Some Observations on the Dependence of Zooplankton Grazing on the Cell Size and Concentration of Phytoplankton Blooms

The amount and the size fraction of particulate material consumed by different zooplankters were studied during the occurrence of two natural phytoplankton blooms. Both the size and quantity of particulate material eaten were determined with a Model B Coulter Counter. Results showed that a Chaetoceros bloom could be readily eaten by Euphausia pacifica but that Calanus pacificus, euphausiid furcilia and Pseudocalanus minutus were unable to derive more than a subsistence level of diet from the same plants. In contrast to the Chaetoceros bloom, a nanoplankter (ca 8ƒÊ diameter) was found to be less available as food for Euphausia pacifica but a better source of food for Calanus pacificus and euphausiid furcilia. These differences have been attributed to the physical availability of the food in terms of its size and shape. Data on the feeding of zooplankton at different concentrations of phytoplankton were best explained by assuming a concentration dependent relationship similar to that proposed by IVLEV (1945) for planktivorous fish. Introduction There are a number of factors which may determine the suitability of a certain phytoplankter as food for a secondary producer. These factors can be divided into those which govern the availability of the food and those which describe the adequacy of the diet once food is available. In the latter case, PROVASOLI et al. (1959) have shown, for example, that although a zooplankter may eat a variety of foods, only certain types (or combinations) are a satisfactory diet for both growth and reproduction. Thus the adequacy of a phytoplankter as food for a secondary producer is largely a nutritional matter. In contrast, the initial availability of a food in situ must be determined to some degree by the size of the food particles and the concentration at which they are present. WALNE (1965) has shown that oyster larvae perferentially assimilate very small flagellates, 2-3 ƒÊ in diameter, while for larger herbivores there appears to be a perference for larger phytoplankters (MULLIN, 1963). Recent information on maximum and minimum food sizes for different zooplankters has been reviewed by JƒÓRGENSEN (1966). The dependence of grazing on the concentration of prey has been described mathematically by IVLEV (1945) and CUSHING (1959). In the following discussion we have used a modification of IVLEV'S equation to describe data obtained during a number of feeding experiments. IVLEV's original equation related ration (r) to food density (p) in the following manner: (1) where R is the maximum ration and h is the proportionally constant. The curve represented by this equation passes through the origin which implies that animals continue feeding down to zero prey density. In fact it appeared from our observations that grazing occurred down to some low prey density, p0, and then ceased. In order to accommodate this observation the above equation was modified to the form: * Received Oct. 24, 1966 Presented at the Eleventh Pacific Science Congress of the Pacific Science Association, held at the University of Tokyo, Japan, August-September 1966, and organized by the Science Council of Japan. * * Fisheries Research Board of Canada, Pacific Oceanographic Group, Nanaimo, B.C.