Light limitation and distribution of chlorophyll pigments in a highly turbid estuary: The Gironde (SW France)

Chlorophyll pigment and suspended particulate matter (SPM) concentrations were measured in two well-differentiated areas in the Gironde Estuary. The maximum turbidity zone (MTZ) was characterized by very high turbidities, low Chl a/SPM ratios and associated low variability of this ratio, and by a highly significant relationship between chlorophyll pigments (chlorophyll a and phaeopigments) and SPM. Seaward of the MTZ, where SPM concentration is lower, the relationship between Chl a and SPM disappeared during phytoplanktonic blooms, the values of Chl a/SPM ratio were higher and there was a high variability of this ratio. Data obtained from stations positioned along the salinity gradient indicate that the spatial distribution of Chl a/SPM ratio in the Gironde is explained by the relation between the mixing depth (Zm) and euphotic depth (Zeu). The estimate of the Zm/Zeu ratio suggests that there is virtually no possibility for phytoplankton primary production in the MTZ but cannot explain the presence of Chl a in very highly turbid areas (> 1 g l^-1). An important percentage of the chlorophyll in the MTZ probably originates from re-suspended microphytobenthos. Photosynthetic activity is reduced as a consequence of light limitation due to high turbidities. This enables the ecosystem to contain a great input of nutrients without eutrophication.