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Zooplankton fecal pellets have long been thought to be a dominant component of the sedimentary flux in marine and freshwater ecosystems, but that view is changing. The last 2 decades have seen publication of > 500 studies using sediment traps, which reveal that zooplankton fecal pellets often constitute only a minor or variable proportion of the sedimentary flux. Substantial proportions of this flux are from organic aggregates ('marine snow') of various origins, including phytoplankton blooms, which sediment directly to the benthos. It now appears that mainly large fecal pellets of macrozooplankton and fish are involved in the sedimentary flux. Smaller fecal pellets of microzooplankton and small mesozooplankton are mostly recycled or repackaged in the water column by microbial decomposition and coprophagy, contributing more to processes in the water column than flux to the benthos. The relative contributions of fecal pellets, marine snow and sinking phytoplankton to the vertical flux and recycling of materials in the water column are highly variable, dependent upon multiple interacting factors. These include variations in productivity, biomass, size spectra and composition of communities in the overlying water columns, and trophic interactions between various components of the plankton and nekton communities at various times, locations and depths. Other factors include differences in sinking rates, sizes, composition and pollutant contents of fecal pellets produced by various sizes of zooplankters, and zooplankton feeding-fecal pellet production interactions in relation to upwelling and El Niño periods, seasonal life-history-related zooplankton vertical migrations and long-term oceanographic regime shifts. There are also suggestions from the geological record that zooplankton fecal pellets may have been important in ancient oceans. The ecological roles of marine snow and phytoplankton aggregates in sedimentary flux also depend on a variety of interacting factors, including sources of origin, degrees of microbial colonization, depth distributions, sinking rates and ingestibility by consumers. Perhaps the major reversal of the previous paradigm on the role of fecal pellets in the sedimentary flux over the last 2 decades has been the realization that much, if not most, of the organic rain from the epipelagic to the abyss is due to direct sedimentation of aggregated phytoplankton, which does not appear to undergo consumption in the water column, and which may be related to seasonality of surface production cycles. Further, there is emerging evidence for benthic responses to sedimented phytodetritus, including apparent synchrony of reproductive cycles of some deep-sea benthic animals with seasonality of sinking of surface blooms. Such episodic input of surface phytodetritus may help resolve apparent discrepancies between average supply and demand of organic matter required to maintain benthic community metabolism. The sedimentary flux of fecal pellets, marine snow and sinking phytoplankton is an important c...

Interactions between toxic phytoplankton and their zooplankton grazers are complex. Some zooplanktcrs ingest some toxic phytoplankters with no apparent harm, whereas others are deleteriously affected. Phycotoxins vary in their modes of action, levels of toxicity and solubility, and affect grazers in different ways. Beyond effects on direct grazers, toxins may accumulate in and be transfcrrcd through marine food webs, affecting consumers at higher trophic levels, including fish, scabirds, and marine mammals. Grazers of toxic phytoplankton include protists as well as metazoans, and the impact of zooplankton grazing on development or termination of toxic blooms is poorly understood. In most interactions of toxic phytoplankters with grazers and other marine food-web components, outcomes are situation-specific, and extrapolation of results from one set of circumstances to another may be inappropriate.

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.

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