Larval Fish Habitats and Deoxygenation in the Northern Limit of the Oxygen Minimum Zone off Mexico

Abstract

The present state of deoxygenation in the northern limits of the shallow oxygen minimum zone off Mexico is examined in order to detect its effects on larval fish habitats and consider the sensitivity of fish larvae to decreased dissolved oxygen. A series of cruises between 2000 and 2017 indicated a significant vertical expansion of low oxygen waters. The upper limit of suboxic conditions (<4.4 μmol/kg) has riseñ 100 m at 19.5°N off Cabo Corrientes and~50 m at 25°N in the mouth of the Gulf of California. The larval habitat distribution was related to the geographic variability of dissolved oxygen and water masses between these two latitudes. One recurrent larval habitat, with Bregmaceros bathymaster larvae as the indicator species, extended throughout the water column off Cabo Corrientes from Subtropical Subsurface Water (suboxic conditions) to the surface (220 μmol/kg). The second recurrent habitat was located between the oxycline (>44 μmol/kg) and the surface in association with the Gulf of California Water, with Benthosema panamense as the indicator species. During the warm El Niño event of 2015-2016, a tropical larval fish habitat (Auxis spp.) associated with Tropical Surface Water appeared to change the larval habitat distributions. These results indicate that some species are resilient to changes of dissolved oxygen and temperature generated by El Niño events and by continuing deoxygenation, although other species with more limited environmental windows could be affected by deoxygenation, probably leading to a change of the pelagic ecosystem over time.Plain Language Summary Dissolved oxygen in the ocean is crucial for the biological cycles of all organisms. There are vast zones in the ocean where the oxygen has dropped to levels that only a few, very well adapted, organisms can live. Dissolved oxygen measurements in the past two decades in the Mexican Pacific have shown that the concentrations have decreased, and water layers with nearly no oxygen are now closer to the surface, potentially impacting larval fish species that are not adapted to extremely low oxygen concentrations. Nevertheless, there is a set of fish larvae that is well adapted to low dissolved oxygen concentrations, principally small forage fish, which have a permanent presence in the poorly oxygenated waters, while some larvae of species with fisheries interest, like small tunids, only live in the well-oxygenated surface layer and are more abundant in warm years. The loss of oxygen in the Mexican Pacific has not had an effect on the small forage fish larvae but can affect those larvae of fish of commercial interest, with less adaptation to low oxygen concentrations.

Changing State of the Climate System

“…Findings since SROCC provide further support that the volume of severely oxygen-depleted water has expanded in some locations of the global ocean (Section 5.3.3.2). For example, vertical expansion of low oxygen zones is reported in the North Pacific at a rate of 3.1 ± 0.5 m yr -1 (Ross et al, 2020), and suboxic waters have increased by 20% at a rate of about 19 m per decade from 1982-2010 in the Arabian Sea (Al-Said et al, 2018;Lachkar et al, 2019), and expanded off the coast of Mexico (Sánchez-Velasco et al, 2019).…”

A Global Ocean Oxygen Database and Atlas for Assessing and Predicting Deoxygenation and Ocean Health in the Open and Coastal Ocean

“…O 2 data are important for assessing the habitat quality of many organisms within the coastal-shelf-open ocean environments. Examples include field-based determination of effects of O 2 on zooplankton (Wishner et al, 2018;Roman et al, 2019), benthos (Levin, 2003;Rabalais et al, 2007b;Levin et al, 2009;Yasuhara et al, 2012Yasuhara et al, , 2017Gammal et al, 2017), fish and shellfish (Keister et al, 2000;Breitburg, 2002;Pollock et al, 2007;Thomas and Rahman, 2010;Keller et al, 2017;Limburg and Casini, 2019;Roman et al, 2019), deep-sea coral along seamounts (e.g., Anderson et al, 2016), and fish habitat (e.g., Stramma et al, 2012;Sánchez-Velasco et al, 2019;Gallo et al, 2020). The effects of low O 2 on the physiological conditions of organisms have been investigated in many laboratory and field studies (e.g., Vaquer-Sunyer and Duarte, 2008;Wu, 2009;Neilan and Rose, 2014;Casini, 2018, 2019) and are summarized by the metabolic index (ability to meet temperature-dependent O 2 demands) and related indices (P crit ) for species in a variety of systems (Chu and Gale, 2017;Deutsch et al, 2020;Howard et al, 2020;Seibel et al, 2021).…”

Temperature and dissolved oxygen concentration in the Pacific Ocean at the northern region of the oxygen minimum zone off Mexico between the last two PDO cool phases

“…Strong stratification has been reported in these surface water masses, with a thermocline/oxycline that varies in depth from ~ 20 m in summer to ~ 70 m in winter, influenced strongly by mesoscale and interannual processes ( Luyten et al, 1982;Godínez et al, 2010;Kurczyn et al, 2012). Below these surface water masses, the water column is dominated by the Subtropical Subsurface Water (StSsW) between ~100 and 400 m depth, which is characterized by DO concentration < 4.4 µmol kg -1 (Stramma et al, 2008;Schmidtko et al, 2017;Sánchez-Velasco et al, 2019), and North Pacific Intermediate Water (NPIW) below to 1000 m depth, where DO increases again to > 4.4 µmol kg -1 (Karstensen et al, 2008;Paulmier and Ruiz-Pino, 2009;Cepeda-Morales et al, 2013).…”

“…Ocean warming and deoxygenation (loss of dissolved oxygen) are two processes that are occurring simultaneously in the interior ocean (Schmidtko et al, 2017;Oschlies et al, 2018;Stramma et al, 2010). The temperature and dissolved oxygen (DO) are key parameters that affect the cycles of carbon and nutrients, and the life cycles of marine organisms from plankton to top predators (Lavin et al 2006;Prince and Goodyear, 2006;Paulmier and Ruiz-Pino, 2009;Sánchez-Velasco et al, 2019). Variations in the area and intensity of spawning, migration routes, species connectivity, and metabolic changes in organisms can modified the structure and function of marine ecosystems (Chávez et al, 2003;Díaz and Rosenberg, 2008;Cavole et al, 2016;Gallo and Levin, 2016).…”

“…It is known that the shallow OMZs are present in regions of high productivity, where microbial respiration is high below the euphotic zone, and the deep circulation derived from polar regions is sluggish, forming steady-state low-oxygen zones in the water column (Gilly et al, 2013). Although there is no consensus on the DO threshold defining a shallow OMZ, it has recorded as its upper limit the depth of 44 µmol kg -1 oxypleth located below the oxycline/thermocline depth, where a drastic drop of DO and the temperature was displayed (Stramma et al, 2008;Schmidtko et al, 2017;Sánchez-Velasco et al, 2019).…”