Reproductive success of “herbivorous” zooplankton fed algal and nonalgal food resources

Abstract

Many species of zooplankton are traditionally considered to be herbivores despite their ability to capture nonphotosynthetic microorganisms. Several recent studies indicate that protozoa are ingested by crustacean zooplankton, but an important aspect of these trophic interactions that has generally been overlooked is the contribution of heterotrophic food organisms to growth and(or) reproduction. We conducted a series of experiments to examine the ability of two crustacean zooplankters to supplement their diets with protozoa and to utilize these foods for reproduction. The cladoceran, Daphnia pulicaria, and the copepod, Diaptomus oregonensis, both ingested bacteria, ciliates, and heterotrophic nanoflagellates in the presence of algal food. Total young produced by D. pulicaria increased in treatments with added ciliates, heterotrophic nanoflagellates, bacteria, or algae relative to the baseline level of algal food. D. oregonensis females produced abundant young only in the treatments with ciliates or extra algae. The heterotrophic flagellates were ingested at high rates by both D. pulicaria and D. oregonensis, but did not contribute significantly to reproduction in the copepod. This suggests that some foods may be utilized (vs. just ingested) differently by these two common crustaceans. Reproduction was not significantly correlated with the total amounts of carbon, nitrogen, or phosphorus ingested by D. pulicaria or D. oregonensis, suggesting that nutrient content does not sufficiently differentiate the quality of different taxa as food.