Enhancing ecological complexity in soft-bottom coastal ecosystems: the impact of introducing hard substrates

Coastal ecosystems globally face pressures, with natural coastal habitats being replaced by engineered structures. While hard structures for navigation-purposes and coastal defense can negatively impact native communities, they can also be applied in ecological restoration as artificial reefs. This way substrates may facilitate establishment of biogenic (shellfish) reefs and provide habitat heterogeneity in soft-sediment ecosystems. In a 1.5-year experiment, we introduced six different types of natural or biodegradable hard substrates in a subtidal soft-sediment system. We compared the substrates with surrounding soft sediment and evaluated differences among substrate types using biodiversity indices, community composition analyses, and food web indicators. This offers a comprehensive understanding of how the introduced hard substrates affect ecosystem complexity. Overall ecological complexity indicators were higher in the second year of the experiment. We found a significant increase in species richness, community composition, and link density (number of feeding interactions per species) on hard substrates compared to the surrounding sediment. However, Shannon diversity index and food web connectance (measure of food web saturation) did not differ between substrates and the surrounding sediment. Although differences among types of hard substrates were small, certain species were uniquely associated with specific substrates with wood and granite performing best. While there was no establishment of epibenthic shellfish reefs within 1.5 years, we conclude that introducing hard substrates effectively enhances ecological complexity in soft-bottom coastal ecosystems within that timeframe. We recommend the use of substrates with a historical ecological relevance in the system for scaling up introduction of substrates for ecological restoration.