Several field studies have demonstrated that shallow waters on the inner-shelf can commonly experience low oxygen conditions, especially in upwelling ecosystems. However, the frequency and intensity of hypoxia events, as well as the physical and biological drivers of such conditions in coastal shallow waters (<30m deep) are still scarcely studied. We used high frequency records obtained between Nov-2015 and Oct-2016 to characterize shallow (15-23m deep) near-bottom dissolved oxygen (DO) at 6 sites (Valparaiso, Quintay, Quisco, ECIM, Matanzas, Pichilemu) exposed to contrasting levels of upwelling intensity and spread over a distance of 170 km along central Chile. Here we analyze short-period oscillations in oxygen concentration and their potential drivers. High-frequency fluctuations were observed at all sites, particularly during spring and summer months. DO at all sites fluctuated between 0.2 mg/l and 13 mg/l, and daily ranges in DO reached values around 11 mg/l. A strong diurnal signal was detected at five of the six sites, with the lowest oxygen concentrations occurring at night and peaks of oxygenated waters during the afternoon. Valparaiso showed a strong semidiurnal signal instead, with peaks of maximum DO occurring early morning (6:00) and afternoon (15:00). Days with hypoxic events (< 2.8 mg/l for > 20 min) were observed along the whole study period, however they were more frequent during spring and summer months and the number of days with hypoxic events differed markedly among sites (from 5.23% of days at the site Quintay to 43.28% of days at Matanzas). At all sites, hypoxic events occurred significantly more often at night hours than daytime, suggesting that community respiration plays a role in driving already low DO levels to hypoxia. Our observations thus showed that shallow waters of central Chile frequently experience low DO and hypoxia conditions, with an important diurnal cycle, and with large variability in frequency and duration of occurrence among sites. Among-site variability is not related to upwelling intensity in a simple manner. Site-specific DO regimes could depend on coastal morphology, local circulation, phytoplankton biomass or river outflow proximity.
|Estación Costera de Investigaciones Marinas, Pontificia Universidad Católica de Chile, Santiago, Chile
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