Seasonal dynamics of growth in the Barents sea seaweeds: Endogenous and exogenous regulation

Measurements of changes in the area of the thallus were carried out for 15 widespread macroalgae from the Barents Sea. The experiments lasted 10 to 14 days and the area was estimated with an image analysis system. The experiments were conducted under both in-situ conditions in the sea and controlled laboratory conditions (10 degrees C, 5-10 W m(-2), light-dark cycles of 12:<(12)over bar>, L:(D) over bar or 24:(0) over bar, L:(D) over bar, and an excess of nutrients) for each of the four seasons of the year over a period of two years. The relative growth rates decreased from spring to autumn under both natural and laboratory conditions. The increase in surface area of most of the algae was as a rule at the maximum in the early spring when irradiance and photoperiod increased rapidly while water temperature (near 0 degrees C) was still far below the optimum for growth. At the beginning of the polar day the growth rates started to decline prior to a period that would seem to be more favourable for growth of the seaweeds. Growth rates of all species decreased significantly in the early autumn. In nature there was no growth in young plants in November and further into winter. In winter, after transfer to the laboratory, some algae (Palmaria palmata, Ulvaria obscura, Laminaria digitata and Alaria esculenta) showed an increase in growth under experimental conditions, whereas others, for instance, Fucus serratus, did not grow. Therefore, the first group of species was in the field in a phase of 'forced rest' in the middle of winter. They were ready for growth throughout the polar night, but could not grow because of the low light levels. Others were in a phase of 'deep rest' in winter. All data supported the existence of circannual growth rhythms in algae of the Barents Sea.