Communities of benthic invertebrates occupy an important position in the 

 estuarine system. They convert the direct and indirect products of primary 

 production to animal protein, which is passed to higher trophic levels through 

 predation. 



The burrowing and feeding activities of benthic invertebrates alter 

 sedimentary structures and return to the water column nutrient materials that 

 have been deposited or bound in the sediments. These activities also disturb 

 bottom sediments, which sometimes make the area unsuitable (excessively 

 turbid) for animals that feed by filtering. Benthic invertebrates are 

 dependent on the overlying water for oxygen and food and to remove wastes. 

 Major foods are phytoplankton, particulate organic material of plant origin, 

 bacterial flora on detritus, benthic diatoms, living plants, and living 

 animals. Of these food sources, particulate organic matter and detritus are 

 particularly important to benthic invertebrates in estuaries. Some feed on 

 only one type of food, whereas others feed on many types (Sanders et al 1962). 

 The food consumed by a particular animal may change with the animals' size or 

 age (Sanders et al. 1962). A small individual may be able to consume only one 

 type of food, but as it grows it may eat larger and more diverse food 

 particles . 



Benthic invertebrates live on all types of substrata and are affected in some 

 way by most types of habitat disruption. Most (nearly 95%) of the animals 

 occur within 5 cm (2 in.) of the water-sediment interface (Stromgren et al. 

 1973), because organic material (i.e., food) is more concentrated there. 



Many species that are considered infaunal leave the sediment at night and swim 

 in the water column (e.g., bloodworms and sandworms; Dean 1978a and b) and 

 crustaceans that are thought to be planktonic (e.g., amphipods) enter the 

 substratum during the daylight hours (Thomas and Jelley 1972). 



Larval stages of many benthic invertebrates live in the water column, (see 

 "Zooplankton" above) usually up to 5 to 6 weeks in Maine (personal 

 communication from E. Bousefield, National Museum of Canada, Ottawa, Canada; 

 June, 1977). Other invertebrates, such as predatory snails, amphipods and 

 isopods, develop directly into small adults and rarely enter the water column. 

 Pelagic larvae have the ability to disperse widely. However, natural 

 mortality of pelagic larvae is high, possibly as high as 957o to 99%. 

 Invertebrates with pelagic larvae produce large numbers of young, thus 

 ensuring the survival of their species. 



The primary factor influencing the distribution and abundance of benthic 

 species is the nature of the substratum. Pelagic larvae metamorphose into 

 juveniles when they come in contact with a suitable substratum and some (e.g., 

 clams and mussels) can delay metamorphosis for a limited time until a suitable 

 substratum is located. The period of metamorphosis and settlement is probably 

 the most critical period in the life of a benthic organism (Thorson 1966). 

 Predation on newly settled juveniles sometimes is severe. 



Substratum requirements are related to feeding type and mode of living. 

 Deposit feeders (e.g., clams) are most abundant in fine sediments of silt and 

 clay containing organic detritus. The deposit feeders pass the sediment 

 through their bodies, digest the bacteria on it, and excrete fecal material 

 which is colonized again by bacteria and reconsumed (Young 1971). Organisms 



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