Trophic structure and biomass distribution of macrobenthos on sheltered and semi-exposed rocky shores of Tsitsikamma Marine Protected Area

Abstract

The trophic structure and biomass of macrobenthos on both wave-sheltered, rocky intertidal shores and semi-exposed ones at seven localities in the Tsitsikamma Marine Protected Area were compared. In the Cochlear zone and entire intertidal (Cochlear- and Balanoid zones combined) biomass values of invertebrates and filter-feeders were significantly (P < 0.05) higher at   semi-exposed shores, while grazer biomass was greater at sheltered sites: a negative relationship (P < 0.01) and a moderate to weak correlation (r > 0.42) was also recorded between filter-feeder biomass and grazer biomass. This negative relationship  reflected changes in standing biomass of the two dominant filter-feeders (Octomeris angulosa and Perna perna) against that of the dominant grazer (Scutellastra cochlear). On sheltered shores the biomass of trophic groups varied significantly (P<0.05) across the intertidal gradient studied, with values of grazers and algae decreasing with  increased aerial exposure. Similar, but less  substantial (P > 0.05) variations were recorded at semi-exposed sites, where the median shell length of the brown mussel P. perna decreased upshore (P < 0.05). Multivariate analyses of square-root transformed biomass values separated most of the sheltered Cochlear zone stations from semi-exposed ones. This study supported the concept that the structure of low-shore communities is determined largely by differences in wave action.

Key words: rocky shore, invertebrate, algae, communities, wave exposure, biomass, trophic structure.