In Situ Observations of Biological and Environmental Parameters by Means of Optics—Development of Next-Generation Ocean Sensors With Special Focus on an Integrating Cavity Approach

To meet the requirements of increasing environmental awareness in aquatic ecosystems, optical techniques offer a fast and reliable opportunity for a wide range of applications providing high-resolution measurements. In this respect, important parameters which have to be addressed include phytoplankton biomass and taxonomic composition, total suspended matter, dissolved organic matter, as well as hazardous substances, e.g., polycyclic aromatic hydrocarbons (PAHs). Requiring comparable low 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 water's inherent optical properties or on measuring the fluorescence properties of different constituents. This contribution presents the objectives and measurement principles of two new optical sensor developments in this respect. A special focus lies on an integrating cavity approach for hyperspectral absorption measurement. This approach overcomes two common problems in classical absorption measurement of seawater: 1) usually low concentration of absorbing material in the water negatively affecting accurate measurements of untreated samples; and 2) errors introduced by light scattering of particles requiring empirical corrections to obtain good accuracy. To combine these advantages with the possibilities of automated, long-term high-frequency measurements, an integrating cavity was adapted for flow-through operation. First field results obtained by the resulting Hyperspectral Absorption Sensor (HyAbS) in the North Sea and off the Norwegian coast are evaluated and compared with discrete measurements. The second development is a matrix-fluorescence sensor with flexible wavelength configuration for the detection and characterization of dissolved substances, such as fluorescent dissolved organic matter (FDOM) and PAHs. Here, the measurement principle of the sensor and first field results from a related, laboratory-based method will be presented as a preliminary work necessary for the development of the final in situ sensor. Furthermore, future plans for both instruments as well as a possible combination will be discussed. In summary, both approaches have the potential to be multiparameter instruments for high-resolution measurements of environmental parameters.