Zooplankton abundance in 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, zooplankton sampling was done in the area of the deep basins around the Outer Banda Arc and in the shallow waters (50 m) in the northernmost part of the Arafura Shelf. Cruises were in the southeast monsoon (August 1984) and in the northwest monsoon (February 1985), both by R.V. ‘Tyro’ and R.V. ‘Samuder’. Surveys involved in total 70 stations, with hauls from a depth of ca 150 m or, on the shelf, from near-bottom to the surface. The area of the Aru Basin was sampled by both ships and mean catches (mesh size 200 versus 300 μm) did not differ significantly. Variation in catch size due to patchiness and to diel vertical migration was large, up to 200% in the 0–150 m layer. All catches were measured on displacement volume and analysed on zooplankton groups. Part of the samples were freeze-dried and carbon/ nitrogen contents determined. For August 1984, in the upwelling season, pooled data gave a mean displacement volume of 25 cm 3·m −2 (1.1 gC·m −2) around the arc and of 32 cm 3·m −2 on the shelf. During February 1985, in the more oligotrophic season, values had dropped to 9 cm 3·m −2 (0.4 g C·m −2) and 6 cm 3·m −2. Mean zooplankton density in the 0–50 m layer during August was significantly lower in the basins than on the shelf (0.41 versus 0.94 cm 3·m −3) but during February about equal (0.16 versus 0.19 cm 3·m −3). All major zooplankton groups decreased in abundance by a factor of 2 to 4 from August to February, numbers of Luciferidae even by a factor of more than 10. In both seasons, copepods numerically predominated, constituting on average 80% of the catch in the basins and 64% on the shelf. In the 0–150 m layer during August, the copepods Calanoides philippinensis and Rhincalanus nasutus reached numbers of ca 4000·m −2 west of the Aru Basin/Arafura Shelf, i.e. downstream of the main upwelling area with surface chlorophyll concentrations of 3 to 4 mg·m −3. On the shelf and in February these species were virtually absent in the layer sampled. Guesses on secondary production in the eastern Banda Sea and Aru Basin come to an annual value of ca 24 g C·m −2, about 5% of the primary production in this area.