MACROALGAE IN MARINE SOFT-SEDIMENT COMMUNITIES - EFFECTS ON BENTHIC FAUNAL ASSEMBLAGES

Algal removal experiments were conducted in 1984 and 1986 to measure the effects of algal cover on the structure of an intertidal benthic assemblage. Cover by the green alga, Ulva expansa (Setchell) Setchell et Gardner, had no affect on total density or species richness of small fauna, compared to removal plots, during the algal season in either year, although diversity indexes (h' and D) were slightly greater in removal plots. Epifauna had consistantly greater densities in algal plots, while infaunal densities were not different in 1984, and greater in algal plots in 1986. Mobile, sediment-water interface feeding taxa were the numerically dominant infauna, and along with predators, exhibited the same patterns as infauna in general. Sedentary sediment-water interface feeders had lower densities under algal cover, while the density of deep deposit-feeders was not different between treatments. Large bivalves, principally the tellinid Macoma nasuta (Conrad), were significantly reduced by algal cover in both years. Post-algal season samples in the 1986 experiment revealed that after a season of algal cover, the densities of small, mobile, sediment-water interface feeding infauna were significantly greater, and those of sedentary species, particularly bivalves and tubicolous suspension-feeders were significantly lower, in plots which had supported an algal mat. The increased abundance of small deposit-feeding fauna was likely a result of an increase in food resources due to in situ burial and decomposition of macroalgae. These experiments indicate that, like other submerged aquatic vegetation (SAV), macroalgae can play an important functional role in structuring benthic faunal assemblages. However, in contrast to other SAV, the presence of macroalgae, particularly laminar forms such as Ulva, can have a mixed effect on the density of benthic fauna. Mobile sediment-water interface feeding species can have greater densities, while the densities of sedentary species, especially bivalves and tube-dwellers, can be much lower with algal cover.