ENHANCED ZOOPLANKTON ABUNDANCE IN THE LEE OF AN ISOLATED REEF IN THE SOUTH CORAL SEA - THE ROLE OF FLOW DISTURBANCE

The effect of flow disturbance on the distribution and abundance of zo oplanktonic particles was investigated around an isolated, steep-sided reef, in the south Coral Sea. North-flowing current >0.3 m s(-1) caus ed doming of isotherms by 20-30 m in the flow-disturbed region at the north-western side of the island. The nutricline and the chlorophyll m aximum were at 80-100 m in the free stream and 50-70 m in the flow-dis turbed region. Over all depths combined, chlorophyll and nutrients wer e 1.4 times greater in the disturbed region. There was a strong correl ation between the depth of the mixed layer, the depth of the chlorophy ll maximum and the depth of the peak abundances of zooplankton between 300 and 1000 mu m equivalent spherical diameter (esd; measured with a n optical plankton counter). Slopes of the log-normalized abundance of 19 particle size classes between 300 and 2500 mu m esd indicated that all particle size classes were more strongly represented in the flow- disturbed region than in the free stream, but that the difference was relatively greater for small plankton. This indicates increased produc tion by smaller zooplankton. Using size and taxonomic information from image analysis of net-collected samples, the relative composition of zooplankton did not differ between regions and was dominated by the ca lanoid copepods Pleuromamma and Acartia, and the cyclopoid copepod Onc aae. Multi-dimensional scaling showed that the particle size community was not significantly different within or between nights, but most fl ow-disturbed stations were significantly different from the free-strea m stations, consistent with current how. Nutrient uplift into the phot ic zone in an oligotrophic tropical ocean has a significant impact on zooplankton size structure, and ultimately fish production.