Workpackage 3 - Benthic-pelagic coupling

Coastal systems are strongly shaped by hydrodynamics, sediment structure, and resuspension, and are characterized by intense benthic–pelagic coupling that regulates the turnover of nutrients and organic matter. Physical processes control the exchange between the water column and sediments by driving advective porewater flows and by resuspending and redistributing organic particles. These particles act as transient hotspots that link pelagic production with benthic mineralization, creating heterogeneous microhabitats where microbial processes are concentrated and dynamically reorganized. A particular focus is on coastal hypoxia in the form of hypoxic waves, as well as on marine heatwaves, whose occurrence, intensity, and frequency are tightly coupled to these physically driven exchange processes and which strongly modulate the availability of oxygen and substrates, as well as microbial turnover.

WP3 aims to quantitatively assess the transformation of organic matter and the turnover and removal of nutrients through a combination of experimental approaches and in situ observations. The coupled cycles of nitrogen, carbon, phosphorus, and sulfur across the sediment–water interface will be resolved in space and time, explicitly linking external inputs (e.g., nutrient loads and particle fluxes) to benthic processing and export to deeper water layers. A central objective is to understand how physical transport processes regulate the availability of electron donors and acceptors, thereby controlling microbial reaction rates and pathwaysparticularly under dynamic oxygen and temperature conditions, including hypoxic waves and heat stress.

Research questions:

How do coastal hypoxia events develop, and what role do sandy sediments and organic particles play in this process?
How are nitrogen retention and loss regulated in coastal sediments?
What role do hypoxic events play in nitrogen losses?
How do oxygen deficiency events influence phosphorus dynamics in sediments, and what role do motile microorganisms play in this context?
What controls sulfur oxidation in coastal sands?

Methods

We use an integrated approach combining field measurements, experimental determinations of volumetric process rates, and reactive transport modeling to mechanistically resolve benthic-pelagic coupling. To quantify exchange between the water column and sediments, we deploy benthic landers that measure oxygen consumption and nutrient fluxes in situ. In addition, sediment cores collected by divers are investigated in controlled experiments to determine process rates of nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA).

Respiration rates are quantified both in situ and in the laboratory using incubations and high-resolution sensor techniques. We combine nutrient analyses with microsensors and optodes, molecular biological methods (sequencing, enrichment/cultivation), as well as isotope and radiotracer techniques. Oxygen dynamics are systematically manipulated using mass-flow-controlled gas mixtures to simulate natural variability, such as hypoxic events. Continuous phosphate measurements are conducted using autonomous sensors, complemented by ion chromatography and analyses of sulfur cycle components.

The measured process rates are embedded within the hydrodynamic and biogeochemical context of long-term mooring observations (WP1) and autonomous sampling (WP2). This enables a mechanistic and temporally high-resolution assessment of how episodic events and continuous forcing shape metabolic activity and benthic–pelagic exchange. All data are integrated into reactive transport models (WP4) to quantify and predict the interactions between physical transport and microbial reactions across scales and estimate their impacts on coasts and society (WP5).

Instruments

Chamber lander (Unisense)
modified free-falling particle camera (UVP6, Hydroptic) for high resolution measurements of particle abundances in the water column
Eddy Covariance Lander (Nortek ADV, Pyroscience AquapHOx) for oxygen flux measurements in coastal habitats
S2B Lander (in-house development) to measure oxygen profiles, current velocities, light
Membran-inlet Massspectroscopy (Bay Instruments) and Isotope Ratio Mass Spectrometry to measure microbial rates
Lab-on-a-chip Phosphate Sensor (Clearwater)
Recirculation flume (1 m) with particle image velocimetry setup (ILA 5150, Optolution)
Microsensors (optodes and electrochemical sensors), planar optodes for spatially resolved measurements of oxygen distributions.