Resilience of fish assemblages in the Breede Estuary,South Africa,to environmental perturbations

Biogeographical transitional areas are often characterised by fluctuating environmental conditions and high variability in species composition and abundance. A 10-year summer sampling programme was undertaken in the permanently open Breede Estuary, situated between the winter and bimodal rainfall regions and in the cool/warm-temperate biogeographical transition zone. This allowed for the collection of data highlighting inter-annual changes in the fish community of a transition zone estuary and relating these changes to selected environmental variables (salinity, temperature and flow). Although the abundance of individual species varied on an inter-annual basis, the ranking of fish taxa in the Breede Estuary varied little over the 10-year period, indicating a fairly stable species composition. Multivariate analysis showed that the fish communities sampled in summer within each reach (upper, middle, lower) remained similar, regardless of year, and this was largely driven by salinity patterns which showed little inter-annual variation during the low-flow summer period. The Breede Estuary is, however, subject to regular winter and spring flooding, where up to 50% of the annual rainfall may occur over a few days. Although the magnitude of flooding prior to sampling had little effect on species abundance, changes in the frequency or return-time of events did; such that when two floods occurred in succession species abundance and richness declined.