Nutrient distribution in the upper 300 m of the eastern banda sea and northern Arafura sea during and after the upwelling season, August 1984 and February 1985

Abstract During the Indonesian-Dutch Snellius-II Expedition monsoonal differences in nutrient profiles were studied around the Outer Banda Arc (R.V. ‘Tyro’) and the Aru Islands (R.V. ‘Samudera’). During the cruises in the southeast monsoon (August 1984) coastal upwelling on the Arafura Shelf was most conspicuous. Aru Basin water from a depth of 100 to 150 m intruded eastwards over the bottom of the shelf up to depths of 40 to 50 m, both north and south of Aru. High chlorophyll concentrations in the Arafura Sea were evidently not due to river runoff but to enrichment of the upper, less saline layer by vertical mixing with the nutrient-rich deeper water. In the eastern Banda Sea and in the Aru Basin, patchy nutrient enrichment of the mixed layer occurred by doming and entrainment. At stations northwest of the Aru Islands and southwest of the Kai Islands nutrient profiles were shifted upwards into the photic zone. A minimum estimate of the mean daily new production based on vertical NO 3 and PO 4 fluxes during August 1984 came to 0.5 g C·m −2 , compared to a net daily primary production of 1.9 g C·m −2 . In the northwest monsoon (February 1985), nutrient concentrations in the upper 100 m were much lower. The mixed layer was nitrate-depleted with very low NO 2 and NH 4 values. Nutrient concentrations at isothermal surfaces of 18, 15 and 12°C (depth range 150 to 300 m) did not markedly differ between the upwelling and the oligotrophic season. During both cruises especially H 4 SiO 4 concentrations were significantly higher in the Aru Basin than west of the Outer Banda Arc. Using silicate as tracer, it was calculated that the enrichment by Aru Basin water of some stations visited during the Indopac Expedition (1976) in the Molucca Sea also involves the 150 to 300 m water layer.