Effects of nutritional deficiency on the seasonal distribution of lipids substances in eutrophic lakes

The seasonal distribution of particulate lipids coupled with N-nutrient availability was studied in eutrophic Lake Aydat [Massif Central, France]. The concentrations of lipids ranged between 196.9 and 2971.5 micro g. l [-1] [mean +/- s. d = 1090.1 +/- 705.5 micro g. I [-1]]. Lipids were abundant in summer and fall when nitrates were unsufficient reflecting thus an orientation of cell metabolism towards an accumulation of storage products. In such conditions, the heterocystous Cyanobacteria were found to develop due to their competitive advantage of exploiting atmospheric nitrogen. Their lipid metabolism did not seem to be affected at least partially by NO [3] exhaustion

Reflexions sur la theorie des ecosystemes: nouvelles vues pour une meilleure comprehension de la distribution de matiere et d'energie au sein d'un systeme viable

Seasonal periodicity of phytoplankton was studied in a eutrophic lake [Lake Aydat, Massif Central, France], from March to November 1985. Weekly samples were taken from both the euphotic and dysphotic zones [245 samples]. An attempt is made to check the general concepts of the theory of ecological succession using the H-diversity index of SHANNON and WEAVER. The population community of Lake Aydat appears to be constantly juvenile and permanently immature confirming the bias introduced by calculation of H, whose values are strongly influenced by the number of species present. In evaluating correlations between specific diversity [H] and several biotic parameters, some of our results approach the theoretic statements of FRONTIER. However some results contradict the ecological succession theory when specific biomasses H-based were considered. These findings imply that this concept should he further [rethought]

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

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