Containment effects in copepod grazing experiments: A plea to end the black box approach1

Containment effects from enclosing unfiltered seawater and copepods in 500-ml jars to estimate grazing rate were examined six times during 48-h incubations. Crazing rates decreased rapidly for the first 3 h (up to 50%) and then continued to decrease at slower rates. The number of bacteria increased in both control and experimental jars. Variable but significant changes were found in the size distribution of particles and concentrations of adenosine triphosphate and chlorophyll a. Ammonia concentrations were higher in jars with copepods. After 24 h, grazing rates calculated from control jars with NH4+ additions to simulate copepod excretion were about twice those calculated from unamended controls. The grazers removed 2% of the 3-5-pm particles compared to unamended controls and 17% compared to control jars plus NH4+. Early zooplankton grazing experiments were done in the laboratory with individual species of copepods and phytoplankton food (e.g. Fuller 1937; Marshall and Orr 1955; Mullin and Brooks 1967; Frost 1972). These controlled laboratory experiments have progressively increased our understanding of the grazing responses of copepods to food size and concentration. In the natural environment, however, the marine seston is a complex mixture containing phytoplankton, detritus, and inorganic particles of varying proportions, chemical compositions, and size distributions. The main approach to study the in situ grazing behavior of marine copepods has been to enclose a sample of natural particulate material with either mixed zooplankton or selected species of copepod and to measure the change in particle concentration with an electronic particle counter after an incubation period, usually of 24 h (e.g. Poulet 1973; Richman et al. 1977). Although these experiments do not distinguish detritus from phytoplankton and cannot assess the nutritional value of the ingested food, they can give useful information on I This research was supported by NSF grant OCE-786084. 2 Present address: CBCES-Smithsonian Institution, P.O. Box 28, Edgewater, Maryland 21037. particle size selection and grazing rate (water cleared of particles * copepod No.-l or biomass-l * time-l). These measurements of copepod grazing rate are similar to .measurements of growth or metabolic rates for phytoplankton and bacteria in that natural concentrations of seston are enclosed in an environment (jar) which is unlike the in situ environment. Although certain physical parameters such as light and temperature can be controlled, other factors such as turbulence, diffusion, interface area, and substrate level may change both immediately upon enclosure and over the incubation period. These “bottle effects” on rate measurements have been investigated for bacteria (ZoRell and Anderson 1936; R. L. Ferguson pers. comm.), phytoplankton (Sheldon et al. 1973; McCarthy et al. 1974; Venrick et al. 1977), and zooplankton (Mullin 1963; Anraku 1964). The zooplankton studies have dealt mainly with the effect of jar size and agitation on cope.pod grazing rate. Natural particulate matter is composed of not only phytoplankton but also bacteria, Protozoa, detritus, and inorganic particles. Changes in the quantity and quality of these components and their effect on copepod grazing rate have not been examined. We have investigated changes in the composition and biomass of particulate matter and their effect on copepod