Biogeochemical interactions in delta systems: present state and future challenges
Authors:   , Rios Yunes Dunia 
Published: February 2024

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.