REPRODUCTIVE SUCCESS OF HERBIVOROUS ZOOPLANKTON FED ALGAL AND NONALGAL FOOD RESOURCES

Many species of zooplankton are traditionally considered to be herbivo res despite their ability to capture nonphotosynthetic microorganisms. Several recent studies indicate that protozoa are ingested by crustac ean zooplankton, but an important aspect of these trophic interactions that has generally been overlooked is the contribution of heterotroph ic food organisms to growth and (or) reproduction. We conducted a seri es of experiments to examine the ability of two crustacean zooplankter s to supplement their diets with protozoa and to utilize these foods f or reproduction. The cladoceran, Daphnia pulicaria, and the copepod, D iaptomus oregonensis, both ingested bacteria, ciliates, and heterotrop hic nanoflagellates in the presence of algal food. Total young produce d by D. pulicaria increased in treatments with added ciliates, heterot rophic nanoflagellates, bacteria, or algae relative to the baseline le vel of algal food. D. oregonensis females produced abundant young only in the treatments with ciliates or extra algae. The heterotrophic fla gellates were ingested at high rates by both D. pulicaria and D. orego nensis, but did not contribute significantly to reproduction in the co pepod. This suggests that some foods may be utilized (vs. just ingeste d) differently by these two common crustaceans. Reproduction was not s ignificantly correlated with the total amounts of carbon, nitrogen, or phosphorus ingested by D. pulicaria or D. oregonensis, suggesting tha t nutrient content does not sufficiently differentiate the quality of different taxa as food.