The impact of a Reno mattress installation on an adjacent seagrass meadow and its macrofauna in a South African estuary

Armouring of coastlines and the banks of estuaries to protect property against erosion can have a negative impact on natural habitats and their communities. Within estuaries, one of the most threatened habitats is seagrass meadows. The impact of newly installed Reno mattresses on an adjacent Zostera capensis meadow and associated fauna was investigated in the Keurbooms Estuary, South Africa. The impact on Z. capensis was studied by determining percentage cover and the length of blades before and after placement of Reno mattresses. At the impact site, percentage cover and blade length decreased during the installation phase, however, recovered shortly after installation was completed. At the control site no changes in eelgrass percentage cover were noted. Invertebrate and fish species richness, diversity and abundance were compared in the eelgrass meadows before and after the installation. Whilst invertebrate species richness increased significantly after the installation at the Impacted site, there was no significant difference at the Control site. There was also no significant difference in invertebrate diversity and fish species richness and diversity at the Control and Impacted sites before and after installation. This suggests that the installation had no net negative impact at the Impacted site. Within a month of installation, the Reno mattresses were colonised by two invertebrate species (Amphibalanus amphitrite and Littoraria sp.). Although the introduction of Reno mattresses at the study site created a hybrid habitat that continued to provide a habitat for fish and invertebrates, we suggest that the use of these artificial structures should be in combination with natural vegetation (e.g. as a hybrid habitat) and not one that replaces natural habitat. There is limited information regarding the ecological impacts of using Reno mattresses in estuaries and this study provides new information on their ecological efficacy thus providing valuable insights in future coastal erosion control practices.