Editorial : Living Along Gradients: Past, Present, Future

The Baltic Sea is a geologically and evolutionarily young part of the coastal ocean that experienced, in its past, several severe environmental changes. In its present state, the Baltic Sea is characterized by both horizontal and vertical gradients of environmental conditions. As a huge estuary, it shows a west to east/south to north surface salinity gradient from 24 in Kattegat to nearly freshwater in the Bothnian Bay. The vertical salinity and oxygen gradients result in stratification which causes hypoxic and sulfidic anoxic conditions in deep basins. These gradient systems are impacted by natural and anthropogenic changes due to physico-chemical driving forces, varying over time and space. Gradient environments produce an imprint on both the structure and function of the biological systems and influence biogeochemical cycling. Besides, coastal seas in general and the Baltic Sea in particular, experience constant and direct influence from land with consequences to matter and energy cycles, biogeochemical interactions, energy fluxes, and sediment dynamics. “Living along gradients: past, present, future” in the Baltic are today’s very important aspects that rise questions like which of the effects we are detecting occur naturally, and which are driven by human activities. Deciphering past environmental changes and their causes provide keys to understand and simulate possible future scenarios, all of which should rise societal awareness and implementation of appropriate marine and coastal policies. Present-day knowledge on the dynamics of gradient systems, on the processes that affect the coastal sea environment, the results of interaction between coastal seas and society, the detection or reconstruction of past and present changes on time scales from inter-annual to millennial, and future change models are summarized here, with the idea to stimulate scientific exchange on most complex questions, addressing them from different perspectives.