Grazing by oligochaetes and snails on epiphytes

Abstract

SUMMARY. 1. The isotope 33 P was used to assess the effect of grazitig by oligochaetes (mainly Stylaria lacttstris L.) and the snail, Lymnaea peregra (Muller), on epiphytes within an Equisetum fluviatile L. stand. 2. Two 1 m 2 polystyrene enclosures were set up within the emergent macrophyte zone of the lake. At the start of the experiment 33 P‐solution was mixed with the water in both tanks. Algal and animal samples for 33 P‐ analysis were collected during the peak occurrence of epiphytes in June. 3. Phosphorus release rates from the animals through defaecation and excretion were measured in the laboratory. The grazing rate of oligochaetes was 2.2–4.1 mg P m −2 (of bottom) d −1 of which about two ‐ thirds was released and recycled. The oligochaete density ranged from 13,400 to 20,900 m −2 . The snails (25 ind. m −2 ) grazed 1.2‐1.5 mg P m −2 d −1 of whieh about a quarter was released through defaecation and excretion. 4. Daily consumption by the oligochaetes and snails corresponded to 22–45% of the average phosphorus uptake by the epiphytes.