Grazing resistance in nutrient-stressed phytoplankton.

Grazing experiments were performed with the zooplankters Daphnia pulex and Daphnia magna, feeding on phosphorus-saturated and phosphorus-limited cells of two green algae (Scenedesmus subspicatus and Selenastrum capricornutum). P-limited algal cells passed largely intact through the gut and were thus spared from heavy grazing pressure. P-saturated algal cells, in contrast, were efficiently assimilated. Structural and morphological changes in the P-limited cells most probably reduced their digestibility. This phenomenon may be an important factor in zooplankton production and competition, and may serve as an example of a highly efficient strategy of P-limited algae to resist heavy grazing pressure.