A large eddy simulation study of the formation of deep chlorophyll/biological maxima in un‐stratified mixed layers: The roles of turbulent mixing and predation pressure
A large eddy simulation study of the formation of deep chlorophyll/biological maxima in un‐stratified mixed layers: The roles of turbulent mixing and predation pressure
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非分层混合层中深层叶绿素/生物最大值形成的大涡模拟研究：湍流混合和捕食压力的作用
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发表时间：
2017
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通讯作者：
J. T. Siddons
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作者：
D. Lewis;A. Brereton;J. T. Siddons
最近在开放海洋的弱分层边界层中记录的荧光值和湍流能量耗散率的实验测量强调了深部叶绿素最大值（DCM）的形成与湍流混合之间的显着相关性。具体而言，观察到许多 DCM 的深度位于能量耗散率曲线达到其最大值的点下方，但在大约一个标准偏差之内。 DCM 和湍流混合的这种相关性既令人兴奋又令人好奇，因为传统思维倾向于将后者视为深层生物最大值 (DBM) 形成的破坏性而非建设性因素，而 DCM 数据通常被解释为代表。为了研究这一现象，将海洋边界层的三维大涡模拟（LES）与通用营养物-浮游植物-浮游动物（NPZ）型生物模型相结合，以确定可能驱动实验观察的机制。基于多组通用生物参数的 LES-NPZ 模型模拟表明，DCM/DBM 的形成发生在接近实验观察结果的归一化深度。模拟结果支持这样的假设，即 DBM 是由浮游动物捕食压力限制近地表浮游植物生长以及通过边界层平流营养物的垂直混合过程强度下降共同产生的。同时，这些产生了一个水柱区域，其中捕食压力相对较低，营养物聚集相对较高，为 DBM 形成提供了合适的条件。
Recent experimental measurements of fluorescence values and turbulent energy dissipation rates, recorded in weakly stratified boundary layers in the open ocean, have highlighted a significant correlation between the formation of deep chlorophyll maxima (DCM) and turbulent mixing. Specifically, the depth of many DCM are observed to lie below, but within about one standard deviation, of the point at which the energy dissipation rate profile reaches its maximum. This correlation of DCM and turbulent mixing is both exciting and curious, as conventional thinking tends to see the latter as a destructive rather than a constructive agent in regards to the formation of deep biological maxima (DBM), for which DCM data is usually interpreted as a proxy. In order to investigate this phenomenon, a three‐dimensional large eddy simulation (LES) of the ocean boundary layer was combined with a generic nutrient‐phytoplankton‐zooplankton (NPZ) type biological model, in order establish what mechanisms might be driving the experimental observations. Simulations of the LES‐NPZ model, based upon various sets of generic biological parameters, demonstrate DCM/DBM formation occurs at normalized depths close to those seen in the experimental observations. The simulations support the hypothesis that the DBM are generated by a combination of zooplankton predation pressure curtailing phytoplankton growth near the surface, and a decline in the strength of the vertical mixing processes advecting nutrient through the boundary layer. In tandem, these produce a region of the water column in which predation pressure is relatively low and nutrient aggregation relatively high, suitable conditions for DBM formation.