LAUSR

Abstract Estuarine environments are very complex systems whose productivity is defined by multiple biological and physicochemical interactions. Thus, monitoring the biophysical properties of estuarine waters becomes fundamental to our understanding of their ecosystem functions. Remote sensing retrieval of biochemical and biological variables appears as a complementary tool, providing information at a high spatial and temporal resolution and thus allowing us to study processes that cannot be comprehended based only on in situ data. However, the quality of remotely sensed data strongly depends on ground-truthing, using discrete in situ measurements. The aim of this study was to describe temporal (short term) and spatial (across short distances) variations of optically active water components in the Bahia Blanca Estuary (BBE, Argentina), and to characterize their relationship with bio-optical properties. Concentrations of suspended particulate matter (SPM) and chlorophyll-a (Chl-a), as well as the absorption by colored dissolved organic matter ( a cdom (443)), considered as a proxy for the concentration of colored dissolved organic matter, were tightly coupled to tidal forcing, causing significant short-term variations. Along the estuary, a steep gradient of these variables was observed with higher concentrations through the head of the channel. Considering bio-optical properties, absorption by non-algal particles ( a n a p (443)) dominate the absorption budget of these optically complex waters, followed by absorption by phytoplankton ( a p h (443). a ph (443) was closely related to variations in Chl-a, and a nap (443) showed increased values under the influence of sewage discharge. Considering in situ measurements of water reflectance, a high short-term variability was observed in response to tidal forcing, as well as large differences across relatively short distances. For the first time in the BBE, bio-optical properties of seawater were quantified, and their variations described at different scales. These results revealed a highly variable environment from the optical perspective and have large implications for the validation of remote sensing products.