MEPS 171:187-198 (1998)  -  doi:10.3354/meps171187

A model of chlorophyll a destruction by Calanus spp. and implications for the estimation of ingestion rates using the gut fluorescence method

C. J. Stevens1,*, E. J. H. Head2

1Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
2Department of Fisheries and Oceans, Bedford Institute of Oceanography, PO Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada
*Present address: Ocean Sciences Centre, Memorial University of Newfoundland, St. John's, Newfoundland A1C 5S7, Canada. E-mail:

ABSTRACT: Chlorophyll a (chl a) destruction by groups of Calanus spp. with different long-term in situ feeding histories was compared. For copepods that had fed during both pre- and early-bloom conditions, degrees of chl a destruction were relatively constant at high ingestion rates and increased as ingestion rates decreased. Assuming that 2 pools of chlorophyll bleaching enzymes (CBEs) were involved in the destruction of chl a (one derived from the copepods and one from the ingested algae), a new model was developed to describe the kinetics of chl a destruction. In this model, the CBE activity of each pool was described using a Michaelis-Menten expression and the total CBE activity was given by the sum of the 2 expressions. Parameter estimates of Vcmax, the maximum activity of the copepod CBE, were higher for the early-bloom copepods than for the pre-bloom copepods, suggesting that the former had a higher destructive capacity. Estimates of R pmax, the phytoplankton 'CBE activity coefficient' which is analogous to Vmax, were similar between the 2 groups of experiments. This is reasonable since most of the food fed to the copepods was healthy, actively growing diatoms. The model could also describe the kinetics of chl a destruction for Calanus spp. that had fed during late-bloom conditions. For the late-bloom data, Vcmax and R pmax values were higher than for the pre- and early-bloom copepods and phytoplankton. This suggests that the late-bloom copepods and the in situ phytoplankton that they ate had higher destructive capacities, perhaps because the spring-bloom was more advanced. Expressions were derived from the new model to describe the relationship between real ingestion rate (Ir) and apparent ingestion rate (Ia), over a range of Ia values, where the latter are values which would have been determined using gut fluorescence methodology. Correction factors (Ir/Ia) varied by a factor of less than 2 (for Ia values ranging from 0.1 to 100 ng chl a ind.-1 h-1) between different groups of copepods (pre-, early- and late-bloom) and sources of algae (actively growing and senescent). In future it will be important to validate this model under controlled conditions (e.g. using single species of copepods and phytoplankton food) and to assess whether correction factors derived from our model are generally applicable, if results of studies using gut fluorescence methods are to be interpreted properly.

KEY WORDS: Copepod grazing · Chlorophyll a destruction · Feeding history · Michaelis-Menten kinetics

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