The summer characteristics of primary productivity in the Tasman and Coral seas

Abstract

An analysis of productivity data from 150 stations in the area, carried out in the period January-March from 1958 to 1962, showed that the area could be divided into five homogeneous regions, each with characteristic magnitudes, vertical profiles, and diurnal variations. A comparison of these regions with the hydrological structure showed a close parallel to a system of three external water masses which mix to form all the major characteristics of the area.

In the northern Coral Sea is a region of moderately productive waters (25 mgC/hr/m²) mainly of equatorial origin, with sharp vertical stratification at 50 m, and a high productivity index (3.7 mg C/ hr/mg chl.a) in the upper 50 m. The central part of the area is dominated by waters of the west central South Pacific Ocean with a low productivity (11 mg C/ hr/ m², no vertical stratification, and a low productivity index (1 -2). The southern Tasman Sea is dominated by waters of subantarctic origin with a very high productivity (49 mgC/ hr/ m²), a large subsurface maximum at 50 m, a sharp stratification below 50 m, and a high productivity index (3.9). Between these three regions are two others formed by mixing, and these have intermediate characteristics.

In all five regions there are subsurface maxima of chlorophyll a below the layers of maximum productivity, and these sink even deeper as the productivity decreases. This suggests that the phytoplankton sink and lose their ability to photosynthesize as the waters become poorer.

Diurnal variations of productivity in the five regions show an inverse relationship with latitude, the ratio of maximum to minimum productivity decreasing from 3.1 at 15º S. to 2.1 at 45º S, but this decrease is less than would be expected from other values published.