Oceanographic processes in the Perth Canyon and their impact on productivity

Abstract

Submarine canyons are important to continental shelf ecosystems. They have a strong influence on shelf circulation and the distribution of biota. The Perth Canyon is a long, deep canyon on the Western Australian coastline that has attracted attention as a feeding area for pygmy blue whales (Balaenoptera musculus brevicauda). Despite existing on a highly oligotrophic coast, the Perth Canyon has the ability to support sufficient krill to feed these massive mammals. The aim of this study was to examine the physical processes within the Perth Canyon, and consider how these could affect productivity. Research areas included the interaction of the Leeuwin Current and Leeuwin Undercurrent with the canyon, the circulation within the canyon, the effect of wind forcing and the occurrence of upwelling. The oceanography of the Western Australian coast including seasonal productivity changes was also examined. This study utilised numerical modelling and collection of field data to develop a thorough understanding of the Perth Canyon. The numerical model ROMS (Regional Ocean Modelling System) was used to simulate a long stretch of coastline in which the Perth Canyon was centrally located. The model forced the Leeuwin Current and Undercurrent using density gradients, and the seasonal Capes Current was then generated by applying a surface wind stress. The simulations showed that primarily the Leeuwin Undercurrent interacted with the canyon. Eddies continually formed within the canyon, which enhanced vertical transport and could contribute to entrapment of passive drifters. The addition of wind had no discernible effect on canyon circulation although vertical velocities increased everywhere and shallow upwelling occurred along the shelf. The field data comprised moored temperature loggers, field cruises, and sundry data from satellite imagery, weather stations and whale observations.The temperature loggers, located on the canyon rim, indicated the range of processes that affect the canyon region. These processes included seasonal changes in the wind, the seasonal changes and meanders of the Leeuwin Current, storms, the near-diurnal sea breeze and inertial period changes, and other internal waves. The temperature loggers also indicated sporadic upwelling at the canyon rims, although this upwelling rarely extended into the Leeuwin Current. The field cruises gathered CTD, ADCP, nutrients and acoustic backscatter data. The water masses near the canyon were identified from their temperature, salinity and oxygen signatures. The deep chlorophyll maximum exhibited high spatial variability around the canyon. The circulation, in conjunction with the simulated circulation from ROMS, reiterated that eddies filled the canyon below its rims, and suggested that passive drifters would aggregate within the head. The acoustic backscatter reinforced this, showing that biota concentrated near the head of the canyon, which is where the whales were most often sighted feeding. The conclusions of this study were that the canyon is a region of enhanced productivity where upwelling is enhanced and aggregation of passive drifters is likely. Upwelling occurs more strongly when the Leeuwin Current is weakened or has meandered out of a region. Seasonal changes in productivity in the canyon conform to seasonal productivity arguments for the entire coastline, which accounts for the seasonal aggregation of blue whales. The physical processes in the Perth Canyon are variable and all are important to this marine ecosystem.