Assessment of dissolved oxygen(DO) and oxygen minimum zone - Ocean Acidification & Hypoxia

Home > Marine Carbon > Ocean Acidification & Hypoxia > Assessment of dissolved oxygen(DO) and oxygen minimum zone

Research Background

Due to the ongoing global warming phenomenon, there is a continuous decline in oceanic oxygen concentrations, which holds significant implications for the physical and biogeochemical processes occurring in the upper and middle layers of the oceans. In the middle ocean, temperature continues to rise, and anthropogenic activities result in the excessive transport of nutrients to coastal waters, stimulating the growth of phytoplankton. This, in turn, leads to the shallowing of the depth of the minimum oxygen zones (OMZs; Dissolved Oxygen (DO)＜60 μmol kg⁻¹) in the middle ocean. Such changes pose a threat to the stability of the ecosystem and the sustainable development of fisheries, particularly within the coastal marine ecosystem.

The presence of the OMZ presents a substantial risk to the growth and development of fish populations. The anticipated expansion of the hypoxic zone in the future will not only directly impact the survival and biological habitats of species with relatively higher tolerance to hypoxic conditions, such as certain fish and crustaceans residing near the upper boundary of the OMZ, but may also result in the contraction of favorable habitats for both pelagic and benthic fish and invertebrates. Consequently, this will alter the survival and distribution patterns of aquatic organisms within the ecosystem.

Hence, the identification and delineation of the location and extent of the OMZ play a crucial role in assessing ecosystem stability and mitigating potential disasters. The initial depth of OMZ formation, referred to as the depth of hypoxia (DOH), serves as a vital parameter for understanding the impact of OMZs on marine life. DOH allows for the determination of whether an OMZ has formed, the extent to which it expands longitudinally within the water column, and the evaluation of its influence on the middle-level marine ecosystem. Moreover, it aids in identifying and predicting changes in the favorable habitats of aquatic organisms.

In conclusion, the changes in DOH, occurring within the context of global climate change, have significant implications for aquatic organisms. These impacts are manifested in alterations to plankton communities in the upper ocean and can potentially result in complex and substantial ecosystem responses.

unfold

Figures

Fig. 1 (left) Flowchart of satellite inversion of DOH in the California Stream. (Top right) Map of DOH in the California stream system from 2003 to 2020. (Bottom right) From 2003 to 2020, the evolution characteristics of DOH retrieved by satellites can be divided into three stages: the period of normal climate (2003~2013), the period of climate oscillation (2014~2016) and the period of climate recovery (2017~2020).

Scientific Progress

The typical oxygen minimum zone (OMZ) dataset includes the depth of hypoxia (DOH, with the unit of m) which is the shallowest depth at which the waters become hypoxic. The dataset has a high spatial resolution of 0.25° x 0.25° and covers the California Current System (CCS) from January 2003 to December 2020. Validation of the DOH inversion model showed an overall coefficient of determination of 0.82 and a root mean square error of 37.69 m (n = 80).

The dataset used the satellite-derived net community production (NCP) which was calculated by daily MODIS/Aqua standard Level 3 products for sea surface temperature (SST), photosynthetically active radiation, monthly sea surface chlorophyll a, and selected satellite-derived SST and NCP as input indicators. Based on the OMZ formation mechanism in the eastern subtropical North Pacific Ocean, we developed the nonlinear polynomial regression inversion model for the satellite-derived DOH in the CCS. This model was used to reconstruct the complete OMZ field in the CCS for the 2003–2020 period and showed that (1) there was a significant shallowing trend in the DOH from 2003 to 2013 in the coastal region of the CCS. Combined with the DO profile records from previous studies and literature records, these findings showed that the shallowing pattern for the DOH existed for almost 40 years. (2) The strong climate oscillation events in the northeastern Pacific over the 2014–2016 period caused rapid variations in environmental parameters. The shallowing trend in the DOH was reversed to a deepening pattern due to warming as a result of regional climate change. There was also a temporary slowing of the expansion of the OMZ. (3) During 2017–2020, effects of the climate oscillation event on the CCS gradually disappeared and the DOH recovered slightly, but it had not returned to the significant shallowing characteristics that existed prior to 2014. In the context of current global warming and increasing stress imposed by human activities, the rising SST, intensifying upwelling-favorable winds and excess nutrient discharge might imply that the depth of the upper layer of the OMZ should be shallower in the future. The shallowing DOH will affect aquatic organisms and lead to corresponding responses by the phytoplankton community in the upper layer, which may ultimately lead to significant and complex changes in the whole marine ecosystem. This dataset expands the high spatiotemporal resolution OMZ data available in the CCS and helps to assess and predict the stable development of ecosystems and fisheries economies.

unfold

References

Zhang, Yifan, Yan Bai, Xianqiang He, Teng Li, Zhiting Jiang, and Fang Gong. 2023. “Three Stages in the Variation of the Depth of Hypoxia in the California Current System 2003–2020 by Satellite Estimation.” Science of The Total Environment 874 : 162398. https://doi.org/10.1016/j.scitotenv.2023.162398.

unfold

Research Background

Figures

Scientific Progress

References

Links