Marine areas around the world face dramatic changes related to the release of greenhouse gases. Some of these changes — such as marine heat waves already proven devastating to coral reefs — are directly related to the temperature regimes experienced by the organisms. In addition, there is a whole list of other changes that occur concurrently.
These changes are not, however, the same in all marine waters, nor are their effects on ecosystems. Coastal areas and regional seas, for instance, often have their own unique characteristics that will shape the way they respond to climate change. The “baseline” of environmental pressures upon which climate-change-related effects operate also differs between regions.
The Baltic Sea provides an example of the importance of regional ecosystem properties in shaping responses to the effects of climate change. These variations in responses apply to the Baltic Sea in comparison with other seas, but also to local differences within the Baltic Sea. In the front-page article of this issue of Acid News, Anu Vehmaa highlights acidification in the Baltic Sea, and describes the unique characteristics of its buffering system, as well as aspects of geographic variation in the buffering system. The importance of variable photosynthetic activity, which can seasonally be extremely high, is also discussed in relation to its implications for pH dynamics.
There are several other examples of environmental changes that are characteristic of the Baltic Sea. Eutrophication is the most notorious one. Eutrophication is evidently not caused by climate change, but by excess nutrients from e.g. unsustainable agriculture practices and burning of fossil fuels. However, due to increased precipitation and warmer winters, the eutrophication problem will get worse unless efforts to cut nutrients are successful. In the wake of increased precipitation, an ongoing freshening (decrease in salinity) is also predicted to continue, at the expense of marine animals and plants already at the limit of their salinity tolerance levels.
There is also an unfortunate interplay between eutrophication and salinity, namely oxygen depletion. In short, the water column in parts of the Baltic Sea is permanently stratified into a deeper, more saline, layer, and a surface layer of less saline water. As oxygen from the surface cannot penetrate to the deeper layer, oxygen will eventually be depleted from this layer. The rate at which the oxygen is depleted is dependent on the amount of organic matter, which increases with increasing eutrophication. In addition to this salinity-dependent stratification, shallower waters also become stratified in summer, due to temperature-dependent stratification. Unfortunately, increased stratification is one of the projected changes related to climate change. In addition, the ongoing warming of the Baltic Sea is in itself bad news for oxygen conditions, as less oxygen can be bound at higher temperatures, and oxygen consumption increases.
The consideration of regional or local conditions does not prevent universal approaches to protect the marine environment from the effects of climate change. The obvious approach is immediate mitigation of greenhouse gases. Other measures include protection and restoration of biodiversity for increased resilience. However, continued, regionally adopted measures are needed to mitigate other, interacting, stressors, such as eutrophication in the Baltic Sea.
Further reading: Baltic Sea Environment Proceedings No. 137 (HELCOM; https://www.helcom.fi/wp-content/uploads/2019/10/BSEP137.pdf), https://www.smhi.se/en/research/research-news/climate-change-makes-reduc..., and https://www.bsag.fi/en/baltic-sea/climate-change/