MICROENVIRONMENT OF CHIRONOMID LARVAE IN THE LITTORAL AND PROFUNDAL ZONES OF LAKE MAARSSEVEEN-I, THE-NETHERLANDS

The relation between the vertical distribution of Chironomidae larvae and the oxygen conditions of their habitat was investigated in the littoral and profundal zone of mesotrophic Lake Maarsseveen I, The Netherlands. Oxygen microelectrodes were used to study seasonal variations in oxygen concentration profiles at the sediment-water interface and to determine the oxygen consumption of the various sediment types. The maximum oxygen penetration depth was 4 and 2 mm in the littoral and profundal zone, respectively. The larvae of Cladotanytarsus mancus (littoral) and Tanytarsus bathophilus (littoral and profundal) inhabit the top layer of the sediment and, therefore, live close to the overlying oxygen-rich water. The larvae of Polydilum gr. bicrenatum, Stictochironomus sticticus (both littoral) and Chrinomus anthracinus (profundal) occur deeper in the sediment and are, therefore, surrounded by anoxic sediment. At comparable temperatures, the silty profundal sediments of L. Maarsseveen I consume three times as much oxygen as the sediment in the sandy littoral zone. The surface of the burrow of S. sticticus, depending on the temperature, consumes approximately the same amount as, or considerably less oxygen than the larva living in it, but the burrows of the Ch. anthracinus larvae consume approximately three times as much oxygen as the larvae. Ch. anthracinus larvae, therefore, live in a less favourable oxygen micro-environment than S. sticticus larvae, as they have to compete more for the available oxygen with the surrounding sediment. In general, no differences were found between the specific haemoglobin content of the larvae of superficial and deeper living species; size (biomass) of the larvae seems to be a more important factor (large larvae burrowing deeper). The exceptionally high haemoglobin content of the small larvae of P. gr. bicrenatum may be an adaptation to a life in the anoxic layers of the sediment.