Biomasse et production primaire du phytoplancton et du microphytobenthos de deux biotopes sableux (Golfe de Fos, France)

The primary production (phytoplankton and microphytobenthos) of a coastaL shallow water ecosystem (0.5 m) was investigated in two sandy areas, differing by their wave exposure. The physical and chemical parameters, the functional chlorophyll and pheopigments as well as the primary production were measured monthly, throughout a two-year study. The bivalve Ruditapes decussatus was the dominant species in the sheltered station (RD), whereas Donax trlmcu/us inhabited the other station (DT) which was exposed to the open-sea waves. Microphytic biomasses were determined after extracting chlorophyll a and pheopigments in 90% acetone. by spectrophotometric measurements before and after acidification. Gross hourly primary production was estimated by measuring the oxygen balance in light and dark enclosures. The mlcrophytobenthos was considered in the upper centimetre. The main characteristics of the water. temperature. salinity and dissolved oxygen were more variable in the sheltered area; the redox potential was generally lower in this sediment and the vertical profile revealed a "redox potential discontinuity"• which usually did not appear in the open site. In the water column (phytoplankton seasonal fluctuations were similar in both stations. The annual average values of Chla and primary production were equivalent (1.40±0.56 and 1.94±078 mg Chla.m2: 19.60±11.09 and 20.29±8.45mgCm- 2.h- 1 in RD and DT respectively). On the other hand. for the microphytobenthos. the autumn blooms were more pronounced in the sheltered station (RD) than in the wave-exposed one (DT). The annual average value of the biomass (ChIa in upper cm of sediment) was Il times higher at the RD than the DT station (42.89 ± 6.86 and 3.87 ± 1.84 mg Chla.m• 2) and the annual average of primary production 4 limes higher (19.94±6.13 and 4.77 ± 3.12 mgC m-2 .h-1). The benthic biomass was 30 times higher at the sheltered station (RD) than the planktonic one. whereas the primary production values were similar. In the wave-exposed station (DT), the benthic biomass was only twice that of the planktonic one and the planktonic primary production 4 times higher than the benthic one. The P/E ratios (assimilation numbers) indicated a better efficiency at the planktonic level [14 mgC(mg ChIa) -1.h-1 for the sheltered site and 10.46 for the open station]. They were only of 0.46 and 1.23 mgC(mg Chla) -1 - h-1 for the benthos. Such results revealed that the primary production did not exhibit the same difference between the two stations as the chlorophylI A correlation analysis gave evidence of strong relationships between both stations for the pelagic parameters, but no correlations were found between the two sites at the benthic level. When considering the primary producers (plankton + benthos) as a whole. the sheltered station was richer and more productive than the wave-exposed one. The waters were similar in both sites. indicating a common origin, but the productivity was more important in the sheltered area. due to the richness of the benthic microflora