International Council for the Exploration of the Seas Theme H: Ecological Carrying Capacity in Shellfish Aquaculture 1 Not to be cited without prior reference to the author ICES CM 2008/ H:12 Phytoplankton depletion by mussel aquaculture: high resolution mapping, ecosystem modeling and potential indicators of ecological carrying capacity Peter J. Cranford 1 , William Li 1 , Øivind Strand 2 and Tore Strohmeier 2 1 Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, NS Canada B2Y 4A2. Contact: cranfordp@mar.dfo-mpo.gc.ca; tel: +1 902 426 3277, fax: +1 902 426 2256 2 Institute of Marine Research, Nordnesgaten 50, 5817 Bergen, Norway Abstract Mussels held in suspended culture have an exceptional capacity to filter the water column and reduce suspended particle concentrations. However, seston depletion is only of concern if the phytoplankton are cleared faster than they can be replaced by tidal exchange and primary production. The occurrence of significant phytoplankton depletions over extended periods and different spatial scales is directly linked to concepts of production and ecological carrying capacity owing to food limitation and alterations in ecosystem structure, material fluxes and pathways and nutrient cycling. Knowledge on ecosystem interactions with shellfish aquaculture supports the growth of a sustainable industry and the development of an ecosystem-based management approach. The scale and magnitude of phytoplankton depletion was documented at mussel aquaculture farms in Canada and Norway using a computer controlled, towed undulating vehicle (BIO-Acrobat) that collects geo-referenced CTD and chlorophyll a data. Rapid synoptic surveys with intensive horizontal and vertical sampling permitted high resolution 3-D mapping of phytoplankton variations over farm to coastal ecosystem scales. Phytoplankton depletion by mussels is size-specific and it is expected that in areas where mussels control phytoplankton biomass, that picophytoplankton (0.2 to 3.0 μm) will dominate. This hypothesis was tested, and confirmed, by measuring total and picophytoplankton biomass in Prince Edward Island embayments, where the risk of phytoplankton depletion varies greatly owing to regional differences in water flushing, bay volume and culture biomass. Key words: mussel culture; phytoplankton depletion; picophytoplankton; carrying capacity; ecosystem models; indicators Introduction Aquaculture is the fastest growing food-producing sector in the world and is the only means of filling the growing gap between consumer demand and seafood production from traditional capture fisheries. While there is a need for the continued worldwide expansion of aquaculture to fill this gap, industry development needs to be promoted and managed in a manner that minimizes negative environmental impacts (FAO, 2008). Unlike finfish culture, which requires the addition of feed and chemical additives, mussel farming relies entirely on natural food supplies. Environmental concerns are related primarily to how the cultured mussels interact within the ecosystem. Mussels live in dense colonies and have an exceptional capacity to filter large volumes of water to extract food (phytoplankton and other suspended particulate matter). Filter- feeding by mussels naturally results in some local reduction (depletion) of their phytoplankton food supply. However, if the mussel culture is consuming phytoplankton faster than they can be replaced by tidal flushing and phytoplankton growth, then the mussels will become food limited and production will be less than maximal for that site. This is referred to as exceeding “production carrying capacity”. If the spatial scale of phytoplankton depletion includes a significant fraction of the coastal inlet, then this effect on the base of the marine food web raises