Enhanced zooplankton abundance in the lee of an isolated reef in the south Coral Sea: The role of flow disturbance

Abstract

The effect of flow on the distribution and abundance of zooplanktonic particles was investigated around an isolated, steep-sided reef, in the south coral Sea. North-flowing current >0.3 m s 1 caused 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 maximum were at 80-100 m in the free stream and 50-70 m in the flow-disturbed region. Over all depths combined, chlorophyll and nutrients were 1.4 times greater in the disturbed region. There was a strong correlation between the depth of the mixed layer, the depth of the chlorophyll maximum and the depth of the peak abundances of zooplankton between 300 and 1000 μm equivalent spherical diameter (esd; measured with an optical plankton counter). Slopes of the log-normalized abundance of 19 particle size classes between 300 and 2500 μm esd indicate that all particle size classes were more strongly represented in the flow-disturbed region than in the free stream, but that the difference was greater for small plankton. This indicated increased production by smaller zooplankton. Using size and taxonomic information did not differ between regions and was dominated by the calanoid copepods Pleuromamma and Acartia, and the cyclopoid copepod Oncaae. Multi-dimensional scaling showed that the particle size community was not significantly different within or between nights, but most flow-disturbed stations were significantly different from the free-stream stations, consistent with current flow. Nutrient uplift into the photic zone in an oligotrophic tropical ocean has a significant impact on zooplankton size structure, and ultimately fish production.