Microalgae on seagrass mimics: Does epiphyte community structure differ from live seagrasses?

The role of seagrasses in regulating epiphytes is a question of central importance for understanding structuring processes in seagrass communities. This study tests the hypothesis that seagrasses are not simply bare substrata for microalgal attachment, but rather influence community composition by altering competitive interactions between different microalgal groups. The effects of wave exposure, depth, seagrass species and seagrass type (mimic vs. live blades) on eelgrass (Zostera marina L.) and shoalgrass (Halodule wrightii Ascher) epiphyte community structure were examined using manipulative field experiments. Relative abundances of major microalgal groups were determined using high-performance liquid chromatographic measurements of diagnostic photopigments. Exposure to wave energy, depth, and seagrass species did not affect epiphyte total biomass. However, epiphyte biomass was significantly greater on live than mimic blades. Diatom biomass was higher under conditions of low wave energy, in deep habitats and on mimic blades. Cyanobacterial biomass was higher in high energy habitats and on live seagrass blades. Although diatoms had a significantly higher biomass on mimic blades, their biomass contribution relative to cyanobacteria was higher on live seagrass blades. The differences in epiphyte community structure on live vs. mimic seagrass blades suggest that competitive interactions between seagrass and epiphytes may result in selection against cyanobacteria or for diatoms. Another possibility is that seagrasses modify the microenvironment on blade surfaces in a way that alters the outcome of competitive interactions between major algal groups (i.e., diatoms and cyanobacteria).  1998 Elsevier Science B.V.