Context-dependent influence of mussel aquaculture on benthic nutrient cycling

Mussel aquaculture is globally widespread and widely regarded as one of the most sustainable forms of aquaculture. While it can provide many beneficial ecosystem services, such as biofiltration and habitat provision, it can have significant impacts on surrounding coastal environments. Some of these environmental effects, including impacts on nitrogen cycling and benthic nitrogen removal, remain understudied. This study aimed to assess the environmental conditions that influence benthic nitrogen cycling and ecosystem functioning beneath and adjacent to mussel farms. Our specific focus was to assess the potential of the presence of benthic mussels (which had dropped off mussel lines above) to enhance the ecosystem service of inorganic nitrogen removal. We measured benthic oxygen and nutrient fluxes, including nitrogen removal, beneath and outside two mussel farms in New Zealand, assessing seasonal (spring vs. autumn) variability and the importance of environmental drivers, including the density and biomass of benthic mussels. Our results revealed that sediments with benthic mussels beneath farms had higher rates of oxygen consumption and nutrient release than sediments outside farms without mussels. Nitrogen removal rates, however, were variable and context-dependent. We also provide evidence that mussels farms may increase benthic biodiversity (i.e., macrofaunal taxa). Benthic mussels were not an important driver of nitrogen removal and mussel count was negatively related to denitrification efficiency. We found no evidence for the contribution of benthic mussels associated with aquaculture in controlling eutrophication in this study area. Bivalve aquaculture consenting and environmental monitoring should acknowledge the context-dependency of both positive and negative ecological effects, and how they may be influenced by farm location and age, and the accumulation of material on the benthos over time.