The effect of bathymetric modification on water age in the Elbe Estuary

Abstract

The transport time of substances is a physical factor that influences the completeness of biogeochemical reactions in the estuary. Since hydrodynamic changes induce changes in transport time, river discharge and its seasonal variability strongly determine the transport time of riverine water and its fluctuations. A factor that leads to a permanent change in hydrodynamics is man-made bathymetric modification. However, the impact of such modification on transport time has never been quantified. Here we show for the Elbe Estuary (Germany), an example of a partially to well-mixed alluvial estuary, that the impact of typical, decadal man-made bathymetric mod- ification on the transport time of riverine water is much smaller than the effect of the natural variability in river discharge. In this study, we used riverine water age to determine transport time. We found the age difference due to river discharge variation to be in the order of days to weeks, depending on the location within the estuary. In contrast to the strong influence of discharge, we found the age difference between scenarios which differ by the effect of 40 years of man-made bathymetric modification to be in the order of hours with a maximum of 38 hours, depending on location and discharge. Overall, riverine water age increases by approximately 7 % in the more strongly impacted bathymetry, suggesting that transport time is only slightly affected by the considerable depth differences of several meters in large parts of the estuary. With regard to the summer oxygen minimum zone, which regularly develops in the estua- rine freshwater section, we therefore expect the physical influence of the realistic modifi- cation via a change in transport time to be small. Nevertheless, the increase in transport time of land-borne material potentially poses an additional stressor to the dissolved oxygen dynamics in the estuary.