Spatial and temporal coherence between Amazon River discharge, salinity, and light absorption by colored organic carbon in western tropical Atlantic surface waters

Abstract

The temporal evolution and spatial distribution of surface salinity and colored detrital matter (cdm) were evaluated within and adjacent to the Amazon River Plume. Study objectives were as follows: first, to document the spatial coherence between Amazon discharge, salinity, cdm, and the nature of the salinity‐cdm relationship; second, to document the temporal and spatial variability of cdm along the trajectory of the low‐salinity Amazon Plume, and third, to explore the departure of cdm from conservative mixing behavior along the plume trajectory into the open ocean. Time series (2003–2007) of surface salinity estimated using the Advanced Microwave Scanning Radiometer–Earth Observing System and corresponding satellite cdm absorption (acdm) data documented a plume of freshened, colored water emanating from the Amazon. Salinity and acdm were generally coherent, but there were regions in which spatial patterns of salinity and acdm did not coincide. Salinity was oppositely phased with discharge, whereas acdm was in phase but lagged discharge and typically remained high after maximum discharge. Along the river plume trajectory, acdm was inversely correlated with salinity, yet there was considerable deviation from conservative mixing behavior during all seasons. Positive anomalies in a linear relationship between salinity and acdm corresponded to areas of enhanced satellite‐retrieved net primary productivity, suggesting the importance of phytoplankton biomass or its subsequent remineralization as a source of cdm. Negative anomalies tended to predominate at the distal sections of the plume trajectories, an observation consistent with the process of photo‐oxidation of cdm over observed time scales of days to weeks.