Estuaries are among the world’s most productive ecosystems, but recent natural and anthropogenic changes have stressed these ecosystems. Tools to assess estuarine pelagic habitats are important to support and maintain healthy ecosystem function. In this work, we demonstrate that estuarine pelagic habitats can be identified by a simple ratio, termed the LE ratio, that takes into account the tidal excursion along a channel (a Lagrangian length scale) and the distance along that channel (an Eulerian length scale). To develop and assess this concept, numerical simulations of the 1D advection–dispersion equation of a conservative tracer and tidal excursion estimates based on data were used to formulize a conceptual model and to define exchange zones within a tidal channel. This conceptual model was then used to predict the extent of pelagic habitats in a terminal channel network in the Sacramento–San Joaquin Delta. Exchange zones mapped onto these channels were found to be in good agreement with independent estimates of residence time. Sensitivity analyses of the numerical model suggest that productive pelagic habitats can be expanded by a factor of 2 by either increasing dispersion or increasing spring–neap variability in mean tidal velocity. Such changes can also enhance flushing in upper channel reaches. These findings are relevant for tidal marsh restoration projects that aim to expand beneficial aquatic habitats by varying exchange or residence time over the spring–neap cycle, because this variability may interact synergistically with varying rates of phytoplankton growth due to spatiotemporal changes in environmental conditions.

河口是世界上生产力最高的生态系统之一，但是最近的自然和人为变化使这些生态系统受到压力。评估河口浮游生物栖息地的工具对于支持和维持健康的生态系统功能非常重要。在这项工作中，我们证明了可以通过一个简单的比率（称为LE）来识别河口中上层生境比率，其中考虑了沿通道的潮汐偏移（拉格朗日长度标尺）和沿该通道的距离（欧拉长度标尺）。为了发展和评估这一概念，使用保守示踪剂的一维对流扩散方程的数值模拟和基于数据的潮汐偏移估计来形成概念模型并定义潮汐通道内的交换区。这个概念模型随后被用来预测萨克拉曼多-圣华金三角洲末端通道网络中的浮游生物栖息地的范围。发现映射到这些通道上的交换区域与驻留时间的独立估计值非常一致。数值模型的敏感性分析表明，通过增加平均潮汐速度的离散度或增加春季至春季的变异性，可以将生产性中上层生境扩大2倍。这样的变化还可以增强上游通道的冲刷。这些发现与潮汐沼泽恢复项目有关，该项目旨在通过在春季-春季周期内改变交换或停留时间来扩大有益的水生生境，因为由于环境条件的时空变化，这种变化可能与浮游植物的生长速率产生协同作用。