Strategic Research Fund for the Marine Environment (SRFME) research projects have contributed significantly to our understanding of the marine environment of south-western Australia. Cold-core eddies tend to spin on the seaward side of the low-nutrient Leeuwin Current.
PHYSICAL OCEANOGRAPHY OF THE SOUTH WESTERN AUSTRALIAN SHELF
Introduction
The Leeuwin Current responds to the very large-scale dynamics of the Indian and Pacific oceans, and is strongest in the La Nina phase of the El Nino cycle. A known consequence of the Leeuwin Current is the high level of correlation between the Southern Oscillation Index (SOI, a measure of the El Nino cycle), Fremantle sea level and lobster larval recruitment on the WA coast (Pearce and Phillips, 1988).
The Leeuwin Current
Year-to-year changes in the Leeuwin Current affect the year-to-year changes in Fremantle sea level. Interannual changes in the eddy field of the Leeuwin Current are also derived from satellite altimetry data.
Seasonal chlorophyll a pattern and eddy-driven cross-shelf transport Off the southwest Western Australian coast, seasonal blooms of sea-surface chlorophyll a,
The reason for the increased productivity and high chlorophyll concentration of the warm-core eddy has been closely investigated using a coupled physical-biological numerical model (Greenwood et al. 2006). By specifying the appropriate density and nutrient properties during eddy formation, the model can successfully distinguish productivity in warm and cold eddies.
Decadal and climate-change signals on the WA coast
There is an extension of the warm season in the annual cycle, especially in southern latitudes in the Abrolhos and Rottnest. The mechanism for increased salinity in the coastal region off WA still needs further research.
Indian Ocean Dipole
Dominant EOF loadings for sea level anomaly (top panel), and temperature anomaly along the two XBT sections (bottom panel: right, IX1; left, IX12). The unit for sea level deviation load is cm and the contour interval is 2 cm.
7IND
The locations of the XBT sections are indicated by black dots in the upper panel (from Feng and Meyers, 2003).
NDIAN/CEAN334
Inshore Dynamics
The monthly mean current profiles in areas A and C (Fig. 2.26) show significant changes related to the relative influence of the nearshore wind and the offshore Leeuwin Current. Near-surface temperatures from transect stations (Fig. 2.28) and derived from satellite data (Fig. 2.29) show a similar annual cycle to mooring data reflecting the seasonal presence of water in the Leeuwin Current (offshore). and heat exchange with the atmosphere (especially at the coast).
Wave Propagation and Dissipation
Time series of the significant wave height for each two-hour measurement period, calculated from the observed pressure at each of the DSPs, is shown in Figure 2.38. An example of the model predicted significant wave height is shown in Figure 2.39 for the offshore forcing conditions: Hs=3.7m, T=15s and dir=71°.
Sediment dynamics
In this section, we investigate spatial, seasonal and inter-annual variation of sediment mobility on the shelf. From the circulation velocities at the bottom, the bottom friction was calculated as a function of the sediment grain size (Johnson 1966; Li and Amos 2001).
Summary and conclusions
Our sediment modeling suggests that, for the SW shelf region during winter, sand-sized sediments are mobile between 50 and 60% of the time. Large-scale environment of the poleward-flowing Leeuwin Current, Western Australia: longshore steric height gradients, wind stresses and geostrophic flow, Journal of Physical Oceanography.
COASTAL AND SHELF PELAGIC COMMUNITY STRUCTURE: PATTERN AND PROCESSES
- Introduction Investigator / Institution
- Temporal and Spatial Variability in Biophysical Oceanography Across the Continental Shelf and Slope
- Nearshore Sediment/Water Column Exchange Processes Investigator / Institution
The Leeuwin Current is highly seasonal, peaking in autumn and winter and diminishing in summer when it flows against the prevailing 'sea breeze' from the south, which drives counter currents flowing from the north to the LC over the shelf - Ningaloo Current to the north and Cape Current to the south. The overall objectives of SRFME were to describe seasonal cycles in water column physical structure, nutrient profiles and plankton communities and their dynamics along the Leeuwin Shelf and Current and to assess the mechanisms driving these patterns. Summary of Biophysical Oceanography surveys undertaken during the 3-year field program off Two Rocks, WA.
The findings from these student components are reported in Part I of the SRFME final report. The microbial remineralization of organic matter and the flux of released nutrients back to the water column can be an important part of the coastal nutrient budget. This study provides the first attempt to quantify nutrient resupply from sediments to the water column in WA and improve our understanding of the physical, chemical and biological processes that control the magnitude of these important nutrient processes.
Coast
Phytoplankton Community Structure Investigator / Institution
Phytoplankton community composition was assessed at five stations along the Two Rocks Transect (Fig. 3.1), with samples obtained monthly to quarterly (depending on station) over approximately 3 years (February 2002 to December 2004). Chlorophytes (indicated by chlorophyll b and lutein) also decreased with distance from the coast and were rarely present at stations D and E, usually only during the winter months (Fig. 3.14a). Zeaxanthin also showed a strong seasonal cycle with a maximum during the summer months, again at all locations except the most coastal station (A; Fig 3.14a).
Differences in the phytoplankton composition between the surface and the chlorophyll maximum depth were most notable at stations D and E (Fig. 3.14a,b). In contrast, the two most coastal stations (A, B) showed similar phytoplankton compositions in both surface and chlorophyll maximum depths (Fig. 3.14a,b), possibly because the water column was generally well mixed. In contrast, chlorophytes (indicated by chlorophyll b) increased in abundance at chlorophyll maximum depth, particularly at stations D and E (Fig. 3.14b).
Bio-optics and Remote Sensing Investigators / Institution
Much of the catchment, inland from Jurien Bay, has been cleared for agricultural purposes. The first station on the sampling transect lies within the harbor area, near the outlet of the Leschenault Inlet which drains the Colliery Four Basin catchment. The composition of only the diagnostic pigments for the outermost station of the Two Rocks transect is shown in Fig.
One of the measurements made during this project is the absorption coefficients of the various components of the water column. Absorption coefficients for CDOM (aCDOM), detrital (ad) and phytoplankton (aph) components are determined for each sample, indicating the contribution of the different components to the total absorption of the water column at the different stations and locations. At Bunbury, the innermost station (B1) is within the harbor area and close to the outlet of.
Mesozooplankton Investigators / Institution
We observed that offshore copepods were more abundant in winter than in summer, consistent with the seasonal pattern of phytoplankton abundance. Mesozooplankton diversity, based on the Shannon index, increased from coast to coast and was higher at all stations in winter compared to summer, except for lagoon station A (Table 3.5). Grazing rates were calculated using a modification of the Frost (1972) equations that allowed the productivity of phytoplankton, phytoplankton-grazing microzooplankton, and phytoplankton-grazing mesozooplankton and microzooplankton to be estimated from the same set of experiments (Koslow et al in prepare).
Mesozooplankton grazing on both total phytoplankton and on the microzooplankton was lowest at station A and increased with distance from the coast (Fig. 3.38). The egg production method is based on the fact that somatic growth of cephalopod ceases in mature females, and it is therefore assumed that the growth rate is equivalent to egg production. Estimates of secondary production using the egg production method were found to be lower than estimates based on the AARS method and the empirical model (Fig. 3.34).
Bioacoustics Investigator / Institution
Acoustic remote sensing allows scientists to map a cross-section of the ocean and view the interaction of marine life with the seafloor and oceanographic features (Sutor et al. 2005). Also, physical and biological samples are critical for interpreting acoustic signatures and training algorithms for multi-frequency remote sensing. This cross-section of the water column provides a window into subsurface marine life and its spatial relationships with oceanographic conditions and seafloor features.
The inset of a satellite image of the ocean temperature variation of the Two Rocks transect, with orange as high temperature and blue as low temperature, gives the sea surface context of the acoustic data. The visual vertical and horizontal resolution of acoustic backscatter (predicted from large biological scatterers) provides a unique spatial context that complements the interpretation of station data. Principal component analysis was used to identify acoustic echo patterns along the Two Rocks transect.
TAPS
COASTAL AND SHELF BIOGEOCHEMISTRY AND MODELLING
- Regional Nutrient Dynamics
- Phytoplankton Dynamics: Biomass and Production
- Phytoplankton dynamics: Investigating the Underlying Biophysical Mechanisms
- A regional 3-D hydrodynamic-coupled biogeochemical model
A conceptual model of the nutrient sources and sinks on the shelf is shown in Figure 4.1. This deepening of the mixed layer can mix nutrients (Fig. 4.4) up into the euphotic zone. Productivity in the surface waters of the southwest Australian shelf, Leeuwin Current and offshore region appears to be highly nitrogen limited.
The highest silicate concentrations (up to 4 µM) are located in the far north of our domain, the source waters of the Leeuwin Current (Fig. 4.12). As such, the exchange between the Leeuwin Current water and the shelf will most likely remove the nutrients from the shelf. We investigated the processes responsible for the seasonal productivity cycle at station E on the outer edge of the Leeuwin Current.
In the winter of 2003 (Fig. 4.22C), the relatively warm, low-salinity water of the Leeuwin Current is confined to the 'shelf break' (station D). Seasonal characteristics of the mixed surface layer in the Australasian region: implications for primary production regimes and biogeography.
BENTHIC ECOSYSTEM STRUCTURE
The distribution of invertebrates was in turn linked to the nature of the algal assemblages, with the number of species varying depending on the algal habitat type. As a result, the SRFME study sought to address this gap in current knowledge of the mechanisms that structure rocky reef communities. The approach of the SRFME study followed the only sound basis for future research, which is to gain a clear understanding of the distribution and abundance of key ecological components in the coastal ecosystem.
The geographical scope of the work included the areas between Green Head in the north and Cape Naturaliste in the south. At the largest scale (hundreds of kilometers), three main "regions" were selected, Perth in the center of the project area, Jurien about 200 km north and Geographe Bay about 200 km south of Perth (Figure 5.2). Water quality impacts in the three regions also include inputs from estuarine and river systems.
