Feeding rates of Calanoides carinatus: a comparison of five methods including evaluation of the gut fluorescence method

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Abstract
A comparison was made of ingestion rates of the copepod Calanoides carinatus estimated from the disappearance of chlorophyll (by fluorometry and HPLC), disappearance of cells, egg production, and from the gut fluorescence technique. Three measures of gut turnover time were also compared. Our primary goal was to determine whether the gut fluorescence method underestimated ingestion and if so, to determine if the problem was due (i) to pigment digestion or (ii) to incorrect estimates of gut passage time. Experiments used as food the diatom Thalassiosira weissflogii at 3 concentrations. The first 4 methods yielded the same result: grand means were 2.7 .mu.g chl d-1 at 8000 cells ml-1, 1.5 .mu.g chl d-1 at 3000 cells ml-1 and 0.84 .mu.g chl d-1 at 1500 cells ml-1. The gut fluorescence technique underestimated ingestion, yielding rates of 1.0, 0.92 and 0.58 .mu.g chl d-1 when calculated following the traditional approach of multiplying gut pigment content by gut evacuation rate (GER). The underestimate was not due to pigment digestion because pigment budgets showed that all pigment was accounted for, 106% on average. Thus we suggest that the problem was associated with using GER as a measure of gut passage time (GPT). GPT estimated from the GER was 16.7 min, but when estimated from (i) measurements of fecal pellet production rates and from (ii) the ratio, pigment egestion rate/mean gut pigment content, results averaged 10.2 min. Using this value, ingestion rates were equal to the other techniques at the intermediate and low food concentrations, and 80% of the other techniques at the high concentration.