Effect of leaf movement on epiphytic algal biomass of seagrass leaves

We tested the hypothesis that the movement of seagrass leaves affects the accumulation and composition of algal epiphytes by comparing the standing crop and composition on tethered (T) and untethered (UT) artificial seagrass units (ASUs) over 4 and 8 wk. We also tested whether any effect of leaf movement on standing crop was dependent on the degree of water movement by repeating the experiment over 8 wk at sheltered and exposed sites. To eliminate the possibility of inter-regional differences between exposed and sheltered sites and to test for interactive effects of light availability and leaf movement on epiphyte biomass, we repeated the experiment at a single location, but at different depths, to achieve different hydrodynamic and light conditions. In the first experiment, T ASUs accumulated between 4 and 12 times the biomass of algal epiphytes on UT ASUs. Fewer species were found on UT leaves after 4 wk, but by 8 wk, there were similar numbers of species on both treatments. The degree of exposure did not influence the effect of leaf movement on epiphyte biomass, but depth did, with a significant but smaller difference between treatments at the deeper site. We conclude that leaf movement has a clear and significant potential to affect the accumulation of epiphytes on seagrass leaves and that this effect may be comparable to, if not more important than, nutrient and light limitation in some circumstances. The movement of seagrass leaves result in up to a 7-fold increase in light penetration due to reduced epiphyte standing crop. If the effect of leaf movement on epiphyte biomass is through physical erosion, then estimates of the contribution of epiphytes to carbonate production and organic loading to sediments must factor in an erosion loss term.