Biogeochemical aspects of the New Zealand sector of the Southern Ocean

Abstract

The main focus of this study was an investigation of phytoplankton processes across the Southland Front off the South Island of New Zealand. The Southland Front is part of the Subtropical Convergence, a major circumglobal oceanic frontal system. A suite of measurements of temporal and spatial changes in hydrography, macro-nutrient levels, phytoplankton biomass, community structure (phytoplankton pigments) and primary production were undertaken between September 1996 and March 1999. Seasonal patterns of algal dominance and succession varied both between water masses and interannually. Despite lower macro-nutrient levels than in the water masses further offshore, neritic waters exhibited the greatest phytoplankton biomass (as expressed by chlorophyll a concentrations) in all seasons. It became apparent that in summer phytoplankton growth in coastal waters is limited by nitrate and/or phosphate and by silicate in offshore waters. Subtropical and subantarctic waters exhibited a pronounced subsurface chlorophyll a maximum in spring and summer. The reasons for the formation of this feature in subtropical and tropical waters are known, but the mechanisms responsible in subantarctic water are not clear. Using surface primary production and solar irradiance measurements, water column integrated areal production was calculated and extrapolated to annual primary production. Primary production, particularly during the spring phytoplankton bloom, was then converted into macro-nutrient uptake and compared to the measured macro-nutrient drawdown in spring. For a finer temporal resolution, measurements of changes in biomass (chlorophyll a) and macro-nutrients over the annual cycle were also carried out weekly at the local beach and revealed the timing, magnitude and duration of the spring bloom and the associated consumption of macro-nutrients. These results were compared to those of the coastal stations along the Southland Front transect and proved useful to help with estimating annual primary production along this transect. A third study area included transects across the Southern Ocean between New Zealand and the Ross Sea, with an opportunity to study oceanic boundaries other than the Subtropical Convergence and in particular, apply phytoplankton pigment analysis. It was demonstrated that levels of phytoplankton pigments and hence the phytoplankton species composition change significantly when major oceanic fronts are crossed, and led to the assignment of 'pigment signatures' to specific water masses.