High-Resolution In Situ Characterization of the Hydrochemical Variability Along the Fluvio-marine Transition of the Río de la Plata Estuary, Argentina

Abstract

Estuaries are characterized by steep biogeochemical gradients that are often overlooked by discrete water sampling which is especially critical for large ecosystems like the Río de la Plata estuary (RLP). To overcome this limitation, high-resolution in situ measurements of salinity, temperature, pH, turbidity, chlorophyll a (Chl-a), colored dissolved organic matter (CDOM) complemented by laboratory analysis of Chl-a and suspended particle (SPM) grain size distribution were employed to characterize the fluvio-marine transition along seven legs covering 1500 km in the RLP. In situ records performed each 400 m permitted to detect the anthropogenic discharge of CDOM in the upper freshwater Buenos Aires–La Plata metropolitan sector (1.7–3.6 mg C l−1) and the sharp turbidity increase at the beginning of the saline intrusion (S: 0.2–2) resulting in a consistent turbidity maximum zone formation (TMZ: 180–350 NTU). High-resolution data at the TMZ also revealed a significant positive covariation of Chl-a with turbidity indicating the retention of freshwater algae. The relative increase of phaeopigments in the South TMZ sector supported the detrital signature of the Paraná River load contrasting with the less turbid and fresher signal of the North corridor influenced by the Uruguay River flow. CDOM showed an opposite pattern and decreased with turbidity at the TMZ suggesting partial colloid precipitation from the surface’s 50-cm layer sampled by the boat, followed by a rapid recovery and a linear decrease with salinity (2.9 ± 0.7 to 0.3 ± 0.4 mg C l−1). Turbidity and SPM grain size measurements also reflected the North–South asymmetry of the estuary with lower turbidities enriched in finer fractions along the Northern corridor. A consistent transport pathway of partially mixed, turbid waters enriched in Chl-a, CDOM, and clays concentrated in the TMZ with intermediate salinities was traced downstream along the South shore as a significant transport pathway of suspended solids to the sea.