Interets et signification ecophysiologique de l'estimation de l'activite nitrate reductase phytoplanctonique en milieu lacustre eutrophe

ABSTRACT

As part of the study of the eutrophic lake Aydat [Massif Central. FRANCE], we estimated the phytoplanktonic nitrate reductase activity [NRA] over 1 year [1985]. We have also looked for the ecophysiological significance of the relationships established between the NRA and several biotic and abiotic parameters of the lake. Weekly samples were gathred from several depths in both the throphogenic and tropholytic zones. The samples were filtered throught GF/C whatman filters presoaked in distilled water during 24 h. The PNRA ranged between 0 and 5.80 mg NO[2]-1[-1]-h[-1]. The majority of the relationships between the PNRA and several abiotic and biotic parameters corresponded to trends reported in the literature. However, PNRA was negatively and highly significantly correlated to nitrate concentrations [r = 0,485 for 28 d.f., p <0.01]. This finding contradicts the generally accepted idea about the inductive character of the NRA enzyme system. This discrepancy may be explained by the interpretation of FINLAY [1985], about the important role the ciliates [i.e. Loxodes which is a dominant member of the ciliate community during our investigations] play in utilization of nitrates during summer in the aerobic/anaerobic interface of the lake