Abstract
Macroalgae living at high latitudes experience considerable variation in the availability of light energy and nutrients. Perennial species may smooth these variations by utilising internal energy and nutrient reserves. These reserves also enable growth during the unfavourable season. The northern Baltic Fucus vesiculosus exhibited vegetative growth throughout the year. The lowest winter growth rate was 1/3 of the highest growth rate in summer. The mannitol content of F. vesiculosus, probably used as an energy reserve, followed the annual cycle of solar radiation. The mannitol reserve was not utilised completely. The lowest winter concentrations of mannitol were 1/3 of the highest summer concentrations. The highest internal nitrogen concentrations were measured in spring, just before the spring bloom depleted the dissolved inorganic nitrogen of seawater. The internal nitrogen reserves declined steadily during the summer period to their minimum in August–September. Our results indicate that F. vesiculosus stores nitrogen for rapid summer growth in nitrogen depleted conditions and mannitol for growth during the dark winter period. Baltic F. vesiculosus is thus well adapted to the conditions of northern latitudes, where nutrients and light energy are not simultaneously available.
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