Eco-engineering of modified shorelines recovers wrack subsidies

Wrack (stranded phyto-detritus) from terrestrial and marine sources is an important source of carbon and nutrients for many intertidal habitats. In urbanised estuaries, seawalls may act as a barrier to the transport of wrack between terrestrial and marine habitats and, where they reduce the width and habitat complexity of the intertidal zone, negatively impact on wrack accumulation and retention on estuarine shorelines. We assessed differences in the accumulation of wrack between natural (sandy beach, rocky reef and rockpools) and armoured stretches of shoreline, and the efficacy of eco-engineering interventions (man-made rockpools and planted saltmarshes) in enhancing the accumulation and retention of wrack. Surveys conducted at 4 sites in Kogarah Bay, New South Wales, Australia, on 3 dates during autumn-winter 2016 revealed that on natural shorelines most wrack accumulated at the high intertidal mark. Placement of seawalls restricted wrack accumulation to the low intertidal zone, in front of the seawall. The cover of wrack was significantly less in the low intertidal zone in front of seawalls than either the high or low tidal elevations of the natural sandy beaches and rockpools. The man-made rockpools in the low tidal zone had higher cover and biomass of marine and terrestrial wrack than the low intertidal zone of the seawalls, and both these and the planted saltmarshes also supported the accumulation of wrack at the mid intertidal elevation. Experimental deployments of marked seagrass wrack revealed that the eco-engineered habitats enhanced wrack retention over a 7 day period compared with the seawalls. However, the artificial rockpools also retaining more wrack than the equivalent natural habitat. Our results suggest that the structural complexity of eco-engineered habitats can be effective at trapping and retaining wrack, with potential flow-on effects to benthic assemblages through increased food and shelter resources. When designing eco-engineering interventions for modified shorelines, scientists and managers should consider not only their impacts on biodiversity but also on key ecosystem functions.