Contemporary and future stresses on estuaries: examples from the Yellow River Delta, the Vietnamese Mekong Delta, and the German Bight

Abstract

Formed where the water from rivers meets the sea, processes in estuaries are driven by the mixing between freshwater and seawater. Being home to diverse plant and animal communities, which have adapted to this unique environment, estuaries are one of the most productive ecosystems in the world. Providing a multitude of ecosystem services, estuaries are also of high economic value and contribute to human well-being. Besides providing habitats to aquatic species, estuaries are a source of food and raw materials while also cycling nutrients and contributing to coastal protection by damping the damaging effects of extreme events (e.g., storm surges). In addition, estuaries ensure safe navigation to and from ports and are used for recreational activities. However, with many of the world’s largest cities located on estuaries, they are directly exposed to impacts from human activity, such as overexploitation of resources or pollution. Being located in low-lying coastal areas, estuaries are also vulnerable to sea-level rise while simultaneously being impacted by climate change-induced alterations in hydrology. The combination of human-driven and climate-induced changes may lead to the degradation or loss of estuarine ecosystems and the services they provide. In order to minimize negative impacts and to promote a sustainable management of estuaries, it is thus important to investigate how estuarine environments respond to drivers of contemporary and future changes.

Since no two estuaries are alike, examples from the Yellow River Delta (China), the Mekong Delta (Vietnam), and the German Bight are presented in this thesis. Major drivers, which impact these focus regions, include: sand mining/dredging, damming, climate change-induced alterations in hydrology, and sea-level rise. The aim of this thesis is to improve the understanding of how the selected estuaries are impacted by predominant contemporary and projected future drivers. This is accomplished by addressing different research questions with a focus on: (i) improving methods for assessing the present-day impact of selected drivers, (ii) improving projections by addressing less visible impacts and by integrating recently identified relevant processes, and (iii) applying different scenarios of plausible future developments in estuarine environments.

(i) In a first study focusing on the Vietnamese Mekong Delta, improved methods were used to gain new insights into the intensity of regional sand mining activity. It was shown that the regional extraction of sand from the Mekong riverbed, which is driven by socio-economic developments in the region, is significantly higher than the river’s natural supply of sand. These findings have strong implications for the stability of the Vietnamese Mekong Delta, which is already subject to riverbank and coastal erosion under present-day conditions.

(ii) Projections of future developments in estuaries were improved by addressing less visible impacts in numerical models and by integrating previously unaddressed processes. A second study concentrating on the Vietnamese Mekong Delta was used to project, for the first time, the morphodynamic evolution of the Mekong in response to a combination of major drivers, including sand mining, damming, climate change-induced alterations in hydrology, and sea-level rise. In a third study, which focuses on the North Sea, it could also be shown that the morphological evolution of intertidal flats in the Wadden Sea has a significant impact on the tidal dynamics in the region when considering future sea-level rise.

(iii) By applying numerous plausible scenarios of future developments, the second study concentrating on the Mekong was able to identify the operation of hydropower dams as the major driver for future morphodynamic changes in the region, followed by sand extraction. Furthermore, this approach enabled to investigate the local interactions between different drivers. By combining different rates of sea-level rise with various rates of vertical accretion in the intertidal flats of the Wadden Sea, several plausible scenarios were also addressed in the study focusing on North Sea tides. If no vertical accretion is assumed in the intertidal flats, sea-level rise will lead to enhanced tidal asymmetry in the German estuaries Elbe, Weser, and Ems, potentially leading to increased sediment import. In contrast, tidal asymmetries resemble present-day conditions if intertidal flats are able to keep up with sea-level rise.