Evidence for the contribution of ciliates to denitrification in a eutrophic lake

Abstract

High levels of nitrate reductase (NR) activity were found during a field survey in the epilimnion and metalimnion of a temperate lake (Lake Aydat, France) during summer stratification, when nitrates were analytically undetectable (< 0.5 mg L(-1)). We hypothesized that the NR activities were due to phytoplankton in the epilimnion, due to the ciliate Loxodes at the mid-depth oxic/anoxic interface and preferentially due to bacteria in the anoxic hypolimnion of the lake. In support of the hypothesis, a significant negative correlation was detected in the metalimnion between the abundance of Loxodes and nitrate concentrations, indicating nitrate use by the ciliate, and significant positive correlations were found between bacteria and nitrite concentrations at depth. The correlations are corroborated by additional evidence from chlorophyll a/NRA ratios, nitrite dynamics, and lake circulation patterns. Other ciliates besides Loxodes did not appear to significantly contribute to NRA potential. The data suggest that facultatively anaerobic ciliates such as Loxodes may be significant contributors to denitrification in eutrophic planktonic ecosystems.