The sea is largely affected by the inflow of numerous rivers, which bring sand, silt, and shells, which in turn form numerous bays, limans, and narrow spits. Because of these deposits, the sea bottom is relatively smooth and flat with the depth gradually increasing toward the middle. Also, due to the river inflow, water in the sea has low salinity and a high amount of biomass (such as green algae) that affects the water colour. Abundant plankton result in unusually high fish productivity. The sea shores and spits are low; they are rich in vegetation and bird colonies.