Summer planktonic copepod communities of Australia's North West Cape (Indian Ocean) during the 1997-99 El Nino/La Nina

Abstract

The community composition of pelagic copepods near Australia’s North West Cape (21° 49′S, 114° 14′E) was studied during the austral summers of 1997/98 and 1998/99. Most sampling occurred at a shallow (20 m) shelf station and a deeper (90 m) shelf-break station, though on four occasions a set of eight stations were occupied on a 36 km cross-shelf transect. During the El Nino conditions prevalent during the austral summer of 1997/98, episodic upwelling occurred causing intermittent high primary production. During the El Nino conditions of 1997/98, there was a little difference between stations in the spring (October-November), but communities differentiated later in the sampling season (December-February) with a more characteristic inshore community developing at the shelf station. In the La Nina conditions of 1998/99, the community at the shelf break was invariant, but the shelf community was mainly offshore copepods as a result of seasonal downwelling during the spring that was later replaced by an inshore community of more widely distributed species. Over 120 species of copepods were identified, of which the most speciose families were the Corycaeidae (22 spp.), Oncaeidae (>20 spp.), Paracalanidae (15 spp.) and the Oithonidae (11 spp.). Cross-shelf transects confirmed the existence of a distinct inshore community of copepods, dominated by small species of Paracalanidae and Oithonidae, and which was at least twice as abundant as those at the shelf break. In both summers, there was an onshore-offshore gradient in community composition, with the inshore stations characterized by small paracalanid and oithonid species.