Assessing the impact of beach nourishment on the intertidal food web through the development of a mechanistic-envelope model

Abstract

Beach nourishment, the placement of sand onto a sediment-starved stretch of coast, is widely applied as a soft coastal protection measure because of its reduced ecological impact relative to hard coastal protection. In order to predict effects on the intertidal sandy beach ecosystem, we developed a simulation model that integrates species envelope-based projections for the dominant macrobenthos species and mechanistic food web modules for higher trophic levels. Species envelopes were estimated by using Bayesian inference of species' biomass relationships according to the three determining abiotic variables: intertidal elevation, median grain size and total organic matter, obtained from multiple sampling campaigns along the Belgian coast. Maximum potential abundance of higher trophic levels represented by birds, shrimp and flatfish were estimated based on their derived trophic relationship with macrobenthos. After validation, we demonstrated that unlike nourishment slope, sediment grain size strongly determines beach-level species richness and production, with strong deterioration in species richness after nourishment with coarse sediment (>300 μm). Patterns for higher trophic levels do not follow the changes in macrobenthos abundance and biomass. Synthesis and applications. The optimal grain size range for nourishment of fine-grained beaches is 200-300 μm. This modelling approach shows that the impact assessment of beach nourishment needs to include the evaluation of different species richness and biomass variables. Focusing solely on the potential abundance of species from higher trophic levels might lead to deceptive conclusions due to the dominance of opportunistic prey species. The optimal grain size range for nourishment of fine-grained beaches is 200-300 μm. This modelling approach shows that the impact assessment of beach nourishment needs to include the evaluation of different species richness and biomass variables. Focusing solely on the potential abundance of species from higher trophic levels might lead to deceptive conclusions due to the dominance of opportunistic prey species.