contrasts with C. glacialis and C. hyperboreus, which had a lower size-limit near 10 µm. In addition to size and relative concentrations of phytoplankton and heterotrophic protists, prey and/or predator behavior is suggested to play an important role for copepod feeding.

We compared seasonal studies of ciliates and heterotrophic dinoflagellates conducted in Disko Bay (West Greenland, ϳ69ЊN) and the Kattegat (Denmark, ϳ56ЊN). In both systems, ciliates and heterotrophic dinoflagellates were important components of the plankton. Their biomass was minute in the winter (October to April) in Disko Bay compared to the Kattegat, but from May to August/September, the biomass and composition of the ciliate and heterotrophic dinoflagellate assemblages were similar in the two systems. The seasonal biomass pattern was unimodal and bimodal for Disko Bay and the Kattegat, respectively. To evaluate top-down versus bottom-up control, experimentally derived maximum estimates of protozooplankton growth rates and copepod predation capacities from the study sites were applied to biomass data. This analysis showed that the effect of copepods was significant but that ciliates and heterotrophic dinoflagellates could effectively exploit prey during periods when top-down pressure was relaxed. In Disko Bay, a high copepod biomass in spring is primarily caused by migration of an overwintering copepod population from deep waters into the photic zone prior to the spring bloom. We suggest that ''regulation windows'' for the protozooplankton are present even during the spring bloom when copepods occur at their peak levels because of food saturation. Bottom-up regulation occurred during the winter and occasionally when copepod predation pressure relaxed, but it was difficult to separate food limitation from the effect of temperature. Multiple regression analysis supports the notion that ciliate and heterotrophic dinoflagellate biomass changed seasonally according to both top-down and bottom-up regulation, as well as to temperature control. Protozooplankton growth estimates were also used to calculate the fraction of primary production processed by the ciliates and heterotrophic dinoflagellates. When assuming complete algivory, 32-55% and 20-60% of the annual primary production was consumed by ciliates in Disko Bay and the Kattegat, respectively. Furthermore, because heterotrophic dinoflagellates were found to be as important grazers as ciliates in both systems, it was concluded that protozooplankton at high latitudes are also important in the cycling of primary production and should be considered if carbon and nutrient cycling in these systems is to be understood.During the past decade it has been shown that ciliates and heterotrophic dinoflagellates are abundant in arctic marine waters; these protists have higher growth capacities than copepods from the genus Calanus, which traditionally have been considered the principal grazers associated with high latitudes (Andersen 1988;Nielsen and Hansen 1995;Hansen et al. 1996;Sherr et al. 1997;Levinsen et al. 2000a). Furthermore, adult and nauplii of the Calanus species have been shown to preferentially feed on ciliates and dinoflagellates (Barthel 1988;Ohman and Runge 1994; Levinsen et al. 1 To whom correspondence should be addressed. Present address: Unive...

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The succession of pelagic ciliates and heterotrophic dinoflagellates was investigated in Disko Bay, West Greenland, from April 1996 to June 1997. In terms of biomass they contributed equally. Their annual distribution was bimodal with maxima in May/June and August. Summer levels were similar to those reported at lower latitudes. Winter dynamics were different. Due to 2 mo of polar darkness, food concentrations for ciliates and heterotrophic dinoflagellates were below threshold concentrations for growth. Nonetheless, small populations persisted throughout the winter. Both protozoan groups had a concentration of ~0.25 × 10 3 cells l -1. The succession patterns showed that all individual species peaked at different times during the summer months. However, diversity of the winter community was not different from that observed during summer. Even in the polar winter, the autotrophic ciliate Myrionecta rubra and the obligate mixotrophic ciliate Laboea strobila were recorded. Different strategies for winter survival of protozoans are discussed.

Grazing experiments were conducted with Calanus spp. and Balanus cf. crenatus nauplii incubated with natural plankton from Disko Bay, West Greenland during the post-spring-bloom period. Both copepod and barnacle nauplii were feeding on different types of protists although at different rates. Calanus spp. nauplii preferred large ciliates and dinoflagellates whereas flagellates ~5 µm in diameter and Myrionecta rubra were hardly ingested at all. B. cf. crenatus nauplii preferred diatoms and also consumed the small flagellates at relatively high rates. Compared to Calanus spp. nauplii, B. cf. crenatus nauplii ingested ciliates and dinoflagellates at low rates suggesting a more herbivorous feeding mode than the more predaceous copepod nauplii. The daily grazing impact of the nauplii community in Disko Bay was estimated using the weight-specific mean clearances from the grazing experiments and field biomass values of different categories of prey and nauplii. These calculations showed that the grazing impact by the nauplii community on all prey categories was generally modest (1.3 to 9.2%). However, the dominant part of the total food intake by Calanus spp. nauplii in the surface water was composed of ciliates and dinoflagellates, most of which were phagotrophic KEY WORDS: Nauplii · Copepod · Barnacle · Grazing · Predation · Arctic · GreenlandResale or republication not permitted without written consent of the publisher