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

This paper describes the interaction of physics, chemistry and biology in c ontrolling phytoplankton biomass and production for a region along 152 degr ees E between 40 degrees S and 50 degrees 30'S in summer 1990. The physical structure in the upper 500 m included a warm core eddy in the north, a col d core feature at 46 degrees 30'S, and two major fronts, the Subtropical Co nvergence and the Subantarctic Front, between 49 degrees S and 50 degrees 3 0'S. In the region between the warm core eddy and the southern fronts, Suba ntarctic Mode Water was capped by a high salinity layer with a subsurface s alinity maximum. Surface nitrate increased from zero in the north to more t han 15 mu M in the south in a series of steps across surface fronts associa ted 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), 3 00 mgC m(-2) d(-1), and 0.27 d(-1), respectively). Surface nitrate seems un likely to be seasonally depleted south of 46 degrees 30'S, and this region shows characteristics similar to other high nutrient but low chlorophyll re gions. Episodic wind mixing appears to play an important role in summer res upply of nitrate to the mixed layer in the central third of the transect. O ver the southern half of the transect, changes in surface pH and derived pC O(2) as a function of nitrate concentrations were consistent with DIC uptak e at Redfield ratios. The derived surface pCO(2) values showed undersaturat ion by 50-100 mu atm throughout the transect. (C) 1998 Elsevier Science Ltd . All rights reserved.