Competition between the tropical alga, Halimeda incrassata, and the seagrass, Thalassia testudinum

Abstract

In this study, we experimentally investigate the interspecific interactions operating between the dominant seagrass and an abundant, native rhizophytic macroalga within a mature seagrass community. Treatments consisted of density manipulations of the dominant seagrass species, Thalassia testudinum, and of the most common representative of the rhizophytic algae, Halimeda incrassata. Evidence of interaction was measured by changes in relative short-term productivity and biomass of both algae and seagrass over a 4-month period. There was an asymmetrical effect of density manipulation: the presence of seagrass decreased the size of algal thalli by 20.4% and the macroalgal growth rate by 33.3%, but the presence of macroalgae had no significant impact on seagrass growth rate and decreased the mean size of short-shoots of T. testudinum by only 10.3%. These results support a competitive interaction theory for T. testudinum and rhizophytic macroalgae. Nutrient and light limitation were investigated as possible underlying mechanisms of the interaction. Seagrass leaf tissue C:N was significantly lower ( P=0.06) in algal removal treatments whereas no significant treatment effect was demonstrated for substrate-level irradiance, suggesting that competition for N was the mechanism of the interaction.