A circannual rhythm controls seasonal growth in the kelpsLaminaria hyperboreaandL. digitatafrom Helgoland (North Sea)

The seasonal growth patterns of the keps Laminaria hyperborea and L. digitata were found to be controlled by an endogenous circannual clock. This result was obtained by growing experimental sporophytes in a tank system under constant conditions of daylength, temperature and nutrient supply. L. hyperborea exhibited a free-running growth rhythm at 12 h light per day, with a period of 32·8–40·0 weeks, while the free-running rhythm in L. digitata occurred at 8 h light per day with a period of 26·8–33·6 weeks. Non-permissible daylength regimes for free-running growth cycles were 8, 16 or 24 h light per day in L. hyperborea, with continuous growth in short days and growth arrest in long days. In L. digitata, continuous growth occurred at 16 h light per day. The growth rhythms in both species were synchronised by experimental daylength cycles simulating the natural year with a period of 12 months, and also by cycles with periods of 6 or 3 months, or by shifting the phase of a daylength cycle.