Biophysical oceanography of tidally-extreme waters of the southern Kimberley coast, Western Australia

The continental shelf of the South Kimberley region of Western Australia is wide (~300 km) and interactions with the Indian Ocean culminate in extreme tides. This results in some of the largest tropical tides in the world (reaching 11 m on spring tide) measured in King Sound (KS), a large embayment which opens to the Indian Ocean. This remote area of Australia is very rich in natural resources and under increasing developmental pressures, but despite this the marine environment has been poorly studied. In Austral autumn (April/May) 2010, we examined phytoplankton biomass and production across four onshore-offshore transects from coastal waters (also within KS) to the 1000 m isobath, and their responses to physical and chemical environmental variables associated with tidal forces during a four week voyage aboard the RV Southern Surveyor. Using temperature and salinity relationships we found three distinct water masses; KS, Kimberley Shelf, and Kimberley Shelf break waters. The highest chlorophyll-a biomass in the study area was measured in a band along the 200 m isobath (27–46 mg Chl a m⁻²), and associated with greater concentrations of nutrients (332–392 mmol NO₃ ^- m⁻², 650–823 mmol Si m⁻², 21–39 mmol PO₄ ^3- m⁻²). Phytoplankton biomass inside KS was dominated by large diatoms (as determined from accessory pigment analyses), shifting to a community largely comprised of picophytoplankton offshore. Rates of primary production (PP) on the shelf, via measurements of phytoplankton carbon (¹⁴C) uptake from photosynthesis versus irradiance incubations, decreased from coastal areas (222–560 mg C m⁻² d⁻¹) to offshore waters (45–78 mg C m⁻² d⁻¹). In KS, PP was highest (1692 mg C m⁻² d⁻¹) despite low standing stock nutrient concentrations (0.02–1.5 mmol NO₃ ^- m⁻²) and relatively high concentrations of suspended particulate material reducing light availability (K_d = 0.27 m⁻¹).