Littoral Pelagic Coupling Horizontal Migration of Zooplankton

The spatial distribution of predators, such as fish, can have important consequences for the spatial distribution of their preys, which can modify the expected outcome of direct predator-prey interactions. The phenomenon is described as 'behavioral cascade' in opposition to the classic 'trophic cascade' concept that implies direct effects of fish on the abundance of lower trophic levels (e.g., phytoplankton) due to predation on the intermediate levels (e.g., zooplankton and macroinvertebrates). In the temperate region, submerged plants help stabilize trophic interactions between fish predators and cladoceran and macroin-vertebrate preys by offering a physical refuge. In deep lakes vulnerable zooplankton migrate to the bottom layers during the day, thus minimizing the predation risk by fish, and migrate upwards during the night to graze with lower chances of being eaten ('diel vertical migration'). Shallow lakes typically lack a hypolim-netic refuge that would favor vertical migration. Instead, the perception of predation risk through chemical cues from potential predators (fish and pelagic macroinvertebrates, such as the phantom midge Chaoborus) may lead the large-bodied zooplankton to perform 'diel horizontal migration' (DHM) into the littoral zone, particularly into the submerged and floating-leaved plants. These zooplank-ters move back to the pelagic zone at night to graze on phytoplankton when the predation risk decreases. Also, emergent plants may occasionally be important for the migration pattern of vulnerable zooplankton. In temperate lakes, submerged plants would offer little refuge for zooplankton at both extremes of the nutrient (and turbidity) gradient. In low-nutrient lakes, clear water and the low density of plants enhance fish predation on zooplankton, whereas the refuge effect is weak under hypertrophic conditions because of the scarcity of submerged plants and often high densities of benthi-zooplanktivorous fish. Accordingly, macro-phyte refuges seem more efficient under eutrophic clear-water conditions and in small and dense plant beds, zooplankton being more concentrated in the edges of the plant patches during the day.

The consequent plant-mediated maintenance of a herbivorous zooplankton population that can effectively graze on algae has consequences for the pelagic habitat and even ecosystem impacts in the temperate region, such as the enhancement of water transparency.

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