Biogeochemical properties and fate of dissolved organic matter in wet deposition: Insights from a mariculture area in North Yellow Sea

Regulation of the colloidal organic matter in coastal waters by scallop farming

To investigate the influence of scallop farming on the biogeochemical characteristics of colloidal organic matter (COM, 1 kDa-0.7 μm), surface and bottom seawater samples collected from a bay scallop mariculture area (MA) and its adjacent waters were size-fractionated and analyzed for absorption and fluorescence characteristics. Compared to inshore shallow water area and non-mariculture area (NMA), COM in MA exhibited the highest proportion of a350 in bulk dissolved organic matter, while the contribution of its 100 kDa-0.7 μm high molecular weight fraction (HCOM) was the lowest. Protein-like components, including tryptophan-like C1 and tyrosine-like C2, predominated in the fluorescent substances of HCOM; while humic-like components, including microbial humic-like C3 and terrestrial humic-like C4, dominated in the fluorescent substances of 1-3 kDa low molecular weight fraction of COM (LCOM). COM transformation was influenced by scallop farming via selective filter-feeding and enhanced degradation of microorganisms. Compared with NMA, phytoplankton production mainly affected the a350 and protein-like substances of HCOM in the surface seawater of MA, and its contributions increased by 2.4% (a350), 5.6% (C1), and 1.8% (C2) respectively. Meanwhile, microbial degradation significantly influenced component C1 of HCOM in the bottom seawater of MA, reducing its contribution by 33.2%; its impact on the contributions of humic-like fluorescent substances of LCOM were decreased by 2.7% for C3 and 1.1% for C4. These variations suggest that scallop farming promotes the production and transformation of protein-like substances in HCOM (mainly C1), potentially leading to an accumulation of humic-like substances in LCOM due to altered microbial degradation dynamics. Photodegradation, particulate organic matter settling, sediment release and colloidal aggregation/disaggregation also influence the transformation of size-fractionated COM.

Organic carbon in wet deposition of an urbanized coastal bay, North China: Flux, sources and biogeochemical implications

The process of atmospheric organic carbon (OC) entering the ocean through wet deposition plays a crucial role in the global carbon cycle. To gain insights into the biogeochemical dynamics of OC at the land-sea margin, we conducted an extensive four-year investigation on precipitation OC in Jiaozhou Bay (JZB). The results showed that the volume-weighted mean concentration of particulate OC (POC) and dissolved OC (DOC) in precipitation were 0.38 and 2.06 mg C L-1 with an average wet deposition flux of OC for 2666.5 mg C m-2 yr-1. The source of POC in precipitation is predominantly by the C3 plant emission and burning and fossil fuel combustion. Wet deposition contributed 986.6 t yr-1 of OC of which 506.3 t yr-1 of bioavailable DOC, which could have significant implications for carbon cycle in the JZB. This study could enhance the understanding of the marine atmospheric OC in coastal areas.

Promoting effect of raft-raised scallop culture on the formation of coastal hypoxia

The coastal waters around the Yangma Island are an important mariculture area of raft-raised scallop and bottom-seeded sea cucumber in the North Yellow Sea, China. Large-scale hypoxia in the bottom water of this area has caused the death of a large number of sea cucumbers and heavy economic losses. To find out the formation mechanism of hypoxia, the data obtained in each August during 2015-2018 were analyzed. Compared with the non-hypoxic year (2018), the temperature, trophic index (TRIX) and dissolved organic carbon (DOC) in the bottom water were relatively higher, and the water column was stratified causing by continuous high air temperature and low wind speed meteorological conditions in the hypoxic years (2015-2017). These sites with the coexistence of thermocline and halocline, and the thickness of thermocline >2.5 m and its upper boundary >7.0 m deep, were prone to hypoxia. Spatially, the hypoxic place was highly consistent with the scallop cultivating places, and the DOC, TRIX, NH₄⁺/NO₃^- and apparent oxygen utilization (AOU) at the culture sites were higher, indicating that organic matter and nutrients released by scallops may lead to local oxygen depletion. In addition, the bottom water of the culture sites had higher salinity, but lower turbidity and temperature, indicating that the slowed water exchange caused by scallop culture was a dynamic factor of hypoxia. All sites with AOU >4 mg/L at the bottom had hypoxia occurrence, even if there was no thermocline. In other words, stratification promoted the formation of hypoxia in coastal bottom water, but it was not indispensable. The raft-raised scallop culture could promote the formation of coastal hypoxia, which should arouse the attention for other coastal areas with intensive bivalve production.

The joint effects of atmospheric dry and wet deposition on organic carbon cycling in a mariculture area in North China

In this research, the atmospheric dry and wet deposition fluxes of particulate organic carbon (POC) over the coastal waters around the Yangma Island in North Yellow Sea were investigated. Combining the results of this research and previous reports about the wet deposition fluxes of dissolved organic carbon (DOC) in precipitation (F_DOC-wet) and dry deposition fluxes of water-dissolvable organic carbon in atmospheric total suspended particles (F_DOC-dry) in this area, a synthetic assessment of the influence of atmospheric deposition on the eco-environment was conducted. It was found that the annual dry deposition flux of POC was 1097.9 mg C m^-2 a^-1, which was approximately 4.1 times that of F_DOC-dry (266.2 mg C m^-2 a^-1). For wet deposition, the annual flux of POC was 445.4 mg C m^-2 a^-1, accounting for 46.7 % that of F_DOC-wet (954.3 mg C m^-2 a^-1). Therefore, atmospheric POC was mainly deposited through dry process with the contribution of 71.1 %, which was contrary to the deposition of DOC. Considering the indirect input of organic carbon (OC) from atmospheric deposition, that is, the new productivity supported by nutrient input from dry and wet deposition, the total OC input from atmospheric deposition to the study area could be up to 12.0 g C m^-2 a^-1, highlighting the important role of atmospheric deposition in the carbon cycling of coastal ecosystems. The contribution of direct and indirect input of OC through atmospheric deposition to the dissolved oxygen consumption in total seawater column was assessed to be lower than 5.2 % in summer, suggesting a relatively smaller contribution to the deoxygenation in summer in this region.

Atmospheric deposition as a direct source of particulate organic carbon in region coastal surface seawater: Evidence from stable carbon and nitrogen isotope analysis

To assess the source characteristics of coastal aerosols and evaluate the contribution of atmospheric deposition to particulate organic matter in surface seawater, total suspended particulates (TSP) were collected at a shore-based site on the south coast of North Yellow Sea from December 2019 through November 2020. The samples were analyzed for total organic carbon (TOC) and nitrogen (TN) as well as stable carbon and nitrogen isotope (δ13C and δ15N). The results showed that the annual mean concentrations of TOC and TN were 5.36 ± 4.74 and 5.12 ± 6.52 μg m-3, respectively. δ13C fluctuated between -25.1 ‰ and -19.2 ‰ with an annual mean of -24.0 ± 1.0 ‰ and a significant seasonal variation (P < 0.05) characterizing by the enrichment in winter (-23.4 ± 0.6 ‰) compared to other seasons, which was probably related to the massive coal combustion. Besides, δ15N ranged from 7.9 ‰ to 21.1 ‰ with an annual mean of 12.5 ± 2.9 ‰ and a less pronounced seasonal pattern (P = 0.23). The Bayesian isotope-mixing model showed that, annually, the most important source of TSP was biogenic and biomass source (55.5 ± 10.8 %), followed by fossil fuel combustion (31.9 ± 9.0 %), while the marine contribution was less (12.6 ± 2.3 %). For TOC and TN, the dominated sources were fossil fuel combustion (47.7 ± 3.4 %) and biogenic and biomass source (57.3 ± 11.7 %), respectively. Furthermore, the model results indicated that the contribution of atmospheric deposition to suspended particulate matter in surface seawater was 18.0 ± 11.0 %, 17.1 ± 6.7 % and 10.2 ± 2.0 % in autumn, spring and summer, respectively. For particulate organic carbon in surface seawater, the contribution of atmospheric deposition was 35.2 ± 3.5 % in spring, highlighting the huge impact of atmospheric deposition on particulate carbon cycling in coastal waters.

Atmospheric wet deposition serves as an important nutrient supply for coastal ecosystems and fishery resources: Insights from a mariculture area in North China

To determine the ecological effects of atmospheric wet deposition of dissolved nutrients on the coastal waters around the Yangma Island, rain and snow samples were collected and analyzed at a shore-based site for one year. The wet deposition fluxes of dissolved inorganic nitrogen and phosphorus (DIN and DIP) and dissolved organic nitrogen and phosphorus were 69.2, 0.136, 13.3 and 0.143 mmol m^-2 a^-1, respectively. In summer, the new production fueled by wet deposition accounted for 19.3 % of that in seawater and 16.4 % of the amount of particulate organic carbon ingested by the scallops cultivated in the study area, indicating the potential contribution of wet deposition to fishery resources. Meanwhile, precipitation increased the seasonal average DIN/DIP ratios in surface seawater by 17.7 %, 16.3 %, 23.4 % and 6.5 % in spring, summer, autumn and winter, respectively, which could change the composition of ecological community and cause obvious negative impact on the ecosystem and mariculture.