Controls on phytoplankton production in the Australasian sector of the subtropical convergence

Abstract

This paper describes the interaction of physics, chemistry and biology in controlling phytoplankton biomass and production for a region along 152°E between 40°S and 50°30′S in summer 1990. The physical structure in the upper 500 m included a warm core eddy in the north, a cold core feature at 46°30′S, and two major fronts, the Subtropical Convergence and the Subantarctic Front, between 49°S and 50°30′S. In the region between the warm core eddy and the southern fronts, Subantarctic Mode Water was capped by a high salinity layer with a subsurface salinity maximum. Surface nitrate increased from zero in the north to more than 15 μM in the south in a series of steps across surface fronts associated with mesoscale features. Phytoplankton column biomass, estimated column production (based on P vs I incubations) and estimated mixed layer growth rates were moderate throughout the transect (typically 30 mg Chl. a m -2, 300 mgC m -2 d -1, and 0.27 d -1, respectively). Surface nitrate seems unlikely to be seasonally depleted south of 46°30′S, and this region shows characteristics similar to other high nutrient but low chlorophyll regions. Episodic wind mixing appears to play an important role in summer resupply of nitrate to the mixed layer in the central third of the transect. Over the southern half of the transect, changes in surface pH and derived pCO 2 as a function of nitrate concentrations were consistent with DIC uptake at Redfield ratios. The derived surface pCO 2 values showed undersaturation by 50–100 μatm throughout the transect./