Denitrification in nitrate‐rich streams: Diurnal and seasonal variation related to benthic oxygen metabolism

Abstract

Seasonal variation of chlorophyll content, photosynthesis, 0, respiration, and denitrification was measured under light and dark conditions in the sediment of a nutrient-rich Danish lowland stream. Exponential growth of benthic microalgae was observed in early spring (April-May) and photosynthetic capacity persisted until fall. The benthic algae were a major C source for heterotrophic activity as indicated by a close correlation between 0, respiration and Chl content in the sediment. Denitrilication activity was related to Chl content, NO,-availability, and 0, conditions. Diffusion from the overlying water was always the major NO,-source for denitrification. Under lighted conditions, photosynthetic 0, production increased the oxic zone and reduced denitrification activity by up to 85% in spring. A simple diffusion-reaction model allowed denitrification rates to be estimated from 0, respiration rates and concentrations of O2 and N03-in the stream water. Throughout the season, estimated denitrification rates correlated well with those actually measured. The model demonstrated that denitrification activity was controlled primarily by the thickness of the oxic surface layer which served as a diffusion barrier for NO,-to the denitrification zone.

Inferring nitrogen removal in large rivers from high‐resolution longitudinal profiling

“…Benthic autotrophs control removal both directly via uptake and indirectly by providing organic matter to support heterotrophic N removal (Christensen et al 1990;Heffernan and Cohen 2010). Our results suggest significantly higher total removal rates in heavily vegetated reaches.…”

“…We predicted greater removal with decreased specific discharge: discharge (Q) divided by channel width (w), as a result of a greater benthic surface area for N processing per unit flow. We also predicted greater removal rates in heavily vegetated reaches in direct response to enhanced assimilatory uptake (Roberts and Mulholland 2007), and indirectly in response to greater supply of labile carbon to fuel dissimilatory removal (Christensen et al 1990;Heffernan and Cohen 2010) and effects of vegetation on residence time .…”

“…Small differences in sampling path and sampling velocity may be sufficient to create significant uncertainty in parameter estimates. More likely, however, these differences arise because heterotrophic pathways are not time invariant, responding to diel variation in dissolved oxygen or other solutes (Christensen et al 1990).…”

Nitrogen dynamics in rural streams : differences between geomorphologic units

“…Most studies focussed on nitrogen-poor desert streams (Fisher 1977, Grimm & Fisher 1984, Grimm 1987, Valett et al 1990, Valett et al 1994, Holmes et al 1996 or on mountain streams (Mulholland 1992, Mulholland et al 2000, Peterson et al 2001), but few works focussed on nitrogen dynamics and microbial activities in disturbed streams (Hill 1983, Christiensen et al 1990, Hill et al 1998, although human activities in rural streams have altered the nutrient budgets and sediment grain size characteristics (Vitousek et al 1997) that both modify nitrogen dynamics. In the same way, most results have been obtained for deep sediments (from 20 to 100 cm deep) (Triska et al 1990, Valett et al 1990, Wroblicky et al 1998 or for different type of substrates (rocky bottom, debris dams) (Coleman & Dahm 1990, Munn & Meyer 1990), but few studies focussed on nitrogen dynamics and microbial activities in the first centimetres of sandy bottom sediments.…”

“…2006, 42 (1), [43][44][45][46][47][48][49][50][51][52] Nitrogen dynamics in rural streams : differences between geomorphologic units** Some effects of geomorphology on the processes that occur in shallow sediment can be predicted from bibliographic sources (Table 1): hydrological exchanges will be reduced in pools because of fine sediment deposition (Mermillot-Blondin et al 2000), nitrification will follow the same trend due to oxygen depletion (Hall 1986, Kemp & Dodds 2001, ammonium produced by ammonification will accumulate inside sediments (Lefebvre et al 2004), while nitrate will disappear because of a high denitrification (Dahm et al 1987). Reverse predictions can be made for riffles and sand bars, where vertical exchanges and oxygenation are higher (Hendricks 1993), nitrification stimulated by the available oxygen, ammonium will be both oxidized and flush out of the sediments (Lefebvre et al 2004), while nitrate content will be similar or slightly higher than in surface water (Christiensen et al 1990). …”