Effects of bioturbation on recruitment of algal cells from the seed bank of lake sediments

Effects of different bioturbators on recruitment of several nuisance algae, Anaboena spp. (Cyanophyta), Microcystis spp. (Cyanophyta), and Gonvostomum semen (Raphidophyta), from sediment to water were studied in a long-term laboratory experiment. Natural sediment, where macrofauna larger than 1 mm had been removed, was added to 18 aquaria. To each of six aquaria, individuals of Asellus aquaticus (Isopoda) or Chironomus plumosus (Arthropoda) larvae were added, and six aquaria were left as bioturbation-free controls. Recruitment of Anabaena, Microcystis, and G. semen from the sediment was detected using inverted traps that were sampled once a week during 8 weeks. The activities of the isopod A. aquaticus increased recruitment rates of all algal groups investigated, whereas chironomids had a less pronounced effect. With respect to Anabaena, increased recruitment rate was expressed as a promotion of growth in the pelagic habitat. To our knowledge, these results are the first to demonstrate that bioturbating invertebrates affect the recruitment of phytoplankton resting stages. Moreover, our results suggest that recruitment rate might be more pronounced in littoral areas, which are often dominated by A. aquaticus, rather than in profundal areas of a lake, generally dominated by chironomids. Hence, with respect to algal dynamics, the strength of the coupling between the benthic and pelagic zones might vary both spatially and temporally, depending on composition of the benthic invertebrate community and the ontogenetic development of the individuals within it.