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.