Chloropyll a (satellite data)

Chlorophyll a concentration calculated on the basis of an algorithm using sea reflectance values calculated for selected spectral channels based on measurements of bottom-up radiation measured by a satellite radiometer. The concentration values in the near-surface layer of water currently available on the website are calculated using the SatBalt_v1 algorithm, the functional form of which is presented below

$$log(chl\: a[mg\:m^{-3}])=2-6.972*X+5.13*X^2$$

where $$X=log(R_{rs}(531nm) / R_{rs}(645nm))$$

In the SatBałtyk System service distributions of chlorophyll a concentrations in the near-surface layer of water are given in milligrams of chlorophyll-a per cubic meter [mg Chl a m^-3].

 Validation (accuracy assessment)

Accuracy of chlorophyll a concentration determination in the near-surface layer of the Baltic Sea using the SatBalt_v1 algorithm was assessed on the basis of the difference in concentrations calculated using the algorithm during cloudless situations and the values obtained from measurements made in situ during research cruises on r/v Oceania and monitoring carried out under ICES in the period 2010-2014. Note, however, that due to the spotty nature of the spatial distributions of chlorophyll a, point measurements may deviate from the values averaged over the entire pixel with dimensions of approximately 1 x 1 km. The error histogram is shown in the figure beside. The systematic error of the values calculated in this way is 35%, while the statistical error is estimated at 150%. Chlorophyll a concentrations have a logarithmic distribution and therefore it is better to express the error in logarithmic statistics The statistical error (logarithmic error factor) is 2.22, and the lower and upper error limits are -55% and 220%, respectively. The logarithmic bias was -28%.

Chlorofil a (model EcoSat)

Chlorophyll a concentration calculated on the basis of phytoplankton biomass determined using the EcoSat ecohydrodynamic model. Maps of chlorophyll a concentrations with a resolution of 1 km are obtained by spatial interpellation of the results of the EcoSat model, whose spatial resolution is from about 5.5 km. Calculations using the model are performed on 18 layers of variable thickness, which allows to calculate distributions of vertical concentrations of chlorophyll a at different depths. In the SO SatBałtyk service, distributions of chlorophyll a concentrations on the surface and at various depths are given in milligrams of chlorophyll-a per cubic meter [mg Chl a m-3], four times a day.

Validation (accuracy assessment)

The accuracy of determining the concentrations of chlorophyll a in the Baltic Sea using the EcoSat model was assessed on the basis of the difference in concentrations calculated using the model and measurements made in the monitoring carried out by ICES in the years 2010-2014. However, it should be remembered that due to the spotty nature of the spatial distribution of chlorophyll a, point measurements may differ from the averaged values in the numerical grid of the 5.5 x 5.5 km model. It can overestimate errors, but there is no other way to assess model accuracy. The statistical error, expressed as the standard deviation of these differences, was estimated at 2.57 mg m-3 in the open waters of the Baltic Sea. The bias (average difference) is 0.25 mg m-3. Chlorophyll a concentrations have a logarithmic distribution and therefore it is better to express the error in logarithmic statistics (figure). The statistical error (logarithmic error factor) is 2.36, and the lower and upper error limits are -57.7% and 136.4%, respectively. The logarithmic bias was 1.5%.

Links to the parameter in SatBałtyk System:

Chlorofil a (SatBalt_v1) 

Chlorofil a (model EcoSat) (for registered)