Drift algae-epiphyte-seagrass interactions in a subtropical Thalassia testudinum meadow

Abstract

Determination of the percentage cover and residence time of drift algae over permanently marked 0.25 m2 plots at 8 study sites indicated that drift algal cover was relatively low (<20% at most sites) and did not remain in the same location longer than 2 to 3 mo. Experimental maintenance of high drift algal cover for 2 to 3 mo produced a ca. 25% reduction in above-ground biomass compared to plots free of drift algae, but the magnitude of the decline in biomass did not increase with an additional 3 mo of algal cover. Below-ground biomass and shoot density, however, were not affected by the presence of drift algae, suggesting that a change in plant morphometrics contributed to reduced above-ground biomass and limited long-term negative impacts as root reserves were not depleted. We investigated the effects of drift algal cover on short-term growth of Thalassia testudinum at 2 different sites in 2 different seasons. In neither instance did we observe any decrease in growth rates of T. testudinum in the presence of drift algae. The lack of a negative effect of drift algae on seagrass growth is likely due to the clonal nature of the seagrass plants, their ability to share resources among shaded and unshaded short shoots, and the availability of stored reserves in robust roots and rhizomes. We also conducted growth experiments assessing the effects of epiphytes on growth of T. testudinum. Unlike drift algal cover, the presence of encrusting calcareous epiphytes was shown to have a negative impact on growth of T. testudinum. This is likely due to a combination of decreased light levels as well as a reduction in exchange of materials across the boundary layer at the leaf surface. Collections of seagrass leaves from under drift algae and from areas free of drift algae indicated that filamentous epiphyte loads are decreased in the presence of macroalgae. Our results suggest that temporary, moderate cover of macroalgae may benefit seagrass by reducing epiphyte loads if the epiphyte cover negatively impacts the seagrass.