Study system and sampling
Lake Mývatn is composed of two basins (North and South basin) that are connected by two narrow channels and vary in a range of abiotic and biotic conditions . The lake is spring fed, with geothermal hot springs (up to ca. 23°C) feeding the north-east of the lake and cold-water springs (ca. 5°C) feeding the south-eastern parts. Most part of the lake follows the ambient temperature, which in summer is around 12-13°C The lake is shallow (1 – 4 m), but with some deeper areas (up to ca. 7 m) caused by historical diatomite mining in some parts of the North basin . Productivity, as well as benthic, epibenthic and pelagic invertebrate abundance and community structure, also varies through space (Phillipset al. in review; Bartrons et al. 2015). Based on the combination of water temperature and depth, as well as vegetation and substrate, the habitats occupied by stickleback have previously been classified as warm, rocky shore, cladophorales, pondweed and mined ). In addition, long-term monitoring data shows that stickleback population density varies in both space and time with the North basin having higher densities than the South, and with periodically strong dispersal from the North to the South basin (Phillips et al. in review).
The stickleback population of Mývatn has been surveyed each year since 1991 as part of an ongoing long-term monitoring of population demographics . This sampling is done during the third week of June and August each year by laying five unbaited minnow traps at pre-determined locations over two 12 hr periods (see Millet et al. 2013 for details). During monitoring, stickleback are counted to estimate catch-per-unit-effort (CPUE) and frozen for later analysis. For phenotyping and genotyping, a random subset of individuals (ca. N = 100 per site for each of the day and night catches) have been stored since 2009. To study patterns of spatial divergence, we used stickleback from 9 sites collected in June of 2012 due to the availability of detailed ecological data for this time point.