A 5-year study of the influence of the northeast and southwest monsoons on copepod assemblages in the boundary coastal waters between the East China Sea and the Taiwan Strait

Abstract

Danshuei River in the northwestern tip of Taiwan discharges into the boundary coastal waters between the East China Sea and the Taiwan Strait. We conducted a 5-year study between October 1998 and September 2003 to assess the influence of the northeast (NE) and the southwest (SW) monsoons on copepod assemblages in the area. A total of 110 copepod species were identified. Temora turbinata, Paracalanus aculeatus, Acrocalanus gibber, Parvocalanus crassirostris and Oithona rigida comprised 80% of the copepod assemblages that consisted of coastal species from the East China Sea and species associated with the Kuroshio Branch Current. The effect of the NE monsoon was observed during a short period of each winter when species such as Calanus sinicus were transported into the study area by the China Coastal Current. In summer, species such as A. gibber, Acrocalanus gracilis and Canthocalanus pauper may be transported into the study area from Southern Taiwan by the combined effect of the SW monsoon and the Kuroshio Current. Influence of the NE monsoon on the copepod assemblages in terms of introduction of species from the Yellow Sea and the East China Sea during winter was observed every year but was unlikely to represent an important carbon source into the region due to its short-term nature. Local species such as T. turbinata, P. crassirostris and O. rigida form a major component of the copepod assemblage. The high copepod diversity in the area was caused by the year-round presence of many local species and the aggregation of different species from subtropical, tropical, and temperate water masses. Monsoon-driven water currents and the Kuroshio Branch Current appeared to mask the effect of river discharge in the region.