A comparison of the fauna in eelgrass and erosion control structures in a warm temperate Southern African estuary

Abstract Estuaries are important coastal systems offering nursery and foraging grounds for many species. With the increasing threat from climate change and sea-level rise, the armouring of coastlines, to withstand coastal erosion, has become common practice. The transformation of soft sediment habitats to hard, artificial habitats can lead to changes in species diversity, composition and distribution. As there is limited information regarding the ecological impacts of using Reno mattresses (wire mattresses filled with rocks) to combat erosion, this study investigated the faunistic differences of these structures compared to natural eelgrass (Zostera capensis) habitat in the Keurbooms estuary, South Africa. Benthic macroinvertebrate and fish abundances and assemblages were assessed seasonally from winter 2018 to spring 2019. Macroinvertebrates were assessed using standard core and suction sampling whereas the non-destructive method of mini Baited Remote Underwater Video Systems (BRUVs) was used to sample fish. Fish assemblages differed significantly between habitats and across seasons with greater abundances and diversity at the artificial structures compared to eelgrass habitat. Invertebrate taxa displayed some overlap between habitats, however, three higher taxonomic groups were only recorded within Reno mattress habitat with only one taxon unique to the eelgrass habitat. This study provides new information on the ecological efficacy of Reno mattress structures used in coastal management which can inform future coastal erosion control practices.