Zooplankton distribution and migration in low-oxygen modewater eddies

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 µmol O2 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 µmol O2 kg-1) OMZ just below the mixed layer could serve to identify zooplankton groups that may be negatively or positively affected by on-going ocean deoxygenation. In spring 2014, a detailed survey of a suboxic anticyclonic modewater 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 (Sv, shipboard ADCP, 75kHz) within the shallow OMZ of the eddy was evident compared to the nighttime distribution outside the eddy. Acoustic scatterers were avoiding the depth range between about 85 to 120 m, where oxygen concentrations were lower than approximately 20 µmol O2 kg-1, indicating habitat compression to the oxygenated surface layer. This observation is confirmed by time-series observations of a moored ADCP (upward looking, 300kHz) 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 have been identified to be followed by zooplankton in response to the eddy OMZ: 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 O2 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 µmol O2 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.东热带北大西洋（Eastern Tropical North Atlantic, ETNA）存在一个分布于约300~600米深度的中层海洋低氧区（mesopelagic oxygen minimum zone, OMZ）。该区域的氧浓度极少低于40 µmol O2 kg-1，但在未来全球变暖的情景下，其氧浓度预计会下降。近期发现的中尺度涡旋在混合层下方形成了一个浅表层亚缺氧（氧浓度<5 µmol O2 kg-1）低氧区，这一发现或可用于识别受当前海洋脱氧作用产生正向或负向影响的浮游动物类群。2014年春季，研究团队在佛得角海洋观测站（Cape Verde Ocean Observatory, CVOO）附近对一个亚缺氧反气旋模式水涡旋（anticyclonic modewater eddy, ACME）开展了详细调查，采用声学与光学剖面法、分层多网拖网及水文测量等多种手段。多网采样数据显示，该涡旋的总区域积分浮游动物丰度提升了约1.5倍。夜间，当大量声学散射体上升至150米以浅水层时，与涡旋外夜间分布相比，涡旋浅层低氧区内的平均体积反向散射强度（Sv，船载ADCP，75kHz）出现显著下降。声学散射体规避了约85~120米的深度区间——该区间氧浓度低于约20 µmol O2 kg-1，这表明浮游动物的栖息空间被压缩至含氧表层水层。2010年CVOO观测站系留ADCP（朝上安装，300kHz）的连续观测数据也证实了这一观测结果。不过，部分从表层水层向中层海洋迁移的昼夜垂直迁移（diurnal vertical migration, DVM）活动仍穿过了该浅层低氧区。基于分层多网拖网、水下视觉剖面仪（Underwater Vision Profiler, UVP5）以及ADCP数据，研究人员已确定浮游动物响应涡旋低氧区的四种策略：i）规避浅层低氧区并向表层水层压缩栖息空间（如多数哲水蚤目桡足类、磷虾类）；ii）日间迁移至浅层低氧区核心，但夜间在表层水层偿还氧债（如管水母类、尖角剑水蚤属（Oncaea spp.）、真哲水蚤类桡足类）；iii）昼夜均栖息于浅层低氧区内（如介形类、多毛类）；iv）从更深的含氧水层向表层进行昼夜垂直迁移并往返，途经浅层低氧区。对于策略i、ii和iv，表层可栖息空间的压缩可能会提升猎物-捕食者的相遇概率，使得浮游动物与微型游泳生物更易被捕食，进而可能使涡旋表层成为更高营养级生物的觅食热点。针对海洋脱氧的长期影响，我们预计当氧浓度降至约20 µmol O2 kg-1以下时，中层海洋低氧区将出现浮游动物的规避行为。由于低氧区内的浮游动物与微型游泳生物的呼吸作用，以及溶解态和颗粒态有机物向低氧区的主动通量均会下降，这可能会对低氧区的氧消耗速率形成正反馈效应。