亞熱帶貧營養鹽海域中束毛藻固氮作用在異營性細菌置換率及有機碳傳輸效率方面之潛在衝擊力

Abstract

水體中的細菌生產力（BP）與初級生產力（PP）可分別視為水體內二氧化碳來源及沉降的過程，因此探討水域內BP：PP ratio變化的控制機制，在瞭解海洋生物源有機碳的過程(例如:生物幫浦)中極具重要性。本研究於2001年在蘇澳外海黑潮海域進行夏、秋兩季航次調查，分別就所量測到的BP、PP及各物理、化學水文參數，與固氮藍綠藻Trichodesmium contortum的豐度（[Tricho#]）一併做比較。結果顯示，夏季上層水體（0-30 m）硝酸鹽平均濃度（NO3da；0.08 ± 0.08 mM）明顯較秋季低，但卻觀察到豐富的[Tricho#]（128-1,166 trichos L-1）。夏天的PP值（21 ± 7 mgC L-1 d-1）及浮游植物置換率（Pm = PP/phytoplankton biomass ; 0.48 ± 0.19 d-1）至少是秋天的1.5倍以上。BP（夏季: 2.5 ± 0.6 mgC L-1 d-1；秋季: 2.4 ± 0.3 mgC L-1 d-1）並未顯示夏、秋兩季間的差異，而夏季的細菌置換率（Bm = BP/bacterial biomass ; 0.11 ± 0.03 d-1）則明顯高於秋季約57﹪。夏季0-30m[Tricho#]的平均豐度與NO3da 之間的明顯負相關以及高水體透光率指出了NO3的缺乏及較充足的陽光可能引發T. contortum的生長。Pm及Bm與T. contortum豐度呈現正相關性，推測細菌的生長可能藉由固氮藍綠藻產生的NH4+及溶解態有機氮而提高，進而使水體內的再生性NH4+供藻類獲取，導致PP與Pm增加；但BP:PP及Bm：Pm ratio與T. contortum豐度間卻呈現負相關性，猜測高的T. contortum豐度可能潛在地降低了藻類與細菌對NH4+的競爭效應。筆者推論在開放性海洋中，T. contortum的存在可促使BP:PP ratio降低，進而增加有機碳往較高營養階層以及運送至系統外（例：深海）的傳送效率。

Bacterial production（BP）, primary production（PP）and abundance of N2-fixing cyanobacterium, Trichodesmium contortum（[Tricho#]）were measured in the North Pacific west-boundary Kuroshio Current in summer and autumn, 2001. The warm（28-29 ℃）upper water column（0-30 m）in summer was low in nitrate（NO3da；0.08 ± 0.08 mM）with T. contortum（192-2,115 trichos L-1）observed at all 5 sampling stations. Averaged NO3 in the upper 30m were higher（0.39 ± 0.27 mM）in autumn but with no T. contortum observed except 1 station with a [Tricho#] of 962 trichos/L in surface waters. PP（21 ± 7 mgC L-1 d-1）and algal turnover rate（Pm；0.48 ± 0.19 d-1）in summer were at least 1.5-folds of those recorded in autumn. BP（summer, 2.5 ± 0.6 mgC L-1 d-1；autumn, 2.4 ± 0.3 mgC L-1 d-1）showed no seasonal difference but with bacterial turnover rate（Bm；0.11 ± 0.03 d-1）57% higher in summer. The negative trend of [Tricho#] v.s. NO3da and higher transmittance in summer indicate that NO3 depletion and better light availability might trigger the development of T. contortum. Bm changed positively with [Tricho#] suggesting that bacterial growth might to enhanced by the supply of “new” inorganic（i.e. NH4+）and/or dissolved organic nitrogen from N2-fixing cyanobacterium. From the negative relationship of BP：PP ratio（7-21%）v.s. [Tricho#], it is deduced that high [Tricho#] might potentially reduce the magnitude of NH4+ competition between algae and bacteria. In open ocean, organic carbon transfer efficiency, either to higher trophic levels or to system exportation, can be greatly enhanced at low BP：PP ratio driven by the prevalence of N2-fixing cyanobacterium.