Sea surface temperatures of the Leeuwin Current in the Capes region of Western Australia: potential effects on the marine biota of shallow reefs

Abstract

The Leeuwin and Capes Currents have been shown to influence marine assemblages along the Western Australian coast. In this study we examined potential relationships between the sea surface water temperature (SST), as a consequence of the Leeuwin and Capes Currents, and the distribution of fishes and algae. Data were collected from locations that spanned the temperate Capes coast (33°30' to 34°25'S). Fish assemblages were measured using diver operated stereo-video and stereo baited remote underwater video. Algae were harvested from quadrats. Mean SST at the most southerly region was 18.5°C while regions on the west coast of the Capes were generally one degree warmer.Seventy three species of fishes were recorded belonging to 36 families. Two species were classified as tropical and one species as sub-tropical (Cirrhilabrus temmincki, Plectorhinchus flavomaculatus and Choerodon rubescens respectively). Forty four percent of species from the Capes were classified as either tropical, subtropical or subtropical-temperate. The remainder were of temperate or widespread distributions. Two hundred and five species of algae were recorded belonging to 49 families. All species were regarded as temperate with the exception of the geniculate red alga, Rhodopeltis borealis. Eleven range extensions were recorded for algae: two were southward of the current known range (Champia compressa and Rhodopeltis borealis) and the remaining nine were northward or westward extensions.Within the 120 km of the Capes coast studied, regions with warmer waters did not have higherabundances of fishes of tropical, subtropical or subtropical-temperate origin than cooler waters. However, the most southerly region was different in terms of algal assemblage structure with water temperature the most influential of environmental variables, relative to exposure, substratum and depth.The large proportion of fish species with tropical, subtropical and particularly subtropicaltemperate distributions recorded is consistent with other studies and may be due to the influence of the Leeuwin Current. The range extensions for algal species may be due to the effects of the Leeuwin and Capes Currents but may also be due to the paucity of algal collections from some parts of the Capes region. Other factors such as topographic complexity, depth and other habitat structure variables may also be influencing marine assemblages. The findings of the work support the notion that there is a large transition zone between biogeographic provinces within which the Capes region is positioned.