Photosynthetically Active Radiation, Attenuation Coefficient, Depth of the Euphotic Zone, and Water Turbidity in the Neva Estuary: Relationship with Environmental Factors

The coefficient of attenuation of photosynthetically active radiation (Kd(PAR)) and the depth of the euphotic zone (Deu) are widely used to study biogeochemical processes and eutrophication in marine ecosystems. However, determination of Kd(PAR) and Deu in estuaries is hampered by the simultaneous influence of many environmental factors. In this study, we analyzed the relationship between water turbidity (Turb), Kd(PAR), Deu, and environmental variables in the Neva Estuary, the largest estuary in the Baltic Sea. The summer values of Kd(PAR) and Deu were significantly higher than in the open waters of the Baltic Sea and varied in the range of 0.5–9.1 m −1 and 0.5–8.5 m, respectively. Mixed-effects regression analysis showed that the concentration of suspended mineral matter primarily determined the Turb. This variable fluctuated widely due to frequent wind-induced resuspension of bottom sediments and the periodic construction of port infrastructure in the shallow upper reaches of the estuary. Deu was determined by the depth of the water area, concentration of chlorophyll a , and concentrations of suspended mineral and organic matter. The average efficiency of using PAR energy for gross primary production (PP) was about 2%. However, PP did not depend on the amount of radiation incident on the water surface, but was mostly determined by underwater light and nutrient conditions. The study showed that more research on the impact of environmental variables on underwater light conditions in different regions is needed to predict the impact of climate change and anthropogenic factors on phytoplankton productivity in coastal areas.