Differential effects of mangrove cover and engineered structures on benthic macrofauna and nekton assemblages in hybrid living shorelines

Abstract

Hybrid shoreline stabilisation with rock fillets has facilitated mangrove establishment at eroding estuarine shorelines in New South Wales, Australia. To assess the biodiversity benefits of installing hybrid mangrove shorelines, this study compared mangrove structure, benthic macrofauna and nekton assemblages in these hybrid mangroves to nearby natural fringing mangroves across multiple estuaries. Hybrid mangroves (6–10 years old) showed higher densities of trees and seedlings but comparable macrobenthos abundance to natural mangroves. However, differences in macrobenthos composition, especially in crab and polychaete species, were noted across estuaries and between natural and hybrid mangroves with varying mangrove cover. Mangrove cover had limited effect on the utilisation of hybrid mangroves by nekton species. Mainly the freshwater prawn (Macrobrachium australiense) and demersal species, such as glassgoby (Gobiopterus semivestitus), juvenile yellowfin bream (Acanthopagrus australis), and flat-tail mullet (Gracimugil argenteus), were found in the hybrid mangroves. Overall, nekton was more abundant at hybrid mangroves than at natural fringing mangroves, possibly due to the hydrodynamic shielding and additional refuge provided by rock crevices. When planning hybrid living shorelines, it is crucial to consider the temporal lag in recovery and the potential trade-offs in ecological processes and functions, which can vary depending on the local species pool and environmental conditions. If mangroves cannot naturally establish at eroding shorelines, installing rock fillets to facilitate mangrove establishment could be an effective alternative to reduce lateral erosion while providing habitats for estuarine fauna.