The Relationship Between Ingestion Rate of Deposit Feeders and Sediment Nutritional Value

Abstract

Food resources of marine deposit feeders have been a subject of intense investigation for many years, but it has been only recently that some of the details have become apparent. Two general hypotheses have dominated research in this field: first, non-living organic carbon including detritus cannot be utilized directly by deposit feeders, but must first he transformed into microbial biomass by bacteria, fungi, and other microbes; and second, the resulting microbial biomass represents a major food resource for the deposit feeders (for example, Newell 1965; Fenchel 1970; Hargrave 1976; Lopez et al, 1977; Gerlach 1978). These two hypotheses are by no means independent and each has been used to support the other. However, as I have pointed out previously (Canunen 1980a), in order to evaluate the importance of a potential food resource it is necessary to consider both the rate at which it is ingested and the efficiency with which it is assimilated. In terms of nonliving organic carbon, we have an abundance of data for deposit feeders on ingestion rates (Canunen 1980b), but very little quantitative data on assimilation efficiencies; the situation for microbial carbon is exactly the inverse, with numerous studies of deposit feeder assimilation efficiencies (for example, Zhukhova 1963; Hargrave 1970; Kofoed 1975; Yingst 1976; Lopez et al. 1977; Cammen 1980a), but very few of ingestion rates (Wetzel 1977; Jensen and Siegismund 1980; Canunen 1980a). Yet, somehow the qualitative data showing that assimilation efficiency is generally low for non-living organic carbon or detritus and the data showing high assimilation efficiencies for microbes have been combined to suggest that the major food source for marine deposit feeders must be the microbes—the information on ingestion rates has been largely ignored. Recently, however, several studies have taken into account the ingestion rates of microbes by the deposit feeders (Wetzel 1977; concluded that bacteria can be only of minor importance in the carbon budgets of the animals. In fact, no one has ever shown that any deposit feeder uses bacteria for a major fraction of its nutritional requirements. Thus, the question of how much non-living organic matter contributes to deposit feeder nutrition has become much more important. Unfortunately, it is a difficult question to address since the high ingestion rates make even a low assimilation efficiency significant (Cammen 1980a; Levinton 1980) and with current methods we cannot measure a low assimilation efficiency with any degree of accuracy.