Lüning, K., 1980. Critical levels of light and temperature regulating the gametogenesis of three laminaria species (Phaeophyceae). Journal of Phycology, 16, 1-15. Kain, J.M., 1979. A view of the genus Laminaria. Oceanography and Marine Biology: an Annual Review, 17, 101-161. Fife Nature Records Centre, 2018. St Andrews BioBlitz 2014. Occurrence dataset: https://doi.org/10.15468/erweal accessed via GBIF.org on 2018-09-27. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details. Mizuta H, Hayasaki J, Yamamoto H (1998) Relationship between nitrogen content and sorus formation in the brown alga Laminaria japonica cultivated in southern Hokkaido, Japan. Fish Sci 64:909–913

The fraction of the fertile sporophytes capable of releasing spores varied between 70 and 100% from November to May, but in June and August none of the collected fertile sporophytes released viable spores, and in October it was estimated that only 10% of the total population released viable spores, despite the fact that more than 40% of the total population were bearing sori. At each water change, the number of fertile sporophytes in each treatment was registered. The number of fertile sporophytes (defined as sori presence) in each treatment, and the total area of sori on each sporophyte were registered measuring the length and width of the fertile area. At the end of the experiment, a tissue disc of 9.29 cm 2 was stamped from the sorus tissue of 10 individuals, and the spores were released and counted as described above, with the modification that 25 mL seawater was used instead of 10 mL. Tissue samples vegetative growth of the gametophyte to form either a larger single-celled female gametophyte or, in the case of male gametophytes, a few small cells, and Kain, J.M., 1975a. Algal recolonization of some cleared subtidal areas. Journal of Ecology, 63, 739-765.

The experimental tanks (360 × 34 × 12 cm, each containing 100 L) were set up and connected in parallel to the same single header tank from which all water was distributed and collected. Consequently, each tank had its own water supply from the header tank providing a unidirectional flow in each tank of app. 500 L h −1. The setup ensured a total mixing of the water, providing all treatments with the same nutrient regime. Lee & Brinkhuis (1988) studied the interaction of light and water temperature on the development of Saccharina latissima gametophytes and juvenile sporophytes in Long Island Sound and found the following. The DM content of the tissue discs was determined by drying the discs at 105 °C until the weight was stabilized, and hereafter calculated as percentage of fresh biomass: dry weight (DW)/fresh weight (FW) × 100. Hereafter, the dry disc was finely milled and homogenized before further analysis.

A decrease in the level of water flow is unlikely to have a detrimental effect because the species often grows in areas of low water movement where it may form extensive loose-lying populations (Burrows, 1958).The external egg is fertilized by the motile sperm and the resultant zygote eventually develops into the new sporophyte. The intact sporophytes were stored in a plastic bag during transportation to the lab (1–2 h). In the lab, the number of fertile sporophytes was recorded and the ratio of fertile to non-fertile sporophytes was calculated. Hereafter, 10 randomly selected fertile sporophytes were submerged in saltwater at 10 °C and stored for 1–2 days until spore release was initiated. In June, August, and November, the 10 selected fertile sporophytes were stored in saltwater from 4 to 6 days. Guiry, M.D. & Nic Dhonncha, E., 2002. AlgaeBase. World Wide Web electronic publication http://www.algaebase.org,

If the gametophytes become fertile, male gametophytes develop antheridia that produce sperm. The females develop oogonia in which the egg develops (Birkett et al., 1998). This egg is subsequently discharged. After the egg has emerged, the cell wall closes behind it and forms a cushion on which the egg is seated (Bisalputra et al., 1971). Davison IR, Stewart WDP (1984) Studies on nitrate reductase activity in Laminaria digitata (Huds.) Lamour. I. Longitudinal and transverse profiles of nitrate reductase activity within the thallus. J Exp Mar Biol Ecol 74:201–210 The morphology of S. latissima was highly influenced by various abiotic conditions, such as salinity that strongly affected the size of the field sporophytes, since specimens got smaller across the Baltic Sea following the salinity gradient from West to East ( Lüning, 1990; Nielsen etal., 2014; Bruhn etal., 2016). Also, an effect of SST on morphology was determined, both along latitudes and the salinity gradient. Higher temperatures increase biofouling of the distal part (e.g. Bruhn etal., 2016), and thus might also have affected the frond length in our study. Still, we observed that sporophytes grew wider in colder regions and therefore revealed smaller length:width. However, it has to be noted that different growth activity across latitudes ( Diehl etal., 2021) and also seasonal timing and age ( Sjøtun, 1993) might influence the morphological appearance. Even though sampling depth has been shown to affect frond length of cultivated S. latissima ( Forbord etal., 2020), depth did not correlate with size in our study. Furthermore, wave exposure has long been known to strongly influence the morphology of S. latissima (e.g. Gerard, 1987; Lüning, 1990; Visch etal., 2020; Zhu etal., 2021). In more exposed habitats, sporophytes of S. latissima form narrow blades with solid stipes, while they exhibit broad blades with hollow stipes in more sheltered habitats. Our observations are in accordance with those descriptions, since, for example, the sampling location in ANS was situated in a protected bay, while HLG and RUN_r were both rather wind- and wave-exposed habitats. Regardless of their latitudinal distribution, it is reasonable to assume that the morphology of S. latissima appeared to be mostly affected by hydro-dynamic forces. 4.2 Biochemical profiles along gradientsConolly N.J. & Drew, E.A., 1985. Physiology of Laminaria. III. Effect of a coastal eutrophication on seasonal patterns of growth and tissue composition in Laminaria digitata and L. saccharina. Marine Ecology, Pubblicazioni della Stazione Zoologica di Napoli I, 6, 181-195. Although application of COI has some limitation, it revealed in our study for the Portuguese samples (AMO) a unique mitochondrial haplotype. These results are consistent with the genetic differentiation based on SNPs of the nearby population (CAS) given in Guzinski etal. (2020). Overall, the consistencies between SNPs and COI detected here indicate some deep evolutionary roots that emphasize the unique genetic composition of warm edge populations. 4.4 Habitat-specific signatures in Saccharina latissima? Centre for Environmental Data and Recording, 2018. Ulster Museum Marine Surveys of Northern Ireland Coastal Waters. Occurrence dataset https://www.nmni.com/CEDaR/CEDaR-Centre-for-Environmental-Data-and-Recording.aspx accessed via NBNAtlas.org on 2018-09-25. A change of two ranks on the wave exposure scale ( view glossary) e.g., from Exposed to Extremely exposed for a period of oneyear. Further details Lüning K (1988) Photoperiodic control of sorus formation in the brown alga Laminaria saccharina. Mar Ecol Prog Ser 45:137–144