Abstract
Two semi-analytical algorithms, Generalized Inherent Optical Property (GIOP) and Garver-Siegel-Maritorena (GSM), were evaluated in terms of how well they reproduced the absorption coefficient of phytoplankton (aph(λ)) and dissolved and detrital organic matter (adg(λ)) at three wavelengths (λ of 412, 443, and 488 nm) in a zone with optically complex waters, the Upper Gulf of California (UGC) and the Northern Gulf of California (NGC). In the UGC, detritus determines most of the total light absorption, whereas, in the NGC, chromophoric dissolved organic material (CDOM) and phytoplankton dominate. Upon comparing the results of each model with a database assembled from four cruises done from spring to summer (March through September) between 2011 and 2013, it was found that GIOP is a better estimator for aph(λ) than GSM, independently of the region. However, both algorithms underestimate in situ values in the NGC, whereas they overestimate them in the UGC. Errors are associated with the following: (a) the constant a*ph(λ) value used by GSM and GIOP (0.055 m2 mgChla−1) is higher than the most frequent value observed in this study’s data (0.03 m2 mgChla−1), and (b) satellite-derived chlorophyll a concentration (Chla) is biased high compared with in situ Chla. GIOP gave also better results for the adg(λ) estimation than GSM, especially in the NGC. The spectral slope Sdg was identified as an important parameter for estimating adg(λ), and this study’s results indicated that the use of a fixed input value in models was not adequate. The evaluation confirms the lack of generality of algorithms like GIOP and GSM, whose reflectance model is too simplified to capture expected variability. Finally, a greater monitoring effort is suggested in the study area regarding the collection of in situ reflectance data, which would allow explaining the effects that detritus and CDOM may have on the semi-analytical reflectance inversions, as well as isolating the possible influence of the atmosphere on the satellite-derived water reflectance and Chla.
Highlights
Ocean color remote sensors onboard satellites, such as the early coastal zone color scanner (CZCS) and the current Moderate Resolution Imaging Spectroradiometer (MODIS), have provided information on oceanographic structures and processes at different scales in the oceans, explaining a series of biological and ecological processes [1]
Betancur-Turizo et al [29] analyzed the spatial and temporal variability of light absorption properties in the study area. They observed that there was a strong temporal variability with a spatial pattern that allowed the definition of two bio-optical regions named Upper Gulf of California (UGC) and Northern Gulf of California (NGC), with particular characteristics that indicated that these regions were very different when evaluating the individual contribution by phytoplankton, detritus, and chromophoric dissolved organic material (CDOM) to total light absorption
In the UGC, ad(λ) contribution to total light absorption was most of the time higher than 40% followed by aCDOM(λ), whereas in the NGC, a co-dominium between aph(λ) and aCDOM(λ) was observed most of the time
Summary
Ocean color remote sensors onboard satellites, such as the early coastal zone color scanner (CZCS) and the current Moderate Resolution Imaging Spectroradiometer (MODIS), have provided information on oceanographic structures and processes at different scales in the oceans, explaining a series of biological and ecological processes [1] The data from these sensors has been used in studies on ocean dynamics, biogeochemistry, and global climate change [2,3,4,5]. These sensors have greatly improved the world’s understanding of the properties of light absorption by water and particulate and dissolved material [6,7,8,9,10,11,12], and they have emphasized the importance of observing the properties routinely from space.
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