Contribution of bioturbation by the red swamp crayfish Procambarus clarkii to the recruitment of bloom-forming cyanobacteria from sediment

The development of cyanobacterial blooms in a small eutrophic pond was monitored along with the potential effect of bioturbation by the red swamp crayfish Procambarus clarkii discussed as well with respect to the recruitment of cyanobacteria from sediment. Cyanobacterial blooms were observed during the early spring and summer. The spring bloom was dominated by Aphanizomenon flos-aquae. Its population density reached the maximum level in late March, thereafter decreasing rapidly and becoming lower than the detection limit from May. When the water temperature exceeded 20 °C in late May, the population density of Microcystis spp. began to increase, and a bloom was formed from July to early August. Anabaena spp. also contributed to the formation of the summer bloom. The population densities of both Microcystis spp. and Anabaena spp. began to decline in mid August. Crayfish were sampled using baited traps from April to November. No sample was obtained in April, whereas crayfish were captured constantly from May to November. They were distributed widely throughout the pond from June, although a large number of crayfish were captured most effectively at a particular point in the pond. The first captures in late May were dominated by males. The sex ratio of the captures was almost 1:1 from June to September, and fell in favor of females from October. The sex ratio reached a minimum (0.2:1) in mid November, when an extremely large number of crayfish were captured at a distinctly warm point. Next, the potential ability of crayfish to promote the recruitment of cyanobacteria from the sediment was examined by performing an incubation experiment. The presence of crayfish in containers of the pond sediment increased the densities of cyanobacteria such as Microcystis spp. and Anabaena spp. However, population densities of cyanobacteria began to decline after the crayfish was removed. Overall, bioturbation by crayfish seemed to be somewhat important in the dynamics of cyanobacterial blooms in the study pond; in addition, their effect varied with sex and season. Males may play an active role in the initiation of the bloom in late spring. Conversely, females may contribute to the extension of bloom in late autumn. Both males and females contribute equally to the maintenance of the bloom from summer to autumn.