The combined influences of intensive mussel aquaculture and watershed nutrient inputs on nitrogen dynamics in Tracadie Bay, Prince Edward Island, Canada, were examined using a nitrogen budget and an ecosystem model. Budget calculations, and inputs and parameters for the model were based on extensive field data. Both approaches showed that mussel aquaculture has a dominant influence on all aspects of the nitrogen cycle and dramatically alters pathways by which nitrogen reaches the phytoplankton and benthos. A large proportion of phytoplankton production is supported by land-derived nitrogen and this anthropogenic input is important for sustaining existing levels of mussel production. The amount of nitrogen removed in the mussel harvest is small compared with agricultural nitrogen inputs and the amounts excreted and biodeposited on the seabed. Mussel biodeposition greatly increases the flux of nitrogen to the benthos, with potentially serious eutrophication impacts. Results from the observation-based nitrogen budget and dynamic model were compared and both support the above conclusions. However, the ability of the model to test different scenarios and to provide additional information (e.g. fluxes) over a finer spatial scale led to insights unattainable with a nitrogen budget. For example, food appears to be less available to mussels at the head of the Bay than at the mouth, despite the lower density of grow-out sites in the former location. The number of fundamental ecosystem processes influenced by the mussels and the complexity of their interactions make it difficult to predict the effects of mussels on many ecosystem properties without resorting to a model.

This study evaluated monthly accumulation rates and types of marine debris washed ashore at a recreational beach in Point Pleasant Park, Halifax Harbour, between April and September 2005. Black Rock Beach is 70 m long and a total of 2129 marine debris items were collected and sorted, representing a mean accumulation rate of 355 (±68 SE) items month-1. The total weight of debris items was only 10.8 kg (mean 2 kg ±0.4 SE), however eighty-six percent of this debris was plastic material. The types of litter found included: tampon applicators, condoms (i.e., sewage-related debris [SRD]); plastic fast food packaging, confectionary wrappers, Styrofoam fragments, plastic bottles and caps, items of clothing, soft drink cans, cigarettes and cigarette holders (i.e., recreational or land-based debris); packing bands, nylon rope and nets (i.e., shipping- or fishing-related debris). These items were generated by recreational use of the park (52%), sewage disposal (14%) and from shipping and fishing activities (7%). It is suggested that a significant reduction in marine debris at recreational beaches may arise by improving public awareness of the environmental and aesthetic impacts of marine litter and future improvements to the municipal sewage disposal system.

Marine bivalves have been designated ecosystem engineers owing to their capacity to control estuarine water quality, particle dynamics, and primary production. Globally, bivalves have higher production than any other cultured animal. Large populations of natural, invasive, and cultured bivalves are suggested to cause changes in coastal ecosystem function through suspension-feeding of particles and biodeposition of waste materials. Association of bivalves with particle depletion is a trophic tenet of coastal ecosystems, but there are no previous observations of this process except at small scales. Using airborne hyperspectral remote sensing, we show direct evidence of aquaculture impacts at the ecosystem scale (kilometres), documenting significant depletion of phytoplankton through a blue mussel (Mytilus edulis) farm in eastern Canada, compared with dispersion in circulation model results without mussels. Understanding of factors controlling primary production and ecosystem processes in the coastal zone is critical in light of growing reliance on this region for development and resource extraction worldwide.

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