Higgins, Harry W., Denis J. Mackey and F. Brian Griffiths

CSIRO Marine Research, Australia, GPO Box 1538 Hobart, Tasmania, Australia 7001, Tel: +61 3 6232 5222, Fax: +61 3 6232 5123, E-mail: Harry.Higgins@csiro.au,

Phytoplankton communities and primary production in the Western Equatorial Pacific and Bismarck Sea: Effects of the ENSO phase, macronutrients and suspended sediments

Phytoplankton community dynamics in the Western Equatorial Pacific (WEP) and Bismarck Sea were investigated as part of the Australian JGOFS and TROPICS programs. Phytoplankton community structure and productivity in the WEP, and the region of the Bismarck Sea remote from riverine discharge, are subject to the physical forcing of a deep thermocline and nutricline, and barrier layers produced by shallow haloclines, which limit upwelling. A Deep Chlorophyll Maximum (80-100m) develops at the base of the surface nitrogen-limited oligotrophic warm pool where macronutrients increase sharply at the nutricline, which also generally coincides with the depth of the permanent thermocline. However, in areas of the Bismarck Sea influenced by the discharge of northern New Guinea rivers, the physics and phytoplankton community structure are complex. Apart from diel variations, phytoplankton biomass and production in the WEP are relatively constant from day to day. Longer-term changes result from weak intermittent upwelling due to current shear or westerly wind bursts, or uplift of the thermocline and nutricline due to sustained physical forcing. Light attenuation in the Bismarck Sea is greater than in the WEP, due mainly to the sediment and solute discharge from the rivers of northern New Guinea.

However, algal pigment analysis revealed photoacclimation in the Bismarck Sea was similar to that in the clear stratified waters of the western equatorial Pacific. We suggest there are physiological limits to the adjustment of carotenoid : chlorophyll a ratios and further forcing would lead to changes in ecotype, species or algal class through photoadaption.