SEASONAL DYNAMICS AND BURROWING OF LITTORAL CHIRONOMID LARVAE IN RELATION TO COMPETITION AND PREDATION

The seasonal dynamics and burrowing in the sediment of the four larval instars of the chironomid Stictochironomus sticticus were followed at two sampling sites in the sandy littoral zone of an oligo-mesotrophic lake. This information was related to the densities and burrowing pat tern of other chironomid taxa, and the occurrence of predation by brea m. A clear relation between Stictochironomus larval instar and burrowi ng depth was found, with later instars burrowing deeper. In late May, synchronous settlement of the first generation took place; a less sync hronous second generation settled in August. The timing of the develop ment of Stictochironomus differed among the two sampling sites because the transition from the first to the second instar occurred two weeks later at one of the sites, where the settling density was higher. Thi s timing difference, most likely caused by intraspecific density effec ts, persisted throughout the sampling period, even in the second gener ation. Bream foraged in the sediment from the middle of July. At one s ite, the Stictochironomus larvae were in the fourth instar then, burro wing deeper than 2 cm; at that depth they are out of reach of the brea m. At the other site, many larvae were still found in surface sediment layers due to the delayed development. Consequently, predation losses were high there; although peak densities of the earlier instars were at least twice as high, fourth-instar larval densities were almost the same at the two sites. Stictochironomus was found to be temporally an d spatially segregated from the principle other chironomid taxa, Clado tanytarsus mancus and Polypedilum sp.; these did not reach high densit ies before larvae of the first Stictochironomus generation had almost completely disappeared from the top 1 cm of the sediment. Cladotanytar sus became very abundant from July, when bream began to feed in the se diment; bream is not able to feed on this species due to its small siz e and protecting sand-tube.