Abstract
Different methods are available for retrieving chlorophyll-a (Chl-a) in inland waters from optical imagery, but there is still a need for an inter-comparison among the products. Such analysis can provide insights into the method selection, integration of products, and algorithm development. This work aims at inter-comparison and consistency analyses among the Chl-a products derived from publicly available methods consisting of Case-2 Regional/Coast Colour (C2RCC), Water Color Simulator (WASI), and OC3 (3-band Ocean Color algorithm). C2RCC and WASI are physics-based processors enabling the retrieval of not only Chl-a but also total suspended matter (TSM) and colored dissolved organic matter (CDOM), whereas OC3 is a broadly used semi-empirical approach for Chl-a estimation. To pursue the inter-comparison analysis, we demonstrate the application of Sentinel-2 imagery in the context of multitemporal retrieval of constituents in some Italian lakes. The analysis is performed for different bio-optical conditions including subalpine lakes in Northern Italy (Garda, Idro, and Ledro) and a turbid lake in Central Italy (Lake Trasimeno). The Chl-a retrievals are assessed versus in situ matchups that indicate the better performance of WASI. Moreover, relative consistency analyses are performed among the products (Chl-a, TSM, and CDOM) derived from different methods. In the subalpine lakes, the results indicate a high consistency between C2RCC and WASI when aCDOM(440) < 0.5 m−1, whereas the retrieval of constituents, particularly Chl-a, is problematic based on C2RCC for high-CDOM cases. In the turbid Lake Trasimeno, the extreme neural network of C2RCC provided more consistent products with WASI than the normal network. OC3 overestimates the Chl-a concentration. The flexibility of WASI in the parametrization of inversion allows for the adaptation of the method for different optical conditions. The implementation of WASI requires more experience, and processing is time demanding for large lakes. This study elaborates on the pros and cons of each method, providing guidelines and criteria on their use.
Highlights
Sustainable management of the quality of inland and coastal waters has emerged as a growing demand due to human-driven nearshore activities and climate change that substantially threaten the aquatic ecosystems [1,2]
For Lake Trasimeno, we considered three different initial values for Chl-a based on a pre-fit analysis in Water Color Simulator (WASI)
This study thoroughly studied the effectiveness of Case-2 Regional/Coast Colour (C2RCC), WASI, and OC3 for the estimation of constituents in four Italian lakes representing a relatively wide range of bio-optical conditions
Summary
Sustainable management of the quality of inland and coastal waters has emerged as a growing demand due to human-driven nearshore activities and climate change that substantially threaten the aquatic ecosystems [1,2]. Eutrophication mainly caused by increased agricultural and industrial activities introduces major problems to the ecosystem health, aquaculture and fisheries activities, recreation, and tourism [3,4]. In this context, spatially and temporally explicit information on the water quality parameters is central in furthering our understanding of ecosystem services and health as well as environmental impact assessment [5,6]. Field sampling and analyzing the samples in the laboratory are costly and time consuming To overcome these issues, remote sensing techniques are pursued as complementary to in situ measurements, in open oceans and coastal waters.
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