Origin and Dynamics of Dissolved Organic Matter in a Mariculture Area Suffering From Summertime Hypoxia and Acidification

Abstract

Based on six cruises from March to September in 2016, we investigated monthly distributions of dissolved organic matter (DOM) and ancillary water chemistry parameters in a mariculture area in the Northern Yellow Sea, where summertime hypoxia and seawater acidification were observed. The most severe oxygen depletion (hypoxia covered approximately one-third of the aquaculture area) and the largest pH decrease (8.07 ± 0.05 in surface layer vs. 7.66 ± 0.07 in bottom layer) were revealed in August. Concentration of dissolved organic carbon (DOC) and the absorption properties of chromophoric DOM (CDOM) were used to characterize DOM. Results showed that DOM mainly originated from marine in situ processes. In March, a DOM pool with the lowest DOC concentration of 211 ± 23 µmol L −1 and nearly uniform optical characteristics were presented in the well-mixed water column. In August, however, DOC increased to 361 ± 29 µmol L −1 in the surface layer and 342 ± 25 µmol L −1 in the bottom layer. Two non-linear relationships between the absorption coefficient at 355 nm [a CDOM (355)] and the absorption spectral slope over 275-295 nm (S 275−295) were revealed. According to modeling results, the non-linear relationships were mostly caused by the conservative mixing of a refractory CDOM pool with a freshly produced CDOM pool. Apparent oxygen utilization in August was positively related to DOC, but not to a CDOM (355) and S 275−295 , presumably due to multiple sources of CDOM in bottom waters. Both a CDOM (355) and S 275−295 respond largely to decreasing pH; however, they would be less affected by ocean acidification since this process leads to a limited pH decline.

Physical Controls of Summer Variations in Bottom Layer Oxygen Concentrations in the Coastal Hypoxic Region off the Northeastern Shandong Peninsula in the Yellow Sea

“…This finding indicates that the water body in the study area was vertically well mixed in spring and autumn, which is consistent with the cruise observations (Yang & Gao, 2019; ZHAI ET AL. Zhang, Gao, et al, 2018). Consequently, the resulting surface-bottom temperature differences were positive in both summer and winter (Figure 6b).…”

“…The full-depth heat content over a unit horizontal area at the online observation site was expressed as the SST and bottom layer temperature, respectively. Considering the vertical structure of the temperature profiles in the study area (Figure 5; Zhang, Gao, et al, 2018), the real full-depth heat content Q was always between Q S and Q B following Q B < Q < Q S . Q S and Q B defined the upper limit and lower limit of Q at any time.…”

Comparing Subsurface Seasonal Deoxygenation and Acidification in the Yellow Sea and Northern East China Sea Along the North-to-South Latitude Gradient

“…Recently, studies relating the concurrency of bottom hypoxia and CO 2 acidification were conducted in many other coastal zones. For example, summertime hypoxia and acidification (pH < 7.7) covered almost one-third of an aquaculture area in southern nearshore waters of the North Yellow Sea (Zhang et al, 2018). In the central Bohai Sea, the near-hypoxic (DO < 90 µmol O 2 kg −1 ) and concurrent low pH T (∼7.7) and arag (∼1.3) were observed in late summer (Zhai et al, 2019;Song et al, 2020).…”

Biogeochemistry of dissolved inorganic nutrients in an oligotrophic coastal mariculture region of the northern Shandong Peninsula, north Yellow Sea

“…Through the field investigation in the coastal waters of the northern Shandong Peninsula from 2015 to 2017, it was found that there was a hypoxia zone existent in the bottom water in summer, and aerobic decomposition of marine autogenous organic matter could be the main biochemical reason for the formation of hypoxia (Yang et al, 2018;Yang and Gao, 2019). In addition, phytoplankton reproduction could be the most important factor affecting the biogeochemical cycle of organic matter (Yang et al, 2018;Zhang et al, 2018;Yang and Gao, 2019). However, as a key factor affecting phytoplankton reproduction, the biogeochemical cycle of nutrients in this area has not yet been studied.…”