Biogeochemical interactions in delta systems: present state and future challenges

Abstract

Estuarine intertidal mudflats play a crucial role in the remineralization of organic matter, nutrient recycling, and the filtration of pollutants and nutrients from terrestrial sources. Despite their importance, the global extent of mudflats is decreasing due to a range of direct and indirect anthropogenic stressors. This thesis was motivated by the urgent need to expand our understanding of the present state of biogeochemical dynamics in intertidal mudflats. Our primary aim was to determine the extent to which mudflats contribute to estuarine nutrient retention and how this contribution is influenced by seasonal fluctuations and biological activity. Furthermore, we sought to understand the potential modifications that may occur in the biogeochemical functioning of mudflats as a resulting of sand starvation and increased sediment resuspension. Since intertidal and subtidal areas experience distinct daily dynamics, it was necessary to determine the different biogeochemical processes occurring in each zone. We quantified the biogeochemical dynamics in both intertidal and subtidal areas of the Easter Scheldt tidal bay and discovered that intertidal sediments exhibited higher productivity and removed significantly higher amounts of nutrients compared to subtidal sediments. Conversely, subtidal sediments played a more prominent role in blue carbon storage. The relevance of this study becomes evident when considering the ongoing loss of intertidal areas due to sand starvation and the potential loss resulting from sea level rise. A decline in intertidal area would effectively diminish the nutrient removal capacity the Eastern Scheldt, compromising its biogeochemical functionality. Part of our study focused on biogeochemical dynamics and the macrobenthic community of intertidal mudflats in the Western Scheldt estuary. We discovered distinct differences among the fresh, brackish, and marine habitats. The estuarine filter became apparent, with freshwater sediments acting as a sink of phosphorous (P) and nitrogen (N), the brackish sediments as a sink of only N, and the marine as a source of both nutrients. Regarding abiotic and biotic influences on nutrient dynamics, we observed that both the local nutrient dynamics and the macrobenthic community were highly influenced by salinity, while temperature exerted an overarching effect on these dynamics as well as on the biological activity. Additionally, the bioturbating and bioirrigating activity of macrofauna played a crucial role in shaping the observed nutrient dynamics at each site, facilitating enhanced OM mineralization and microbial processes. The sediment of intertidal mudflats serves as a nutrient reservoir; however, occasional disturbances such as sediment resuspension can release porewaters and the nutrients within into the water column. While sediment resuspension is a natural process, our understanding of its influence on nutrient dynamics and nutrient concentrations in coastal waters and sediments remains limited. We investigated the effect of sediment resuspension in mudflats of both the Eastern and Western Scheldt and found that sediment resuspension did indeed release nutrients into the water column. However, the magnitude of this release depended on site exposure and the time elapsed since the last resuspension event. Interestingly, we found consistent differences in the response of phosphate to erosion between the estuary and the tidal bay suggesting that some nutrients may exhibit site-specific responses to sediment resuspension. These finding carry significance as it is anticipated that that storm frequency and strength will increase in the coming decades, leading to a greater release of nutrients with potential consequences for nutrient dynamics in coastal systems. This thesis underscores the critical importance of intertidal mudflats as active centres of biogeochemical transformations and nutrient removal, positioning them as invaluable components for maintaining the functionality of estuarine ecosystems. Moreover, emphasis is made on the potential consequences of the worldwide loss of mudflats on the ecosystem services provided by coastal systems.