Abstract
Current water color remote sensing algorithms typically do not consider the vertical variations of phytoplankton. Ecolight with a radiative transfer program was used to simulate the underwater light field of vertical inhomogeneous waters based on the optical properties of a eutrophic lake (i.e., Lake Chaohu, China). Results showed that the vertical distribution of chlorophyll-a (Chla(z)) can considerably affect spectrum shape and magnitude of apparent optical properties (AOPs), including subsurface remote sensing reflectance in water (rrs(λ, z)) and the diffuse attenuation coefficient (Kx(λ, z)). The vertical variations of Chla(z) changed the spectrum shapes of rrs(λ, z) at the green and red wavelengths with a maximum value at approximately 590 nm, and changed the Kx(λ, z) from blue to red wavelength range with no obvious spectral variation. The difference between rrs(λ, z) at depth z m and its asymptotic value (Δrrs(λ, z)) could reach to ~78% in highly stratified waters. Diffuse attenuation coefficient of downwelling plane irradiance (Kd(λ, z)) had larger vertical variations, especially near water surface, in highly stratified waters. Three weighting average functions performed well in less stratified waters, and the weighting average function proposed by Zaneveld et al., (2005) performed best in highly stratified waters. The total contribution of the first three layers to rrs(λ, 0−) was approximately 90%, but the contribution of each layer in the water column to rrs(λ, 0−) varied with wavelength, vertical distribution of Chla(z) profiles, concentration of suspended particulate inorganic matter (SPIM), and colored dissolved organic matter (CDOM). A simple stratified remote sensing reflectance model considering the vertical distribution of phytoplankton was built based on the contribution of each layer to rrs(λ, 0−).
Highlights
The frequent occurrence of algal blooms in eutrophic lakes is a worldwide problem that threatens ecological and environmental safety, drinking water resources, and human activities
Under the regulation of self-buoyancy and environmental conditions, algae can move up and down in the water column, and exhibit a vertically inhomogeneous distribution of chlorophyll-a (Chla(z), z refers to water depth, Table 1)
The results showed that the Kd (λ, z) spectrum had highest values at z = 0.2 m in waters with low Chla(z) vertical variation, but had highest values at z = 0.1 m with large vertical discrepancy in stratified waters (Figure 4)
Summary
The frequent occurrence of algal blooms in eutrophic lakes is a worldwide problem that threatens ecological and environmental safety, drinking water resources, and human activities. Under the regulation of self-buoyancy and environmental conditions (e.g., light intensity, wind, waves), algae can move up and down in the water column, and exhibit a vertically inhomogeneous distribution of chlorophyll-a (Chla(z), z refers to water depth, Table 1). The Gaussian and shifted Gaussian models have often been used to illustrate the vertical non-uniform profiles of phytoplankton in oceanic waters [1,2,3,4,5,6]. Four vertical distribution classes (vertically uniform, Gaussian, exponential, and hyperbolic) of Chla(z), with the maximum value on the water surface, were observed in a shallow eutrophic lake (i.e., Lake Chaohu, China) with a large content of cyanobacteria [7].
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