Photosynthetic Production Determines Bottom Water Oxygen Variations in the Upwelling Coastal South China Sea Over Recent Decades

Abstract

Dissolved oxygen (DO) in seawater is fundamental to marine ecosystem health. How DO in coastal upwelling areas responds to upwelling intensity under climate change is of particular interest and vital importance, because these productive regions account for a large fraction of global fishery production and marine biodiversity. The Yuedong upwelling (YDU) in the coastal northern South China Sea can be served as a study case to explore long-term responses of DO to upwelling and climate due to minor influence of riverine input. Here, bottom water DO conditions were recovered by sedimentary C28Δ22/Δ5,22 ratios of steroids in three short cores, with lower ratio value indicating higher DO concentration. The ratio records showed oscillations in varying degrees and exhibited no clear trends before ∼1980s, after which, however, there occurred a persistent decreasing trend or basically remained at lower values. Thus, inferred DO variations by the C28Δ22/Δ5,22 ratio records are not compatible with regional YDU-involved physical processes under climate change, such as southwesterly wind-induced onshore advection of reduced-oxygenated source waters from outer shelf and oceanic warming that would rather lead to less oxygenation in bottom waters in recent decades. Intriguingly, the alcohol records of n-C20:1/C28Δ5,22 and br-C15/C28Δ5,22 ratios, indicative of the relative strengths between biogeochemical oxygen consumption (i.e., by zooplankton and microbes) and photosynthetic oxygen production (i.e., by phytoplankton), changed almost in parallel with the C28Δ22/Δ5,22 records in three cores. Accordingly, we propose that net photosynthetic oxygen production outweighs source water– and warming-induced increasing deoxygenation in the study area. This study may suggest an important biogeochemical mechanism in determining bottom water DO dynamics in shallow coastal upwelling regions with minor contribution of riverine input.

Production of short-chain n-fatty acids in coral reefs in the southern South China Sea since the Late Miocene

“…We notice the good positive correlations among n-C 14-18 FAs (and alcohols), zooplankton-derived n-C 22:1 FAs and bacteria-derived br-FAs in Well NK1 (Supplementary Table S1). Similar scenarios have also been documented in three shallow-water cores (water depths <35 m) in the northern SCS, showing strong correlations of phytoplankton-sterols with zooplankton-produced n-C 20:1 alcohols and n-C 14-18 alcohols (Zhu et al, 2021). These occurrences may reflect the close coupling among different groups of marine biotas (i.e., phytoplankton, zooplankton and bacteria) through food chains and their coupled contribution to the production of n-C 14-18 FAs and alcohols in the euphotic shallow-water zone.…”

“…The n-C 14-18 FAs (and n-alcohols) are generally used to reflect total marine OM input, including phytoplankton, zooplankton and bacteria (Mudge and Norris, 1997;Gogou and Stephanou, 2004;Treignier et al, 2006;Hu et al, 2009;Strong et al, 2012Strong et al, , 2013Zhu et al, 2014Zhu et al, , 2021Guo et al, 2019); whereas n-C 22:1 FAs and br-FAs are usually specified to inputs of zooplankton and bacteria, respectively (Dalsgaard et al, 2003;Volkman, 2006). The abundant presence of these biomarkers in Well NK1 coral reefs (Fig.…”

“…As mixotrophic organisms, reef-building corals obtain nutrients from autotrophic photosynthesis by endosymbiotic coralline algae and heterotrophic predation on external various food resources (i.e., phytoplankton, zooplankton and bacteria) in surface waters (Wood, 1993(Wood, , 1995Anthony and Fabricius, 2000;Houlbrèque and Ferrier-Pagès, 2009). Under the condition of autotrophy, coral autotrophic production by symbiotic algae (i.e., dinoflagellats) correlates in concert with surface production by asymbiotic phytoplankton (i.e., diatoms) due to the common factor of nutrients in the euphotic zone (e.g., He et al, 2013;Li et al, 2015;Zhu et al, 2021). In the case of heterotrophy, coral heterotrophic production also relates in coupling to surface production due to the large dependence of corals feeding on asymbiotic phytoplankton (and phytoplankton-governed secondary consumers, i.e., zooplankton and bacteria) in surface waters (Wood, 1993(Wood, , 1995.…”

“…The dominance of short-chain n-FAs occurs also in marine geological archives (i.e., sediments) likely due to their ubiquity in various marine biotas, including phytoplankton, zooplankton and bacteria (Dalsgaard et al, 2003;Volkman, 2006). In view of this character, short-chain n-FAs are widely applied to indicate total marine organic matter (OM) input (Mudge and Norris, 1997;Gogou and Stephanou, 2004;Treignier et al, 2006;Hu et al, 2009;Strong et al, 2012Strong et al, , 2013Zhu et al, 2014Zhu et al, , 2021Guo et al, 2019). Thus, given the incorporation of various marine organisms into reef-building corals through autotrophic and heterotrophic pathways (Anthony and Fabricius, 2000;Houlbrèque and Ferrier-Pagès, 2009), short-chain n-FAs are likely feasible to document the production in mixotrophic reef-building corals and their reef buildups.…”