Attenuation of sunlight measured from moored radiometers to assess depletion of suspended particles caused by bivalve aquaculture

Diego A. Ibarra, Allan Cembella, Jon Grant

Limnol. Oceanogr. Methods 10:1051-1069 (2012) | DOI: 10.4319/lom.2012.10.1051

ABSTRACT: Bivalve suspension-feeding can produce horizontal gradients of particulate suspended matter, or seston, which may impair bivalve growth among other impacts to the coastal ecosystem. We proposed a method to assess the concentration of seston at different locations along a shellfish farm by means of measurements of the depth averaged diffuse attenuation coefficient of downwelling irradiance at 490 nm, K̄, from at least two autonomous buoys equipped with a vertical array of irradiance sensors. Two approaches were compared. First, horizontal gradients of chlorophyll plus phaeopigments (Chl) were calculated from K̄ using an empirical algorithm derived from water samples. In the second approach, gradients of particulate suspended matter were calculated after correcting K̄ for the attenuation due to water and riverine colored dissolved organic matter (CDOM), estimated from continuous in situ measurements of salinity. The method was assessed in a mussel farm in Ship Harbour (Nova Scotia, Canada). The proposed method is relatively insensitive to the angular distribution of downwelling irradiance, biofouling, frame-shading, and wave focusing; but it cannot be easily applied in places with strong and sustained sediment resuspension. This method can complement and validate current modeling studies of seston depletion, and it can assist managerial activities and legislative requirements of shellfish aquaculture. This method can also be used to assess gradients of other ecologically relevant substances (i.e., CDOM, phytoplankton, and seston) in applications associated with sewage discharges, river runoff, harmful algal blooms, suspension-feeding invasive bivalves, and other horizontally variable phenomena in the coastal ocean.