AbstractTo meet the requirements of increasing environmental awareness in aquatic ecosystems, optical techniques offer a fast and reliable opportunity for a wide range of application providing spatio-temporal measurements. Within the work package 5 of NeXOS innovative, compact, and cost efficient multifunctional sensor systems for optical measurements of marine environmental parameters will be developed. Addressed parameters include phytoplankton biomass and taxonomic composition, total suspended matter and chromophoric dissolved organic matter (CDOM), as well as hazardous substances, e.g. polycyclic aromatic hydrocarbons (PAHs). Requiring comparable less effort, optical methods are a convenient and noninvasive way to derive information on the optical active substances in different water bodies. Various approaches and devices are available, either aiming on the determination of the wateŕs inherent optical properties or on measuring the fluorescence properties of the different constituents. This contribution summarizes the objectives and approaches within the optical sensor development of the NeXOS project with a special focus on an integrating cavity approach. This approach overcomes the two common problems occurring in classical absorption measurement of seawater: (i) usually low concentration of absorbing material in the water negatively affecting accurate measurements of untreated samples, and (ii) errors introduced by light scattering on particles, which require empirical corrections of the measurements to obtain good accuracy. An example for a device based on an integrating cavity is the point-source integrating cavity absorption meter (PSICAM). Because of its advantages, an adaptation of the PSICAM for continuous flow-through operation is of interest to provide high resolution measurements of absorption coefficients with good accuracy. The continuous observation system is planned to be used stand-alone or in combination with unattended systems, e.g. FerryBox.- In this contribution, the setup and first field results of the flow-through-PSICAM (ft-PSICAM) are reviewed. Furthermore, absorption coefficients as proxies for chlorophyll-a and total suspended matter are evaluated. Within the optical sensor package of NeXOS, a further objective is the development of a matrix-fluorescence sensor with flexible wavelength configuration for the detection and characterization of dissolved substances, such as CDOM and PAHs. Here, objectives and first approaches will be presented with special focus on CDOM fluorescence as main linkage to the ft-PSICAM. Finally, future plans for both instruments as well as their combination will be presented. In summary, both approaches have the potential to be multi-parameter instruments for high-resolution measurements of environmental relevant parameters.