Modelling effects of seaweed aquaculture on phytoplankton and mussel production. Application to Strangford Lough (Northern Ireland)

Coupled macroalgal-phytoplankton and shellfish models were used to investigate the effect of macroalgae cultivation on phytoplankton and mussel growth in Strangford Lough, Northern Ireland. The study focused on the kelp species Saccharina latissima for which observational data were available from rope deployments at an experimental seaweed farm. The calibrated model gave good agreement with observed nutrient and chlorophyll concentrations as well as matching observation of kelp growth rates and frond area. Timing of spring phytoplankton growth and subsequent nutrient drawdown in the model was found to have a large effect (~50% change) on predicted kelp frond area in summer. The model was used to investigate the effect of light shading and nutrient competition between farmed S. latissima and phytoplankton (a key food source for shellfish) for a range of farm sizes. Simulations showed that kelp farming had minor impacts in general but that very intensive farming over an extensive area could lead to significant decreases in phytoplankton and mussel biomass. Simple criteria were developed for estimating the size and density of macroalgal farming that would lead to significant nutrient competition with phytoplankton. Modelling in the strong nutrient competition regime with a hypothetical very high intensity farm occupying 22% of the Lough surface, gave decreases in spring/summer phytoplankton chlorophyll concentrations of 11% (nutrient competition only) and 23% (nutrient competition and light shading). The impact on harvested dry weight of mussels is similar but smaller, with decreases of 4% (nutrient competition) and 10% (light and nutrient competition). It should be noted these are based on model compartment averages and localised changes, not resolved by the model, may be higher.