The effect of temperature and photoperiod along the year on the composition of 5 macroalgal species representative of the sublittoral system of Kongsfjorden was studied in laboratory conditions. These species were the ochrophytes Saccharina latissima and Alaria esculenta, the rhodophytes Phycodrys rubens and Ptilota gunneri, and the chlorophyte Monostroma aff. arcticum (only present from summer to early autumn). Three different seasons were compared: early March, early August and late September. Two temperatures were tested (4 and 8°C) as well as continuous irradiation (CL - simulating summer) and 12:12 h light:dark photoperiod (simulating equinoxes). Total carbon was not affected by warming or photoperiod, except in S. latissima in autumn under CL and 4°C, being 40% higher than with photoperiod. Nitrogen content generally increased by photoperiod, the highest being found in 12:12 L/D in all seasons. Carbohydrates were maximal in CL conditions in all species, indicating that their synthesis is light-stimulated, mainly in the equinoxes. In general, warming did not induce significant changes in carbohydrate content. Lipid content was affected by photoperiod only in brown algae. In S. latissima lipids presented maximum values in CL in the autumn equinox, while in A. esculenta was under photoperiod in summer. Protein content did not change with warming or photoperiod in three out of the five species, and only in S. latissima and P. gunneri proteins were higher in CL than under photoperiod in the autumn equinox. In summary, the results indicate that only isolated changes in the composition of these representative macrophytes may be expected under warming conditions in near-future scenarios, while a general increase in carbon, carbohydrates and proteins under CL was observed mainly in the autumn equinox. The relation of this increase with light utilisation performance will be discussed.
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