Absorption per unit of chlorophyll

The mean specific (i.e. per unit of chlorophyll a) coefficient of light absorption by all phytoplankton pigments at selected depths. The absorption of solar radiation by phytoplankton stimulates the photosynthesis of organic matter in the sea, and is thus the main factor responsible for supplying marine ecosystems with energy. The magnitude of light absorption is given by the mean specific coefficient of absorption of PAR (i.e. 400-700 nm) by phytoplankton at selected depths. The unit of the mean specific absorption coefficient at selected depths is [m² (mg chl a)^-1]. The definition of the mean specific coefficient of light absorption by all (i.e. photosynthetic and photo-protecting) phytoplankton pigments at a given depth z (Majchrowski et al., 2000): $$\langle a_{ph}^* (z) \rangle= \frac{1}{300}\int_{400nm}^{700nm} a_{ph}^* (z,\lambda)d\lambda$$ where $$a_{ph}^* (z, \lambda)$$ - the spectral specific absorption coefficient at a given depth z.

Methodology

Mean specific coefficients of light absorption (in the PAR range, i.e. the wavelength interval  λ = 400-700 nm) by phytoplankton at selected depths in the Baltica are calculated using the DESAMBEM model (Woźniak et al., 2008). The input data for this model are the concentration of chlorophyll a just below the surface and the daily dose of surface irradiance. Vertical profiles of concentrations of the principal groups of photosynthetic and photo-protecting pigments in phytoplankton are calculated (Majchrowski et al., 2007). Using the spectral reconstruction based on a total of 24 elementary absorption bands described by Gaussian functions (Ficek et al., 2004) and calculated concentrations of the main groups of pigments at a given depth, the DESAMBEM model determines the “unpackaged” (i.e. scattered) spectral coefficients of light absorption by phytoplankton. There then follows the “packaging” (i.e. the determination of absorption coefficients for pigments contained in phytoplankton cells) of the absorption coefficients using the spectral function of the packaging effect. This yields the spectral coefficients of absorption in vivo for the main groups of pigments and, after summing them, for the phytoplankton. Finally, the absorption coefficient is averaged over the wavelength and is divided by the concentration of chlorophyll a at a given depth. The relevant formulas for calculating averaged coefficients of light absorption per unit of chlorophyll a by phytoplankton in the Baltic can be found in the articles by Woźniak et al. (2008) and Ficek et al. (2004).

Fig.1 Comparison of empirical a^*_ph,meas and modelled a^*_ph,calc values of the mean absorption coefficients per unit of chlorophyll a at different depths z in the euphotic layer of the Baltic. Absorptions per unit of chlorophyll a^*_ph,meas were measured during research cruises in 2010-2014, while the absorptions  a^*_ph,calc were calculated using the DESAMBEM algorithm – one input datum was the concentration of chlorophyll a at the surface measured from on board ship.

The validation for the averaged coefficient of light absorption per unit of chlorophyll a by phytoplankton was carried out for surface waters and for selected depths in the Baltic. Measurements of 154 spectral coefficients of light absorption by phytoplankton in the surface layer of the Baltic and more than a dozen depth profiles were performed in 2010-2014 during cruises, mainly in the southern Baltic. The spectral coefficients of light absorption by phytoplankton were measured using a non-extractive, spectrophotometric method on a UNICAM UV 4-100 spectrophotometer equipped with an RSA-UC40 (LABSPHERE) integrating sphere. The chlorophyll a concentration was measured by HPLC.

The next part shows the validation of mean absorption coefficients per unit of chlorophyll a at different depths, determined using the DESAMBEM algorithm on the basis of input data obtained from measurements of surface chlorophyll a from on board ship. The standard error factor x (see the table below) characterizing the method of calculating the absorption coefficient per unit of chlorophyll a is fairly small: it varies from 1.284 at 5 m depth to 1.427 at 3 m. One can assume that the standard error factor x for this magnitude in the whole water column does not exceed 1.427. The logarithmic statistical error therefore ranges from σ- = -29.9% to σ+ =42.7%.

Table 1 The systematic and statistical errors of estimated averaged absorptions per unit of chlorophyll a at selected depths z in the Baltic in arithmetic and logarithmic statistics when the chlorophyll a concentration, measured from on board ship, is the input datum for the DESAMBEM algorithm.

Links to the parameters in SatBaltic System: