Study On The Zooplankton Functional Groups In The Yellow Sea

Zooplankton plays a vital role in the marine ecosystems. The variations of zooplankton species composition, biomass and secondary production will change the structure and function of the ecosystems. How to describe this process and make it easier to be modeled in the Yellow Sea ecosystem is the main purpose of this paper. The biomass and secondary productivity are the basis of the food web in the marine ecosystem. Who are the main contributors in the biomass and secondary productivity of zooplankton? Which species take the roles to affect the structure and function of the ecosystem? It is very hard to describe in the temperate continental shelf area, such as in the Yellow Sea, where the species composition, biomass and secondary production changed seasonally. Therefore, when modeling the key process of ecosystem food production in the Yellow Sea, an approach which is both precise and easy must be applied. After adequately analyzing the structure of zooplankton community and features of physical oceanography, the zooplankton functional groups approach, which is considered to be a good method of linking the structure of food webs and the energy flow through ecosystems, is used in the Yellow Sea ecosystem modeling.According to the size spectrum, feeding habits and trophic functionality, the zooplankton could be classified into 6 functional groups: giant crustacean (GC), large copepods (LC), small copepods (SC), chaetognaths, medusae and salps. The GC, LC and SC groups which are the main food resources of fish are defined based on the size spectrum. Medusae and chaetognaths are two gelatinous carnivorous groups, which compete with fish for food. The salps group, acting as passive filter-feeders, competes with other species feeding on phytoplankton, but their energy could not be efficiently variations and geographical distributions of each zooplankton functional group, the ecoregions related to zooplankton functional groups, the secondary production of zooplankton, the impact of chaetognaths group feeding on zooplankton and trophic ecology of Calanus sinicus were studied in this paper.The mean zooplankton biomass was 2.1 g dry weight m–2 during spring, to which the GC, LC and SC contributed 19, 44 and 26%, respectively. High biomasses of the LC and SC were distributed at the coastal waters, while the GC was mainly located at offshore stations. In summer, the mean biomass was 3.1 g dry weight m–2 which was mostly contributed by the GC (73%), and high biomasses of the GC, LC and SC were all distributed in the central part of the Yellow Sea. During autumn, the mean biomass was 1.8 g dry weight m–2 which was similarly constituted by the GC, LC and SC (36, 33 and 23%, respectively) and high biomasses of the GC and LC were occurred in the central part of the Yellow Sea, while the SC was mainly located at offshore stations. The GC and LC dominated the zooplankton biomass (2.9 g dry weight m–2) in winter, each contributing 57% and 27% and they as well as the SC were all mainly located in the central part of the Yellow Sea. The chaetognaths group was mainly located in the central and northern part of the Yellow Sea during all seasons, but contributed lower to the biomass compared with other groups. The small medusae and salps groups were distributed unevenly with sporadic dynamics, mainly along the coast line and at the northern part of the Yellow Sea. No more than 10 species belonging to the respective functional group dominated the zooplankton biomass and controlled the dynamics of the zooplankton community.During spring, the Yellow Sea can be divided into 4 zooplankton ecoregions. The high biomass of zooplankton was mainly distributed at the coastal waters near the south shore of Shandong peninsula, which corresponded to the first ecoregion. The LC and SC were the dominated functional groups in the first ecoregion. In summer, autumn and winter, the Yellow Sea can be classified into 3, 4 and 3 zooplankton ecoregions, and the high biomass were all mainly distributed in the central part of the Yellow Sea, which all corresponded to the first ecoregion. The GC and LC were the dominated functional groups in the first ecoregion in these three seasons. The Yellow Sea Cold Bottom Water (YSCBW) plays a vital role in the distribution mode of GC, LC and SC. The geographical distribution mode of each zooplankton ecoregion in different season had important ecological meaning in the Yellow Sea ecosystem.In terms of size spectrum, the main food zooplankton of higher trophic levels can be divided into 5 classes of 0.16–0.25 mm, 0.25–0.5 mm, 0.5–1 mm, 1–2 mm and >2 mm using sieves onboard. The secondary production of each size class was estimated by using physiological method. The results showed that the estimated production rate of zooplankton was enormously high in May 2007 (91.9 mg C m–2 d–1), followed in order by June (75.6 mg C m–2 d–1), September (65.5 mg C m–2 d–1), August (42.3 mg C m–2 d–1), March (35.9 mg C m–2 d–1) and December (27.9 mg C m–2 d–1). On the basis of these rates, the integrated production of zooplankton was 18.9 g C m–2 year–1 in the Yellow Sea. The 0.16–0.25 and 0.25–0.5 mm classes which correspond to the SC group together comprised 58–79% of the production in the study area. The P/B value of 0.16–0.25 and 0.25–0.5 mm (0.091–0.193 d–1) were higher than other size classes.Sagitta crassa, S. nagae, S. enflata and S. bedoti were the dominated species of chaetognaths group. The production of these four species as well as feeding impact of them on zooplankton secondary production was estimated in this paper. The results showed that the biomass and estimated production rate of chaetognaths were in the range of 98–217 mg m–2 and 1.22–2.36 mg C m–2 d–1. The proportion of chaetognath biomass was 6.35–14.47% of the zooplankton biomass, while chaetognath production rate was 2.54–6.04% of the zooplankton production. S. crassa and S. nagae were the absolutely dominated species, controlling the dynamics of chaetognath community. The feeding rate of chaetognath ranged from 4.24–8.18 mg C m–2d–1, and feeding impact on biomass and secondary production of zooplankton were 0.94% and 12.56%. In winter, the biomass and production of zooplankton were only 0.4 g C m–2 and 0.026 g C m–2d–1, while the feeding impact peaked in winter at 1.4% and 20.94%. Therefore, the structure of zooplankton community may be significantly affected by chaetognath in winter. On the basis of feeding rate of different body length groups, we can conclude that chaetognaths mainly feed on the species in SC group throughout the year. And besides small copepods, the species in LC group, such as C. sinicus, was largely feed by chaetognahs. Because C. sinicus was in diapause, so C. sinicus community would be affected significantly by chaetognaths in summer.The ingestion rate of C. sinicus (2.08–11.46 and 0.26–3.70μgC female–1 d–1 in spring and autumn, respectively) was positively correlated with microplankton carbon concentrations. In the northern part of the Yellow Sea, feeding on microplankton easily covers the respiratory and production requirements, whereas in the southern part in spring and in the front zone in autumn, C. sinicus must ingest alternative food sources. Low ingestion rates, no egg production and the dominance of the fifth copepodite (CV) stage indicated that C. sinicus was in quiescence inside the YSCBW area in autumn. C. sinicus ingested ciliates preferentially over other components of the microplankton. The EPR (0.16–12.6 eggs female–1 d–1 in spring and 11.4 eggs female–1 d–1 at only one station in autumn) increased with ciliate standing stock. Gross growth efficiency (GGE) was 13.4% (3–39%) in spring, which was correlated with the proportion of ciliates in the diet. These results indicate that ciliates have higher nutrient quality than other food items, but the low GGE indicates that the diet of C. sinicus is nutritionally incomplete.The feeding impact of medusae group on zooplankton, the ecological effects of salps group in the ecosystem, long term variations of seasonal and spatial distribution and composition patterns of zooplankton functional groups, especially under the climate changes and human perturbations, should be considered in the future.