The intensification of coastal hypoxia off central Chile: Long term and high frequency variability

Abstract

Hypoxia is a phenomenon where dissolved oxygen (DO) is reduced to levels that are low enough to strongly affect ecological and biogeochemical processes. This occurs within the continental shelf off central Chile (36°S), influenced by seasonal coastal upwelling (Spring-Summer). Monthly measurements of DO and other oceanographic variables in the water column (1997−2021) over the 92 m isobath along with high-resolution and near-surface observations (POSAR buoy), are analyzed to examine incidences of hypoxia and understand the physical and biogeochemical processes modulating DO vertical distribution and its temporal variability. On average, the percentage of the water column with DO levels below 89 (hypoxia) and 22 (severe hypoxia) μmol L−1 reaches 68% (i.e., hypoxic waters are found below 30 m) and 44% (below 50 m depth), respectively, but during the upwelling season, as much as 87% (below 12 m depth) and 81% (below 17 m depth) of the water column exhibits these levels. On the sub-seasonal scale during upwelling season six hypoxic events lasting at least 2 days occur at 10 m depth. There is a strong seasonal correlation between the volume of the seawater presenting hypoxia and upwelling favorable winds. Furthermore, there is a high DO interannual variability partially related to the El Niño Southern Oscillation (ENSO). Over 2 decades, it is estimated that DO concentration in surface and subsurface layers decreases (up to 21 μmol L−1 decade−1) as waters get colder (up to 0.29°C decade−1). Remarkably, the volume of hypoxic and severe hypoxic waters over the shelf has increased more than 2 times since 1997 and shows a significant positive correlation with the upwelling index. These preliminary findings indicate that the increase in local DO consumption is partially associated with upwelling intensification. Given the clear evidence of wind intensification in coastal upwelling ecosystems and thus the increase in hypoxic events, the coastal zone may be highly vulnerable to hypoxia, impacting biological resources and biogeochemical cycles.

Environmental DNA reveals temporal variation in mesophotic reefs of the Humboldt upwelling ecosystems of central Chile: Toward a baseline for biodiversity monitoring of unexplored marine habitats

“…Importantly, while nutrient pulses do alter phytoplankton community composition (relative abundances, e.g., Ferreira et al., 2020 ) and some macroalgal growth rates (Nielsen & Navarrete, 2004 ), no studies in Chile (or California) have unambiguously reported that seasonal variation in nutrients can propagate up the local food webs to alter species composition (through direct observations or eDNA), at least not in the better studied shallow subtidal and intertidal communities. Therefore, one of the factors that could affect the composition of invertebrates and especially fish assemblages is the frequent occurrence of hypoxic events at waters deeper than 30 and especially 60 m, that is the penetration of cold water with an oxygen level below 2 μm/L, especially in spring and summer months (De La Maza & Farías, 2023 ). Sensitivity to hypoxia varies widely among marine taxa (Diaz & Rosenberg, 1995 ; Miller et al., 2002 ; Rosenberg et al., 1991 ).…”

Biogeochemical Response of the Water Column of Concepción Bay, Chile, to a New Regime of Atmospheric and Oceanographic Variability

“…On the other hand, the frequency and intensity of drastic dissolved oxygen decline (<20 µM O 2 ) events in many coastal marine environments have increased worldwide since the 1960s [34,[142][143][144][145]. In fact, a recent long-term study demonstrated the intensification of the hypoxia over the continental shelf off CB, where hypoxic and severe hypoxic waters (<89 and <22 µmol O 2 L −1 ) have increased more than 2-fold in volume since 1997 [146]. The projected decrease in oxygen concentrations [32] and the increase in water column stratification [33], as well as local eutrophication [34], might lead to more frequent and intense events of oxygen and other electron acceptors (e.g., NO 3 − and NO 2 − ) reduction.…”

Emergent constraint on oxygenation of the upper South Eastern Pacific oxygen minimum zone in the twenty-first century

“…In the SEP, potentially observable OMZ metrics include the oxycline depth along the coast at a few sites where long-term monitoring has been performed [28][29][30] , although these coastal sites mostly sample the water column above the OMZ and may monitor anthropogenic signals from different sources difficult to interpret. Regarding relevant timescales for oxygen variability in SEP, it appears obvious to consider the interannual timescale as the El Niño Southern Oscillation (ENSO) is the main mode of interannual variability at global scale.…”