Nitrate reductase activity of phytoplankton populations in eutrophic Lake Aydat and meso-oligotrophic Lake Pavin: A comparison

Abstract

From April 1993 to May 1994, samples were taken monthly from the epilimnion, metalimnion and hypolimnion of two lakes: the eutrophic Lake Aydat, and the meso-oligotrophic Lake Pavin. Nitrate reductase (NR) activity was measured according to Hochman's (1986) modified procedure, to estimate potential nitrate assimilation by algal cells. Although the inorganic nitrogen (Ni) source was essentially in the oxidized form in both lakes (77 ± 24% and 81 ± 7% respectively in Lake Aydat and in Lake Pavin), specific NR activity (sNR) was 10.5 times higher in Lake Pavin than in Lake Aydat. Moreover in this latter ecosystem the distribution of specific NR activity was sporadic. In Lake Pavin, NR activity was correlated with nitrate concentration in the medium, nitrogen nutrient ratios (NO3/Ni, NH4/Ni and NO3/NH4) and biomass of some diatoms and Chlorophyceae. However, in the eutrophic ecosystem the temporal changes in NR activity were more complex. The specific NR activity seemed to be independent of the available inorganic nitrogen substrate but linked to the species composition of the natural phytoplankton community. As in Lake Pavin, the distribution of NR activity seemed to be linked to the biomass of some diatoms. However, this was insufficient to explain the sporadic distribution of NR activity. As ammonification and NH4 regeneration could occur rapidly and irregularly, NH4 could be preferentially assimilated.