Abstract
To date, several algorithms for the retrieval of cyanobacterial phycocyanin (PC) from ocean colour sensors have been presented for inland waters, all of which claim to be robust models. To address this, we conducted a comprehensive comparison to identify the optimal algorithm for retrieval of PC concentrations in the highly optically complex waters of Lake Balaton (Hungary). MEdium Resolution Imaging Spectrometer (MERIS) top-of-atmosphere radiances were first atmospherically corrected using the Self-Contained Atmospheric Parameters Estimation for MERIS data v.B2 (SCAPE-M_B2). Overall, the Simis05 semi-analytical algorithm outperformed more complex inversion algorithms, providing accurate estimates of PC up to ±7 days from the time of satellite overpass during summer cyanobacteria blooms (RMSElog < 0.33). Same-day retrieval of PC also showed good agreement with cyanobacteria biomass (R2 > 0.66, p < 0.001). In-depth analysis of the Simis05 algorithm using in situ measurements of inherent optical properties (IOPs) revealed that the Simis05 model overestimated the phytoplankton absorption coefficient [aph(λ)] by a factor of ~2. However, these errors were compensated for by underestimation of the mass-specific chlorophyll absorption coefficient [a*chla(λ)]. This study reinforces the need for further validation of algorithms over a range of optical water types in the context of the recently launched Ocean Land Colour Instrument (OLCI) onboard Sentinel-3.
Highlights
It has recently been estimated that there are as many as 117 million lakes on Earth covering approximately 3.7% of the planet’s non-glaciated land surface [1]
Using ±1 day matchups, Chl-a (RMSElog = 0.394, Biaslog = 0.329, Mean Absolute Percentage Error (MAPE) = 151%, Median Absolute Percentage Error (MdAPE) = 86.4%, Symmetric Mean Absolute Percentage Error (SMAPE) = 68.5%) and PC concentrations (RMSElog = 0.272, Biaslog = 0.147, MAPE = 77%, MdAPE = 50.8%, SMAPE = 48.2%) could be retrieved with from MEdium Resolution Imaging Spectrometer (MERIS), this study can attest to the good performance of Gons05 and Simis05 algorithms for pigment retrieval in Lake Balaton
With the recent launch of ESA’s Sentinel-3 Ocean and Land Colour Instrument (OLCI), it is an opportune time to explore the historic archive of data from satellite instruments, such as MERIS, to investigate and improve retrieval of inland water quality parameters
Summary
It has recently been estimated that there are as many as 117 million lakes on Earth covering approximately 3.7% of the planet’s non-glaciated land surface [1]. Some species of cyanobacteria can fix dissolved dinitrogen gas into organic nitrogen, allowing them to outcompete other phytoplankton species and thrive in conditions with a low nitrogen to phosphorus ratio [7,8,9,10,11,12]. Other adaptations such as the ability to store excess nutrients like phosphorus [13], low light requirements [14], increased growth rates at higher temperatures [15] and buoyancy regulation mechanisms [16] further allow cyanobacteria to prosper in warmer, nutrient enriched waters
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