The Hillarys Transect (1): Seasonal and cross-shelf variability of physical and chemical water properties off Perth, Western Australia, 1996-98

Abstract

A 27-month study of the water properties across the continental shelf off Perth, Western Australia (the "Hillarys Transect"), has provided the first systematic interdisciplinary climatology of the physical, chemical, optical and biological cycles across the shelf. This paper describes the main features of the seasonal and cross-shelf variability of the physical oceanography and chemistry, while companion papers discuss some of the links between the biology and physics of the region. The oceanography is dominated by the seasonally varying Leeuwin Current flowing southwards along the shelf break and outer shelf, and the northwards inshore Capes Current which is driven by the net southerly wind stress between about October and March (the austral summer). As a result of the poleward boundary current, there is no large-scale upwelling comparable with the Humboldt and Benguela Current systems. Water temperature and salinity in the shallow coastal waters are largely influenced by air-sea heat flux processes, while advection plays a more important role along the outer shelf; as a consequence, seasonal variations in the inshore temperature and salinity are much larger than those offshore. Cross-shelf exchange of water and plankton is effected by (1) large-scale meandering of the Leeuwin Current, (2) horizontal mixing as tongues of Leeuwin Current water penetrate across the shelf, (3) cascading of high-density coastal water offshore along the seabed and (4) sporadic summer upwelling onto the outer shelf (including the wake effect north of Rottnest Island). Nutrient and chlorophyll concentrations are low in comparison with other typical west coast situations. While there is some indication of a seasonal cycle, the relatively short sampling period and high patchiness have precluded definitive patterns being described and longer-term sampling may be required to resolve this. The effects of smaller-scale temperature and chlorophyll variability on satellite remote sensing measurements (both "within pixel" and "between pixel") in these coastal waters have been quantified using the underway (horizontal) and profile (vertical) data from the surveys. The project has demonstrated the great potential of using remote sensing information for regular monitoring of the Western Australian continental shelf waters provided that adequate in situ validation measurements are also undertaken. Crown Copyright © 2006.