Abstract:Abstract. The ratio between underwater quantum irradiance (q) and irradiance (E) for derlov's [ 1976] oceanic and coastal water types and for 11 Estonian and Finnish lakes was studied. This ratio was found to depend on the depth in the water body and the transparency of the water. The ratio q/E for the photosynthetically active radiation (PAR) region of the spectrum may differ from its value in air by up to 24%. The results of the present paper can be used to convert the underwater radiation data from units o… Show more
“…Quantum PAR irradiance Q PAR is given in PAR quanta per unit area and unit time; transformation to PAR irradiance in units of power (E PAR ) depends on the spectral distribution of irradiance. For white light, which can here be assumed for the atmosphere, we have Q PAR [µmol m -2 s -1 ] = 4.60 × E PAR [W m -2 ]; in water (and ice) the spectrum of light is different and the factor 4.60 is replaced by a factor in the range 4.8-5.5, large values for more turbid waters (Reinart et al 1998). The correction factor of 5.0 can thus be taken as a representative one in the present study.…”
A field programme on light conditions in ice-covered lakes and optical properties of lake ice was performed in seven lakes of Finland and Estonia in February–April 2009. On the basis of irradiance measurements above and below ice, spectral reflectance and transmittance were determined for the ice sheet; time evolution of photosynthetically active radiation (PAR) transmittance was examined from irradiance recordings at several levels inside the ice sheet. Snow cover was the dominant factor for transmission of PAR into the lake water body. Reflectance was 0.74–0.92 in winter, going down to 0.18–0.22 in the melting season. The bulk attenuation coefficient of dry snow was 14–25 m–1; the level decreased as the spring was coming. The reflectance and bulk attenuation coefficient of snow-free ice were 0.1–0.4 and 1–5 m–1. Both were considerably smaller than those of snow cover. Seasonal evolution of light transmission was mainly due to snow melting. Snow and ice cover not only depress the PAR level in a lake but also influence the spectral and directional distribution of light
“…Quantum PAR irradiance Q PAR is given in PAR quanta per unit area and unit time; transformation to PAR irradiance in units of power (E PAR ) depends on the spectral distribution of irradiance. For white light, which can here be assumed for the atmosphere, we have Q PAR [µmol m -2 s -1 ] = 4.60 × E PAR [W m -2 ]; in water (and ice) the spectrum of light is different and the factor 4.60 is replaced by a factor in the range 4.8-5.5, large values for more turbid waters (Reinart et al 1998). The correction factor of 5.0 can thus be taken as a representative one in the present study.…”
A field programme on light conditions in ice-covered lakes and optical properties of lake ice was performed in seven lakes of Finland and Estonia in February–April 2009. On the basis of irradiance measurements above and below ice, spectral reflectance and transmittance were determined for the ice sheet; time evolution of photosynthetically active radiation (PAR) transmittance was examined from irradiance recordings at several levels inside the ice sheet. Snow cover was the dominant factor for transmission of PAR into the lake water body. Reflectance was 0.74–0.92 in winter, going down to 0.18–0.22 in the melting season. The bulk attenuation coefficient of dry snow was 14–25 m–1; the level decreased as the spring was coming. The reflectance and bulk attenuation coefficient of snow-free ice were 0.1–0.4 and 1–5 m–1. Both were considerably smaller than those of snow cover. Seasonal evolution of light transmission was mainly due to snow melting. Snow and ice cover not only depress the PAR level in a lake but also influence the spectral and directional distribution of light
“…where k is the wavelength of light, h is the Planck's constant 6.625 9 10 -34 J s, and c 0 is the speed of light 3 9 10 8 m s -1 (Reinart & Arst, 1998). K d(k) was defined as the slope of the least-square regression of ln [E d(z,k) /E s(0-,k) ] with respect to depth over the depth interval just below the water surface to Z 1% , where most of the attenuation of solar energy takes place (Kirk, 2003).…”
The light field and its relationship with biogeochemical variables were investigated in the Solimões, Negro, Amazon, Madeira, Uatumã, Trombetas, and Tapajós Rivers. In high suspended sediment rivers, total suspended matter is the primary control on light attenuation (r = 0.8), with colored dissolved organic matter (CDOM) being secondary (r = -0.6) due to scattering and absorption, respectively. Photosynthetically active radiation was the lowest (\100.0 lmol m -2 s -1 at the depth of half Z 1% ) and was limited to depths of less than 1.0 m and confined to red light. In low suspended sediment rivers, CDOM is the primary control on light attenuation (r = 0.9). The concentrations of chlorophyll a (Chla) and CDOM cause variations among these rivers. High CDOM rivers, Negro and Uatumã, are depleted (\0.5% of incoming irradiance) of blue and green light at the depth of half Z 1% . The light spectra of low CDOM and higher Chla waters, such as the Tapajós, Uatumã, and Trombetas Rivers at rising water stage, are restricted to green and red wavelengths, and marked by high absorption at 620 and 670 nm, due to the presence of Cyanophyceae.
“…The short-wave surface irradiance flux Q S is obtained generally from an atmospheric radiative transfer model and is converted from W m −2 to the units of µE m −2 s −1 with the constant factor 1/0.215 (Reinart et al, 1998). ε PAR is the coefficient determining the portion of PAR in Q S .…”
Section: The Environmental Parameters Affecting Biological Ratesmentioning
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