Dead zone or oasis in the open ocean? Zooplankton distribution and migration in low-oxygen modewater eddies
Dead zone or oasis in the open ocean? Zooplankton distribution and migration in low-oxygen modewater eddies
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DOI：
10.5194/bg-13-1977-2016
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发表时间：
2016-01-01
期刊：
影响因子：
4.9
通讯作者：
Fiedler, Bjoern
中科院分区：
文献类型：
--
作者：
Hauss, Helena;Christiansen, Svenja;Fiedler, Bjoern
北大西洋热带东部（ETNA）在大约300 - 600米深度处有一个中层海洋低氧区（OMZ）。在此处，氧浓度很少低于40微摩尔O₂/千克，但在全球变暖的未来预测情景下预计会下降。最近发现的中尺度涡旋在混合层下方含有一个浅的亚缺氧（<5微摩尔O₂/千克）的低氧区，这有助于确定可能受到持续海洋脱氧影响（负面或正面）的浮游动物类群。2014年春季，在佛得角海洋观测站（CVOO）附近对一个亚缺氧的反气旋模态水涡旋（ACME）进行了详细调查，将声学和光学剖面测量方法与分层多层网捕捞和水文测量相结合。多层网数据显示，该涡旋的特征是浮游动物总面积积分丰度增加了约1.5倍。在夜间，当大量声学散射体上升到上部150米时，与涡旋外的夜间分布相比，涡旋浅低氧区内75千赫兹的平均体积后向散射（Sv）明显大幅降低（船载声学多普勒海流剖面仪，ADCP）。声学散射体避开了大约85至120米的深度范围，此处氧浓度低于约20微摩尔O₂/千克，这表明栖息地压缩到了富氧表层。2010年在CVOO系泊点的一个ACME过境期间，一个系泊的ADCP（向上观测，300千赫兹）的时间序列观测证实了这一观察结果。然而，从表层到中层海洋的部分昼夜垂直迁移（DVM）仍穿过浅低氧区继续进行。基于垂直分层的多层网捕捞、水下视觉剖面仪（UVP5）和ADCP数据，已确定浮游动物针对涡旋低氧区采取的四种策略：（i）避开浅低氧区并在表层压缩（例如大多数哲水蚤、磷虾）；（ii）在白天迁移到浅低氧区核心，但在夜间在表层偿还O₂债务（例如管水母、长腹水蚤属、真哲水蚤）；（iii）日夜都栖息在浅低氧区内（例如介形虫、多毛类）；（iv）从较深的富氧深度通过浅低氧区进行昼夜垂直迁移到表层然后再返回。对于策略（i）、（ii）和（iv），表层可栖息体积的压缩可能会增加猎物 - 捕食者的相遇率，使浮游动物和小型游泳生物更容易受到捕食，并且可能使涡旋表面成为更高营养级的觅食热点。关于海洋脱氧的长期影响，如果氧含量下降到低于约20微摩尔O₂/千克，我们预计会出现避开中层海洋低氧区的情况。这可能会对低氧区的氧消耗率产生正反馈，因为低氧区内浮游动物和小型游泳生物的呼吸以及溶解和颗粒有机物质向低氧区的主动通量将会下降。
The eastern tropical North Atlantic (ETNA) features a mesopelagic oxygen minimum zone (OMZ) at approximately 300-600 m depth. Here, oxygen concentrations rarely fall below 40 mu mol O-2 kg(-1), but are expected to decline under future projections of global warming. The recent discovery of mesoscale eddies that harbour a shallow suboxic (< 5 mu mol O-2 kg(-1)) OMZ just below the mixed layer could serve to identify zooplankton groups that may be negatively or positively affected by ongoing ocean deoxygenation. In spring 2014, a detailed survey of a suboxic anticyclonic mode-water eddy (ACME) was carried out near the Cape Verde Ocean Observatory (CVOO), combining acoustic and optical profiling methods with stratified multinet hauls and hydrography. The multinet data revealed that the eddy was characterized by an approximately 1.5-fold increase in total area-integrated zooplankton abundance. At nighttime, when a large proportion of acoustic scatterers is ascending into the upper 150 m, a drastic reduction in mean volume backscattering (S-v) at 75 kHz (shipboard acoustic Doppler current profiler, ADCP) within the shallow OMZ of the eddy was evident compared to the nighttime distribution outside the eddy. Acoustic scatterers avoided the depth range between approximately 85 to 120 m, where oxygen concentrations were lower than approximately 20 mu mol O-2 kg(-1), indicating habitat compression to the oxygenated surface layer. This observation is confirmed by time series observations of a moored ADCP (upward looking, 300 kHz) during an ACME transit at the CVOO mooring in 2010. Nevertheless, part of the diurnal vertical migration (DVM) from the surface layer to the mesopelagic continued through the shallow OMZ. Based upon vertically stratified multinet hauls, Underwater Vision Profiler (UVP5) and ADCP data, four strategies followed by zooplankton in response to in response to the eddy OMZ have been identified: (i) shallow OMZ avoidance and compression at the surface (e.g. most calanoid copepods, euphausiids); (ii) migration to the shallow OMZ core during daytime, but paying O-2 debt at the surface at nighttime (e.g. siphonophores, Oncaea spp., eucalanoid copepods); (iii) residing in the shallow OMZ day and night (e.g. ostracods, polychaetes); and (iv) DVM through the shallow OMZ from deeper oxygenated depths to the surface and back. For strategy (i), (ii) and (iv), compression of the habitable volume in the surface may increase prey-predator encounter rates, rendering zooplankton and micronekton more vulnerable to predation and potentially making the eddy surface a foraging hotspot for higher trophic levels. With respect to long-term effects of ocean deoxygenation, we expect avoidance of the mesopelagic OMZ to set in if oxygen levels decline below approximately 20 mu mol O-2 kg(-1). This may result in a positive feedback on the OMZ oxygen consumption rates, since zooplankton and micronekton respiration within the OMZ as well as active flux of dissolved and particulate organic matter into the OMZ will decline.