River oxygen uptake by benthic microorganisms
The factors influencing benthic microbial uptake of river dissolved oxygen (DO) and dissolved organic matter (DOM) were investigated in 4 New Zealand rivers receiving organic waste discharges from dairy factories and a pulp and paper company. A chamber system for in situ use was developed and tested against segment oxygen mass balances in 3 rivers. Benthic oxygen uptake rate (BUR) was found to be strongly affected by circulation velocity, electron acceptor (oxygen) concentration and in some instances electron donor (glucose) concentration. Satisfactory agreement between calculated BUR values and those obtained using the chamber was obtained with matching boundary velocities (0.05 m above bed) in the 2 shallow (<0.5 m) rivers, with low chamber values in the deep (>1.5 m) river. Thus chamber devices must adequately simulate the natural flow and turbulence conditions in order to obtain realistic BUR measurements. Maximum BUR was measured for filamentous ‘sewage fungus’ colonisation at 70.2 g O₂ m⁻² day at 7.0 g O₂ m⁻³. Benthic oxygen penetration depths were estimated from chamber and sediment intrinsic activity measurements and ranged from 0.32 to 32 cm, with minimum values for ‘sewage fungus’ biofilms. Actual measurements of interstitial DO for 2 pumice bed rivers, showed DO present down to at least 10 cm in the Waiotapu River and down to 15 cm in the Tarawera River. The estimated oxygen penetration depths were used to calculate benthic dispersion coefficients. These values ranged from 0.38-16.30 x 10⁻³ cm⁻² s⁻¹ which are several orders of magnitude higher than previously reported maxima. Such high values imply that a high rate of sediment-water exchange was sustained by turbulent mass transfer processes rather than molecular diffusion. Benthic biomass values by ATP extraction ranged from 0.67-4.95 μg ATP cm⁻³, with higher maximum values for the rivers studied than have been reported for sediments in other aquatic systems, and suggesting greater levels of metabolic activity. Benthic biomass involved with sediment-water exchange was shown to be up to 50 times the planktonic biomass present, and generally 5 to 10 times for sites below organic discharges. Longitudinal and depth profile studies of enzyme activities showed that the predominant benthic metabolic activity changed in relation to distance from the organic discharge and with depth beneath the sediment surface, presumably reflecting changes in organic constituents and concentrations in the overlying flow. Removal of organic wastes from the river water in shallow systems was shown to be largely the net effect of 2 processes: (i) active benthic microbial removal of DOM and (ii) passive settling particulate organic material. It is concluded that lotic river environments may sustain higher levels of benthic biomass and nutrient transformation than do lentic environments as a result of turbulent sediment-water exchange processes. Substantial differences in the level of activity will occur between rivers with an erosional environment, which promote epilithic metabolic processes, and those with a depositional environment where detrital processes predominate.
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