Using qualitative network models to assess the influence of mussel culture on ecosystem dynamics

The expansion of the aquaculture industry in the last several decades has raised concerns about potential ecological impacts of the industry. Bivalve culture, particularly mussel farming, relies on naturally occurring plankton and numerous studies have demonstrated top-down control on phytoplankton, increased nutrients through excretion of metabolic wastes and remineralization of faeces and pseudofaeces, and bottom-up effects on predators and scavengers through mussel fall-off. However, results are inconsistent between studies, and hydrodynamic conditions and nutrient availability are thought to play an important role in the magnitude and the direction of the ecological effects of mussel culture on the surrounding ecosystem. We used qualitative network models (QNMs), to outline a general model that integrates these environmental conditions and (1) evaluated the ability of different model configurations to reproduce known responses to perturbations, (2) analyzed the behaviour of key components to contrasting hydrodynamic and nutrient condition scenarios, and (3) identified the most influential features of the derived scenarios. The model that included uncertain linkages to characterize unknown relationships performed best based on predetermined validation criteria; the addition of semi-quantitative information on the relative strength of certain linkages improved accuracy and sign determinacy of outcomes. The presence of suspended mussel culture negatively affected primary producers, zooplankton and deposit-feeders, and had a positive effect on predators and scavengers, especially in low-energy environments. Hydrodynamic conditions were shown to have a major impact on the response of the community to mussel culture, while nutrient availability had a very minor impact.