Diatom colonisation and biofilm metal bioaccumulation: Can Indigenous Knowledge Systems aid the ecological engineering of urban coastlines?

Marine engineered structures alter the coastal ecosystems' functionality, replacing natural substrates with flat surfaces, often negatively impacting biodiversity. While providing coastal protection, artificial structures can hinder the initial colonisation by biofilm and the recruitment of coastal species. Greening the grey infrastructure through nature-based interventions is increasingly used to enhance biodiversity in artificial structures. This study explores the potential of the sedge Cyperus textilis and its Indigenous Knowledge applications as a substrate for coastal eco-engineering at an urbanised site on the southeast coast of South Africa. Diatom succession and metal bioaccumulation on the deployed trial-version designs (imizi structures) were monitored for a month, with samples collected at six, twelve, twenty-four, seventy-two hours, one and two weeks, and one month after deployment. Water quality, including dissolved nutrients and metal concentrations, were assessed near the substrates. Findings indicated that diatom colonisation occurred within twelve hours, with initial species including Fragilaria pulchella, Neofragilaria nicobarica, Navicula sp. and Grammatophora undulata, followed by a significant increase (4.6 times) in species diversity from 5 to 23 diatoms after one week. Metal bioaccumulation of aluminium, iron, zinc, manganese and arsenic was higher in the biofilm developing on the substrate compared to its surrounding environment (imizi substrate and water), suggesting the potential bioremediating capabilities of the biofilm on the nature-based material. These findings indicate the potential suitability of using Indigenous Knowledge-based materials for coastal eco-engineering practices as promoters of primary productivity, with the added potential of the plant C. textilis for bioremediation of toxic metals such as arsenic.