Influence of atmospheric forcing and freshwater discharge on interannual variability of the vertical diffuse attenuation coefficient at 490 nm in the Baltic Sea

Each year, spatial patterns of ocean color in the Baltic Sea differ in temporal evolution and magnitude. We have investigated the interannual variability of the spatially averaged vertical diffuse attenuation coefficient at 490 nm (Kd(490)) in response to atmospheric forcing and river discharge. Our results indicate that atmospheric forcing does not have a significant influence on interannual anomalies of Kd(490) in the Baltic Sea. This is dissimilar to the North Atlantic at the same latitudes, where interannual variability of phytoplankton blooms (ocean color chlorophyll concentration, Chl, and Kd) is to a large degree controlled by local weather. In contrast, in the Baltic Sea the interannual variability of Kd(490) is significantly influenced by river runoff. Higher values of Kd(490) are observed in years with larger inflow of water from rivers. Without access to more detailed information about the concentrations of various optically significant water components we can only speculate about the possible reasons for this relationship. Most likely a combination of several factors causes the observed positive correlation between Kd(490) and river discharge. These factors include development of more intense phytoplankton blooms in response to a larger supply of nutrients delivered by rivers. This conclusion is supported by satellite Chl data. However, an advection of optically important material with river water most likely also contributes to the bserved variability of Kd(490). The diffuse attenuation coefficient plays a critical role in many oceanographic processes. For example, Kd is essential for quantification of radiative heating of the ocean, in models of primary production and other photoprocesses, and in studies discussing water turbidity and water quality. Better understanding of the variability of Kd in the Baltic Sea can improve our knowledge of this marine environment.