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Theoretical considerations of the effects of turbulence around and within sedimentation traps show that the aspect ratio (height:mouth opening) of the traps is important in determining particle retention. Collection efficiency and comparative measures of deposition per unit area in traps of various design were independent of this ratio in cairn water, but an asymptotic relation of increased collection efficiency with higher aspect ratio existed under nonstagnant conditions. Turbulent mixing in a water column could be assessed by comparisons of deposition in collectors exposed simultaneously but having different aspect ratios.

Oxygen uptake by microbial populations on mud, sand, and various types of detritus was measured in short-term experiments in aerated water at 20C. Sample size had no effect on oxygen consumption per unit weight, but stirring increased uptake. A 2% Formalin solution completely stopped biological uptake of oxygen by microorganisms on sand and detritus; lake muds showed various degrees of chemical uptake of oxygen.Microorganisms on dead Phrugmites leaves consumed oxygen at an increasing rate during the first few days of decomposition, followed by a decline to a rate comparable to uptake by freshly collected detritus.Limnaea feces initially consumed oxygen three times more rapidly than detritus, but after 5 days the rates were equal.Detritus consumed up to three orders of magnitude more oxygen per dry weight than sand; uptake rates were inversely related to particle diameter.The logarithm of oxygen uptake was directly related to the logarithm of particle organic content. Particulate oxygen uptake in this and previous studies fell between 0.1 and 10 mg 02 (g organic matter)-l hr-l, a rate inversely related to particle diameter.Oxygen uptake per unit weight by particles of ashed mud and sand exposed for 24 hr to decomposing detritus in a nutrient solution and transferred to fresh solution was inversely related to particle size and similar to rates measured with freshly collected samples. On an areal basis all particles consumed between 0.01 and 1.0 x lo-' mg O2 cm-l hr-I.The negative linear correlation on logarithmic axes of surface area, organic carbon and nitrogen and bacterial plate counts with sediment particle size is similar to that observed for measures of oxygen uptake. Bacteria cover only a few percent of particle surfaces. This may result in the narrow range of measures of microbial community respiration on an areal basis.

'~e p a r t m e n t of Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 451 311420 40th Ave., Apt 101, Edmonton, Alberta, Canada T6J OR5 4Aquaculture and Invertebrate Fisheries Division, Department of Fisheries and Oceans, Biological Station, St. Andrews, New Brunswick, Canada EOG 2x0ABSTRACT: 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.