Distribution and diel migration of macroinvertebrates within dense submerged vegetation

1. We studied vertical and horizontal distribution of macroinvertebrates wi thin a dense stand of Chara spp. in Lake Krankesjon, southern Sweden. Inver tebrates were sampled at three depths within the vegetation and at three di stances from the vegetation edge during day and night in July and August. C orresponding samples of oxygen content of the water were taken. 2. The densities (number of invertebrates per unit plant biomass) of most i nvertebrate taxa were generally lower in the upper layers of the vegetation than in the layers close to the sediment. The densities of several taxa. ( Asellus aquaticus, Cloeon sp. and Polycentropodidae), as well as total dens ity of invertebrates, were higher at the edge than in the innermost parts o f the macrophyte stand, whereas snail densities generally were highest at t he innermost sites. 3. Densities of A. aquaticus, Chironomidae larvae and Helobdella stagnalis generally increased at night. These taxa. appear to undertake a diel vertic al migration within the vegetation, towards, or even down to the sediment i n daytime and up into the vegetation, in some cases to the vegetation surfa ce, at night. Factors underlying the diel vertical migration are discussed, as are their ecological consequences.