Spatial variation in abundance and diversity of the smaller surface and near‐surface eelgrass‐associated intertidal macrobenthos within a warm‐temperate estuarine bay in the Garden Route National Park, RSA

Abstract

ABSTRACT1. Smaller members (o10 mm) of the sediment-surface macrobenthos of Nanozostera capensis meadows across 9 km 2 of the marine Outer Basin of the Knysna estuarine bay (Garden Route National Park, Western Cape, South Africa) were investigated at a series of 24 stations.2. Ordination (nMDS) disclosed the existence of five clusters of stations related to degree of exposure. Relatively sheltered stations were dominated by two endemic species of deposit-feeding microgastropod, and they supported significantly higher macrobenthic densities but lower species diversity and less evenness than relatively exposed stations. The latter were dominated by polychaetes and also possessed more suspension feeders, equivalent to more open seagrass beds in other latitudes. Species richness per station, however, was relatively constant across the whole basin.3. The smaller benthic macrofauna, totalling 82 species, was found to include several animals (o5 mm) hitherto unknown from the region, including one gastropod genus (Cornirostra) not previously known from Africa.4. Species were patchily distributed across all scales from 1 m to 41 km, but variance partitioning showed components of total variance to decrease with increasing spatial scale: sample (46.5%), station (30.0%), site (23.5%).5. These findings are discussed in relation to conservation site selection in rich but faunistically heterogeneous seagrass beds that are impacted by bait collection or other human disturbance.

Hierarchical scales of spatial variation in the smaller surface and near-surface macrobenthos of a subtropical intertidal seagrass system in Moreton Bay, Queensland

“…Not only was there marked variation in identity of the species collectively responsible for aggregate features such as abundance and species richness, but replacing species did not appear necessarily to be ecologically equivalent. To some degree a similar phenomenon has also been seen amongst the smaller free-living macrobenthos of the ecologically equivalent, extensive N. capensis beds of marine outer basin of the Knysna estuarine bay (Barnes, 2010b), although unlike the position at Stradbroke the Knysna dwarf-eelgrass beds spanned a wide range of exposure and seagrass morphology. Considerable variation in species composition between localities, and to a degree in the dominant species, has also been recorded from other components of seagrass systems, e.g.…”

“…Here, the position in subtropical Moreton Bay contrasts with those in equivalent Nanozostera beds in the temperate zone. In the warm-temperate South African Knysna estuarine bay, the dominant species showed clumped dispersion patterns at all spatial scales (Barnes, 2010b). Only where a widespread and moderately abundant species occurred there at low density were dispersions random.…”

“…The obvious co-variant feature is overall faunal abundance. Total population density at the Moreton Bay sites is \15% of the mean value at Knysna (Barnes, 2010b) and \4% of that in East Anglia (Barnes, in prep.). Circumstantial evidence therefore suggests that low population density may be the key: individual animals are scattered thinly and widely across the Stradbroke seagrass beds and correspondingly random dispersion at small spatial scales would not be unexpected.…”

“…The paucity of knowledge of this component of the fauna, however, is very evident and is illustrated by the two most abundant and most widely distributed species being only described in the 1990s. The \2.5 mm gastropod Calopia imitata is a little known and seldom recorded, eastern Australian endemic, belonging to a family regarded as the sister-group of the more abundant, widely spread, and much better known Hydrobiidae (Ponder, 1999) that dominate Nanozostera meadows in the temperate Northern Atlantic and in the Western Cape, South Africa (Barnes, 2010b). Like the hydrobiids, Calopia inhabits estuarine and similarly sheltered lagoon-like areas (Ponder, 1999).…”

“…Olabarria & Chapman, 2001;Kelaher et al, 2007). Little work at all has been done on the\5 mm component of seagrass faunas, so much so that in a recent study of it in the Knysna estuarine bay in the Western Cape-an area intensively researched for over 50 years-several species new to South Africa came to light including one member of a genus previously only known from Australia (Barnes, 2010b). Nevertheless, some information on spatial variation in this fauna is available.…”