The longitudinal distribution of riverine benthos: A river dis-continuum?

We used several techniques to analyze 10-11 collections from each of 13 stations along a mountain stream gradient to examine the hypothesis that there was a statistically significant (p < 0.05) uniform downstream gradient in benthic distribution. Random skewer techniques suggested that there was a significant downstream gradient of individuals per species, but only a weak gradient in species per station, and no gradient in functional guild composition. Ordination and clustering of either taxonomic or functional guild data suggested the existence of four longitudinally-defined groups of stations. Rarefaction curves also implied longitudinal groupings, with differences among groups implying species replacement than changes in organism numbers. Two species replacement indexes confirmed that suggestion. In summary, the distribution of the benthos in the creek may best be characterized as a punctuated gradient where gradual downstream changes in community composition are punctuated by sudden shifts to new community types. This punctuated gradient appears caused by changes in stream physical and chemical characteristics; the rapid nature of the changes hampers the interpretation of longitudinal analyses. Techniques such as random skewers and cluster analysis of rarefied samples will prove helpful in discerning pattern in stream benthos.