PDF(Pubmed)
Abstract：
Decline of the dissolved oxygen in the ocean is a growing concern, as it may eventually lead to global anoxia, an elevated mortality of marine fauna and even a mass extinction. Deoxygenation of the ocean often results in the formation of oxygen minimum zones (OMZ): large domains where the abundance of oxygen is much lower than that in the surrounding ocean environment. Factors and processes resulting in the OMZ formation remain controversial. We consider a conceptual model of coupled plankton-oxygen dynamics that, apart from the plankton growth and the oxygen production by phytoplankton, also accounts for the difference in the timescales for phyto- and zooplankton (making it a \"slow-fast system\") and for the implicit effect of upper trophic levels resulting in density dependent (nonlinear) zooplankton mortality. The model is investigated using a combination of analytical techniques and numerical simulations. The slow-fast system is decomposed into its slow and fast subsystems. The critical manifold of the slow-fast system and its stability is then studied by analyzing the bifurcation structure of the fast subsystem. We obtain the canard cycles of the slow-fast system for a range of parameter values. However, the system does not allow for persistent relaxation oscillations; instead, the blowup of the canard cycle results in plankton extinction and oxygen depletion. For the spatially explicit model, the earlier works in this direction did not take into account the density dependent mortality rate of the zooplankton, and thus could exhibit Turing pattern. However, the inclusion of the density dependent mortality into the system can lead to stationary Turing patterns. The dynamics of the system is then studied near the Turing bifurcation threshold. We further consider the effect of the self-movement of the zooplankton along with the turbulent mixing. We show that an initial non-uniform perturbation can lead to the formation of an OMZ, which then grows in size and spreads over space. For a sufficiently large timescale separation, the spread of the OMZ can result in global anoxia.
摘要：
海洋中溶解氧的下降越来越令人担忧，因为它最终可能导致全球缺氧，海洋动物死亡率上升，甚至大规模灭绝。海洋的脱氧通常会导致形成最小氧气区（OMZ）：大域的氧气丰度远低于周围海洋环境。导致OMZ形成的因素和过程仍然存在争议。我们考虑了浮游生物-氧气动力学耦合的概念模型，除了浮游生物的生长和浮游植物的氧气产生，还解释了浮游植物和浮游动物的时间尺度差异（使其成为“慢-快系统”）以及高营养水平的隐含效应，导致密度相关（非线性）浮游动物死亡率。使用分析技术和数值模拟相结合的方法对模型进行了研究。慢速-快速系统被分解为慢速和快速子系统。然后通过分析快速子系统的分岔结构，研究了慢-快系统的临界流形及其稳定性。对于一系列参数值，我们获得了慢速系统的canard周期。然而,系统不允许持续的弛豫振荡；相反，鸭类循环的爆炸导致浮游生物灭绝和氧气消耗。对于空间显式模型，在这个方向上的早期工作没有考虑浮游动物的密度依赖性死亡率，因此可以表现出图灵模式。然而,将密度依赖性死亡率纳入系统可以导致固定的图灵模式。然后在图灵分叉阈值附近研究系统的动力学。我们进一步考虑了浮游动物的自我运动以及湍流混合的影响。我们证明了初始的非均匀扰动可以导致OMZ的形成，然后扩大大小并在空间上传播。对于足够大的时间尺度分离，OMZ的传播可导致全球缺氧。
