Table_1_The Importance of Mesozooplankton Diel Vertical Migration for Sustaining a Mesopelagic Food Web.docx

We used extensive ecological and biogeochemical measurements obtained from quasi-Lagrangian experiments during two California Current Ecosystem Long-Term Ecosystem Research cruises to analyze carbon fluxes between the epipelagic and mesopelagic zones using a linear inverse ecosystem model (LIEM). Measurement constraints on the model include 14C primary productivity, dilution-based microzooplankton grazing rates, gut pigment-based mesozooplankton grazing rates (on multiple zooplankton size classes), 234Th:238U disequilibrium and sediment trap measured carbon export, and metabolic requirements of micronekton, zooplankton, and bacteria. A likelihood approach (Markov Chain Monte Carlo) was used to estimate the resulting flow uncertainties from a sample of potential flux networks. Results highlight the importance of mesozooplankton active transport (i.e., diel vertical migration) in supplying the carbon demand of mesopelagic organisms and sequestering carbon dioxide from the atmosphere. In nine water parcels ranging from a coastal bloom to offshore oligotrophic conditions, mesozooplankton active transport accounted for 18–84% (median: 42%) of the total carbon transfer to the mesopelagic, with gravitational settling of POC (12–55%; median: 37%), and subduction (2–32%; median: 14%) providing the majority of the remainder. Vertically migrating zooplankton contributed to downward carbon flux through respiration and excretion at depth and via mortality losses to predatory zooplankton and mesopelagic fish (e.g., myctophids and gonostomatids). Sensitivity analyses showed that the results of the LIEM were robust to changes in nekton metabolic demand, rates of bacterial production, and mesozooplankton gross growth efficiency. This analysis suggests that prior estimates of zooplankton active transport based on conservative estimates of standard (rather than active) metabolism are likely too low.

本研究利用在两次加州洋流生态系统长期生态系统研究巡航中从准拉格朗日实验获取的广泛生态和生物地球化学测量数据，采用线性逆生态系统模型（LIEM）分析了表流层与中层水层之间的碳通量。模型测量约束包括14C初级生产力、稀释法微浮游生物摄食率、基于肠道色素的中层浮游动物摄食率（涉及多个浮游动物体型类别）、234Th:238U不均衡和沉积陷阱测量的碳输出，以及微型浮游动物、浮游动物和细菌的代谢需求。通过似然方法（马尔可夫链蒙特卡洛）估计了从潜在的通量网络样本中产生的结果流量不确定性。研究结果突出了中层浮游动物主动运输（即昼夜垂直迁移）在满足中层水层生物碳需求以及从大气中固定二氧化碳方面的重要性。在从沿海浮游生物到近海寡营养条件的水样中，中层浮游动物主动运输占总体碳传输至中层水层的18-84%（中位数：42%），其中颗粒有机碳（POC）的沉降（12-55%；中位数：37%）和俯冲（2-32%；中位数：14%）提供了剩余部分的大部分。垂直迁移的浮游动物通过在深处的呼吸和排泄以及通过死亡损失给捕食性浮游动物和中层水层鱼类（例如鲱鱼和舌形鱼）贡献了向下碳通量。敏感性分析表明，LIEM的结果对浮游动物代谢需求、细菌生产速率和中层浮游动物总生长效率的变化具有鲁棒性。此分析表明，基于保守的（而非主动的）代谢估算的先前对浮游动物主动运输的估计可能过低。