ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on December 13, 2007
ICES Journal of Marine Science: Journal du Conseil 2008 65(3):462-468; doi:10.1093/icesjms/fsm173
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This article appears in the following ICES Journal of Marine Science issue: 4th International Zooplankton Production Symposium: Human and Climate Forcing of Zooplankton Populations [View the issue table of contents]
Seasonal relationships between the copepod community and hydrographic conditions in the southern East China Sea
1 Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, Keelung, Taiwan
2 Fishery Research Institute, Keelung, Taiwan
Correspondence to M-A. Lee: tel:+886 2 24622192 5032; fax: +886 2 24634419; e-mail: malee{at}mail.ntou.edu.tw
Lan, Y-C., Lee, M-A., Chen, W-Y., Hsieh, F-J., Pan, J-Y., Liu, D-C., and Su, W-C. 2008. Seasonal relationships between the copepod community and hydrographic conditions in the southern East China Sea. – ICES Journal of Marine Science, 65: 462–468.We studied the relationship between seasonal variation of the copepod community and hydrographic conditions in the southern East China Sea (ECS). Hydrographic conditions in the southwestern ECS were influenced by the China Coastal Current (CCC) from late autumn to early spring and by the South China Sea Current (SCSC) during the rest of the year, and in the southeastern ECS by the Kuroshio Current (KC) year-round. We identified 141 species of copepods belonging to 54 genera and 27 families. Diversity and evenness of copepods were higher in waters influenced by the KC and the SCSC, and lower in waters with intrusion of the CCC. The CCC transported dominant copepods such as Paracalanus aculeatus, Paracalanus parvus, and Calanus sinicus into the southwestern ECS in late autumn and winter, whereas the SCSC carried Temora discaudata, Oncaea venusta, and P. aculeatus in spring, and Undinula vulgaris, T. discaudata, and P. aculeatus in summer. In the southeastern ECS, the KC brought in tropical copepods having clear seasonal variations throughout the year. We examined the relationships between indicator copepod species and currents and oceanographic conditions, using canonical correlation analysis.
Keywords: China Coastal Current, copepod, diversity, Kuroshio Current, South China Sea
Received 9 July 2007; accepted 28 October 2007; advance access publication 13 December 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The East China Sea (ECS) is located in the western North Pacific between the Yellow Sea and Taiwan Strait (TS). It is one of the largest marginal seas in the world, with high primary productivity resulting from a rich supply of nutrients from terrestrial and marine sources (Walsh, 1991; Wollast, 1993). In the southern ECS, upwelling caused by the Kuroshio Current (KC) is present throughout the year off northeastern Taiwan (Wong et al., 1991). The upwelling system maintains a constant supply of nutrients from deep water to help sustain high primary productivity in the study area (Gong et al., 1997). The high primary productivity has made this area one of the most productive neritic fishing grounds around Taiwan (Chiu, 1991). Hydrographic conditions in the area are dictated by the seasonal monsoons, alternating between the northeasterly monsoon (NE monsoon) in winter and the southwesterly monsoon (SW monsoon) in summer (Jan et al., 2002, 2006). In summer, the prevailing SW monsoon forces the South China Sea Current (SCSC) to flow northwards through the TS to the southwestern ECS. When the SW monsoon wanes in autumn, the China Coastal Current (CCC) blocks the SCSC, causing it to flow southwards. The NE monsoon prevails in winter, and the southwestern ECS is then dominated by the CCC. The intrusion of the KC to the eastern ECS occurs year-round.
Copepods are the most abundant zooplankton in the ocean and the main food source for marine fish larvae; they play an important role in marine foodwebs (Last, 1978; Hunter, 1981). Their abundance and geographic distribution are influenced by hydrographic conditions (Williams et al., 1994; Shih and Chiu, 1998; Lan et al., unpublished data). Some copepods have been suggested to be good biological indicator species for water masses (Zheng et al., 1992; Lan et al., 2004). Copepod composition and diversity associated with hydrographic conditions in the southern ECS have been described for spring (Shih and Chiu, 1998; Shih et al., 2000), summer (Liao et al., 2006), autumn (Zuo et al., 2006), and winter (Lan et al., unpublished data), but a comparison of successive seasons has not been made. Indicator copepod species have been recognized for the Kuroshio Branch Current (Hsieh et al., 2004), CCC (Zheng et al., 1992; Hwang and Wong, 2005), and SCSC (Xu, 2006).
The southern ECS is one of Taiwan’s largest fishing grounds. Although copepods are major food items for fish larvae, and therefore important for the recruitment of fishery resources, our knowledge of the seasonal variations in copepod composition in the southern ECS is lacking. Our study investigated the seasonal variations of the copepod community under the influence of the hydrographic regime caused by the alternation of the NE and SW monsoons in the southern ECS.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Plankton samples were collected by the RV "Fishery Researcher I" using an ORI net with a mouth diameter of 160 cm and mesh size 330 µm during the periods December (late autumn) 2003, and February (late winter), June (late spring), and August (summer) 2004 (Figure 1). A flowmeter and a depth sensor were mounted at the mouth of the net to calculate the volume of filtered water and record the sampling depth. The net was towed obliquely from near-bottom to the surface, or from 200 m to the surface if the depth of the seabed exceeded 200 m. The plankton samples were preserved in seawater with 5–10% formalin.
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Temperature and salinity were recorded with a CTD profiler lowered from the surface to near the bottom at each sampling station. In the laboratory, NOAA AVHRR images with 1.1 km spatial resolution were obtained from the Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, to demonstrate the spatial distribution of hydrographic conditions during the four study periods. In the laboratory, each plankton sample was repeatedly divided with a Folsom splitter until the subsample contained 300–500 copepods. Copepods were sorted, enumerated, and identified to species, whenever possible. The abundance of copepods was expressed as the number of individuals per m3 (ind. m–3).
Shannon’s diversity index was used to calculate the species diversity, and Simpson’s evenness index was used to measure the relative abundance of species at each station. Cluster analysis with normalized Euclidean distances was used to measure the levels of similarity in species composition among the sampling stations, and Ward’s method was used to illustrate the relation of these stations in a dendrogram. The representative species of each current were identified using the indicator value (IV) index (Dufrêne and Legendre, 1997). IVs were calculated from:
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The spatial distribution of SST during the four study periods reveals the warm water of the KC intruding into the southeastern ECS year-round (Figure 2). The southwestern ECS was influenced by the cold water of the CCC from late autumn to spring and was affected by intrusion of the warm waters of the SCSC in late spring and summer. The vertical distributions of temperature (Figure 3) and salinity (Figure 4) revealed lower values in shelf and in offshore waters during each sampling period. The CCC influenced Stations 1–3 (temperature and salinity lower than 20°C and 34) in late autumn, and a halocline at
40 m in late winter. However, in late spring and summer, the shelf waters were affected by the intrusion of a warm current with surface temperature and salinity greater than 28°C and 34.1. Temperature and salinity decreased with depth at stations in the offshore waters. Upwelling was present between Stations 4 and 5 in late spring and summer.
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Copepods were more abundant at Stations 1–3 located in the continental shelf waters than at Stations 4–6 located in the offshore waters throughout the year, except during late spring (Figure 5). In continental shelf waters, copepod abundance increased from late autumn (average abundance: 184 ind. m–3) to late spring (2148 ind. m–3) and decreased in summer (1223 ind. m–3), except for a sharp decline at Station 1 in late spring. In late spring, copepods were abundant in the western upwelling waters (Stations 3–4).
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In this study, we identified 141 species of copepods belonging to 54 genera and 27 families. Diversity and evenness of copepods at Stations 4–6 varied between 3.06 and 3.61, and between 0.93 and 0.97, respectively, and were higher than at Stations 1–3 throughout the year, except at Station 4 in late spring (Table 1 and Figure 6). Fluctuations of diversity and evenness of copepods were greater at Stations 1–3 than at Stations 4–6.
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) of copepods at each station on the four cruises.According to the result of cluster analysis, the copepod species composition for the stations was divided into three groups, CCCG, SCSCG, and KCG, at a linkage distance of 20 (Figure 7). Stations of the CCCG group were influenced by the CCC in late autumn and late winter. The dominant copepod species in late autumn were Paracalanus aculeatus, Paracalanus parvus, Scolecithricella minor, Temora discaudata, and Calanus sinicus, and in late winter P. parvus, C. sinicus, Corycaeus affinis, Oncaea venusta, and P. aculeatus (Table 2). The stations of the SCSCG group were located in the waters with intrusion of the SCSC in late spring and summer. Their dominant species in late spring were T. discaudata, O. venusta, P. aculeatus, Canthocalanus pauper, and Acrocalanus gibber, whereas in summer they were Undinula vulgaris, T. discaudata, P. aculeatus, C. pauper, and A. gibber. The stations of the KCG group were in the offshore waters influenced by the KC with the highest SST. The first five dominant species of the KCG demonstrated seasonal changes: P. aculeatus, A. gibber, Clausocalanus furcatus, Acartia bifilosa, and O. venusta in autumn; P. aculeatus, O. venusta, C. furcatus, Cosmocalanus darwini, and A. gibber in late winter; O. venusta, A. gibber, C. furcatus, Farranula gibbula, and C. darwini in late spring; and O. venusta, C. darwini, Clausocalanus minor, T. discaudata, and P. aculeatus in summer.
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Indicator species of the CCC, KC, and SCSC are listed in Table 3. Seven indicator species (IV > 25%) were observed in the CCC, including C. affinis (98.9%), C. sinicus (82.6%), Euchaeta concinna (78.2%), P. parvus (66.2%), and S. minor (46.3%); 36 in the KC, including Centropages calaninus (63.6%), Temoropia mayumbaensis (54.5%), Lucicutia flavicornis (51.9%), Acartia danae (51.6%), Calocalanus pavo (48.3%); and 49 in the SCSC, e.g. Temora turbinata (99.3%), U. vulgaris (98.3%), Corycaeus andrewsi (97.5%), Macrosetella gracilis (95.1%), and C. pauper (94.0%).
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The results of the CCA analysis revealed the first two roots with a significant relationship (p < 0.01) between the copepods and hydrographic variance, and explained 78.3% of hydrographic variance and 31.5% of the copepod variance. The first two CCA axes appear in Figure 8 to illustrate the distribution patterns of these selected copepods. The first CCA axis (Axis 1) was negatively correlated with water depth and could be used to distinguish the shelf and offshore waters. The second axis (Axis 2) was correlated negatively with the SST and positively with chlorophyll concentration. This suggests that copepods prefer an environment with higher SST but lower chlorophyll concentration or vice versa. Most of the indicator species were positively related to Axis 1 and negatively related to Axis 2, indicating that they preferred the continental shelf waters with high SST caused by the intrusion of the SCSC. Corycaeus affinis and C. sinicus demonstrated a negative association with depth and SST, and they dominated in the CCC.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Hydrographic dynamics in the southern ECS are controlled by the Asian monsoon system. The seasonal variations in hydrographic conditions can be observed widely and immediately by satellite remote sensing (Lee et al., 2005). The strength and spatial distribution of the main currents intruding into the southern ECS during our study period could be observed from satellite images (Figure 2). The KC is a strong, western boundary current, flowing northwards along the east coast of Taiwan, and intrudes onto the ECS shelf, forming the Kuroshio upwelling off northeastern Taiwan (Wong et al., 1991). As a result of the KC intrusion, the southeastern ECS had higher SSTs during our whole study period, and upwelling was observed between Stations 4 and 5 in late spring and summer (Figure 3). Liu et al. (2000) reported that the Kuroshio upwelling provided the ECS shelf with constant nutrient fluxes that are comparable with the total nutrient influxes from slope waters to the Mid- and South Atlantic Bight and larger than the riverine sources. It was demonstrated that the hydrographic and nutrient conditions regulated the phytoplankton biomass and primary productivity in the southern ECS in spring (Liu et al., 1995).
The southwestern ECS is a highly dynamic region because of the seasonal variations of the different water types (Gong et al., 1996). The CCC flows south along the coast of China into the southwestern ECS, when the NE monsoon prevails in winter, and is replaced by the SCSC when the SW monsoon prevails in summer. The CCC is characterized by low temperature and salinity, but high nutrients resulting from river run-off from Mainland China (Liu et al., 2000). The SCSC is an oligotrophic current where phytoplankton growth is limited by nitrogen (Chen et al., 2004). Gong et al. (2000) reported that the lowest primary productivity in waters of the southern ECS shelf was in summer, but elevated primary productivity in the inner shelves in spring was caused by a small spring bloom. Copepod abundance in the waters of the southern ECS shelf especially in spring, except Station 1 (Figure 5), may be the result of the supply of phytoplankton food. Top-down control may be another factor affecting copepod abundance, because we observed a large number of jellyfish and chaetognaths at Station 1 in late spring, and these groups actively prey on copepods (Stuart and Verheye, 1991; Duró and Saiz, 2000).
Copepods were more diverse in the KC and the SCSC than in the CCC, as reported in previous studies (Shih and Chiu, 1998; Dur et al., 2007). Shih and Chiu (1998) reported that the Shannon diversity index of copepods increased eastwards from the CCC (2.25) to KC (3.28) in spring. The factors controlling the seasonal variations of copepod composition in the southern ECS would differ between the shelf and offshore waters. The natural seasonal cycle may play an important role in the offshore waters because it is influenced by the KC throughout the year. However, in the shelf waters, hydrographic conditions are influenced by the cold waters of the CCC in the NE monsoon season and the warm waters of the SCSC or mixed with the Kuroshio Branch Current in the SW monsoon season. Therefore, transport by currents may play a major role affecting the copepod composition in the southwestern ECS.
Several studies have attempted to recognize indicator species for water masses or currents in the waters surrounding Taiwan (Hsieh et al., 2004; Xu, 2006; Dur et al., 2007). Indicators of three main currents in the southern ECS were also found in this study (Table 3). Corycaeus affinis and C. sinicus were found to be good indicator species of the CCC and had a negative association with water depth and SST, based on the results of the CCA analysis (Figure 8). Their occurrence has been reported to coincide with the CCC (Hsieh et al., 2004; Dur et al., 2007). Calanus sinicus is transported from the ECS to the TS and the South China Sea in the winter/spring period (Zheng et al., 1992; Lan et al., 2004). Zheng et al. (1992) reported that C. sinicus and C. affinis were two of the coastal/neritic species with temperature optima lower than 20°C and were carried southwards to the northern part of South China Sea by the CCC in winter. Hsieh et al. (2004) recorded 16 species to be good indicator species of the KBC in spring, and most of them were indicator species of the KC and SCSC. Oithona plumifera, L. flavicornis, Oncaea mediterranea, and Euterpina acutifrons were indicator species of the KC in this study. However, A. gibber, Clausocalanus frucatus, C. minor, Calocalanus plumulosus, O. venusta, C. pauper, and Scolecithrix danae were indicator species of the SCSC in this study. Further work will develop the concept of indicator species for water masses or currents in the waters surrounding Taiwan.
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Acknowledgements |
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We thank the captain and crew of the RV "Fishery Researcher I" and graduate students of the Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, who helped with the collection of zooplankton samples and environmental data. We acknowledge Chang-Tai Shih for help with identification of copepod species, and Roger Harris for comments and suggestions on an earlier draft of this paper. This GLOBEC project (Grant No. NSC94-2611-M-019-009 and NSC95-2611-M-019-016) was financially supported by the National Science Council of Taiwan.
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