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行政院國家科學委員會專題研究計畫成果報告
台灣海峽浮游生物生產力與食物階層動態：
高階肉食性浮游動物在夏季藻華中成為優勢浮游動物
計畫編號：NSC 89-2611-M-110-002
執行期限：88 年 08 月 01 日至 89 年 07 月 31 日
主持人：陳宏遠 國立中山大學海洋生物研究所
hychen@mail.nsysu.edu.tw
1.中文摘要 西元 2000 年夏天在台灣海峽西北角發生一 規模相當的矽藻藻華，藻華區域東西向從閩江河 口向西延伸約 55 公里至海峽中線。由於其表層 水鹽度偏低因此推測此藻華是由閩江河水所造 成，最大亞硝酸濃度達 1.77µM，而最高葉綠素 甲濃度為 22.27 mg m-3，藻華發生時在台灣海峽 的調查發現浮游動物生物量與浮游植物生物量 間並無直接相關，浮游動物生物量乾重與密度在 藻華水域與非藻華水域並無不同，但兩水體間浮 游動物種類組成則相當不同，在藻華水域肉食性 Sagittoidea及Luciferidae為在橈腳類之後之優勢 浮 游 動 物 ； 不 像 在 非 藻 華 水 域 以 橈 腳 類 及 Notiluca 為 優 勢 浮 游 動 物 ，Sagittoidea 及 Luciferidae一般屬食物鏈相對高階的動物，大於 500 微米網採混合浮游生物樣品分析結果亦顯示 藻華水域者 ä15N 值較非藻華水域高。導致大量 高 階 肉 食 性 浮 游動 物在 藻華 水域 成為優勢族 群，而不是可直接利用多量矽藻的橈腳類，真正 的原因需進一步研究，但這樣的現象顯示海洋生 態系統在環境擾動時相關食物階層作用的不直 接性，面對生態系統食物鏈或食物網受海域優養 化或藻華時，研究必須更注意食物階層的互動。 關鍵詞：台灣海峽，優勢，藻華，浮游動物，食 物階層動態 Abstr actAn extensive summer diatom bloom was observed in the northwestern Taiwan Strait. The bloom with a diameter of approximately 63 km extended from the Min Chiang estuary to the mid-strait. Low surface salinity in the bloom water suggested that the bloom was caused by the freshwater discharge from the Min Chiang River. A high nitrate concentration of 1.77 ìM and chlorophyll a of 22.27 mg m-3 were recorded. Distribution of zooplankton biomass in the Strait during the June 2000 investigation, however, was not related to phytoplankton abundance in light of the existence of the bloom. Zooplankton dry weight and density in the bloom were not different from the non-bloom water. Zooplankton community in the bloom, on the other hand, was different from the non-blooming water. Two carnivorous zooplankton
Sagittoidea (12.6-34.9%) and Luciferidae (5.6%) were the second and third most dominant zooplankton, following copepods, in the bloom water, while copepods and flagellated Notiluca dominated the zooplankton community in the adjacent waters. Both Sagittoidea and Luciferidae generally fed high in the food chain. ä15N signature of >500 ìm mix plankton from the bloom area was higher than that from adjacent non-bloom area. It is not clear why the carnivorous zooplankton, not herbivorous zooplankton that feed directly on the blooming plant biomass, became dominant in the phytoplankton bloom. These observations show indirect interactions of marine ecosystems to perturbation and indicate that cautions must be taken when dealing with trophic interactions and integrations in response to eutrophication, such as the spring bloom seen this study.
Keywor ds: Taiwan Strait, domination, spring bloom, zooplankton, ,trophodynamics
II. Intr oduction
South China Sea and East China Sea are two major marginal seas in the rim of the Northeast Pacific. South China Sea harbors a vast body of deep water that is largely oligotrophic. In contrast, East China Sea, with the inflows of the Changjiang and the Kuroshio shelf-break upwellings, is a relatively productive continental shelf. The two systems are connected by the Taiwan Strait, which provides a channel allowing the transport of water from south to north. In summer, seawater from the South China Sea, streamed by the southwest monsoon, floods the Taiwan Strait. The oligotrophic water from the south renders low productivity in the Strait, except along the coasts. In winter, with the northeast monsoon dominates a vast area around Taiwan and coastal China, the nutrient-rich water from the East China Sea invades the Strait and meets the northbound water in the middle of the Strait (Jan 1995). Kuroshio water penetrates into the Taiwan Strait through the northern rim of the East China Sea in winter and constitutes the mainstream of the northbound water in the Strait. Nutrient-laded water from the East China Sea, which forms the China Coastal Current, is limited in general to the
2 coast of China. The China Coast Current and summer upwelling along the Chinese coast bring nutrients to the Strait and support the balk of the primary production in the Strait. Nutrients carried by runoff from the rivers emptying into the Taiwan Strait often support sporadic phytoplankton blooms. The rapid changes of physical and biological factors in the Strait determined at least in part the distribution of marine organisms, including some economically important species such as anchovy (Young et al 1995; Tsai et al. 1997).
Phytoplankton bloom developed when a species suddenly increases in numbers under favorable conditions. As the bloom develops phytoplankton concentration may exceed 108 cell l-1 (Lalli and Parsons 1993). Although dinoflagellate blooms, often called red tide, receive most attention, blooms of diatoms or other microalgae are important oceanic phenomena. These plankters convert inorganic nutrients into organic compounds and transport essential elements along food chains or food web and enhance production in every trophic level. Rivers often carry high nutrients, derived from natural resources or from agriculture fertilizers and sewage. These nutrients enrich coastal waters and increase productivity off the mouth of the river. Phytoplankton blooms often appear at the outlaying area of the river plume, though many factors such as quantity of nutrients entrained, the settling out of river silt, grazing by zooplankton (Strom et al. 2001) and the depth of the mixed layer actually decide the exact position. It may also be disrupted or enhanced by the prevailing oceanic climate (Lalli and Parsons 1993).
The present paper reports the occurrence of an extensive diatom bloom in the northeastern Taiwan Strait, where the Minjiang River enter the sea. Physical, chemical and biological factors including parameters detailing phytoplankton and zooplankton dynamics were measured and used to describe the nature of the bloom that occurred in the summer of 2000. Efforts were concentrated on the interactions of plankton of different trophic positions.
III. Mater ials and Methods
Physical and biological properties were measured in the Taiwan Strait during a cruise (OR585) aboard R/V Ocean Researcher I in June 2000. A total of 50 stations was investigated (Fig. 1). Plankton tows were conducted at 17 of the stations. Seawater was collected at all stations using 20-l Go-Flo bottles attached to a rosette multi-sampler mounted on a CTD (SBE 9/11plus, SeaBird Inc., WA).
Figure 1. Sampling stations in the Taiwan Strait.
Measurements of size fractionated chlorophyll a concentration and primary production were carried out. Chlorophyll a concentration was determined fluorometrically on extracted samples. Plankton were collected in oblique tows using a three-layered plankton net, each with a mesh size of 20, 200 and 500 µm, respectively. The setup allowed simultaneous collection of 3 size classes: 20-200 µm, 200-500 µm and >500 µm. Additional tows using a net with 1000 µm mesh were carried out at selected stations and used as the plankton size class >1000 µm. A quantitative portion of each size-fractionated plankton sample was preserved with neutral formalin for taxonomic examination. The remanding plankton was picked over to collect larval fish and then rinsed with distilled water, blotted dry and kept at -20oC. Larval fish were also washed and frozen.
All frozen samples (plankton, larval fish, fish muscle and stomach content) were freeze-dried, pulverized and frozen until isotope analysis. Quantitative determinations of δ15N were carried out using an ANCA GC-mass spectrometer (Europa Scientific, Crewe, UK). Measurements were made relative to the isotopic compositions of and nitrogen in air (δ15N), where
δ15N = [(Rsample-Rstandard)-1] x 1000 and R = 15N/14N
Analysis of replicates usually showed agreement at the 0.2‰ levels for δ15N or better.
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IV. Results and Discussion
During the OR1-585 cruise we observed a red tide-like phytoplankton bloom extending from the Island of Matsu to approximately 63 km south into the mid-strait. Surface chlorophyll a concentration was as high as 18.5 mg/m3 (Fig. 2) Surface nitrate concentration was 1.77µM (Fig. 3) and salinity ranged between 30.7 to 32 psu. These results indicated that nutrient input from freshwater discharge of rivers might be responsible for the formation of the bloom. Figure 2. Distribution of surface chlorophyll a in
the Taiwan Strait.
Figure 3. Surface nitrate concentrations measured in the Taiwan Strait.
The sizes of phytoplankton in the bloom water were larger than those in the non-bloom water. In a size fractionation analysis, chlorophyll a concentration from the >20 mm size fraction accounted for 79~84% of total pigment concentration. Microscopic analysis showed that phytoplankton density in the bloom was 5000 per ml. The most dominant phytoplankton were Thalassiosira sp. and Skeletonema costatum, both are diatom. In contrast, its adjacent non-blooming water north of 24.5oN, including the water covering the sampling stations 1, 2, 3, 13, 16, 19 and 23, which covered the Chinese coast, Taiwanese coast and the mid-Strait water, was low in chlorophyll a and nutrient concentrations and high in salinity. Surface salinity in these outlaying water ranged between 32.8 to 33.8 psu. Surface nitrate concentration ranged between 0 to 0.1µM, with an exception of 2.62µM at station 16. Surface chlorophyll a concentration, much lower than in the bloom, had a range between 0.37 to 4.55 mg/m3. Contribution of the >20 µm size fraction to total pigment concentration was between 4-70%. The only station with high surface nitrate concentration, station 16 along the Taiwan coast, did not show corresponding high phytoplankton biomass. Phytoplankton there was dominated by cells smaller than 20µm (82% in terms of chlorophyll a concentration).
Zooplankton biomass in the bloom water did not increase with phytoplankton bloom (Fig. 4). In the bloom water, zooplankton biomass when addressed by dry weight was 17.0~18.6 mg/m3; while that in the non-bloom water was 10.9~30.8 mg/m3. Zooplankton biomass seemed to concentrate in the mid-strait. For example, station 3 located at the mis-strait had a zooplankton biomass of 30.8 mg/m3. In a study of zooplankton in the North Arabian Sea, Kidwai and Amjad (2000) found no significant difference in zooplankton biomass between eutrophic and oligotrophic stations.
The food chain length in the bloom, although based on phytoplankton of large body size, was not shorter than the non-bloom water. Stable isotope analysis showed that ä15N signature of zooplankton was no smaller than the non-blooming zooplankton. When the size fraction 200-500ìm of zooplankton were compared, there was no difference in ä15N due to algal bloom; On the contrary, ä15N signatures of >500ìm size fraction in the bloom water were higher than the non-bloom water.
4 Figure4. Distribution of zooplankton (in terms of unit dry weight) in the Taiwan Strait.
Taxonomic analysis of the zooplankton samples showed that carnivorous zooplankton belonging to Sagittoidea (12.6-34.7%) and Luciferidae (5.6%) dominates zooplankton community in the bloom water. The predominance of these highly carnivorous zooplankters in the bloom water contributed significantly to high ä15N values. Relative occurrences of Sagittoidea (3.8-4.3%) and Luciferidae (0.5-0.9%) were low comparatively. It is thus very likely that the reason why zooplankton biomass and density in the bloom water were not significant might be the top-down control of the large carnivorous zooplankton and their predation on the small preys effectively removed the small zooplankton. This suggestion was supported by the isotope analysis of the 200~500 and >500ìm plankton size fractions. In the bloom water the difference between the two size fractions was 2.5‰ ; while the same comparisons in the non-bloom water all yielded differences smaller than 0.5‰ . The difference of 2.5‰ could constitute almost a trophic level (2.8‰ in the Taiwan Strait). These results clearly demonstrated indirect interactions among trophic components of marine ecosystems to perturbation (Hulot et al. 2000) and indicate that cautions must be taken when dealing with trophic interactions and integrations in response to eutrophication, such as the phytoplankton bloom seen this study.
V. Refer ence
Jan, S.（1995）Seasonal change of current fields in the Taiwan Strait. PhD dissertation. National Taiwan University, Taipei.
Kidwai, S. and Amjad, S. (2000) Zooplankton: pre-southwest and northeast monsoons of 1993 to 1994, from the North Arabian Sea. Mar. Biol. 136: 561-571.
Hulot, F. D., Lacroix, G., Lescher-Moutoue, F. and Loreau, M. (2000) Functional diversity governs ecosystem response to nutrient enrichment. Nature 405: 340-344.
Lalli, C. M. and Parsons, T. R (1993) Biological oceanography: an introduction. Pergamon Press, Oxford.
Strom, S. L., Brainard, M. A., Holmes, J. L. and Olson, M. B. (2001) Phytoplankton blooms are strongly impacted by microzooplankton graing in coastal North Pacific waters. Mar. Biol. 138: 355-368.
Tsai, C F., P. Y. Chen, C. P. Chen, M. A. Lee, G. Y. Shiah and K. T. Lee (1997) Fluctuation in abundance of larval anchovy and environmental conditions in coastal waters off southwestern Taiwan as associated with the El Nino-Southern Oscillation. Fish. Oceanogr. 6: 238-249.
Young, S. S., T. S. Chiu and S. C. Shen (1995) Taxonomic description and distribution of larval anchovy (Engraulidae) occurred in the water around Taiwan. Acta Zoologica Taiwanica 6: 33-60. VI. 計畫成果自評 本計畫為「台灣海峽生態系統動態整合研究」 先導計畫之子計畫，目的在探討海峽浮游生物 之生產力及生態系統食物能階間的關係研究 各體型浮游生物（20-200、200-500 及> 500µm） 之生物量與碳氮同位素含量，據以為建立各食 階要素（trophic components）之食物鏈關係， 並配合基礎生產力了解生產力與食物鏈上層 動物間之關聯。計畫配合海科中心 TSnow 計 畫航次採樣，以獲水文資料的支持，本年度研 究中共參加海研一號 569 及 585 航次與海研三 號航次，航次中取得部分營養鹽、分體型葉綠 素甲含量、基礎生產力並採多層採集網集浮游 生物， ANCA-質譜儀測量樣品 15 N 含量，這 些資料可用來比較各體型浮游生物食物鏈階 層（trophic level）時空變化，若配合基礎生產 力或植物生物量（如葉綠素 a），可建立自基 礎生產者至大型浮游動物以體型為基礎的食 物階層生物量，這些資料的建立對於瞭解台灣 海峽的生態係食物網是必須的。本年度計畫結 果基本上已達成此目標，海洋生態研究結果可 能要累積一段時間才能有足夠證據檢討主要 生態問題，本計畫結果擬投稿由黃天福教授所 召集的台灣海峽專刊。
