Abstract
Standard NASA ocean color algorithm OC4 was developed on the basis of ocean optical data and while appropriate for Case 1 oceanic waters could not be adequately applied for the Black Sea waters due to its different bio-optical properties. OC4 algorithm is shown to overestimate chlorophyll concentration (Chl-a) in summer and underestimate Chl-a during early spring phytoplankton blooms in the Black Sea. For correct conversion of satellite data to Chl-a, primary production and other indicators regional algorithms should be developed taking into account bio-optical properties of the Black Sea waters. Light absorption by phytoplankton pigments – a_ph (λ) have been measured in open sea and shelf Black Sea waters in different seasons since 1998. It was shown that the first optical depth was located within the upper mixed layer (UML) for most of the year with the exception of the spring when seasonal stratification was developing. As a result spectral features of water leaving radiance were determined by optical properties of the UML. Significant seasonal differences in Chl-a specific light absorption coefficients of phytoplankton within UML have been revealed. These differences were caused by adaptive changes of composition and intracellular pigment concentration due to variable environment conditions – mainly light intensity. Empirical relationships between a_ph (λ) and Chl-a were derived by least squares fitting to power functions for different seasons. Incorporation of these results will refine the regional ocean color models and provide improved and seasonally adjusted estimates of chlorophyll a concentration, downwelling radiance and primary production in the Black Sea based on satellite data.
Highlights
Visible spectral radiometric data are used widely to assess water productivity (Saba et al, 2011) and to study effect of climate change on ocean productivity (Behrenfeld et al, 2006)
In the deep western part of the sea in August 2011 (Figure 1) Chl-a in the upper mixed layer (UML) were in a range 0.15–0.30 mg·m−3
Vertical Chl-a distribution was similar to that observed in August (Figure 2), but with less variability in Chl-a: Chl-a concentration in the deep Chl-a maximum (DCM) was 3 times higher than in the UML in comparison with 5–10 times differences in August
Summary
Visible spectral radiometric data are used widely to assess water productivity (Saba et al, 2011) and to study effect of climate change on ocean productivity (Behrenfeld et al, 2006). Light absorption by particles (ap (λ)), phytoplankton (aph(λ)), NAP (aNAP (λ)) and CDOM (aCDOM(λ)) have been studied in different regions of the global ocean since the 80-s (Hoepffner and Sathyendranath, 1992; Bricaud et al, 1995, 1998; Cleveland, 1995; Babin et al, 2003) to develop algorithms for assessment of water productivity based on remote sensing. Due to high variability in light absorption and scattering by optically active components, the world ocean needs to be subdivided into various provinces based on regional IOPs, and their features could be used to improve remote-sensing algorithms for each province (Hoepffner and Sathyendranath, 1992; Lutz et al, 1996; Suzuki et al, 1998). NASA standard algorithms are continually being updated (O’Reilly et al, 2000), the latest versions (OC4 for SeaWiFS, and OC3M for MODIS-Aqua /Terra) do not provide an adequate assessment of chlorophyll a concentration (Chl-a) in the Black Sea waters (Suslin and Churilova, 2016) which belong to the Case 2 (Suslin et al, 2007). Berthon et al (2008) underlined that important uncertainties for the retrieval of marine products like Chl-a still persisted in areas (including the Black Sea) where relatively high CDOM absorption and optically active water constituents CDOM and NAP do not co-vary in a predictable manner with Chl-a
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