GUT FLUORESCENCE IN HERBIVOROUS COPEPODS - AN ATTEMPT TO JUSTIFY THE METHOD

Recently the gut fluorescence technique has been critisized because of the possible degradation of chlorophyll into nonfluorescent derivativ es during passage through copepods guts and changes of the gut passage time with food concentration. Here pigment budgets have been calculat ed in 6 experiments with Calanus finmarchicus CIV caught 2 km offshore of the Murmansk Marine Biological Institute (the Barents Sea, Dalnije Zelentsi) in September 1992. Copepods were fed with culture of Platym onas viridis at different concentrations. Gut pigment and ingestion ra te increased with food concentration in a similar way. On average betw een 78% and 89% of ingested chlorophyll was recovered in the guts and faecal pellets. No trend for a greater loss of fluorescence at low foo d concentration than at high was observed. Pigment content of faecal p ellets incubated in filtered seawater decreased by 20-30% in the first 7-12 h and by up to 60% in 48 h. The decline of pigment content was a ccompanied by a rapid bacterial growth (by a factor of 3 in 48 h). Gut passage time increased with decreasing food concentration (from 40 mi n at 9 mug pigm l-1 to 64 min at 0.9 mug pigm l-1). These results toge ther with some data by other authors suggest that the gut fluorescence method can be used to estimate in situ grazing rate providing gut pas sage time is measured properly and there are no losses of faecal mater ial. However, careful consideration should be given to the previous fe eding history of copepods.