A new radiotracer technique involving 14C and 51Cr, for estimating the assimilation efficiencies of aquatic, primary consumers

Abstract

Gravimetric, radiotracer, and indicator methods currently available for estimating assimilation efficiencies, have been reviewed and their associated limitations have been discussed. It was concluded that the basic assumption implicit to gravimetric and indicator techniques, i.e. that all material contained within the faeces is derived from the food, does not generally hold. Radiotracer techniques are not based on this assumption but are time consuming. Consequently a new radiotracer technique analogous to indicator methods has been developed. In this technique the concentration of a non-absorbed indicator is expressed in terms of a radiotracer, C, which can be absorbed but which, at least initially, is only present in the food, rather than expressing it in terms of dry weight.Cr has been used as the nonabsorbed indicator.Use of these two isotopes in conjunction not only enables a distinction to be made between faecal material derived from food, and that derived from metabolic secretions but also facilitates estimation of assimilation efficiences fromsmall samples of faeces only. The new technique requires simply, measurement of the ratioC:Cr in samples of both food and faeces.The applicability of conditions necessary for operation of the new technique has been tested on two species of freshwater gastropod, one feeding on epilithic algae, the other on bacteria, and its effectiveness has been tested by reference to results obtained from another, more conventional method involving C only.

Assimilation of trace elements and carbon by the mussel Mytilus edulis: Effects of food composition

“…gravimetric and radiotracer) or by the ratio method 1:i.e. radiotracer and ash ratio) (Conover 1966;Calow and Fletcher 1972;Bricelj et al 1984). Each method has advantages and disadvantages.…”

“…C assimilation was determined either by the mass balance method (14C retained divided by 14C ingested, using both feces and mussel tissue data) or the i4C : 51 Cr ratio method (Calow and Fletcher 1972) by assuming that 51Cr was inert to the mussels. C AE therefore was calculated as (14c~51wfeccs…”

Rates and routes of trace element uptake in zebra mussels

“…C assimilation in mussels was measured by the 14 C : 51 Cr ratio method (Calow and Fletcher 1972), which assumes Cr is an inert tracer of digestion. The following equation was used: 51 Cr radioactivity on particles during the radioactive feeding.…”

“…1), and because it is not highly assimilated. In fact, its AE was comparable to that of Cr, which is widely used as an inert tracer (e.g., Calow and Fletcher 1972). Although this method has been used in previous work with M. edulis (Wang et al 1995), it is used cautiously here as an index of digestive partitioning because, although it is poorly assimilated, Ag is not fully inert to digestion, and because metal and organic material may follow different pathways to some degree in the gut.…”

Absorption efficiency, ingestion rate, gut passage time and scope for growth in suspension- and deposit-feeding Yoldia hyperborea

“…Method: Most feeding studies on deposit-feeders have used various techniques involving radiolabelled food material to determine IR, GPT and AE , Kofoed et al 1989, Decho & Luoma 1991, Charles et al 1995. The advantage of this approach over gravimetric and indicator methods (sensu Lopez et al 1989) is its increased sensitivity for measuring small changes (Calow & Fletcher 1972). Dual-labelled food is particularly useful as it can be partitioned into an absorbed (organic) and an unabsorbed (conservative) fraction.…”