Diel habitat shifts of macrofauna in a fishless pond

Most studies of pond animals are undertaken during the daytime, despite evidence that many pond organisms demonstrate marked nocturnal changes in behaviour. Nocturnal studies of animals in fishless ponds are particularly rare, probably because diel changes in aquatic animal behaviour are often found to be a response to visual predation by fish. We used non-metric multidimensional scaling (MDS) and analysis of similarity (ANOSIM) to detect patterns of similarity in the community composition of macroinvertebrates and amphibians in samples taken from vegetated and unvegetated areas during the day and night in a fishless Vermont pond. We tested the hypotheses that (i) macrofaunal activity increased at night near the pond surface, and (ii) horizontal movement of the pond community was occurring on a diel cycle. At night, many taxa were more abundant in the surface waters and the community showed a general habitat expansion or shift from the littoral zone towards the edge and central waters. Our results challenge the assumptions that one would make about pond animal habitat use and interactions based solely on daytime studies. Even in fishless systems, where diel changes are unexpected, habitat use and behaviour might change at night and affect the strength and variety of species interactions.