Ephemeral biological production in the oligotrophic North West Shelf, Western Australia

The North West Shelf (NWS) of Australia is an oligotrophic coastal system with generally low phytoplankton biomass (chl-a) and production caused by limited surface nutrient availability and persistent downwelling conditions.... [ view full abstract ]

The North West Shelf (NWS) of Australia is an oligotrophic coastal system with generally low phytoplankton biomass (chl-a) and production caused by limited surface nutrient availability and persistent downwelling conditions. Elevated biomass is observed seasonally during the annual Austral winter bloom (May – July), and also at depth with a characteristic deep cholorophyll maximum (DCM) around 40-70 m. However, recent observations of biological activity following ephemeral events such as tropical cyclones, wind-driven upwelling, and internal waves indicate the potential for enhanced biological production outside of the primary bloom period.

In order to better understand the processes controlling these sub-seasonal trends, high temporal and spatial resolution observations were used in conjunction with shipboard data. Biological and physical instrumentation deployed on Integrated Marine Observing System (IMOS) moorings monitored chlorophyll fluorescence, temperature, salinity, turbidity, and water current velocities. Combined SeaWiFS and MODIS remote sensing observations provided 15 years data for the determination of seasonal patterns and comparison between shelf and continental slope dynamics at different areas along the coast.

Mooring observations of chl-a obtained from the mixed layer and in the DCM showed evidence of stratification with incidence of surface ephemeral peaks during the spring and summer months. Some of the timings of these peaks matched the incidence of short duration upwelling events that were identified using coastal upwelling indices along with cross-shelf transport velocities. Winter chl-a were less variable but exhibited better agreement between surface and depth measurements, which coincided with the seasonal deepening of the mixed layer and the onset of the winter bloom. These observations indicated the importance of the vertical transport of nitrate from below the mixed layer in enhancing surface biological production. Regional distinctions in the depths and locations of the nitracline and DCM suggest the importance of physical features such as bathymetry, shelf width, and influence by internal waves and tidal processes in controlling sub-seasonal phytoplankton dynamics. These previously understudied ephemeral processes are important to better understanding annual biogeochemical cycles and ecological processes in the NWS.