Temporal variability of bottom hypoxia in open and semi-enclosed coastal areas in an upwelling region

Abstract

Hypoxia events driven by the onshore advection of oxygen-poor upwelling waters have become increasingly common along Eastern Boundary Upwelling Systems (EBUS). More frequent or intense drops in nearshore oxygen concentrations can impact the behavior, population dynamics, and geographic distribution of many coastal species. To better understand these effects, it is necessary to determine the periodicity of hypoxia in the inner shelf, its response to upwelling-favorable winds, and the local factors that may modify its intensity and duration. Here, we used a two-year record (March 2017–February 2019) of near-bottom dissolved oxygen (DO) and water temperature from seven sites spanning 260 km of the upwelling coast of Central Chile to characterize the local variability in oxygen concentration and its association with coastal winds. The temporal patterns observed in the inner shelf were compared with 13.5 years of monthly hydrographic profiles (2002–2015) from a mid-shelf station located ca. 30 km offshore. The spatial structure of nearshore hypoxia was inferred from hydrographic data gathered during two surveys conducted in winter 2018 and late summer 2019. Inner-shelf hypoxia (DO$<$2.0 mg/L) occurred predominantly in the austral summer and exhibited substantial among-site differences in its persistence and timing relative to wind-driven upwelling. The mean duration of summertime hypoxia events ranged between 1 and 9 days, except for a site at the innermost section of the Gulf of Arauco, where events could be as long as 62 days. Hypoxic waters appeared at the inner shelf after 1–2 days of sustained upwelling-favorable winds (59% of the hypoxia events occurred after $\le$ 2 days of persistent wind). The seasonality and vertical structure of hypoxia were apparent in the mid-shelf monthly time series, with the hypoxic layer’s upper limit ascending to 20–30 m depths during spring-summer (October–March) and receding to 45–60 m in autumn-winter (May–September). Hydrographic properties of the hypoxic layer pointed to Equatorial Subsurface Water (ESSW) as the source water mass for mid-shelf hypoxia throughout the year. These properties were consistent with those observed on the inner shelf. Among the seven inner-shelf sites, Arauco showed the most persistent (70% of the time spanned by our records) and severe hypoxia, with DO levels below those observed at the mid-shelf during active upwelling. We hypothesize that these conditions respond to a combination of physical phenomena and coastal eutrophication that locally enhance thermal stratification, productivity, and benthic oxygen consumption, thus amplifying the severity of hypoxia and physiological stress for marine organisms inside the Gulf of Arauco.