Observations of vertical turbulent nitrate flux during summer in the Great Australian Bight

Abstract

To assist with developing an understanding of the nutrient supply dynamics in the Great Australian Bight (GAB) the vertical turbulent nitrate flux was calculated at stations along two cross-shelf transects using direct measurements of turbulence and nitrate concentrations. Coincident hydrographic profiling conducted at the onset of the summertime upwelling season revealed upwelling onto the shelf in the eastern GAB (eGAB) and downwelling at the shelf slope in the central GAB (cGAB). Under these conditions, examination of the strength of competing vertical temperature and salinity gradients using the Turner angle suggested a high potential for double diffusive convective mixing. However, corresponding estimates of the mixing efficiency derived from microstructure profiling indicated turbulence was the main mixing process driving vertical fluxes across the region. The average upward nitrate flux between the surface mixed layer depth(MLD) and the base of the euphotic layer (92 m depth) was O(10−6−10−5) mmol N m−2 s−1 in the cGAB, with the peak flux observed at the upper-slope station (~ 400 m total water depth). In the eGAB, the magnitude of flux estimates at the shelf-slope and offshore stations were similar to those measured in the cGAB. An enhanced nitrate flux O(10−4) mmol N m−2 s−1 was observed on the shelf in the eGAB (~ 100 m total water depth) as the result of enhanced turbulence and an increased vertical nitrate gradient. Application of Redfield stoichiometry suggests vertical turbulent nitrate fluxes play a significant role in supporting up to 50% of the primary production in the GAB. We hypothesize that mixing processes resulting from interactions between upwelled and downwelled water masses and new water masses formed on the shelf during summer play an important role in maintaining the bands of subsurface chlorophyll a maxima observed below the MLD at the nitricline during periods of strong stratification. The snapshot provided by this study, emphasizes the need to better understand the variability and influence of mixing processes on nutrient supply and productivity across the GAB marine ecosystem.