What time is the tide? The importance of tides for ocean colour applications to estuaries

Tides can play a major role in transitional water dynamics, being the primary driver of fluctuations in water parameters. In the last decade, remote sensing methods have become a popular tool for cost-effective systematic observations, at relatively high spatial and temporal scales. However, the presence of tides introduces complexities, given that Sun-synchronous satellites will observe a different tidal condition at each overpass, effectively aliasing the daily signal. This can create non-obvious biases when using remote sensing data for monitoring tidally-dominated systems, potentially leading to misinterpretation of patterns and incorrect estimates of periodicities. In this work, we used a six-year Sentinel-2-derived turbidity dataset to evaluate the impact of tidal aliasing on the applicability of a Sun-synchronous satellite to a tidally-dominated system (Tagus estuary, Portugal). Each satellite observation was classified according to tidal phase. Results indicate that tidal processes dominated over seasonal variability, with significant differences observed between turbidity levels of different tidal phases (p < 0.0001). Climatology analyses also revealed significant changes between all-data and per-tidal-phase data (p < 0.001), highlighting the importance of classifying satellite data by tidal condition. Additionally, tidal condition labelling at each Sentinel-2 overpass revealed that not all tidal conditions are observed by a Sun-synchronous satellite, as Low tide and Floods are always observed during Spring tides and High tide and Ebbs observed under Neap tides. Spring Low tides are overrepresented compared to all other tidal conditions. This result is particularly relevant for water quality monitoring based on remote sensing data in tidally-dominated systems.