The depth distribution of Oligochaeta in an Irish quaking marsh

Abstract

The depth distribution of oligochaetes was studied in a quaking marsh at Tinure, Co. Louth, Ireland. The dominant plants were grasses which formed a fibrous mat floating on semi-liquid peat. The water level remained near the surface except for a few weeks in winter and early spring when up to 50% surface cover of water was seen. Depth profiles of pH and redox potential showed seasonal variations with the anaerobic layer approaching the surface in summer. Oligochaetes were the most important group of invertebrates and were represented by 7 species of Naididae, 7 Tubificidae, 2 Lumbriculidae, 13 Enchytraeidae and 1 Lumbricidae. Lumbricids and most species of Enchytraeidae were concentrated at the surface but the enchytraeids Marionina riparia, Cernosvitoviella spp. and Cognettia glandulosa were frequent in the anaerobic layer. Aquatic oligochaetes were most numerous below the surface. Tubificids penetrated further than other groups and reached a peak at 4–6 cm. The effects of lowering the water level were studied in pot cultures where water was maintained at 4 different levels. Enchytraeids always remained near the surface, while aquatic oligochaetes concentrated nearer the water level. It is concluded that enchytraeids are prevented from colonizing most freshwater habitats by their inability to tolerate extended periods of anoxia.