abundance begin before July south of Mass- 

 achusetts (Duncan 1S74; Dobbs 1981). These 

 declines in population abundance are prob- 

 ably the result of biotic interactions 

 such as competition for food and space and 

 the seasonal appearance of vertebrate and 

 invertebrate predators (e.g., fish, epi- 

 faunal gastropods, crabs, and birds). 



While seasonal change in the physi- 

 cal and chemical components of benthic 

 systems contributes to the highly variable 

 spatial-temporal abundance of organisms 

 in tidal flats, several studies have noted 

 the existence of consistent year-to-year 

 trends in benthic community structure 

 in New England and elsewhere (Grassle 

 and Smith 1976; Whitlatch 1977; Coull and 

 Fleeger 1978). The cycle may be attrib- 

 uted to seasonally-programmed reproduc- 

 tive activities of organisms found in dif- 

 ferent geographic areas (Whitlatch 1977) 

 or to the seasonal occurrence of benthic 

 invertebrate and vertebrate predators 

 (e.g., demersal fishes, epifaunal crusta- 

 ceans and gastropods). Other studies have 

 failed to find repeatable seasonal trends 

 in community structure (e.g., Levings 

 1976; Dobbs 1981). The existence of such 

 patterns may be the result of the specific 

 characteristics of the local biotic and 

 abiotic environment controlling the struc- 

 ture of the infaunal populations and com- 

 munities. 



Infaunal interactions result in 

 alterations of their abundance and distri- 

 bution patterns on tidal flats. These 

 interactions may take several forms but 

 may be conveniently separated into direct 

 and indirect effects. The most common 

 form of indirect interaction is habitat 

 modification by one species or trophic 

 group resulting in an adverse impact upon 

 another species or trophic group. The 

 best documented example of this type of 

 interaction is called trophic group amen- 

 salism (Rhoads and Young 1970). First 

 described in subtidal, muddy sediments of 

 Buzzards Bay, Massachusetts, this phenom- 

 enon involves the destabi lization of the 

 surficial sediment by the burrowing and 

 feeding activities of deposit feeders 

 which results in increased sediment resus- 

 pension and subsequent interference with 

 the filtering activities of suspension 

 feeders. This type of interaction is most 

 likely to occur in muddy sediments where 



deposit feeders are abundant and fine sed- 

 iments are easily resuspended, but Myers 

 (1977a, b) has recently reported trophic 

 group amensalism in a shallow water sandy 

 habitat. Biological destabilization of 

 the sediment-water interface by demersal 

 fishes, large epifaunal invertebrates, and 

 meiofauna has also been reported (e.g., 

 Yingst and Rhoads 1978; Boyer 1980), but 

 the predicted effect upon suspension feed- 

 ers has yet to be determined. 



Direct interactions can be either 

 adult-adult or adult-larval effects. 

 Adult-larval interactions occur when 

 infaunal assemblages of adult organisms 

 are dense enough to prevent or restrict 

 recruitment of larvae. Woodin (1976) sug- 

 gested that these interactions occur when 

 suspension and deposit feeders ingest 

 settling larvae or when deposit feeders, 

 through their feeding activities, bury or 

 smother settling larvae. Dense popula- 

 tions of infauna are common in New England 

 tidal flats (e.g., Sanders et al. 1962; 

 Whitlatch 1977; Dobbs 1981) and there is 

 evidence that adult-larval interactions 

 occur. At present, however, we lack con- 

 trolled field studies to document the 

 importance and magnitude of adult-larval 

 interactions in the New England region. 



Adult-adult interactions involve 

 predatory interactions and infaunal organ- 

 isms competing for either space (lateral 

 or vertical) and/or food. Whitlatch (1980) 

 found a general relationship between food 

 and space overlap and sediment organic 

 matter suggesting the importance of ex- 

 ploitive competition for food by deposit- 

 feeding species. In habitats with high 

 levels of organic matter, species that 

 were similar in resource utilization were 

 able to coexist and species numbers were 

 high. In less productive habitats, eco- 

 logically similar species were excluded 

 and species number declined. Grassle and 

 Grassle (1974) documented intraspecif ic 

 effects on egg production in the poly- 

 chaete, Capitella capitata , related to 

 competition for food. Other studies have 

 noted the importance of exploitive inter- 

 actions in limiting the distributional 

 patterns of infaunal organisms (e.g., 

 Levinton 1977; Weinberg 1979). Competi- 

 tion between species for space within sed- 

 iments has been shown in a variety of 

 suspension- and deposit-feeding species 



34 



