Copepod Diversity and Composition As Indicators of Intrusion of The
Total Page：16
File Type：pdf, Size：1020Kb
Zoological Studies 43(2): 393-403 (2004) Copepod Diversity and Composition as Indicators of Intrusion of the Kuroshio Branch Current into the Northern Taiwan Strait in Spring 2000 Chih-Hao Hsieh1,*, Tai-Sheng Chiu1 and Chang-tai Shih2,3,4 1Department of Zoology, National Taiwan University, Taipei, Taiwan 106, R.O.C. 2Taiwan Fisheries Research Institute, Keelung, Taiwan 202, R.O.C. 3Canadian Museum of Nature, Ottawa, Canada K1P-6P4 4Department of Marine Resources, National Sun Yat-sen University, Kaohsiung, Taiwan 804, R.O.C. (Accepted February 13, 2004) Chih-Hao Hsieh, Tai-Sheng Chiu and Chang-tai Shih (2004) Copepod diversity and composition as indica- tors of the intrusion of the Kuroshio Branch Current into the northern Taiwan Strait in spring 2000. Zoological Studies 43(2): 393-403. This study was carried out during the waning of the northeastern monsoon, at which time the Kuroshio Branch Current intrudes into the northern Taiwan Strait. The Kuroshio Branch Current was characterized by high temperature and salinity in contrast to the low temperature and salinity of the China Coastal Current. A front was induced south of the Haitan Island, which resulted in a phytoplankton bloom when the China Coastal Current and Kuroshio Branch Current met. The front is marked by a temperature of 18 ± 2 C and a salinity of 33 ± 0.5 psu. The intrusion of the Kuroshio Branch Current apparently influenced the species° diversity of copepods. Species richness and diversity were higher in the area affected by the Kuroshio Branch Current than that affected by the China Coastal Current. No significant differences in the copepod abundances between the two water masses were observed. Species assemblages were distinct in the two water masses. The China Coastal Current contained only coastal/neritic cold-water species, while the Kuroshio Branch Current consisted of coastal, neritic, and oceanic warm-water species. Indicator species of each water mass were identified and are discussed. http://www.sinica.edu.tw/zool/zoolstud/43.2/393.pdf Key words: China Coastal Current, Kuroshio Branch Current, Indicator, Species diversity, Species composi- tion. The Taiwan Strait is a shallow channel con- Chern 1988). Stagnated by the CCC, the KBC is necting the East China Sea and South China Sea blocked south of the Changyun Ridge when the and serves as an important pathway for faunal northeastern monsoon is strong. Only when the exchange between these two waters in the margin northeastern monsoon wanes in the spring, does of the western North Pacific. It is also a main the KBC flow over the Changyun Ridge or move route for migratory fishes and thus an important northwestward along the local isobaths into the fishing ground. Three main currents, the China northern Strait. A front south of the Haitan Is. is Coastal Current (CCC), Kuroshio Branch Current induced when the northwardly flowing KBC (KBC), and South China Sea Surface Current encounters the retreating CCC. During summer (SCSSC), all driven by the monsoon system, (June to Aug.), the southwestern monsoon causes affect the hydrographic conditions of the Strait the SCSSC to move northward into the Strait in (Jan et al. 2002). When the northeastern mon- place of the KBC. The seasonal variation in circu- soon prevails during winter (Nov. to Mar.), the lation of the Taiwan Strait was described by Jan et CCC moves southward into the Strait; meanwhile, al. (2002). The physical oceanography of the the monsoon also forces the KBC to penetrate into intrusion of the KBC into the Strait during the wan- the Strait through the Penghu Channel (Wang and ing of northeastern monsoon has been well stud- *To whom correspondence and reprint requests should be addressed. Present address: Scripps Institution of Oceanography, University of California-San Diego, California, 92093-0208, USA. E-mail:[email protected] 393 394 Zoological Studies 43(2): 393-403 (2004) ied (Jan et al. 1998); however, the biological 1994). Studies on copepods are crucial for under- effects of this circulation are not known. standing the dynamics of marine ecosystems and The CCC is a neritic water mass character- are necessary for purposes of management and ized by low salinity, low temperature, and high conservation of marine resources. The objectives nutrient due to river runoff from the mainland of our study are to provide basic information about China (Liu et al. 2000). The KBC, which originates the copepod fauna and to determine the effects of from the oceanic Kuroshio Current, has high tem- the Kuroshio intrusion in the Taiwan. perature and high salinity, but counter-intuitively contains a nutrient level comparable to that of the CCC (Chung et al. 2001). It is known that zoo- MATERIALS AND METHODS plankton composition and abundance are affected by the properties of water masses (Boucher et al. Sampling 1987, Kouwenberg 1994, Lopes et al. 1999). Since our samples were collected across both the A cruise was carried out on board the Ocean CCC and KBC, this survey allowed us to test the Research I on 20~25 Mar. 2000 in the Taiwan following hypotheses: (i) distinct species composi- Strait (Fig. 1). Copepods were sampled using a tions occur in the CCC and KBC; (ii) low species standard North Pacific zooplankton net with a diversity occurs in the CCC (a neritic water mass mouth diameter of 45 cm and mesh size of 150 subject to frequent disturbances) and high diversity µm, and samples were preserved in seawater with occurs in the KBC (an oceanic water mass with 5% formaldehyde. A flowmeter was mounted at greater stability); and (iii) equal abundances of the center of the net mouth to record the volume of copepods occur in the CCC and KBC according to water filtered. The net was towed obliquely from their similar nutrient contents. near the bottom to the surface. Vertical profiles of Previous faunal surveys in the Taiwan Strait temperature, salinity, fluorescence, and dissolved have been either confined to a small area or con- oxygen were recorded at each station from the ducted without attention to hydrographic effects. bottom to the surface with a conductivity-tempera- Tzeng and Wang (1992 1993) studied the compo- ture-depth (CTD) profiler (Sea-Bird Electronics, sition and seasonal variation of larval fish fauna in Bellevue, USA) equipped with a fluorometer. The the Tanshui River estuary off northwestern Taiwan; fluorescence value was not calibrated but used as Chiu and Chang (1995) reported diurnal cycling in a reference index of the relative phytoplankton bio- the vertical distribution of larval fishes at a fixed mass. During the cruise, plankton samples were station in the northeastern Strait; Chiu and Chang collected at only 10 of 36 stations, because the (1994) compared the larval fish compositions dur- ship time allocated for our study was limited. ing winter and spring in the northeastern Strait; In the laboratory, copepods were sub-sam- Chiu (1992) and Chiu and Chen (1997) examined pled using a Folsom splitter until the sample size the larval fish composition in relation to the hydrog- was reduced to 300~500 specimens. Copepods raphy in the southern Strait. The copepod fauna (including copepodid stages) were identified to was reported by Tan (1967) around the Tanshui species whenever possible. The abundance of River estuary; Hsieh and Chiu (1998) compared copepods was expressed as the number of individ- the species assemblages and abundances of uals/m3. copepods in the Tanshui River estuary and adja- cent area; Zheng et al. (1982) and Chen (1992) Data analysis reported copepod species in the Strait. A relatively , large-scale survey including benthic, planktonic, Shannon s diversity index was used to mea- and larval fish faunas in the Strait was conducted sure the species diversity of each sampling station. by Zhu et al. (1988), but was limited to the western A correspondence analysis based on the relative Strait and hydrographic effects on biological varia- abundance of the 20 most abundant species was tion were not examined. The present study is thus applied to simultaneously examine the station a preliminary survey of copepods covering the associations and the corresponding species northern Taiwan Strait and analyzes hydrographic (Pielou 1984). For reducing the patchiness effect effects on copepod assemblages. Copepods are a of zooplankton, data were square-root transformed major component of marine zooplankton and play (Krebs 1989). K-means classification based on an important role in marine food webs (Runge the station scores of the correspondence analysis 1988, Poulet and Williams 1991, Williams et al. was applied to distinguish station groups Hsieh et al. -- Copepods as Indicators of the Kuroshio Branch 395 (Wilkinson 1990). By using K-means classifica- value of that species. A Monte Carlo method was tion, the number of groups was decided in used to test the significance of their indicator val- advance (in the current case, the number of ues by randomly permuting the data and recalcu- groups was two), and an iterative algorithm was lating Ind Val. One thousand permutations were used to assign stations to the groups by maximiz- made to compare the observed Ind Val and recal- ing the variance (of input variables) between culated Ind Val. The result was deemed significant groups and minimizing the variance within groups. when the observed Ind Val fell in the 5% upper tail. This procedure results in an objective classification Only individuals identified to species were included of stations. Indicator species of each station in the above analyses. A Mann-Whitney U test groups were extracted by an indicator species was used to test for differences in copepod abun- analysis (Dufrene and Legendre 1997). The indi- dance, species richness, and diversity between the cator value (Ind Val) of each species was comput- station groups affected by the China Coastal ed as follows: Current and Kuroshio Branch Current.