A field investigation into the effects of interspecific competition on the distribution and cover of Ulothrix flacca

Abstract

Ulothrix flacca is a filamentous green alga found in its greatest abundance in the littoral fringe of the rocky intertidal during the winter months. It is much less abundant below this level. This experiment was undertaken tot determine if the lower abundance below the littoral fringe is due to the effects of interspecific competition or physiological limitations. Clearing experiments were performed on the Coast of Maine, USA to eliminate competition. Belts 10 cm wide that ran from the littoral fringe downward into the eulittoral zone were cleared of all attached organisms down to bare rock. A second condition was set up in which only the brown algal canopy was removed from a 60 cm wide belt. Unaltered belts were designated as controls. Squares 0.25 m2 in area were cleared in the littoral fringe and eulittoral zone. U. flacca was able to colonize the cleared areas of the eulittoral zone and reach abundances equal to those found in the unaltered control areas of the littoral fringe. This indicates that physiological limitations do not restrict the abundance of U. flacca in the eulittoral zone. However, evidence of grazing in cleared areas adjacent to high concentrations of the barnacle Semibalanus balanoides suggest that competition alone is not responsible for the limited abundance of U. flacca in the eulittoral zone. Herbivores associated with the presence of barnacles may also have an impact on the abundance of U. flacca, even during the winter months.