Filaments and fronts play a crucial role for a net offshore and downward nutrient transport in Eastern Boundary Upwelling Systems (EBUSs) and thereby reduce regional primary production. Most studies on this topic are based on either observations or model simulations, but only seldom are both approaches are combined quantitatively to assess the importance of filaments for primary production and nutrient transport. Here we combine targeted interdisciplinary shipboard observations of a cold filament off Peru with submesoscale-permitting ($\mathrm{1}/\mathrm{45}$^∘) coupled physical (Coastal and Regional Ocean Community model, CROCO) and biogeochemical (Pelagic Interaction Scheme for Carbon and Ecosystem Studies, PISCES) model simulations to (i) evaluate the model simulations in detail, including the timescales of biogeochemical modification of the newly upwelled water, and (ii) quantify the net effect of submesoscale fronts and filaments on primary production in the Peruvian upwelling system. The observed filament contains relatively cold, fresh, and nutrient-rich waters originating in the coastal upwelling. Enhanced nitrate concentrations and offshore velocities of up to 0.5 m s⁻¹ within the filament suggest an offshore transport of nutrients. Surface chlorophyll in the filament is a factor of 4 lower than at the upwelling front, while surface primary production is a factor of 2 higher. The simulation exhibits filaments that are similar in horizontal and vertical scale compared to the observed filament. Nitrate concentrations and primary production within filaments in the model are comparable to observations as well, justifying further analysis of nitrate uptake and subduction using the model. Virtual Lagrangian floats were released in the subsurface waters along the shelf and biogeochemical variables tracked along the trajectories of floats upwelled near the coast. In the submesoscale-permitting ($\mathrm{1}/\mathrm{45}$^∘) simulation, 43 % of upwelled floats and 40 % of upwelled nitrate are subducted within 20 d after upwelling, which corresponds to an increase in nitrate subduction compared to a mesoscale-resolving ($\mathrm{1}/\mathrm{9}$^∘) simulation by 14 %. Taking model biases into account, we give a best estimate for subduction of upwelled nitrate off Peru between 30 %– 40 %. Our results suggest that submesoscale processes further reduce primary production by amplifying the downward and offshore export of nutrients found in previous mesoscale studies, which are thus likely to underestimate the reduction in primary production due to eddy fluxes. Moreover, this downward and offshore transport could also enhance the export of fresh organic matter below the euphotic zone and thereby potentially stimulate microbial activity in regions of the upper offshore oxygen minimum zone.

中文翻译：

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由亚中尺度细丝和前沿塑造的秘鲁上涌硝酸盐的命运

细丝和前沿对东部边界上升流系统 (EBUS) 中的净离岸和向下养分传输起着至关重要的作用，从而减少了区域初级生产。大多数关于该主题的研究基于观察或模型模拟，但很少将这两种方法定量结合以评估细丝对初级生产和养分运输的重要性。在这里，我们将秘鲁附近冷灯丝的有针对性的跨学科船载观测与亚中尺度许可相结合（$\mathrm{1}/\mathrm{45}$^∘ ) 耦合物理（沿海和区域海洋社区模型，CROCO）和生物地球化学（碳和生态系统研究的远洋相互作用计划，PISCES）模型模拟​​，以 (i) 详细评估模型模拟，包括新生物地球化学改造的时间尺度上涌水，以及 (ii) 量化亚中尺度锋和细丝对秘鲁上涌系统初级生产的净影响。观察到的细丝含有源自沿海上升流的相对寒冷、新鲜和营养丰富的水域。提高硝酸盐浓度和高达 0.5 m s ^{-1 的}离岸速度在细丝内表明营养物质的离岸运输。长丝中的表面叶绿素比上升流前沿低 4 倍，而表面初级生产高 2 倍。与观察到的细丝相比，模拟展示了在水平和垂直尺度上相似的细丝。模型中细丝内的硝酸盐浓度和初级生产也与观察结果相当，证明使用该模型进一步分析硝酸盐吸收和俯冲是合理的。虚拟拉格朗日浮标沿陆架在地下水中释放，生物地球化学变量沿海岸附近上升流的浮标轨迹进行追踪。在亚尺度允许（$\mathrm{1}/\mathrm{45}$^∘ ) 模拟，43% 的上涌浮游物和 40% 的上涌硝酸盐在上涌后 20 d 内被俯冲，这对应于与中尺度分辨率相比的硝酸盐俯冲增加（$\mathrm{1}/\mathrm{9}$^∘ ) 模拟 14%。考虑到模型偏差，我们给出了秘鲁附近上升流硝酸盐俯冲 30%–40% 的最佳估计。我们的结果表明，亚中尺度过程通过放大先前中尺度研究中发现的营养物质的向下和离岸输出，进一步减少了初级生产，因此可能低估了涡流导致初级生产的减少。此外，这种向下和近海的运输还可以增强透光区下方新鲜有机物的输出，从而潜在地刺激上近海最低氧区区域的微生物活动。