On the causes for salinity variations in the Baltic Sea during the last 8500 years

Abstract

[1] The Baltic Sea became connected to the open sea about 8500 years B.P., and that marked the beginning of the still continuing Litorina Sea stage. There have since been significant long-term variations of the salinity from a maximum of some 10-15% to the present of 7-8%. The salinity variations have been known since the beginning of the twentieth century, and various mechanisms have been proposed to explain these. However, so far, no one has actually tried to quantify and thereby rank the relative importance of the different proposed mechanisms. In this paper we used a simple oceanographic model to quantify the impact of changes in crosssectional areas of the inlets to the Baltic. We found that such changes cannot explain the full variation of the salinity. We propose that the remaining salinity variations (0.5-5%) are explained by changes in net freshwater input in the range 15-60%. The calculated variations in the freshwater supply are in large coherent with known variations of the northern European climate. We show that other possible factors as increased mixing, frontal displacements, higher deep water salinity, and larger depths in Kattegat and larger oscillating flows across the sills due to increased sea level variability are less likely to have made substantial contributions to the salinity variations in the Baltic Sea.

Platichthys solemdali sp. nov. (Actinopterygii, Pleuronectiformes): A New Flounder Species From the Baltic Sea

“…The evolution of opposite reproductive strategies might be a reflection of the different environmental conditions at the time the ancestors of the two contemporary species invaded the Baltic Sea. Momigliano et al (2017a) estimated that the ancestors of the Baltic flounders invaded the Baltic Sea during the early stages of the Litorina sea, when salinity throughout the Baltic Sea was lower than today (Gustafsson and Westman, 2002). At that time, spawning of demersal eggs would have been the only viable reproductive strategy.…”

“…The demersal flounders invaded the Baltic Sea at the beginning of the Litorina Sea stage, a period of marine transgression, which started ∼8,000 years BP transforming the freshwater Ancylus Lake into the large body of brackish water that is the modern Baltic Sea (Björck, 1995;Gustafsson and Westman, 2002); at the very start of the Litorina Sea stage, salinity in the newly formed Baltic Sea was extremely low (Gustafsson and Westman, 2002) and pelagic spawning would not have been possible. The pelagic flounders invaded the Baltic Sea later on, possibly during peak salinity (6,500-5,000 years BP), when salinity averaged at 10-15 psu and pelagic spawning would have been possible in extended areas of the Baltic Sea (Gustafsson and Westman, 2002). However, the spawning grounds of pelagic flounders have likely been contracting since due to the steady decrease in salinity that the Baltic Sea has experienced in the past 5,000 years (Gustafsson and Westman, 2002).…”

“…The pelagic flounders invaded the Baltic Sea later on, possibly during peak salinity (6,500-5,000 years BP), when salinity averaged at 10-15 psu and pelagic spawning would have been possible in extended areas of the Baltic Sea (Gustafsson and Westman, 2002). However, the spawning grounds of pelagic flounders have likely been contracting since due to the steady decrease in salinity that the Baltic Sea has experienced in the past 5,000 years (Gustafsson and Westman, 2002). Today salinity in the Baltic Sea averages at 7-8 psu, and pelagic flounders encounter environmental conditions suitable for reproduction only in the deep waters of the Arkona, the Bornholm, and the Gotland Basins (Figure 1; Nissling et al, 2002).…”

“…As migratory fish, European flounders are commonly found in shallow coastal waters and in estuaries (Summers, 1979); they can enter freshwater systems for extended periods of time, though they do not successfully reproduce in freshwater (Nissling et al, 2002). The European flounder is one of the few marine species that thrive in the low salinities of the Baltic Sea, a large body of brackish water that became connected to the North Sea at the end of the last glaciation, ∼8.5 kya (Gustafsson and Westman, 2002). The transition zone between the North Sea and the Baltic Sea ( Figure 1A) presents a steep gradient in salinity, which rapidly decreases from the typical values of the marine environment (>30 psu) to values as little as 5-7 psu encountered in the shallow coastal waters of the Baltic Proper ( Figure 1A).…”

“…Using a combination of genome-wide SNP genotyping and approximate Bayesian computation modeling, a recent study demonstrated that the two flounder types originated via two distinct colonizations of the Baltic Sea from the same ancestral population (Momigliano et al, 2017a). The demersal flounders invaded the Baltic Sea at the beginning of the Litorina Sea stage, a period of marine transgression, which started ∼8,000 years BP transforming the freshwater Ancylus Lake into the large body of brackish water that is the modern Baltic Sea (Björck, 1995;Gustafsson and Westman, 2002); at the very start of the Litorina Sea stage, salinity in the newly formed Baltic Sea was extremely low (Gustafsson and Westman, 2002) and pelagic spawning would not have been possible. The pelagic flounders invaded the Baltic Sea later on, possibly during peak salinity (6,500-5,000 years BP), when salinity averaged at 10-15 psu and pelagic spawning would have been possible in extended areas of the Baltic Sea (Gustafsson and Westman, 2002).…”