Seasonal and interannual variability of water column properties along the Rottnest continental shelf, south-west Australia

Abstract

Abstract. A multiyear, ocean glider dataset, obtained along a representative cross-shelf transect along the Rottnest continental shelf, south-west Australia, was used to characterise the seasonal and inter-annual variability of water column properties (temperature, salinity, and chlorophyll fluorescence distribution). All three variables showed distinct seasonal and inter-annual variations. Local and basin-scale ocean–atmosphere processes also affected the spatial distributions of the water column properties. The controlling influences for the variability were derived from (a) at the local scale, the Leeuwin Current and dense shelf water cascades (DSWC); and, (2) at the basin scale, the El Niño Southern Oscillation (ENSO). In spring and summer, shallow waters were well mixed due to strong wind mixing and the deeper waters (> 50 m) were vertically stratified in temperature that contributed to the formation of a subsurface chlorophyll maximum (SCM). With the onset of storms in late autumn, the water column was well mixed with the SCM absent. On the inner shelf, chlorophyll fluorescence concentrations were highest in autumn and winter; DSWCs were also the main physical feature during autumn and winter. Chlorophyll fluorescence concentration was higher closer to the sea bed than at the surface in spring, summer, and autumn. The seasonal patterns coincided with changes in the wind field (weaker winds in autumn) and air–sea fluxes (winter cooling and summer evaporation). Inter-annual variation was associated with ENSO events. Lower temperatures, higher salinity, and higher chlorophyll fluorescence (> 1 mg m−3) were associated with the El Niño event in 2010. During the strong La Niña event in 2011, temperatures increased (a marine heat wave), and salinity and chlorophyll fluorescence decreased (

Occurrence and seasonal variability of Dense Shelf Water Cascades along Australian continental shelves

“…The DSWC was also associated with lower dissolved oxygen values (saturation levels ~ 75%) in the bottom layer. This was due to higher productivity of nutrient supply from the sediments 36,37 and strong vertical stratification, due to the presence of DSWC inhibiting vertical mixing (Fig. 8d,h).…”

“…contained a WETLabs BBFL2SLO 3 parameter optical sensor that measured chlorophyll fluorescence (a proxy for phytoplankton biomass), coloured dissolved organic matter (CDOM), and volume backscatter at 660 nm (a proxy for suspended material) and an Aanderaa dissolved oxygen (DO) Optode that collected data simultaneously with the temperature and salinity profiles 35,36 . Examples of these data streams from Two Rocks and Pilbara (Fig.…”

“…The study region is strongly oligotrophic as there are no major terrestrial sources or upwelling for the supply of nutrients and therefore, water column nutrient concentrations are very low. There are no major river inputs resulting in very clear waters with photosynthesis occurring in depths up to 150 m 36 . In contrast to other regions there is no spring phytoplankton bloom; the seasonal chlorophyll concentrations show maxima during winter due to remobilisation of nutrients from sediments 37 .…”

“…In terms of suspended material, at Two Rocks the backscatter signal extended to water depth of 150 m (Fig. 8c) subsequent to resuspension www.nature.com/scientificreports www.nature.com/scientificreports/ due to storm activity 36 . At Pilbara, DSWC contained elevated backscatter values across the shelf (Fig.…”