Macrofaunal community structure in the littoral zone of a freshwater-deprived, permanently open Eastern Cape estuary

Abstract

Spatial patterns in the macrofauna community structure within four distinct zones of the permanently open, freshwater-deprived Kariega Estuary on the southeastern coastline of southern Africa were investigated in March–May 2010. The zones within the littoral zone comprised a band of Zostera capensis (Setchell) exposed at spring low tide and comprising Zone I; a band of mud or sand, lacking vegetation, corresponded to Zone II; stands of Spartina maritima (Curtis) formed Zone III and a belt of Sarcocornia perennis (Miller) constituted Zone IV. Total macrofauna abundance and biomass during the study ranged from 16–816 ind/m2 and from 0.032–390.76 mg wwt/m2 and demonstrated no significant horizontal or vertical patterns (P > 0.05 in both cases). Multidimensional scaling (MDS) indicated that there were no significant spatial patterns in the macrofaunal community structure within the four zones which could be related to the predominance of euryhaline species, including Marphysa sanguinea (estuarine wonder worm), Arcuatula capensis (estuarine mussel), Macoma litoralis (littoral tellin) and Nassarius kraussianus (tick shell)  throughout the estuary. The absence of any distinct spatial patterns in the community structure of the macrofauna appears to be related to the virtual absence of any horizontal patterns in selected physico-chemical variables and availability of submerged macrophytes along the length of the estuary.  Comparisons with  historical data indicate that while there have been no significant changes in the estimates of the total macrofauna abundance and biomass within the estuary over the past three decades,  abundances of selected species have declined by as much as 80%. The decline in the abundances of these species appears to reflect the predominance of hypersaline conditions within the upper reaches of the estuary and intense fishing activities in the lower reaches of the system.

Key words: long-term changes, macrobenthos, permanently open estuary, southern Africa, Upogebia africana.