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 Chla/SPM ratios and associated low variability of this ratio, and by a highly significant relationship between chlorophyll pigments (chlorophyllaand phaeopigments) and SPM. Seaward of the MTZ, where SPM concentration is lower, the relationship between Chlaand SPM disappeared during phytoplanktonic blooms, the values of Chla/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 Chla/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 Chlain 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.