SHORT COMMUNICATION Gut clearance rate constant, temperature and initial gut contents: a review

The influence of temperature (7) and initial gut contents on estimates of copepod gut clearance rate constant (GCRC) is reviewed from literature data. Previous results on the relationship between temperature and GCRC are confirmed, and the inclusion of initial gut contents did not increase the significance of the model. Nevertheless, a significant difference was found between the experiments performed with pre-fed animals and animals from the environment. A high percentage of variability remains unexplained, but part of this is likely to be due to experimental error and differ- ences in the methods used. Since the first articles on the measurement of gut fluorescence (Nemoto, 1968; Mackas and Bohrer, 1976), this method has been widely used to assess ingestion of phytoplankton by copepods. The estimation of ingestion by the gut fluor- escence method requires the measurement of two factors: the gut fluorescence and the gut clearance rate constant (GCRC) (for a complete review of the method and problems, see Morales and Harris, 1990; Morales et al, 1990; Paster- nak, 1994). Although gut fluorescence is relatively easy to measure, GCRC requires a higher number of samples to be analyzed and attempts have been made to establish general rules for the estimation of GCRC. Dam and Peterson (1988) found that GCRC was strongly related to temperature and proposed an equation describing this relationship. Subsequently this equation has been generally accepted and employed. Nevertheless, some other studies have shown that food concentration also has an important influence on GCRC (Dagg and Walser, 1987; Pasternak, 1994) and that the relationship with temperature disappears when food is strongly limited (Irigoien et al., 1996). In addition, some results and simu- lation models show an apparent relationship between initial gut fluorescence and GCRC (Baars and Oosterhuis, 1984; Head, 1986; Irigoien, 1994; Tseytlin, 1994; Perissinotto and Pakhomov, 1996), whereas this relationship has not been reported in other studies (e.g. Batchelder, 1986; Wang and Conover, 1986; Head et al, 1988). This article presents an extensive review of the literature in order to assess the influence of temperature and the initial gut fluorescence on GCRC, and to deter- mine differences in the temperature-GCRC relationship between field and laboratory measurements. Data were collected from articles (Table I) reporting experiments which followed the standard procedure of placing animals in filtered sea water and removing samples for gut fluorescence measurements at different time steps. A first dataset was prepared with data from articles where GCRC, temperature and the initial gut fluorescence were available. Gut fluorescence was standardized by dividing it by the dry weight of the animal (ng Chi equivalent