Production primaire pélagique de l'Etang de Berre en 1977 et 1978. Comparaison avec le milieu marin (Méditerranée nord-occidentale)

Measurements of primary production and photosynthetic efficiency were carried out in the brackish lake “Etang de Berre” near Marseilles (France), which is diluted by the Durance River, and in the area of Carry-le-Rouet (Mediterranean Sea) about 25 km off the Rhône River outlet. Primary production (¹⁴C) estimations were made in Etang de Berre from December 1977 to November 1978. The carbon uptake rates ranged between 38 and 1 091 mg C m^-3 d^-1, with an average surface value of 256 mg; water-column carbon-uptake rates ranged between 240 and 2 310 mg C m^-2 d^-1, with an average of 810, representing 290 g C m^-2 per year and 45 000 tons per year of photosynthetized carbon for the whole lake. Gross photosynthetic production measured by the method of Ryther was studied over a 2 yr period. The values obtained from marine water (Carry-le-Rouet) ranged from 23 to 2 337 mg C m^-2 d^-1, with a weighted average of 319, representing about 110 g C m^-2 per year. The values in brackish water (Etang de Berre) ranged from 14 to 1 778 mg C m^-2 d^-1, with a weighted average of 682, representing 250 g C m^-2 per year and 38 400 tons per year of photosynthesized carbon for the whole lake. The values derived from both methods of primary production measurements are approximately similar. Net production (computed from biomass estimations by Utermöhl's method) was compared with gross photosynthetic production. The net production in marine water did not display significant variations: most values were usually near zero. On the other hand, net production in brackish water exhibited a number of clear variations compared with concentrations of gross photosynthetic production during the whole 2 yr period. This large difference between estimations of gross and net production may be due to grazing, which is high in Etang de Berre, but slower and more constant in seawater. The ratios of primary production: chlorophyll a and gross photosynthetic production: biomass were also studied. In Etang de Berre, the former ratio ranges between 0.57 and 3.75, with an average of 1.44; this is similar to previously reported values. The ratio gross production: biomass in Etang de Berre varies between 0.3 and 4.2, with an average of 1.27, also confirming previous data. The very high values calculated for marine waters in the present study may result from an under-estimation of biomass.