Summary

Climate change is a highly important issue for the Baltic Sea. Using historic data and sedimentary records of previous centuries, implications for a warmer future can hypothesized. There are two very important topics concerning the future of the Baltic Sea as a function of changing microbiological landscapes if climate warming is not meaningfully averted: The increase in deoxygenation and the spread of hypoxic dead zones, and the increase in abundance of, and infections associated with, pathogenic bacteria, especially those of the genus Vibrio. Both aspects are expected to devolve into, at least from a human economic and public health perspective, an ever-worsening state as the climate continues to warm. The mechanisms behind both dynamics are often complex, making precise predictions very challenging, yet, based on the available data and weight of research, such broad trends can be postulated reasonably confidently. It is anticipated that climate warming will drive warmer waters in the Baltic Sea generally, but in deep anoxic waters and sediment of the Baltic Proper particularly. These warmer temperatures will drive increased mineralization of organic nutrients, providing greater soluble nutrient pools, especially of phosphorus. Increases in these nutrients that upwell to surface waters are expected to stimulate ever-more pronounced cyanobacterial blooms especially in late summer, which cause increased deoxygenation in the Baltic Sea. In a similar way, warmer surface waters in combination with cyanobacterial blooms and associated nutrients are closely associated with increased abundance of Vibrio vulnificus, a pathogenic bacterium found in the Baltic Sea in summer responsible for human cases of vibriosis, which are often fatal. Ever-warmer surface waters and cyanobacterial bloom scenarios are expected to also cause increased numbers of V. vulnificus, thus the expectation that continued climate warming will also drive increased cases of Vibrio infection associated with the Baltic Sea appears reasonable. In all, without a significant alleviation of climate warming, abundances and dynamics of microorganisms are expected to be considerable, causing greater spread of both deoxygenation and human infection in the Baltic Sea over the course of the rest of the 21st century.