Martin A. Montes-Hugo

Biogeo-optical modeling of SPM in the St. Lawrence Estuary

Optical and chemical measurements were obtained in the St. Lawrence Estuary during spring of 2000 and 2001 to develop three biogeo-optical models for estimating concentration of suspended particulates (SPM) in optically complex waters where light absorption in the visible range is dominated by coloured dissolved organic matter (CDOM). SPM was computed from remote sensing reflectance (Rrs) calculated at 670 and 550 nm (SPM = 50.35 [Rrs(670)/ Rrs(555)]2.03) and corrected using a linear function related to the absorption coefficient of CDOM measured at a wavelength of 412 nm (aCDOM(412)). The resulting CDOM-unbiased estimates of SPM had an uncertainty of ± 40%, a major improvement with respect to uncorrected measurements (relative error up to 85%) or theoretical calculations assuming a constant SPM and a variable aCDOM(412) (up to 400%). Spatial patterns of unbiased SPM derived from SeaWiFS (Sea-viewing Wide Field-of-view Sensor) during spring 1998 were consistent with general trends previously described based on shipboard data. Satellite observations also highlighted the important association between hydrographic features and horizontal distribution of suspended solids in surface waters of the Estuary.

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

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 (chl CASPER ) 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 chl CASPER values. This was attributed to the smaller interference of detritus (mineral + organic non-living particulates) and chromophoric dissolved organic matter on chl CASPER 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.

Modeling the effects of surface and bottom geometries on LiDAR bathymetric waveforms

LiDAR bathymetric biases due to geometric changes at the air-water and water-bottom interfaces are investigated based on calculations made with a modified version of the waveform simulator Wa-LID. Main assumptions include a homogeneous water column and a spaceborne LiDAR having a footprint smaller than 50 m and a wavelength centered at 532 nm. Preliminary results showed major temporal modifications on second Lidar return (up to 100 cm or 10 ns) due to tilted bottoms. This shift was in average >6-fold the maximum bottom depth bias originated from capillary waves forming at the air-water surface.

Weibull approximation of LiDAR waveforms for estimating the beam attenuation coefficient

Tank experiments were performed at different water turbidities to examine relationships between the beam attenuation coefficient (c) and Weibull shape parameters derived from LiDAR waveforms measured with the Fine Structure Underwater LiDAR (FSUIL). Optical inversions were made at 532 nm, within a c range of 0.045-1.52 m-1, and based on a LiDAR system having two field-of-view (15 and 75.7 mrad) and two linear polarizations. Consistently, the Weibull scale parameter or P2 showed the strongest covariation with c and was a more accurate proxy with respect to the LiDAR attenuation coefficient.

Particle Composition Effects on MERIS-Derived SPM: A Case Study in the Saint Lawrence Estuary

Abstract An empirical optical model for estimating the concentration of suspended particulate matter (CSPM) was developed in the upper part of the Saint Lawrence Estuary based on remote sensing reflectance (Rrs) measurements corresponding to medium-resolution imaging spectrometer (MERIS) spectral channels 7 and 9 (i.e., wavelengths centered at 665 and 708 nm, respectively). Sensitivity of CSPM estimates to changes in mineral content of suspended particulates was investigated based on simulated Rrs values. For June 2012 measurements, CSPM varied with values following a power-type relationship (y = 235.7 x8.321, r2 = 0.7, N = 10). In addition, numerical experiments and analysis of regression type II showed that exponent parameter of this biogeo-optical model decreased as suspended particulates become more enriched in organic matter.

LiDAR Measurements and Applications in Coastal and Continental Waters

The first LiDAR systems applied to aquatic environments were developed for underwater imaging by the \{US\} navy, This pioneer LiDAR system was developed by Radio Corporation of America (RCA) for conducting moreefficient military operations and improving the safety of divers. This primitive LiDAR was characterized by a blue-green laser with a laser pulse a pulse repetition smaller than 0.01 Hz. In the late 60s, LiDAR technologies underwent major progress due to the development of scanners and advanced photodetectors (Avalanche Photo Diode (APD) or Photomultiplier Tubes (PMT) type). It was during this period that detection of submerged targets based on LiDAR systems was first attempted. This capability was originally evaluated in the United States, Canada, Australia and Sweden and was later exapanded to other countries such as China and the Soviet Union.

A MERIS-based model for estimating the concentration of suspended particulates in the ST. Lawrence Estuary

The MERIS-derived remote sensing reflectance (R_rs) ratios R_rs(708)/R_rs(665) and R_rs(753)/[R_rs(665)-R_rs(708)] have been successfully used to estimate chlorophyll concentration in diverse turbid coastal waters of US. Here, the response of these two optical proxies with respect to changes on concentration of suspended particulate matter (C_SPM) is evaluated in the St. Lawrence Estuary for the first time. Preliminary results showed that R_rs(708)/R_rs(665) had a better performance (RMSE = 3.06) than R_rs(753)/[R_rs(665)-R_rs(708)] (RMSE = 7.76) for estimating C_SPM distributions during spring conditions.

Satellite-derived suspended particulates in the Saint Lawrence Estuary: uncertainties due to bottom effects

Historical regional budgets of suspended particulate matter (SPM) in surface waters (i.e., 0–20 m depth) of the St. Lawrence Estuary (SLE) are not very accurate because of the lack of in situ measurements. The aim of this study was to improve the accuracy of SPM budgets in the SLE based on spaceborne ocean color measurements by investigating one kind of uncertainty on satellite-derived SPM concentration and related with the influence of the bottom depth and (or) bottom reflectivity (hereafter bottom effects) on water-leaving radiance. Theoretical results suggest that bias on optically derived SPM concentration due to bottom effects is greater in the lower estuary and can be up to 36.6% when the water depth is ≤10 m, sediments are dominated by sand, and water turbidity is relatively low (beam attenuation coefficient <1 m−1). Sea-Viewing Wide Field of view Sensor measurements and radiative transfer simulations suggest that bottom effects may introduce an uncertainty of 3.4% or 1.1 × 103 tonnes on 1 m vertically integrated SPM mass over the whole estuary. This error increases (up to 14.5% or 2.9 × 103 tonnes) when optically shallow areas were excluded from the regional SPM budget analysis.