Genetic Differentiation in Contrasting Salinity Regimes

Lerviksbäcken (henceforth Lervik, N 57° 04.414′; E 16° 31.246′) is located on the Swedish mainland east coast, and Harfjärden (N 56° 49.063′; E 16° 48.673′) is located on the east coast of the island of Öland (Fig. 1). In a previous study Larsson and colleagues²⁸ analysed genetic variation based on 10 nuclear microsatellite DNA loci (Elu 2, Elu, 6, Elu 19, Elu 37, Elu 51, Elu 64, Elu 76, Elu 78, Elu 86, Elu 276 (Miller and Kapuscinsky 1996^{40 (link)}; 1997^{41 (link)}; Hansen et al. 1999^{42 (link)})) and found a strong neutral genetic differentiation between the populations spawning in Lervik and Harfjärden (pairwise F_ST = 0.226), despite geographic proximity²⁸ . This suggests that gene flow is probably too weak to prevent evolution of local adaptations. Within population neutral genetic diversity was slightly lower for Lervik (gene diversity 0.324 ± 0.187 N = 36) than for Harfjärden (0.456 ± 0.251, N = 41)²⁸ .
The spawning locations differ in their salinity regimes. The stream leading up to the Lervik wetland flows through agricultural land at a very low inclination, and downstream water flux is limited to floods in the spring and rainy periods in addition to being heavily dependent on the water level in the Baltic Sea⁴³ . The spawning area in Lervik is in level with the altitude of the Baltic Sea, and the water level in the sea periodically exceeds that of Lervik (Fig. 1b), allowing backflow of brackish water to enter and temporarily increase salinity in the spawning area (occassionaly up to 7 psu, see Fig. 1c). By contrast, the spawning area in Harfjärden on Öland is fed solely by freshwater and located at an altitude 1 m above the Baltic Sea water level (Fig. 1b), such that it cannot be influenced by backflow of brackish water. In regards to salinity, the two spawning locations thus represent contrasting levels of environmental heterogeneity (Fig. 1c).
Altitude analysis was performed in ArcMap version 10 using geodata (2-meter resolution raster) from the Swedish Authority for Geographic and Geometric Information (Lantmäteriet). Twenty-five height measurement points were placed at random within each wetland and outside the outlet of each wetland (in total 100 datapoints). The wetland in Harfjärden was located 1.07 meters (±0.02 SD) above mean sea level (MSL) and the wetland in Lervik was at 0.24 meters (±0.02 SD) above MSL. Sea level analysis was performed in RStudio version 0.99.903 using hourly measured sea level data for Oskarshamn (years 1961 to 2016) from the Swedish Meteorological and Hydrological Institute (SMHI).