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 2 Australia (the "Hillarys Transect") has provided the first systematic inter-disciplinary climatology of the physical, chemical, optical and biological cycles across the shelf. This paper 4 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 6 biology and physics of the region.The oceanography is dominated by the seasonally-varying Leeuwin Current flowing 8 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 10 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 12 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 14 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) 16 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 18 the outer shelf (including the wake effect north of Rottnest Island).Nutrient and chlorophyll concentrations are low in comparison with other typical west 20 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 22 sampling may be required to resolve this.The effects of smaller-scale temperature and chlorophyll variability on satellite remote 24 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 26 3 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 28 validation measurements are also undertaken.

The effect of the Leeuwin Current on phytoplankton biomass and production off Southwestern Australia

“…The late autumn bloom initially appears anomalous because it is not associated with decreased near-surface temperatures: in fact the shelf was flooded with warm Leeuwin Current water in late April 2003 during the bloom (Figure 11). Nor is it associated with enhanced winds: mean wind speed decreases in autumn relative to summer off southwestern Western Australia, due to the decline in the summer monsoonal winds [Pearce et al, 2006]. Neither is it possible that the autumn bloom is associated with runoff, which is negligible off central and southwestern Western Australia: the region's few, small rivers would not influence the oceanography over the continental shelf in autumn beyond the coastal ''lagoon.…”

“…The transect position was selected to be typical of the open coast off southwestern Western Australia and to be beyond the influence of coastal outfalls, which enhance the local nutrient levels near Perth and of the Perth canyon and Rottnest island, which affect the local circulation [Alaee et al, 1998;Pearce et al, 2006].…”

Long-term changes in temperate Australian coastal waters: implications for phytoplankton

“…), these winds result in more Capes Current water being transported substantially further north. This nutrient supply mechanism may explain some of the cross-shelf variability observed in phytoplankton biomass in the southwest region (Pearce et al 2006). …”

Spawning, larval abundance and growth rate of Sardinops sagax off southwestern Australia: influence of an anomalous eastern boundary current

“…Sea surface temperatures along the Two Rocks transect were extracted from brightness temperatures in AVHRR bands 4 and 5 from satellite images obtained by the Western Aus- tralian Satellite Technology and Applications Consortium (WASTAC) (see Pearce et al 2006). Monthly mean temperatures for the broader study area were obtained from Giovanni (Table 1).…”

“…Sardine eggs < 24 h old were also counted in monthly plankton samples (1996 to 1998) (Table 1) along the Hillarys transect (see Pearce et al 2006) (Fig. 1).…”