New and remineralised nutrient supply and ecosystem metabolism on the northeastern New Zealand continental shelf

Abstract

The nutrient dynamics of the NE New Zealand shelf and adjacent Hauraki Gulf are described, with the aim of relating the along- and cross-shelf physical circulation to co-occurring supplies of new and remineralised nutrients, and ecosystem metabolism. Chemical and biological properties were studied over four research voyages in early spring to late summer in 1996–1997, coincident with the physical oceanography (Zeldis et al. this issue). Upwelling prevailed in early spring to early summer, but between early and late summer upwelling ceased and there was a marked transition toward downwelling and strongly stratified conditions on the shelf. The upwellings caused extensive loading of the inner-shelf water column and Hauraki Gulf bottom waters with nitrate. However, significant increases in phytoplankton biomass were not observed, probably because of dilution of shelf phytoplankton stocks by upwelling, and mismatch of voyages with delayed bloom development. Between spring and late summer, the shelf and Hauraki Gulf evolved strongly from net autotrophic toward net heterotrophic ecosystem metabolism, as indicated by decreasing ratios of new-to-regenerated nitrogen (N) stocks and of chlorophyll- a to phaeopigment, and increases in apparent oxygen utilisation (AOU). The effect of this transition on supply rates of new and remineralised N and silica in shelf waters was examined using a three end-member mixing model. Nitrate was the dominant N species remineralised near-bed, whereas ammonium remineralisation dominated shallower in the water column. Silica trapping and denitrification on the inner shelf were evident from stoichiometries of remineralisation. Vertical and along-shelf patterns in remineralisation were found, apparently related to benthic remineralisation, current speed and sediment type. Remineralised nutrients and AOU were concentrated by strong stratification in bottom waters in late summer, but on a water-column integrated basis, no seasonal change in remineralisation rates was evident. Nevertheless, with the large decrease in advected (i.e. upwelled) nutrients in late summer, the maintenance of significant remineralised nutrient supply appeared to be important for sustaining the chlorophyll- a stocks in late summer over the shelf.