Stratification Has Strengthened in the Baltic Sea – An Analysis of 35 Years of Observational Data

Abstract

Stratification of the water column, consisting of the three layers (upper, intermediate, and deep layer) separated by the seasonal thermocline and the permanent halocline, respectively, is an important factor for the functioning of the brackish Baltic Sea. In the present work, changes in the vertical structure of temperature and salinity, as well as heat content, salt mass, and stratification conditions were estimated on the basis of in situ and remote sensing data in 1982-2016. The seasonal thermocline and the halocline have strengthened in most of the sea by a rate of 0.33-0.39 and 0.70-0.88 kg m −3 , respectively, during 35 years. The upper layer has warmed by 0.03-0.06 • C year −1 and sub-halocline deep layer 0.04-0.06 • C year −1 in most of the sea. The total warming trend in the whole Baltic has been 1.07 • C for 35 years, being approximately twice higher compared to the upper 100 m in the Atlantic Ocean. Average upper layer warming of the sea from May to September has been 0.07-0.08 • C year −1 while in winter, trends were mostly statistically not significant. More rapid warming during summers has occurred in shallower, closed-end areas of gulfs if compared to the rest of the sea. Possible reasons for high warming there might be shallow depths and limited water exchange, stronger stratification, and/or higher turbidity. Sea surface temperature trends estimated by in situ and satellite data agree well. Trends of freshening (−0.005 to −0.014 g kg −1 year −1 ) of the upper layer and increasing salinity (0.02 to 0.04 g kg −1 year −1 ) in the sub-halocline deep layer were detected. Increased salinity in the deep layer is likely caused by the increased lateral import of saltier water from the North Sea. Changes in the upper layer salinity might not be related to the accumulated river runoff only, but decadal changes of vertical salt flux might also contribute. The vertically distinct changes cancel each other and no significant trend in the mean salinity of the Baltic Sea was detected. No remarkable changes have occurred in the cold intermediate layer. In conclusion, different dominating processes have caused distinct long-term trends in the three layers of the Baltic Sea.

Freshening rather than warming drives trematode transmission from periwinkles to mussels

“…Supporting this, in coastal waters with salinity below 15, H. elongata infections were registered only in periwinkles at the bottom of shallow waters (Lauckner 1984a, b). Stratification of the water column in the Baltic Sea often leads to the deeper water layer being higher in salinity and lower in temperature (Liblik and Lips 2019). In contrast to activity, cercarial survival was negatively affected by temperature only.…”

The Globalization of Cultural Eutrophication in the Coastal Ocean: Causes and Consequences

“…In winter, the water column is mixed down to a permanent halocline (40-80 m). In summer, a seasonal thermocline forms above the permanent halocline and a three-layered structure develops with a warm and low salinity surface layer, a higher salinity intermediate layer of cold water, and an oxygen depleted bottom layer of warmer and saltier water (Liblik and Lips, 2019).…”

“…Hypoxic bottom water in the Baltic proper (Gotland Sub-Basin) has been present for at least the last 100 years, but increased in spatial and temporal extent over the last 3 decades (Duncombe, 2018) as the buoyancy of the surface layer above the permanent halocline increased due to increasing temperature and decreasing salinity (Liblik and Lips, 2019). During 1993-2016, the spatial extent of hypoxic bottom water in the main basins of the Baltic (Figure 5) increased from ∼5,000 km −2 (1.3% of the Baltic) to ∼ 70,000 km −2 (19% of the Baltic) (Jokinen et al, 2018;Limburg and Casini, 2019).…”

A decade of dissolved sulphur compounds measurements in the southwestern Baltic Sea

“…However, low-oxygen events (hypoxia or anoxia) occur more frequently within the last decades in the Baltic Sea and so at BE (Lennartz et al, 2014). Probably, climate warming enhances bacterial activities and respiration (Hoppe et al, 2013) and extends the period of stratification (Liblik and Lips, 2019). This overrides the effect of decreasing nutrient inputs in the last decades (Lennartz et al, 2014).…”