which is spent in the water column) and meroplankton (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 and remove 

 particles from the water coluann. Meroplankton in estuaries consist largely of 

 larval stages of benthic invertebrates and fishes that may outnumber the 

 holoplankton for short periods, particularly in the spring. 



The holoplankton are a principal trophic link between the primary producers 

 (phytoplankton and detritus) and aquatic carnivores (e.g., herring). Grazing 

 by zooplankton controls, to a certain degree, the abundance of phytoplankton. 

 Detritus, from dead phytoplankton, emergent wetland vegetation, and 

 macroalgae, (Jeffries 1972) may be an important food source for zooplankton 

 during periods of low phytoplankton production (Riley 1963). Certain 

 zooplankton species may be part of more than one trophic level. For example, 

 filter-feeding herbivores, such as Acartia clausi , are to some extent 

 zooplanktivorous (Hodgkin and Rippingdale 1971) and even cannibalistic (Petipa 

 1966). Some meroplankton (e.g., soft-shelled clam larvae) feed on 

 phytoplankton. Other macroplankton (e.g., fish fry) live on yolk supplies in 

 their egg sacs. 



A close connection may exist between food supply and the initiation of 

 reproduction in zooplankton. This is especially true of copepods, whose 

 spawning and breeding times correlate closely with seasonal phytoplankton 

 blooms (Marshall and Orr 1972). For the most part, zooplankton in estuaries 

 are dependent upon phytoplankton for food (Smayda 1973). In some areas, 

 (e.g., Narragansett Bay, Rhode Island) a delay takes place between the onset 

 of phytoplankton blooms and the initiation of zooplankton reproduction (Martin 

 1964). 



Benthic organisms receive food through (1) the vertical movements of live 

 zooplankton in the water column, (2) the sinking of detritus in the form of 

 fecal pellets, dead organisms, and molted exoskeletons , plus phytoplankton and 

 detrital plant material. Davies (1975) indicates that the primary input to 

 the benthos of a Scottish sea loch is in the form of sinking zooplankton fecal 

 pellets, and this rate of input is about one-third the primary production of 

 the overlying waters. The contribution of fecal pellets may be important in 

 the shallow waters and estuaries of coastal Maine, where dense zooplankton 

 populations are present. Zooplankton also may have an important role in the 

 release of organic matter (see "Organic Matter Cycle," below) and nutrients, 

 including nitrogen and phosphorus (see also "Nutrient Cycle," page 4-48in 

 chapter 4), which then become available to phytoplankton for growth. 



Benthic invertebrates . Benthic invertebrates are either infaunal (live in 

 the bottom sediments; e.g., clams and worms) or epifaunal (live on the bottom; 

 e.g., mussels and scallops). Three groups, the annelids, molluscs, and 

 crustaceans, constitute the majority of estuarine benthic invertebrates in 

 Maine. Annelids are segemented worms; molluscs, such as clams and snails, 

 have shell-enclosed soft bodies; and crustaceans are crablike organisms. 

 Fifteen hundred and eighty-two taxa have been found in coastal Maine. These 

 species are listed by region in appendix E of chapter 4. The commercially 

 important invertebrates (e.g., clams, scallops, and lobsters) are described in 

 detail in chapter 12, "Commercially Important Invertebrates." 



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