Seasonal hydrographic changes along the Gulf of Arauco (37º S) coast

Abstract

The Gulf of Arauco, located in central Chile, is one of the most productive areas in the Southern Humboldt Current System. Here, urban-industrial activities coexist with fishing communities and aquaculture concessions. In this context, the seasonal variability of the wind forcing is one of the main factors impacting its hydrographic variability. Upwelling favorable winds predominate in summer, promoting the incursion of subsurface waters with high nutrients that sustain the high primary production of the gulf. During autumn and winter, the increased freshwater discharges and downwelling-favorable winds drive to a large extent the gulf’s hydrodynamics. We present the seasonal hydrographic variability of the Gulf of Arauco during three contrasting periods (spring 2021, summer, and winter 2022) through in situ and satellite data. Our results show a marked hydrographic seasonality, mainly in the sea surface temperature. Low thermal values (<13°C) covered most of the gulf in spring, from the Biobío River mouth to Punta Lavapie. High temperatures (>15°C) featuring an upwelling shadow characterized the gulf in summer. Low temperatures (<12°C) were detected in winter with a homogeneous spatial distribution. Surface chlorophyll was high (>20 mg m^-3) during spring and summer because of increased solar radiation and upwelling favorable winds predominance. Low salinity (<20 psu) waters, associated with spreading the Biobío River plume, dominated in winter. The dissolved oxygen distribution showed similar variations in spring and summer, where waters with concentrations >5 ml L^-1 were found in the upper 10 m. However, hypoxic waters covered the bottom along the entire nearshore coastal band during summer. We conclude that the Gulf of Arauco exhibits contrasting seasonal hydrographic patterns where the wind, the Biobío River discharge, and solar radiation modulate the water column structure.