LABORATORY STUDY OF THE CHEMICAL-COMPOSITION OF AGING COPEPOD FECAL MATERIAL

Changes in the chemical composition of aging copepod fecal material were investigated by incubating fresh feces in the dark at 5 and 15 °C and following transformations through time (27 days at 5 °C and 11 days at 15 °C) in terms of carbon (C), nitrogen (N), chlorophyll-type pigments and dissolved free amino acids (DFAA). At both temperatures, feces weight specific C and N decreased during incubation, this decrease being much more important at 15 °C (daily C loss of 2.5 μg·mg feces-1·day-1) than at 5 °C (maximum daily loss = 1.5 μg·mg feces-1·day-1). The C:N atomic ratio ranged between 7 and 8; it showed little temporal variation for both temperature conditions. The relative composition of HPLC-analysed chlorophyll-type pigments was rather constant, as illustrated by the Chl a-based pigment ratios. Six pigments were detected: Chl a, Chla-type (possibly allomer), Chl c, Chl c-like, pheophorbide a, pheophorbide a-like. Their concentration generally decreased through time. Significant linear decreases were observed for Chl a (rate = -l.15 ng·mg-1 feces·day-1) and pheophorbide a-like at 15 °C. After a rapid decrease within the first 3-5 days of incubation, the total DFAA concentration measured in the filtrates was stable. Most individual amino acids (aspartic and glutamic acids, asparagine, arginine, leucine and ornithine) rapidly became undetectable or were <5-10% of the initial relative molar composition. A few others, considered as waste products (α-amino butyric acid, valine), accumulated with time. This process was temperature-dependent in <1 wk. After that period, the DFAA concentration and composition became stable, whatever the temperature, even when fecal C and pigments continued to decrease. Addition of chloramphenicol at the beginning of the incubation period generally resulted in larger C and pigment content in the fecal material, especially at 15 °C, as well as higher concentrations of DFAA and a more varied composition of amino acids when analysed on the last day of incubation at both temperatures. C or pigment values obtained in the antibiotic-treated samples could not be statistically differentiated from values of the first sampled day of each series, indicating the important role of bacteria in mediating observed transformations. Comparison of the temporal evolution of C and pigments showed differences in the relative rate of transformation of these two elements, particularly at 15 °C, with pigments decreasing faster than C and causing the C: Chl a ratio to increase with time in feces. © 1990.