Consumers 



Consumers are those animals that feed on the products of primary productivity, 

 as well as on each other. The major marine consumer groups are zooplankton, 

 benthic invertebrates, squid, fish, birds, and marine mammals. 



This section describes each group, its habitat preference, and its role in the 

 system. 



Zooplankton . Zooplankton is the collective term for the diverse 

 assemblage of animals that float or swim weakly in the water column and thus 

 are carried by currents. The principal components of the zooplankton can be 

 classified as follows: holoplankton, if all the life cycle is spent in the 

 water column; and meroplankton, if only a portion of the life cycle is 

 planktonic. The copepods are the most important members of the holoplankton. 

 These small crustaceans are largely filter feeders, which remove particles, 

 principally phytoplankton and detritus, from the water column. Larval stages 

 of benthic invertebrates and fishes make up the meroplankton, a group that may 

 outnumber the holoplankton for short periods, particularly in bays during 

 spring. 



The holoplankton are a principal trophic link between the primary producers, 

 phytoplankton and detritus, and aquatic carnivores, such as jellyfish, comb 

 jellies, arrow worms, larval fish, herring, mackerel, and menhaden. Grazing 

 by zooplankton can be a significant control of phytoplankton populations. 

 Detritus, from unused phytoplankton production or fringing salt marshes 

 (Jeffries 1972) , may be an important food source during periods of low 

 phytoplankton production (Riley 1963). Certain zooplankton species may be 

 part of more than one trophic level. For example, a filter-feeding herbivore, 

 such as the copepod Acartia clausi , is to some extent carnivorous (Hodgkin and 

 Rippingdale 1971), and even cannibalistic (Petipa 1966). Some meroplankton 

 feed on phytoplankton, while others live off their own yolk supplies. A close 

 connection may exist between food supply and the initiation of reproduction in 

 zooplankton. This is especially true of copepods, where spawning and breeding 

 times correlate closely with seasonal phytoplankton blooms (Marshall and Orr 

 1972). The general suitability of the phytoplankton as food, in terms of 

 size, taste, and composition, is also a critical factor (Smayda 1973). As was 

 observed in Narragansett Bay, Rhode Island (Martin 1965), there may be a delay 

 between the onset of the phytoplankton bloom and the initiation of zooplankton 

 reproduction. Zooplankton produce detritus, in the form of fecal pellets and 

 molted exoskeletons , which sink to the bottom and provide food for benthic 

 organisms. Their vertical movements in the water column bring some live 

 animals near the bottom, also resulting in food for the benthos. Davies 

 (1975) has indicated that the primary input to the benthos of a Scottish sea 

 loch was in the form of sinking zooplankton fecal pellets and this rate of 

 input was about one-third the primary production of the overlaying waters. 

 The contribution of fecal pellets may be important in the shallow waters and 

 estuaries of coastal Maine, where dense zooplankton populations occur. 

 Zooplankton also may have an important role in the release of organic matter 

 (see "Organic Matter Cycle" below), nutrients, nitrogen and phosphorus (see 

 "Nutrient Cycle" below ); and excretory products (Corner and Davies 1971), 

 which then become available to phytoplankton for enhanced growth. 



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