Data_Sheet_2_The Importance of Mesozooplankton Diel Vertical Migration for Sustaining a Mesopelagic Food Web.xls

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.

本研究采用了在两次加州流生态系统长期生态系统研究巡航中从准拉格朗日实验中获得的广泛的生态和生物地球化学测量数据，运用线性逆生态系统模型（LIEM）对浮游生物上层与中层之间的碳通量进行了分析。模型测量约束包括放射性碳14（14C）初级生产力、稀释法测定的微型浮游动物摄食速率、基于肠道色素的中型浮游动物摄食速率（针对多种浮游动物体型等级）、234Th:238U 不平衡以及沉积物陷阱测量的碳输出，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）对潜在通量网络样本的结果不确定性进行估计。研究结果表明，中型浮游动物的主动运输（即昼夜垂直迁移）对于满足中层生物的碳需求以及从大气中吸收二氧化碳具有重要意义。在从沿海藻华到近海寡营养条件的九个水样中，中型浮游动物的主动运输占到了总碳向中层转移的18%-84%（中位数：42%），而颗粒有机碳的沉降（12%-55%；中位数：37%）和俯冲（2%-32%；中位数：14%）提供了剩余大部分的碳转移。垂直迁移的浮游动物通过在深水处的呼吸、排泄以及死亡损失给捕食性浮游动物和中层鱼类（如灯笼鱼和舌形鱼）的方式，对向下碳通量作出了贡献。敏感性分析表明，LIEM的结果对浮游动物代谢需求、细菌生产速率和中型浮游动物总生长效率的变化具有鲁棒性。这一分析表明，基于保守估计的标准（而非主动）代谢来估算的浮游动物主动运输的前期估计可能过低。