Table 1.

Summary of sampled populations and characteristics at sampling stations. The sample size (No. of genotyped) varied between 17 and 58 individuals per local population, which accounted for approximately 65–95% of the originally established strains (No. of strains). The column showing month/year refers to the time point when sediment samples were taken. We recorded the coordinates (decimal values), surface salinity (psu) and depth (m) of the ten sampling stations along the salinity gradient

Population	No. of strains	Month/year	No. of genotyped	Longitude	Latitude	Salinity, psu	Depth, m
Bothnian Sea (BS)	42	06/2011	40	18.55E	62.12N	3–6	83
Gulf of Finland (SF)	35	04/2013	32	23.25E	59.88N	4–6	20
Gotland (GD)	37	09/2011	33	19.05E	57.41N	6–8	96
Gdansk (GK)	39	04/2012	33	18.95E	54.56N	7–10	50
Ystad (YS)	25	05/2011	17	13.85E	55.42N	7–10	1
Öresund (RO)	61	03/2009	58	12.73E	55.98N	10–20	14
Arild (AR)	27	05/2011	17	12.34E	56.16N	11–23	1
Båstad (BA)	36	05/2011	33	12.85E	56.43N	11–23	1
Vinga (VI)	57	03/2009	45	11.52E	57.55N	12–35	78
Lysekil (LY)	61	03/2009	46	11.05E	58.25N	12–35	101