Assessing temporal trends in ocean transparency using GlobColour products from the Copernicus Marine Service and accounting for associated uncertainties

Ocean transparency refers to the ability of water to transmit light, which directly influences the depth to which sunlight penetrates the water column. This parameter is crucial for marine ecosystems, as it determines the availability of light for photosynthesis, a process vital for phytoplankton, the primary producers of the ocean. Phytoplankton not only form the base of the marine food web but also contribute significantly to biogeochemical cycles, including the carbon cycle. A decline in transparency can limit photosynthesis by reducing the depth of the euphotic zone, potentially disrupting ecosystems and impacting carbon sequestration. Transparency has traditionally been measured using the Secchi disk, a simple yet effective tool for assessing water clarity. More recently, satellite-derived products have revolutionized the study of transparency, enabling large-scale and long-term observations. But whatever the method employed to measure transparency, robust evaluation of long-term trends depends on properly accounting for the inherent uncertainties of the observations. This issue is especially critical for remote sensing datasets, as the succession of sensors across two decades can introduce biases in long-term trend detection. In this study, we accounted for observational uncertainties in the products by using a Monte Carlo approach, which incorporates the Seasonal Mann–Kendall test to evaluate trend detection reliability and the Theil–Sen slope estimator to quantify trend magnitude. In parallel, seasonal and inter-annual variability in transparency were also examined in relation to in situ data of water discharge, providing insights of riverine influences on transparency dynamics. Study areas include turbid French estuaries (Gironde and Loire), the semi-sheltered Gulf of Morbihan waters, tropical waters around Mayotte, and the oligotrophic North-East Atlantic, representing a broad range of hydrographic and climatic regimes. The results reveal pronounced spatial contrasts: strong variability and decreasing transparency linked to high-flow events in the Gironde estuary, weaker but noticeable trends in the Loire estuary, no significant changes observed in the North-East Atlantic, and stable conditions for clear waters off Mayotte. The gulf of Morbihan exhibited strong variability that prevailed any trend detection from the long-term signal. Resolution analyses highlighted the importance of finer-scale products for capturing local estuarine processes, while coarser resolution provided more robust basin-scale patterns. This study confirms the value of satellite-derived transparency indicators and highlights the need to integrate uncertainties, resolution effects and call for synergies with in situ data to improve water quality assessments.