The influence of upwelling and water mass on the ecological group distribution of zooplankton in Zhejiang coastal waters

Abstract

The Zhejiang region experiences coastal upwelling characterized by low temperatures and high salinities throughout the year but particularly during the summer months. The upwelling in this area brings nutrients from the bottom to the surface, providing nutrients for large numbers of breeding plankton. The large quantities of zooplankton are a major food source for many fish and consequently the offshore waters of Zhejiang are productive fishing grounds. Another effect of the upwelling is that temperate warm water species that normally occur in deeper waters and offshore species are taken along with the current resulting in shifts in local zooplankton species composition and changes to the ecological structure and function of this group. This research was based on data from three oceanographic surveys in the offshore waters of the Yushan fishery during April, July and November of 2010. Using quantitative and qualitative methods, the primary aim of this paper was to examine the influence of upwelling on the ecological distribution of zooplankton in this area. A total of 64 species mainly dominated by copepods were identified which were classified into five major ecological groups including coastal temperate warm water species, offshore temperate warm water species, coastal subtropical water species, offshore subtropical water species and oceanic tropical water species. Coastal temperate warm water species were prevalent in spring, coastal subtropical water species and offshore subtropical water species in summer and subtropical species in autumn. The higher abundance of temperate warm water species such as Calanus sinicus, Paracalanus parvus and Sagitta nagae in summer relative to spring and autumn was primarily caused by the upwelling that transported these species from the bottom to the surface. This phenomenon is very different from the hydrodynamic processes that occur in the East and Yellow China Seas. Additionally, coastal species were the dominant zooplankton group, which means that coastal currents have the greatest influence on species composition in these waters. According to the research of Hu Dun-Xin, the main power of the Zhejiang coastal upwelling comes from the upward movement of subsurface water from the Kuroshio current as it follows the continental shelf. The subsurface water of the Kuroshio current runs southeast to northwest and shifts to the east forming an upwelling when it is blocked by the Chen Island Archipelago. With low temperatures and high salinities, the subsurface water of the Kuroshio current carries temperate warm water species and offshore species. As a result, during periods of coastal upwelling temperate warm water species are carried to the surface from the bottom, producing the unusual phenomenon were the number of temperate warm water species in summer is higher than in spring and autumn, contrary to the situation in other areas that have similar climatic conditions. In terms of abundance, coastal temperate warm water species accounted for 98.79% of the zooplankton in April and also a large proportion in July, whereas coastal subtropical water species were the dominant zooplankton group in October. Upwelling leading to the appearance of large numbers of temperate warm water species is a key factor affecting the ecological composition of zooplankton in these waters. The dominance of coastal species that are influenced by Jiao-Jiang runoff was a prime ecological characteristic of zooplankton in this region.