Effect of organic enrichment and thermal regimeon denitrification and dissimilatory nitrate reductionto ammonium (DNRA) in hypolimnetic sediments of twolowland lakes

We analyzed benthic fluxes of inorganic nitrogen, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates in hypolimnetic sediments of lowland lakes. Two neighbouring mesotrophic (Ca’ Stanga; CS) and hypertrophic (Lago Verde; LV) lakes, which originated from sand and gravel mining, were considered. Lakes are affected by high nitrate loads (0.2 to 0.7 mM) and different organic loads. Oxygen consumption, dissolved inorganic carbon, methane and nitrogen fluxes, denitrification and DNRA were measured under summer thermal stratification and late winter overturn. Hypolimnetic sediments of CS were a net sink of dissolved inorganic nitrogen (-3.5 to -4.7 mmol m-2 d-1) in both seasons due to high nitrate consumption. On the contrary, LV sediments turned from being a net sink during winter overturn (-3.5 mmol m-2 d-1) to a net source of dissolved inorganic nitrogen under summer conditions (8.1 mmol m-2 d-1), when significant ammonium regeneration was measured at the water-sediment interface. Benthic denitrification (0.7 - 4.1 mmol m-2 d-1) accounted for up to 84-97% of total NO3- reduction and from 2 to 30% of carbon mineralization. It was mainly fuelled by water column nitrate. In CS, denitrification rates were similar in winter and in summer, while in LV summer rates were 4 times lower. DNRA rates were generally low in both lakes (0.07- 0.12 mmol m-2 d-1). An appreciable contribution of DNRA was only detected in the more reducing sediments of LV in summer (15% of total NO3- reduction), while during the same period only 3% of reduced NO3- was recycled into ammonium in CS. Under summer stratification benthic denitrification was mainly nitrate-limited due to nitrate depletion in hypolimnetic waters and parallel oxygen depletion, hampering nitrification. Organic enrichment and reducing conditions in the hypolimnetic sediment shifted nitrate reduction towards more pronounced DNRA, which resulted in the inorganic nitrogen recycling and retention within the bottom waters. The prevalence of DNRA could favour the accumulation of mineral nitrogen with detrimental effects on ecosystem processes and water quality.