A COMPARATIVE-STUDY OF COLONIZATION BY BENTHOS IN A LAKE AND ITS OUTFLOWING STREAM

1. The patterns of colonization of Littoral benthos onto hard substrat a on an exposed and a sheltered shore of Lake Purrumbete in Victoria, Australia, and the riffles of its outflow stream (Curdies River) were examined experimentally in winter and summer. The common taxa in the l ake (gastropods, amphipods, isopods, planarians, ostracods) also occur red in the stream, although they were not abundant. The stream fauna w as dominated by insects. 2. Defaunated half-bricks were sampled at eac h of three sites at the three different locations (exposed shore, shel tered shore, stream) in winter and summer at weekly or biweekly interv als, with natural stones also being sampled during the colonization pe riod. Colonization patterns of individual taxa which occurred in both the lake and stream, and stream-only taxa, were compared using ANOVA, and the changes in the assemblage through time and between locations w ere analysed with non-metric multidimensional scaling (NMDS) and analy sis of similarities (ANOSIM). 3. Colonization in the lake was very rap id, with species richness and assemblage composition on bricks after j ust one day matching that of natural stones. Colonization in the strea m was slower, the assemblage composition not matching natural stones a fter 42 days in winter but being comparable after 28 days in summer. T here was considerable species turnover during colonization in the stre am, but little turnover in the lake, with most common taxa back after one day. There was little difference between the two lake shores in co lonization patterns. 4. Taxa that occurred. in both lake and stream sh owed broadly similar patterns of colonization, with early occupancy at high densities. In contrast, stream insects showed a variety of colon ization strategies. 5. These results indicate that rates and patterns of colonization on to hard substrata are quite different in Lake Purru mbete compared with its outflowing stream. The rapid colonization in t his lake indicates great mobility for much of the fauna on hard substr ata, possibly by swimming, benthic crawling or passive drift (even att ached to floating vegetation). This may be an appropriate strategy in lake littoral zones where the amount of inhabitable hard substrata and accompanying food resources may be limited.