Limnetica 38

Spatial variations of denitrification activity and the relative importance of controlling factors were determined using the isotope pairing technique in a seepage chain of Mediterranean flowthrough lakes (Ruidera lakes) and within a given lake in the chain (Colgada lake), all receiving extremely high nitrogen inputs. The range of denitrification rates measured (28-155 μmol N m^-2 h^-1) were comparable with rates measured with isotope pairing in other freshwater systems. While the bulk of total denitrification was mostly based on NO₃^- from the overlying water, coupled nitrification-denitrification seemed to be lower in all Ruidera lakes. At the regional scale, i.e. lake district, inter-variability in denitrification (CV = 37 %) was lower than the intra-variability observed in a single lake (CV = 54 %). In fact, a preliminary meta-analysis of data from published studies suggested that denitrification rate variabilities did not differ statistically between the environmental and regional scales. Dissolved oxygen concentration, dissolved organic carbon content and nitrate from the overlying water were found to be the most important factors affecting the spatial heterogeneity of denitrification rate in the lake complex. At the single-lake scale, only phosphorus explained the variability of denitrification rates observed along its main axis. Our study emphasizes the significance of hotspots in denitrification processes, but also highlights the need for further studies on spatial denitrification given that fact that conflicting controlling factors have been found at different scales. To spatially measure denitrification is complicated due to the intrinsic difficulties of the incubation process and hotspots, which makes modelling indispensable.