Seasonal variations in the Indian Ocean along 110°E. IV. Primary production

Abstract

Mean productivity (light saturated photosynthesis), Ps, for the meridian rose from 50 mg C/(hr m²) in August 1962 to a maximum of 62 in October, then fell to a minimum of 4 in January, whereafter it rose slowly to 25 in April-May, then sharply to 45 in late May, and remained at that level till August 1963. Mean Ps for the year was 37 mg C/(hr m²). The depth of the layer of photosynthetic organisms varied between 130 m in October and 60 m in January, with a mean of 85 m. Maximum Ps occurred at 25 m in 36% of the stations, at 0 m in 29 %, and at 50 m in 24%.

In January-February the whole meridian was occupied by waters of low productivity, approximately 4 mg C/(hr m²) from the centre of the south Indian Ocean. In April-May the Ps remained uniform along the meridian but rose to 24. At other times four latitudinal intervals along the meridian, with distinctive seasonal variations of productivity characteristics, were found. From 9 to 15°S., waters with high Ps (69 mg C/(hr m²)), and sharp stratification at 50 m, caused by equatorial upwelling, occurred from May to October. From 15 to 24 and 24 to 30°S., waters with high Ps (60 mg Cl(hr m²) and (100 m) deep layers of photosynthetic organisms were found during October-November and May-July respectively. From 30 to 32ºS., waters of high productivity (70 mg C/(hr m²)) and a deep layer (100 m) were found in the period July-August.

The daily rate of primary production, Pa, of the whole meridian varied from 0.13 g C/(day m²) in August to 0.08 from October to early May, rising sharply in late May to 0.18 and again in early August to 0.27. The depth of the euphotic layer varied between 76 m in October and 63 m in July-August, with a mean of 68 m.