Ecological restoration of artificial seawalls via ecological engineering approaches and the potential impact of global warming on a desirable ecosystem function

Abstract

Shoreline hardening is a process that modifies and replaces the natural shoreline with coastal and marine infrastructures (CMI) partly to protect populated coastal regions from natural processes such as wave action, flooding, erosion, and sea level rise. Unfortunately, traditional CMI have poor habitat heterogeneity, biodiversity, and ecosystem functions. This study aimed to investigate the effectiveness of various eco-engineering approaches for ecological restoration of artificial seawalls in three regions in Hong Kong – Ma Liu Shui, Sai Kung, and Lung Kwu Tan/Tuen Mun – and assess the global-scale effect of ocean warming on the biofiltration of habitat-forming bivalves, which is regarded as a desirable ecosystem function in restoration programs.

The hard engineering approach was investigated on both vertical and riprap seawalls in the three regions by retrofitting multiple industrial-sized precast eco-engineered concrete features to the mid intertidal zone (i.e., panels of two surface designs on vertical seawalls; armouring unit and tidal pool on riprap seawalls). After 24 months, the features harboured significantly higher alpha diversity of epibiota than the controls and exhibited higher unique taxa numbers, leading to distinctive community compositions. However, the two eco-features for each seawall type showed little difference in alpha diversity. Beta diversity of the enhanced seawalls was also substantially greater than other unmodified seawalls of the same type within each region.

Oyster baskets filled with bags of live Hong Kong oysters (Magallana hongkongensis) and cured oyster shells were retrofitted to the lower intertidal zone on riprap seawalls of the three regions and tested as a soft engineering approach. In each region, over 100 taxa were recorded during the 18-month monitoring period, of which at least 97% were recorded once or more in the oyster baskets, more than double of the controls. Furthermore, the oyster baskets had much higher unique taxa numbers than the controls (>55% versus <3% of all taxa recorded). However, both epibiotic diversity and biofiltration capacity were similar between the live-oyster set and the oyster-shell set, possibly due to high mortality of the live oysters.

A meta-analysis was conducted to examine the global-scale effect of ocean warming on the biofiltration of Mytilidae and Ostreidae, two important bivalve taxa for restoration. The annual mean biofiltration efficiency in 2100 was projected to be slightly boosted compared with 2019 as ocean warming intensified, except for tropical oysters. However, various degrees of biofiltration inhibition were projected in the warmest three consecutive months, regardless of geographical location and taxon, while the tropical oysters would still be worse off. These projections have significant implications for the planning, management, and outcome of restoration programs involving these bivalves.

Overall, the results of this study demonstrated that large-scale implementation of industrial-sized eco-engineered features of both the hard and soft approaches could effectively enhance alpha and beta diversity and ecosystem functions on artificial seawalls. However, it is important to stress that eco-engineering technology should not be considered as a surrogate for natural habitats or an excuse for reclamation, because natural rocky shores with highly complex habitats often display higher biodiversity that eco-engineering cannot attain.