Ecological Characteristics Of Planktonic Ostracods In The Nansha Islands Waters In Spring And Autumn

The south-subdivision region surrounding the Nansha Islands is located in the tropics with characteristic geography, landscape, climate and ecological environment very different from other parts of the South China Sea. Numerous islands are scattered in this area and lies close to continental slop and deep sea basin. Surface oceanic circulation driven by stable and strong monsoon in this region is highly complex, creating number of diversity hotspots in the marine ecosystem. Planktonic ostracods, the sensitive indicators in the marine environment, are important component of marine zooplankton, ranking second only to the copepods in waters surrounding the Nansha Islands. The study of their biology and ecology can provide valuable information on fish baits and its sustainable utilization, and the basic environmental conservation and ecosystem health. However, despite of a few large-scale ecological investigations carried out in the northern and middle parts in South China Sea, little efforts were reported from the Nansha Islands waters, especially those focused on the planktonic ostracods.This study aims to analyze and compare the community structure of planktonic ostracod assemblages between Nansha reef area and open water area, based on, which the distribution pattern influenced by ocean current movement was discussed.During April and September in 2013, 27 sampling stations were set up in Nansha Islands Waters, and planktonic ostracods were collected at depths from 200 m to the surface using medium plankton net. Samples were treated and counted according to the standard methods.The results show the following findings,(1) A total, of 45 planktonic ostracods species were identified, belonging to 19 genera, 4 subfamilies, and 2 families. Twenty-one species only occurred in spring and 38 species only in autumn, and 14 species occurred in both seasons.(2) The dominant species were different between two seasons; Euconchoecia maimai, Porroecia porrecta, and Conchoecetta acuminate dominated in spring, while Cypridina name, Euconchoecia bifurata, Porroecia spinirostris, Microconchoecia curta, Metaconchoecia macromma dominated in autumn. All dominant species are either eurythemal-euryhaline or warm-hyperhaline.(3)There are no remarkable differences in the density of planktonic ostracods between two seasons. In spring, density ranged from 0.03 to 8.25ind/m3, averaging 1.4 ind/m3; in autumn, density ranged from 0.05 to 18.92 ind/m3 with an average of 4.11ind/m3.(4) High density occurred in certain limited area due to ocean circulation. In spring, three high density areas were located in the west, northwest and southeast corners of Nansha Islands waters, respectively. In autumn high density area occupied the vast east part.(5) Species richness, diversity and evenness were 0.59, 1.37 and 0.55, respectively, with much higher values in spring than in autumn.(6) Planktonic ostracods can be divided into two communities in both seasons characterizing, reefs and open waters, and seasonal variation in community distribution was impacted strongly by ocean current change.(7) The tropical cyclonic and anticyclonic circulation driven by monsoon can be regarded as the dominant factor to influence the structure of planktonic ostracod community.