River plumes, Coral Reefs and mixing in the Gulf of Papua and the northern Great Barrier Reef

Abstract

The mean annual freshwater discharge in the Gulf of Papua, principally from the Fly, Kikori and Purari rivers, is estimated to be 13000 m3 s−1. Water from these rivers forms a low salinity surface layer in the Gulf, where it has a residence time of about two months and flows generally eastward. A small fraction of this water intrudes through Bligh Entrance on the Great Barrier Reef continental shelf. Horizontal patchiness and mixing of these intruding waters with shelf waters are considerably enhanced by secondary circulation (wakes) around coral reefs. The wakes shapes, visible in enhanced LANDSAT imagery, are similar to those around cylinders and plates in laboratory experiments at low Reynolds numbers. Topographically enhanced mixing may explain why the cross-shelf gradients of temperature and of the structure of fish communities are smaller in the northern region than in the central region of the Great Barrier Reef.