A Lagrangian-to-Eulerian Metric to Identify Estuarine Pelagic Habitats
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A Lagrangian-to-Eulerian Metric to Identify Estuarine Pelagic Habitats Paul R. Stumpner 1
&
Jon R. Burau 1 & Alexander L. Forrest 2,3
Received: 13 February 2020 / Revised: 19 October 2020 / Accepted: 23 October 2020 # This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020
Abstract 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. Keywords Lagrangian . Eulerian . Pelagic habitat . Tidal excursion . LE ratio . Marsh restoration
Introduction Ecological theory supports the idea that habitat complexity (i.e., spatiotemporal differences in environmental conditions) and suitability (the capacity to support aquatic organisms) in estuaries result from the interaction between spatially variable landscape features (e.g., morphology and elevation) and fluctuating physical transport rates (e.g., exchange and residence times) (Peterson 2003). Habitat complexity promotes Communicated by David K. Ralston * Paul R. Stumpner [email protected] 1
US Geological Survey, California Water Science Center, 1280 Terminal St, West Sacramento, CA 95691, USA
2
Civil and Environmental Engineering, University of California – Davis, One Shields Avenue, Davis, CA 95616, USA
3
Tahoe Environmental Research Center, University of California – Davis, 291 Country Club Dr., Incline Village, NV 89451, USA
diversity in estuarine species, which are uniquely adapted to tolerate variable conditions (Moyle et
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