Seasonal changes in benthic fluxes of dissolved oxygen and ammonium associated with marine cultured Atlantic salmon

Benthic fluxes of dissolved oxygen and ammonium were measured at bi-weekly to monthly intervals during 1990-91 proximate to and under an array of pens holding Atlantic salmon Salmo salar Linn. in L'Etang Inlet, a macrotidal embayment in the Bay of Fundy, Canada.Hierarchical clustering of data indicated that the 7 stations could be divided into 3 groups (3 stations under the pen array, 2 at the perimeter of the array and 2 away from pens).Average rates of oxygen uptake and ammonium release for the 3 stations under the pens were 4 and 27 times higher, respectively, than values at the 2 stations distant from the cages.Maximum average rates of ammonium release (38 mm01 m-2 d-') in late July and oxygen uptake (99 mm01 m-' d-') in early September for stations under the pens coincided with maximum water temperatures and sediment sulfide accumulation, respectively.Negative redox (Eh) potentials (c 0 mV) and reduced numbers of benthic polychaetes Capitella spp.also occurred in sediments under pens between mid-July and September.Values of > 100 mM S= in sediment pore water during September could have been toxic to benthic fauna as well as to heterotrophic bacteria that produce substrates utilized by sulfate-reducing bacteria.