The availability of light in the water column is essential for the life of marine organisms. Water color allows conclusions to be drawn about water transparency and light availability. Several studies show that there is a global decline in phytoplankton or that there are regionally different developments of ocean color towards bluish or greenish. Coastal areas and their changes with respect to light availability in the water are not considered much in the studies mentioned above. Coastal waters are sensitive to changes in abiotic conditions, which have not been adequately addressed with respect to spectral light observations. Coastal Ocean Darkening examines the past and future 100 years for potential reductions in light availability in coastal waters, through a combination of historical observations, hyperspectral measurements, biodiversity studies, and model scenarios. Radiometer data at the Time-Series Station Spiekeroog provide measurements of apparent optical properties from which water transparency and color can be inferred. This allows a detailed investigation of possible trends in these parameters in the Wadden Sea, where changes in environmental conditions are of particular interest with respect to biodiversity and primary productivity.