Mobile phosphorus in the seabed sediments of the northern Baltic Proper, Baltic Sea: hypoxic conditions limit large-scale phosphorus release

Baltic Sea is a geologically young semi-enclosed brackish-water body whose water exchange with the ocean has gradually declined. Approximately 85 million people live in the Baltic Sea's catchment area, resulting in significant human impact on the basin's ecosystem. Eutrophication due to anthropogenic discharge of nutrients is considered the most serious environmental problem, leading to greater growth of phytoplankton and algae, deterioration of water quality, and lack of oxygen in near-bottom water masses. As a result of recent large-scale nutrient input, phosphorus has accumulated into the seabed sediments from where it has been remobilizing and releasing into the water column under favorable conditions. Marine sediments contain phosphorus in various components, i.e. fractions, but not all of them are affected by remobilization. The release of phosphorus from sediments is affected by different oxygen conditions in the bottom water layer. Therefore, understanding the principles of phosphorus release and the distribution of phosphorus fractions in seabed sediments is extremely important.

Phosphorus fractions, porewater chemistry, and their vertical distribution were studied from the sea-bottom sediments from three locations in western Estonia in the northern Baltic Proper. The amount of mobile phosphorus fraction (mobile in hypoxic and anoxic conditions) stored in the surface sediments of the northern Baltic Proper is lower than expected, which indicates that most of the mobile phosphorus fraction has already been released back into the water column. In two out of three locations, the content of mobile phosphorus fraction in the sediment surface has decreased close to the natural background, which on average is less than 200 mg/kg (dw). Constant hypoxic conditions prevail at the sediment-water interface in all three locations. In the near future, oxygen levels can only increase in these areas due to Major Baltic Inflow (MBI) events, which introduce dense, salty, oxygen-rich water into the Baltic Sea. During sufficiently large inflow events, oxygen-rich water can reach areas previously characterized by stable hypoxic conditions, temporarily altering the deep-water oxygen levels. Under oxygen-rich conditions, organic material (including organic phosphorus) begins to decompose. The sediments in the study area contain up to 32.8% (dw) organic matter, holding substantial amounts of organic phosphorus. The released phosphorus is converted to a mobile phosphorus fraction, which becomes mobile again when oxygen conditions return to hypoxic.