Regenerated primary production dominates in a periodically upwelling shelf ecosystem, northeast New Zealand

Abstract

We present the first field measured primary production values for an intermittently upwelling shelf ecosystem in the Hauraki Gulf, northeast New Zealand. Phytoplankton uptake rates of 13C and 15N were used to determine the relative importance of new (15NO3− ) versus regenerated nutrients (15NH4+ and 15N-urea) to productivity, and to test the hypothesis that the upwelling system supports high export of organic material, acting as a net sink for CO2. From early spring to early summer (1996–1997), deep NO3−-rich waters upwelled onto the shelf and into the inner gulf, producing a short-lived low-level predominantly NO3−-fuelled bloom (surface chlorophyll-a (chl-a) >1.0mgm−3) in early spring. From late spring onwards, mixed layer NO3− concentrations were depleted to often lower levels than NH4+ and urea (∼0.1mmolm−3), with mid-water chl-a maxima associated with the top of the nitracline fuelled predominantly by regenerated nutrients. By late summer, upwelling had ceased, with intrusions of low-nutrient subtropical oceanic surface water producing oligotrophic conditions. NH4+ and urea uptake nearly always exceeded NO3− uptake throughout the euphotic zone, even within the nitracline, with relative preference index (RPI) estimates giving RPIUrea>RPINH4+>RPINO3−, and low uptake f ratios (generally f<0.3), supporting the physiological preference of phytoplankton for reduced forms of nitrogen. Despite periods of upwelling producing relatively high levels of integrated NO3− (typically 25–150mmolNO3−m−2) across the shelf from early spring to early summer, the highest integrated total production (1266mgCm−2d−1) and specific uptake rates of C/chl-a were obtained in late summer. In the inner gulf and on the mid-shelf, these higher late summer production rates coincided with low integrated chl-a biomass and NO3− inventories. There was no correlation of chl-a with C uptake in either surface waters (r2=0.0115), or in the euphotic zone (r2=0.0232) for the compiled data set, raising potential issues of using chl-a data in this region for modelling primary production. Caveats and problems associated with using the new production paradigm, f ratios and e ratios to calculate carbon export from highly dynamic coastal systems are discussed. Contrary to our hypothesis of NO3−-fuelled production in this upwelling system through the spring-to-summer transition, we found relatively high primary production levels were supported by regenerated nutrients most likely derived from the breakdown of NO3− sourced in the nitracline. The resulting pattern of persistent low nutrient uptake f ratios (f<0.3), punctuated only by a brief episode of new production in early spring (f=0.53, outer shelf), resulted in new production accounting for 10–20% of total inorganic nitrogen production, producing low organic material export. In situ oxidation was the primary fate of carbon on the northeast New Zealand continental shelf, where 80–90% of the carbon fixed by phytoplankton was remineralised in the euphotic zone.