Functional-guild compositional structure and patchiness in subtidal Nanozostera macrobenthos across three contrasting estuaries

Two permanently-open inlets, Knysna estuarine bay and Keurbooms Lagoon, and the only temporarily-open Swartvlei estuary are three adjacent warm-temperate water bodies that differ in their size, morphology and hydrography but which nevertheless all support extensive beds of the endangered seagrass, Nanozostera capensis. To investigate whether their varied environmental conditions influence the compositional structure of the functional guilds of seagrass-associated macrobenthos, the relative abundance and patchiness of the 28 identifiable guilds occurring subtidally along their main channels were examined, including in relation to the spatial patchiness in abundance displayed by their whole macrobenthic assemblages. Two markedly different functional-group structurings were apparent, without intermediate states: one overwhelmingly dominated by local but highly abundant, epifaunal microgastropods (Alaba pinnae, 'Assiminea' capensis and 'Hydrobia' knysnaensis) that feed on leaf-associated periphyton, and the other in which this guild although present was insignificant. It was not apparent why, unusually from a global or even South African perspective, some beds were microgastropod dominated but others not. Spatial variation in microgastropod abundance appeared largely responsible for the patchiness displayed by the whole benthic assemblages, although the abundances of all guilds were significantly patchy. Where abundant, the microgastropod guild obscured underlying relationships between the functional compositional structure of the different estuarine systems and their various corresponding regions, 'mouth', 'lagoon' and 'estuary'. Here regional biogeography might be trumping physiographic, hydrographic and ecological considerations, but the ranges of the endemic microgastropods concerned are particularly data-deficient creating incertainty.