Spatial-temporal bio-optical classification of dynamic semi-estuarine waters in western North America

The use of standard ocean colour reflectance based algorithms to derive surface chlorophyll may have limited applicability for optically dynamic coastal waters due to the pre-defined coefficients based on global datasets. Reflectance based algorithms adjusted to regional optical water characteristics are a promising alternative. A class-based definition of optically diverse coastal waters was investigated as a first step towards the development of temporal and spatial constrained reflectance based algorithms for optically variable coastal waters. A large set of bio-optical data were collected as part of five research cruises and bi-weekly trips aboard a ship of opportunity in the west coast of Canada, to assess the spatial and temporal variability of above water reflectance in this contrasted coastal environment. To accomplish this, in situ biophysical and optical measurements were collected in conjunction with above-water hyperspectral remote sensing reflectance (R-rs) at 145 stations. The concentrations of measured biophysical data varied considerably; chlorophyll a (Chla) (mean = 1.64, range: 0.10-7.20 mu g l(-1)), total suspended matter (TSM) (3.09, 0.82 20.69 mg l(-1)), and absorption by chromophoric dissolved organic matter (CDOM) (a(cdom) (443nm)) (0.525, 0.007-3.072 m(-1)), thus representing the spatio-temporal variability of the Salish Sea. Optically, a similar large range was also found; particulate scattering (b(p)(650nm)) (1.316, 0.250-7.450 m(-1)), particulate backscattering (b(bp)(650nm)) (0.022, 0.005-0.097 m(-1)), total beam attenuation coefficient (c(t)(650)) (1.675, 0.371-9.537 m(-1)) and particulate absorption coefficient (a(p) (650nm)) (0345, 0.048-2.020 m(-1)). An empirical orthogonal function (EOF) analysis revealed that R-rs variability was highly correlated to by (r = 0.90), b(bp), (r = 0.82) and concentration of TSM (r = 0.80), which highlighted the dominant role of water turbidity in this region. Hierarchical clustering analysis was applied to the normalized R-rs spectra to define optical water classes. Class 1 was defined by the highest R-rs values, particularly above 570 nm, indicating more turbid waters; Class 2 was dominated by high Chla and TSM concentrations, which is shown by high R-rs at 570 nm as well as fluorescence and absorption peaks; Class 3 shows strong fluorescence signatures accompanied by low TSM influence; and Class 4 is most representative of clear waters with a less defined absorption peak around 440 nm. By understanding the bio-optical factors which control the variability of the R-rs spectra this study aims to develop a sub-regional characterization of this coastal region aiming to improve bio-optical algorithms in this complex coastal area. (C) 2017 Elsevier Ltd. All rights reserved.