Food chains structured by strong top-down influences can produce trophic cascades. One of the best known of these occurs in some mid-latitude lakes, where the food chains have been explored systematically with experiments. The basal link between the herbivores and plants comprises zooplankton and phytoplankton. The large zooplankton in these lakes filter phytoplankton from the water at a high rate; they are among the most voracious herbivores on Earth. In lakes with low densities of small planktivorous fish, the large species of zooplankton grow to high densities and greatly reduce the density of phy-toplankton. Such lakes have clear water by mid-summer. In lakes with enough planktivorous fish to suppress zooplankton densities, the effect of the zooplankton upon phytoplankton is cancelled and the lake becomes opaque green with phytoplankton by mid-summer. This is a cascade from the planktivorous fish to the phyto-plankton. In lakes with many large piscivorous fish, such as wall-eyed pike, the cascade extends to four links, the planktivorous fish population is suppressed, and the zooplankton is released from predation. The resulting intense herbivory suppresses the phytoplankton and yields a clear lake. Piscivores are sought after by sportsmen, and heavy fishing pressure has led to the undesirable consequence of dense algal blooms (Figure 2).
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