The significance of shore height in intertidal macrobenthic seagrass ecology and conservation

Benthic faunal assemblages of an intertidal seagrass bed were sampled at three shore heights (LWN, MLW, LWS) at the mouth, mid-point and head of the Steenbok Channel in South Africa's premier seagrass site, the warm-temperate Knysna estuarine bay, Garden Route National Park. Faunal abundance, species richness, species diversity, and proportion of rare species were relatively uniform along the Channel, as were faunal abundance, species diversity and proportion of rare species down the shore. Overall species richness per station, however, was significantly lower at LWN than at either MLW or LWS, although the distribution of species richness over the shore did not depart from random at one of the three sites. Overall faunal abundance and those of individual component species were dispersed patchily through the bed. The nature of the faunal assemblages present, however, varied significantly throughout the bed, both along the Channel at each shore-height horizon, and down the shore at each site. LWN assemblages formed a unit distinct from those at MLW and LWS. Overall, the shore-height axis accounted for 47% and the along-shore axis 28% of total assemblage variation. Faunal assemblages were randomly structured at each nodal intersection point of the down-shore and along-shore axes, but were significantly non-randomly assembled down the shore at each site and along the Channel at each shore-height horizon except at the LWN level. Components of total assemblage variance were largest at the smallest scale investigated (1m). Higher seagrass horizons are not just progressively more impoverished versions of lower ones and although macrofaunal assemblage composition is heavily influenced by shore height much of the ecological structure of faunal assemblages seems little affected by tidal horizon. These findings are discussed in relation to conservation of the vulnerable seagrass Nanozostera capensis at Knysna, a system affected by subsistence exploitation of its intertidal benthic invertebrates. © 2011 John Wiley & Sons, Ltd