Salinity dynamics of the Baltic Sea

Abstract

Abstract. In the Baltic Sea, salinity and its large variability, both horizontal and vertical, are key physical factors in determining the overall stratification conditions. In addition to that, salinity and its changes also have large effects on various ecosystem processes. Several factors determine the observed two-layer vertical structure of salinity. Due to the excess of river runoff to the sea, there is a continuous outflow of water masses in the surface layer with a compensating inflow to the Baltic in the lower layer. Also, the net precipitation plays a role in the water balance and consequently in the salinity dynamics. The salinity conditions in the sea are also coupled with changes in the meteorological conditions. The ecosystem is adapted to the current salinity level: a change in the salinity balance would lead to ecological stress for flora and fauna, as well as related negative effects on possibilities to carry on sustainable development of the ecosystem. The Baltic Sea salinity regime has been studied for more than 100 years. In spite of that, there are still gaps in our knowledge of the changes in salinity in space and time. An important part of our understanding of salinity is its long-term changes. However, the available scenarios for the future development of salinity are still uncertain. We still need more studies on various factors related to the salinity dynamics. Among others, more knowledge is needed, e.g., from meteorological patterns at various space scales and timescales as well as mesoscale variability in precipitation. Also, updated information on river runoff and inflows of saline water is needed to close the water budget. We still do not understand the water mass exchange accurately enough between North Sea and Baltic Sea and within its sub-basins. Scientific investigations of the complicated vertical mixing processes are additionally required. This paper is a continuation and update of the BACC (Baltic Assessment of Climate Change for the Baltic Sea Region) II book, which was published in 2015, including information from articles issued until 2012. After that, there have been many new publications on the salinity dynamics, not least because of the major Baltic inflow (MBI) which took place in December 2014. Several key topics have been investigated, including the coupling of long-term variations of climate with the observed salinity changes. Here the focus is on observing and indicating the role of climate change for salinity dynamics. New results on MBI dynamics and related water mass interchange between the Baltic Sea and the North Sea have been published. Those studies also included results from the MBI-related meteorological conditions, variability in salinity, and exchange of water masses between various scales. All these processes are in turn coupled with changes in the Baltic Sea circulation dynamics.

Global climate change and the Baltic Sea ecosystem: direct and indirect effects on species, communities and ecosystem functioning

“…For the deep benthic communities, climate change effects are also not straightforward. If salinity declines, the majority of marine species will suffer, but according to the latest analyses undisputable evidence is lacking for a future decline in the salinity of the Baltic Sea (Lehmann et al, 2022;Meier et al, 2022b). Improvement of oxygen conditions may first promote higher zoobenthos biomasses, but eventually increasing stratification will weaken benthic-pelagic coupling and reduce food availability for benthic organisms.…”

“…Freshwater vascular plants will be favoured by freshening of the Baltic Sea, but they cannot replace the marine macroalgae on rocky sublittoral because they only grow on soft substrates. On the other hand, salinity projections are still uncertain (Lehmann et al, 2022), and even if salinity declined, Fucus vesiculosus may be able to adapt to salinity changes (Rothäusler et al, 2018).…”

“…As with other species groups, projecting the fate of macroalgae and vascular plants is challenging. This is caused by the uncertainties in projections concerning salinity and stratification (Lehmann et al, 2022), discrepancy on which physicochemical factors determine the distribution of invertebrates, unknown adaptation capabilities of algae and plants, and uncertainties concerning future trophic interactions within macroalgae and vascular plant communities.…”

“…The marine ecosystem therefore receives excess nutrients and other elements and contaminants from the land via rivers, through the air, and by leaking from the sediments of the Baltic Sea. Furthermore, the irregular inflows of more saline and oxic North Sea water, which at specific basin-wide weather conditions enter the Baltic sea through the Danish Straits (Matthäus and Schinke, 1994;Lehmann et al, 2022) and influence the state and functioning of the Baltic Sea.…”

Intraspecific Trait Variability of a Diatom and a Dinoflagellate Along a Salinity Gradient

“…Salinity level strongly varies temporarily and spatially in estuarine and coastal areas, while inland lakes and open oceans have a more stable and narrower salinity range. The Baltic Sea is characterized by strong salinity fluctuations, and salinity is predicted to decline in the future (Meier et al, 2012) due to increased freshwater inflow from the atmosphere and land (Lehmann et al, 2021). Vuorinen et al (2015) predicted a salinity of less than 7 psu for the entire Baltic Sea by 2060, a bottleneck for many freshwater and marine species, including phytoplankton (Olli et al, 2019).…”