Variability of bio-optical properties in nearshore waters of the estuary and Gulf of St. Lawrence: Absorption and backscattering coefficients

The inherent optical properties and the optically significant constituents in nearshore zones of the Estuary and Gulf of St. Lawrence (EGSL), eastern Canada, were systematically investigated. Dry-mass concentrations of dissolved organic carbon (DOC), phytoplankton pigments, and suspended particulate matter (SPM) were determined together with the absorption coefficients of chromophoric dissolved organic matter, a(cdom); non-algal particles, a(nap); phytoplankton, a(phy); and particulate backscattering coefficient, b(bp). The sampling design allowed analysis at different spatial scales and considered some intra-regional differences in distinctive zones (subregions) of the EGSL's north shore (separated by -10(2) km), as well as within subregions (samples separated by -10(0)-10(1) km). Temporal analysis focused on seasonal variability (sampling time separated by -10(1)-10(2 )days). Optical indices, such as the spectral slopes of a(cdom) and a(nap), pigment ratios, mass-specific absorption and backscattering coefficients, and particulate matter fractioning (inorganic and organic) allowed a detailed characterization of the dissolved organic matter pool and provided useful information about particulate matter assemblages. The a(cdom) was highly correlated with DOC and was found to have a conservative mixing behavior as the dominant process controlling its distribution, although differences among and within subregions were noted and related to differences in local river endmembers and seasonality. The a(cdom) also dominated the absorption budget, even in long wavelengths of the visible range (e.g., 550 nm). Despite the highly light-absorptive characteristic of the waters, phytoplankton biomass was not negligeable, as shown by the chlorophyll -a (Chla) concentrations means that were almost always greater than 1 mg m(-3). Analysis of the spectral shape of a(phy) and pigment ratios revealed a seasonal modulation in the composition of phytoplankton assemblages and photo acclimation. The optical properties of particulate matter showed a very dispersed relationship when compared to dry-mass concentration proxies in the study area, but the extremely low SPM-specific b(bp) values encountered were explained by the organic-rich characteristic of the nearshore zones of the EGSL. Short-term variability (atmospheric events) and the hydrodynamical regime also resulted in substantial variability in the optical properties of these areas. Overall, we provide a general parametrization for the relationships among the investigated inherent optical properties and dry-mass concentrations, which, in turn, will provide a baseline for tuning and developing regional remote sensing algorithms for the retrieval of biogeochemically relevant constituents of the water.

Automated summary

The inherent optical properties and optically significant constituents in the nearshore zones of the Estuary and Gulf of St. Lawrence were investigated. The study provided a detailed characterization of dissolved organic matter and particulate matter, and revealed the dominant control processes and seasonal variations. The findings contribute to the development of remote sensing algorithms for monitoring biogeochemically relevant constituents in the water.