黑潮流域海底山之克赫波(Kelvin-Helmholtz billows)對海洋上層生態系統的影響

Abstract

克赫波可以驅動流經貧營養鹽黑潮在海底山周邊小尺度的海水混合。本研究目標主要探討不同之克赫波强度(或大小)，包括「間歇小型」的小波狀況(small billow case; SBC)與「穩定大型」的大波狀况(large billow case; LBC)之克赫波對本海域硝酸鹽垂直通量的影響，並進一步瞭解其在海底山周遭生態系，不同環境條件下對超微型浮游生物及浮游動物組成之影響(第一章)。克赫波所造成之亂流動能耗散率 (turbulent kinetic energy dissipation rate; ε = O (10-7–10-6) W kg-1) 及渦流擴散率 (eddy diffusivities; Kρ = O (10-4–10-3) m2 s-1) 明顯高於無克赫波(時)，利用在此波內 Kρ 所估算之平均硝酸鹽通量最大值為 10.0 mmol m-2 day-1，此值遠高於黑潮流域之平均值(第二章)；在較淺層形成的克赫波所攜入的硝酸鹽通量，將豐富次表層海域的無機營養鹽濃度；而靠近海底山頂較深的克赫波，則將會更有效率的從更深層水體中垂直向上傳輸豐富的硝酸鹽。另一方面，海底山周遭海域的超微浮游生物主要以異營性細菌為主(>50%; 第三章)；然而，由於聚球藻生物量的增加，在海底山測站超微浮游生物的結果顯示出與近岸海域相似的生物碳量；此結果建議在貧營養鹽水體的黑潮流域，其海底山及其周圍海域有類似沿岸海域海水的特性。另外，在冬季航次時黑潮有較強勁流速，其浮游動物生物量(SKC; 104.5 ml 100m-3)較夏季航次黑潮流速較弱時(WKC; 33.7 ml 100m-3)高出60%以上，但此差異可能主要是受到季節性的影響所致(第四章)；另外，由浮游動物豐度的分布結果，顯示出海底山地形所形成的「阻塞效應」(blocking effect)，會將它們聚集在海底山周遭與其側翼；然而，較強的黑潮海流時(例如SKC)可消除阻塞效應，並將浮游生物快速帶往下游。而在海底山周遭亦觀測到仔稚魚個體早期發育階段，顯見海底山有利於作為魚類產卵和繁殖的棲地。整體而言，本研究顯示黑潮流域海底山之克赫波，對海底山生態系统之養分循環與垂直混和，對此海域的生產力與下游能量具有重要貢獻，並顯示海底山在貧養鹽黑潮海域扮演生命綠洲的角色。Kelvin-Helmholtz (KH) billows can drive microscale turbulent mixing around seamounts in the oligotrophic Kuroshio. This study sought to describe the influence of billow intensity, i.e., “intermittent and small” (small billow case; SBC) and “steady and large” billows (large billow case; LBC), on vertical nitrate fluxes, and to illustrate the responses of picoplankton and zooplankton assemblages to the varying environmental conditions on seamounts (Chapter 1). KH billows led to turbulent kinetic energy dissipation rates (ε = O (10-7–10-6) W kg-1) and eddy diffusivities (Kρ = O (10-4–10-3) m2 s-1) that were significantly stronger than those outside the billow depths. The mean nitrate flux estimated using Kρ in the billow depths had a maximal value of 10.0 mmol m-2 day-1, which was much higher than estimates for the open ocean (Chapter 2). The nitrate flux associated with the shallow KH billows contributed to subsurface enrichment by entraining existing nutrients, while the flux of deeper billows closer to the summit were more effective in vertically transporting nitrates directly from the deeper water. On the other hand, the picoplankton around the seamount area was consistentlydominated by heterotrophic bacteria (>50%; Chapter 3). The on-seamount station showed similar carbon biomass in comparison with those in onshore areas due to the increase in Synechococcus biomass. This suggests that a small area along the Kuroshio features coastal water-like properties near and around the seamount in oligotrophic waters. In addition, the zooplankton biomass was over 60% larger during the winter cruise when the Kuroshio’s velocity was strong (SKC; 104.5 ml 100m-3) than in the summer cruise when the Kuroshio was weaker (WKC; 33.7 ml 100m-3), and this variation was more likely influenced by the sampling season (Chapter 4). Furthermore, the distribution pattern of zooplankton concentrations revealed a seamount-induced “blocking effect”, which aggregates them within the impeding seamount and its flanks. However, a strong Kuroshio current can dismantle the blocking effect and quickly sweep the plankton away (i.e., SKC). Moreover, the high relative contributions of very early ontogenetic stages of larval fish underscored the importance of the seamount as an isolated habitat good for spawning and as a source of fish recruits. Overall, this study showed that KH billows make important contributions to seamount ecosystems, particularly in the cycling and vertical mixing of nutrients to make them available for local production and potential downstream transport. At the same time, this study exemplified the role of seamounts as the oasis of life in oligotrophic areas.