The IOW's contribution to DArgo2025 focusses on:

• Sensor-based nitrate measurements by BGC-Argo Floats. The main work area is the Baltic Sea, where no German Argo activities took place so far.

The only commercially available nitrate sensor for BGC-Argo Floats so far is SUNA sensor from Satlantic (Sea-Bird Scientific, USA). In order to extend sensor options, the OPUS nitrate sensor by TriOS GmbH (Germany) will be characterized and used on floats alongside the SUNA sensor for comparison. The aim is to obtain a realistic assessment of the OPUS sensor for global deployment and thus the potential diversification of the available BGC-Argo sensors. Through the float deployments in the Baltic Sea, the German Argo activities are extended to this marine region and the IOW becomes involved as a new Argo actor. Furthermore, the Baltic Sea as a deployment area allows the validation and improvement of the data processing and quality control routines in an environment showing additional optically active, absorbing substances, which also benefits the accuracy of nitrate measurements in the deep global ocean.

• The coordination of the biogeochemical work with the DArgo2025 project partners.

The Argo data management system with its 11 globally distributed data centers, 8 core- and BGC-Argo parameters and 4 main file types per float shows a not always intuitive complexity. Nevertheless, it is operational, successfully serves a large number of users every day, and is constantly evolving according to the latest needs and scientific findings. The aim is to integrate the work of the project partners into this Argo context and to ensure a maximum impact of the gained knowledge on an international level. For this purpose, the biogeochemical work is coordinated nationally between the project partners (IOW, ICBM, GEOMAR and BSH) following the Argo philosophy and matched with the international BGC-Argo.