The relation of energy and organic carbon in aquatic invertebrates1

The physical and nutritional energy values of aquatic invertebrates were studied in relation to organic carbon content. The ratio of energy content to the weight of organic carbon increased with increasing carbon. The dependence is described in regression equations which permit prediction of the cncrgy content with an accuracy of about +5% (P > 0.95 ) . Even the use of the mean conversion factor of 46 kJ g-l organic carbon yields considerably more accurate results than the use of a general conversion factor for ash-free dry weight. Accurate energy values of animals are essential for studies of animal energetics. Determination of the energy content of animals with the aid of bomb calorimeters now used requires samples relatively large in comparison with the small size of most freshwater invertebrates. Most scientists use dry weight or ash-free dry weight to estimate biomass. As the energy content of dry or ash-free dry biomass varies considerably from species to species (e.g. Cummins and Wuycheck 1971)) and within one species from season to season (e.g. Schindler et al. 1971; Wissing and Hasler 1971; Snow 1972), the use of general energy equivalents for dry or ash-free dry weight does not yield very accurate values. As noted earlier by Platt and Irwin ( 1973)) the energy equivalent for biomass expressed in terms of organic carbon should theoretically vary less than the energy equivalent for biomass expressed in terms of dry or ash-free dry weight. The use of 41.9 k J g-l C ( = 10 kcal g-l C ) as an approximate conversion factor has been recommended by Winberg (1971a). I-Iowever, new methods have made it possible to determine organic carbon rapidly and accurately even from submicrogram samples unsuitable for calorimetry. To obtain more accurate general energy equivalents for biomass expressed in terms of organic carbon, we have studied the relation between energy content and organic carbon in 1 This work was supported by the Academy of Finland and the University of Helsinki. freshwater invertebrates from various taxonomic groups. We thank C. and F. Roberts for checking the language of our manuscript. Materials and methods Material was collected in November and December 1974 from the mesohumic, oligotrophic lake P%j%vi and the meromictic, eutrophic lake Lovojgrvi (Daphnia, Corda portions of ash by combustion at 950°C in an oxygen flow in the presence of a cupric oxide catalyst. The resulting carbon dioxide was measured with a nondispersive infrared gas analyzer (Hartmann and Braun, Uras 2). The combustion peak areas on the recorder chart were measured with a planimetcr. The carbon analyzer was calibrated against oxalic acid and CaCOs. Both gave equivalent calibration coefficients, so the oxidation of oxalic acid was probably complete. The 95% confidence limits for the linear calibration were -+0.7% (the statistical treatment and terminology follow Snedecor and Cochran 1967). Nitrogen was determined from 0.6-3.1mg portions of the homogenized material, digested with a solution of 5 ml of concentrated I-12S04, 2 ml of 11202, and 0.2 g of Dewarda’s alloy. Ammonia was then distilled with a Biichi 1500 distillation apparatus. Thereafter 2 ml of NaKC4O 46.2 4 0.6%, n = 5), suggesting that the weight loss was due to a decrease in water lcvcl. Thus no carbon loss was apparent from samples dried at 60°C as compared to living material, in agreement with the finding of Schindler et al. ( 1971) that heat-drying at 110°C and freeze-drying gave similar energetic values for freshwater zooplankton. The effect of ash on the determination of energy content by calorimetry was studied by adding various amounts (O-80% ) of CaC03 to bcnzoic acid. The resulting decrease in the energy content was 0.69 kJ g-l CaC03 ( n = 22)) fairly close to the value (0.58 kJ g-l CaC03) obtained by Paine ( 1966). We corrected our results for the observed decrease, assmning that all the measured inorganic carbon was derived from the decomposition of CaC03. Ta bl e 1. As h, ca rb on , an d ni tro ge n le ve ls , en er gy co nt en ts , an d en er gy -to -c ar bo n ra tio s, w ith 95 % co nf id en ce lim its , in 19 sp ec ie s of fre sh w at er in ve rte br at es . AF D W -a sh -fr ee dr y w ei gh t.