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

摘要

Mussels held in suspended culture have an exceptional capacity to filter the water column andreduce suspended particle concentrations. However, seston depletion is only of concern if thephytoplankton are cleared faster than they can be replaced by tidal exchange and primaryproduction. The occurrence of significant phytoplankton depletions over extended periods anddifferent spatial scales is directly linked to concepts of production and ecological carryingcapacity owing to food limitation and alterations in ecosystem structure, material fluxes andpathways and nutrient cycling. Knowledge on ecosystem interactions with shellfish aquaculturesupports the growth of a sustainable industry and the development of an ecosystem-basedmanagement approach. The scale and magnitude of phytoplankton depletion was documented atmussel aquaculture farms in Canada and Norway using a computer controlled, towed undulatingvehicle (BIO-Acrobat) that collects geo-referenced CTD and chlorophyll a data. Rapid synopticsurveys with intensive horizontal and vertical sampling permitted high resolution 3-D mapping ofphytoplankton variations over farm to coastal ecosystem scales. Phytoplankton depletion bymussels is size-specific and it is expected that in areas where mussels control phytoplanktonbiomass, that picophytoplankton (0.2 to 3.0 ìm) will dominate. This hypothesis was tested, andconfirmed, by measuring total and picophytoplankton biomass in Prince Edward Islandembayments, where the risk of phytoplankton depletion varies greatly owing to regionaldifferences in water flushing, bay volume and culture biomass.Key words: mussel culture; phytoplankton depletion; picophytoplankton; carrying capacity;ecosystem models; indicators