The relative irradiance (given as a percentage) expresses the ratio of the downward irradiance of PAR at a given depth E_d(z) to the irradiance of PAR measured just below the water surface E_d(0). PAR stands for “photosynthetically active radiation”, i.e. the radiation from the 400 – 700 nm wavelength range. Just below the water surface (depth z=0 m) the value of this parameter is equal to 100% and falls with increasing depth. The relative irradiances given here are for the following depths z: 0, 1, 3, 5, 10 and 20 m. Other parameters associated with the calculation of primary production in the sea are also given for these same depths.

The definition of relative irradiance, also known as irradiance transmission T(z) at specified depths (Woźniak et al., 2008):

$$T(z)=\int_{400nm}^{700nm}f_e(\lambda, z)d\lambda$$

where

$$f_e$$ – the spectral function of the distribution of downward PAR irradiance with respect to the surface

$$K_d$$ - the coefficient of diffusional attenuation of downward irradiance

Methodology

The relative irradiance at specified depths is calculated using the DESAMBEM model (Woźniak et al., 2008). The model is used to compute the light field in the water, which enables the PAR irradiance to be determined at particular depths. From this, the relative irradiance at various depths can be worked out.