A Comparison between Local and Global Spaceborne Chlorophyll Indices in the St. Lawrence Estuary

Abstract

Spaceborne chlorophyll indices based on red fluorescence (wavelength = 680 nm) and water leaving radiance (Lw) in the visible spectrum (i.e., 400–700 nm) were evaluated in the St Lawrence Estuary (SLE) during September of 2011. Relationships between chlorophyll concentration (chl) and fluorescence were constructed based on fluorescence line height (FLH) measurements derived from a compact laser-based spectrofluorometer developed by ENEA (CASPER) and using spectral bands corresponding to the satellite sensor MERIS (MEdium Resolution Imaging Spectrometer). Chlorophyll concentration as estimated from CASPER (chlCASPER) was relatively high NE of the MTZ (upper Estuary), and nearby areas influenced by fronts or freshwater plumes derived from secondary rivers (lower estuary). These findings agree with historical shipboard measurements. In general, global chl products calculated from Lw had large biases (up to 27-fold overestimation and 50-fold underestimation) with respect to chlCASPER values. This was attributed to the smaller interference of detritus (mineral + organic non-living particulates) and chromophoric dissolved organic matter on chlCASPER estimates. We encourage the use of spectrofluorometry for developing and validating remote sensing models of chl in SLE waters and other coastal environments characterized by relatively low to moderate (<10 g·m−3) concentrations of detritus.

Keywords

• phytoplankton
• chlorophyll
• ocean color
• fluorescence
• detritus
• CDOM
• Saint Lawrence Estuary