Incorporating spatial variation of nitrification and denitrification rates into whole‐lake nitrogen dynamics

Abstract

Despite dramatic increases in nitrogen (N) loading to fresh waters and growing scientific attention on the changing N cycle, measurements of nitrification and denitrification rates in lakes are lacking. In particular, we know little about how these processes vary spatially within a lake, and how this potential spatial variation contributes to a lake's N dynamics. We measured sediment nitrification and denitrification rates at 40 sites in Gull Lake, Michigan (USA), and found that the shallow edge sediments (<2 m deep) of the lake were hot spots of N transformation. Nitrification rates were comparable in sediments at all depths, while sediment denitrification rates were highest in the shallow edge habitat, and lowest in the profundal sediments (<2 m and >10 m deep, respectively). We scaled‐up our sediment transformation rates across the lake to illustrate spatial variability in nitrification and denitrification. For whole‐lake nitrification, the contribution of shallow edge, littoral, and profundal sediments followed in proportion to lake surface area of each habitat. In contrast, the contribution of each of these areas to whole‐lake denitrification was not proportional to their respective surface areas, and instead was equal across the 3 habitat types, with each area contributing roughly 30% of the total N loss via denitrification. Spatially representative characterization of nitrification and denitrification in lentic ecosystems requires incorporation of the spatial variation in these transformations with a particular focus on littoral sediments, and this is often overlooked in studies of lentic N cycling. Furthermore, anthropogenic changes to lake shorelines that influence N cycling in littoral sediments may have a disproportionate effect on whole‐lake ecosystem function.

Whole-lake spatial variability of organic matter molecular composition and elemental inorganic properties in a small boreal Swedish lake

“…Overall, our molecular characterization of OM in the south basin suggests an enrichment in 471 algal versus allochthonous OM (e.g., higher N-compounds:carbohydrates) in the deeper areas of a 472 deep, simple lake basin, in line with previously reported sediment C:N ratios along lake-basin 473 transects (Kumke et al, 2005;Dunn et al, 2008;Bruesewitz et al, 2012). Given our data on the 474 degradation status of the different OM source-pools, we believe that this trend in OM quality results 475 from preferential degradation of algal versus allochthonous OM in sediments at 476 shallower/intermediate water depth in addition to the known focusing of living, and residues of, 477 authochthonous OM towards deeper sites (Ostrovsky and Yacobi, 1999).…”

The Effects of Turbulence and Carbon Amendments on Nitrate Uptake and Microbial Gene Abundances in Stream Sediment

“…In addition, J NO3 rates were similar to those quantified in a similar flume experiment (O'Connor et al, ). One reason for this could be due to the redox conditions in the flume, where oxic conditions were present in the sediment, differing from the anoxic conditions of the assays (Bruesewitz et al, ), thereby causing increased denitrification rates for the assays. Also, the assays are maintained in slurried conditions, where all bacteria present in the sediment are able to denitrify nitrate in the water and normalization to areal rates assumes that the upper 5 cm of sediment has this same denitrification rate.…”

Control of nitrogen and phosphorus transport by reservoirs in agricultural landscapes

“…This may indicate that shallow littoral sediments have a high denitrification rate due to the co-occurrence of anoxia, nitrate supply, and carbon supply (Bruesewitz et al 2012). Further, in order for denitrification to be maximized at the ecosystem-level over the annual time frame, zones or patches of sediment denitrifier activity must occupy a sufficiently large spatial or temporal extent, and N sources originating in surface water (oxic) must be transferred across the sedimentwater interface into sedimentary (anoxic) N sinks.…”