Abstract
This investigation compares two populations of Fucus vesiculosus L. in which the lower bathymetric limits are determined by insufficient light rather than by the presence of a superior algal competitor. In the tidal reaches of the R. Mersey (NW England), water turbidity is so severe that Fucus experiences darkness when covered by an incoming tide. These plants must therefore photosynthesise only when emergent and are hence exposed to full sunlight. Baltic Sea (SW Finland) Fucus, in contrast, is permanently submerged in an atidal environment and subject to shade conditions that increase with depth. Chlorophyll concentrations and ratios in these populations differ significantly. Significant morphological differences are also present, with Baltic thalli showing distal blade expansion and Mersey thalli proximal expansion. A transplant experiment involving Baltic thalli from different sites and depths suggested that morphological plasticity had a minor role in the observed differences. The adaptive implications are discussed.
