The meaning of Benthic Ecological Quality status through a warm temperate South African estuary and in a contained blind-ending marina

Abstract

Successful management relies on accurate information on local environmental health. A typical macrofaunal assemblage-composition index, the BO2A (Benthic Opportunist Annelids Amphipods) considered to quantify benthic estuarine quality particularly well, was measured throughout the Knysna estuarine system (the one with the highest South African biodiversity and conservation rating, and considered 'largely natural with few modifications'), including in a blind-ending small-boat harbour. All index levels were represented across its intertidal and subtidal areas of bare sediment and seagrass beds, from <0.001 (='high' quality) to 0.240 (='poor' quality). Overall, BO2A did indeed suggest 'good' ecological quality. Nevertheless, only 25% of individual sites displayed 'high' quality; most were rated at the lower levels normally taken to indicate some degree of environmental degradation. The abundance of opportunistic annelids, which averaged 1153 m⁻² (16% of total macrofaunal numbers), did not vary along its longitudinal axis, and neither did the index. BO2A proved directly related to log[p/(1-p)] where p = proportion of just the opportunistic annelids. As in other equivalent marinas, these annelids also dominated the small-boat harbour, and this showed some evidence of faunal impoverishment relative to adjacent backwaters but its benthos displayed little relationship with the degree of faunal confinement. The record of a spadellid chaetognath from the harbour access channel appears to be the first from Africa. As predicted by the Estuarine Quality Paradox, being based on the questionable assumption that the presence of certain animals considered to be opportunistic indicates unnatural, degraded conditions, macrofaunal composition indices do not contribute unambiguous data to the environmental assessment of brackish-water habitats.