Vulnerability of Asterionella formosa to Daphnia grazing: impact of a fungal parasite

摘要

Phytoplankton cells are exposed to zooplankton grazing. Particle size (cell or colony size) of phytoplankton species is an important attribute that determines vulnerability to zooplankton grazing since the range of food particle size of zooplankton like Daphnia is limited (Geller Muller 1981, Sterner 1989). The freshwater pennate diatom Asterionella formosa Hass. forms multicellular colonies that change in size (both in diameter and number of cells per colony) with the seasons (Lund et al. 1963, Bellinger 1977, Hayakawa et al. 1994). Large colonies of A. formosa (8 cells colony~(-1)) are known to be less vulnerable to Daphnia grazing (Lehman Sandgren 1985, Knisely Geller 1986, Kagami et al. 2004); therefore, the vulnerability of A. formosa to grazers may also vary during the seasons. Changes in colony size are triggered by nutrient limitation and temperature (Tilman et al. 1976, Hayakawa et al. 1994), factors that regulate colony size by influencing metabolic processes associated with connection between cells that make up the colony such as the excretion of mucilage pads (Hayakawa et al. 1994).