Channel curvature improves water quality and nutrient filtering in an artificially deepened mesotidal idealized estuary


Estuarine ecology suffers from both physical aspects of human influence, such as dredging, and biogeochemical aspects, such as eutrophication. Apart from being dredged, modern estuaries often manifest rectified geometries deprived of meanders or other nonlinear topographic features. This study has two overarching aims, a theoretical and a practical one. The theoretical objective is to establish an understanding of the effect of physical dynamics induced by channel meanders on the biogeochemical dynamics in a typical estuarine oxygen minimum zone. The practical aim is to clarify whether and how channel curvature can mitigate the consequences of human intervention, such as dredging and eutrophication. To answer these questions, a coupled hydrodynamic and water quality model is applied to a pair of idealized funnel-shaped topographies with dimensions and axial depth distribution similar to the Elbe Estuary, Germany, serving as the prototype estuary in this study. The first topography is symmetric about the channel axis (straight channel), while the second topography contains a small section of sinusoidal meanders in the dredged limnic reach of the estuary. The setups are forced by an M-2 tide and daily salinity and temperature data at the seaward open boundary. Atmospheric and river forcings are based on regional operational and observational data to impose seasonal temperature variability and biogeochemical cycles. The model simulates tidally driven estuarine physics, as well as seasonal estuarine cycles and axial gradients of nutrients, oxygen and plankton that characterize alluvial human-shaped and eutrophied estuaries. The channel meanders in the lower limnic reaches lead to locally enhanced ebb dominance, vertical overturning and increased levels of turbulent kinetic energy. The curvature-induced dynamics decrease turbidity levels by up to 12.5% and increase oxygen concentrations by up to 14% in the area of the oxygen minimum zone, improving the ecological status of the eutrophied estuary. Finally, we assess the sustainability of the ecological benefits of the channel meanders in the face of global warming by applying a simple 2 °C warming scenario to the straight and meandering channel cases. We demonstrate that channel meanders potentially improve estuarine ecology even under increased pressure due to climate change.
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