Phytoplankton depletion by mussel aquaculture: high resolution mapping, ecosystem modeling and potential indicators of ecological carrying capacity
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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 ìm) 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