Full details of the experimental setup are outlined in Gu et al. (2012). Briefly, individuals of Z. marina and N. noltii were collected in the spring 2009 from a northern European location (western Baltic/Kattegat, Hals, Denmark; 56°50′ N, 10°1′ E, 2009, hereafter “northern populations”) and a southern European location (Adriatic Sea; Gabicce Mare, Italy; 43°50′ N, 12°45′

E, late April, hereafter “southern populations”). At both locations, both species co-occur in the intertidal to the shallow subtidal. Summer surface water temperatures BMS-354825 mw ranged from 13 °C to 22 °C (mean 18 °C) in the northern location and 21 °C to 29 °C (mean 25 °C) in the southern location based on in situ records covering the previous six years (Fig. S1). Within each population, ca. 30 shoots (leaf bundles plus attached rhizome) were harvested selleck kinase inhibitor from each of 15 sub-plots (total 450 shoots), which were separated by 10 m, to minimize the chance of collecting shoots from the same genotype

(i.e., clone) (Bergmann et al., 2010). Shoots were transported in coolers filled with seawater and planted in the AQUATRON (a mesocosm facility installed at the University of Münster, Germany) within 48 h of collection. The experimental design is shown in Fig. S2. As described in Gu et al. (2012), the AQUATRON consisted of two temperature-controlled semi-connected water circuits, each with six 700-L mesocosms and a storage tank. All mesocosms contained artificial seawater adjusted to 28 psu (practical salinity units: 1 psu ~ 0.1% salinity) and illuminated under light-saturating conditions (~ 400 μmol photons s− 1 m− 2). Shoots PTK6 were planted

into boxes with a sediment height of 10 cm (details see Fig. S2). Two boxes for each of all four populations were placed into each mesocosm (= 6 independent replicate units per population and treatment condition) (Fig. S2). All shoots were genotyped with four microsatellite loci for Z. marina and five for N. noltii to verify that all genotypes were unique ( Reusch, 2000 and Coyer et al., 2004). Plants were acclimatized for 50 days, during which the water temperature in all mesocosms was slowly raised from 14 °C (collection temperature) to 19 °C (experimental control temperature) (Fig. S3). Following acclimation, a heat wave was initiated in a common-stress-garden approach. Six experimental units were maintained at the control temperature of 19 °C, while the temperature in the remaining six was gradually increased by 1 °C per day, up to 26 °C, and held for 3 weeks; then decreased by 1 °C per day to the control temperature of 19 °C (Fig. S3). The experimental profile mirrored the temperature profile observed during a heat wave in summer 2003 in the shallow waters of the western Baltic Sea (Reusch et al., 2005). Plant performance was estimated by changes in shoot number from the start of the experiment until the midpoint of the heat wave and ca. 1.5 weeks after the end of the heat wave (Fig. S3).