Many phytoplankton species have evolved a variety of different defenses to decrease losses from grazing; morphological features, changes in life-history/behavior, and production of toxins. These defenses may be associated with costs. Therefore, some phytoplankton only express the defense when needed, i.e. when grazers are present.The defense can be induced by direct contact with the grazer, or infochemicals released during grazing activities may function as reliable signals of grazer presence. Morphological defenses were studied in the colony forming prymnesiophyte Phaeocystis globosa, in combination with varying nutrient status, such as nitrogen(N) and phosphorus (P) sufficiency, N deficiency and P deficiency. NP sufficient P. globosa remained mainly as solitary cells in response to infochemicals. The responses were more complex in the nutrient deficient experiments, due to the increased mortality of copepods observed, which may have resulted from lower food quality in nutrient stressed cells. This could affect both grazers and the infochemicals released, which could have been to weak to affect P. globosa. Morphological defenses include formation of digestion resistant gelatinous sheaths, which were examined in the chlorophyte Oocystis submarina. Direct, not indirect, exposure to copepods and cladocerans caused a shift towards cells and colonies with gelatinous sheaths. Thus, infochemicals played no role in these responses. The cyanobacterium Nodularia spumigena has two potential defense mechanisms; morphology (filament size/structure), and toxicity. These defenses are not induced by the direct or indirect presence of copepod grazers. However, N. spumigena increased the mortality of copepods, which was probably related to starvation. This may contribute to the success of N. spumigena blooms, as there could be a shift ingrazing towards other phytoplankton species. The combined effects of direct/indirect copepod exposure and low light conditions on the dinoflagellate Scrippsiella trochoidea life-history (e.g. temporary cyst formation) were examined. Induction of temporary cysts occurred in response to decreased light intensity, but not in response to copepods despite the fact that copepods showed decreased ingestion on temporary cysts. In low light situations, temporary cyst formation can be an effective tool to minimize population losses.The results presented here contribute to the complex understanding of factors influencing phytoplankton-zooplankton interactions.