Benthic biogeochemical processes in the coastal ecosystems of the Yellow Sea

Abstract

Coastal sediments are global hotspots for carbon cycling. The mineralization and preservation of organic matter is an important processe by which coastal sediments exert their strong influence over the global carbon cycle. This thesis addresses the effects of various global change factors (biological invasion, construction of artificial dam) on organic matter mineralization processes in coastal sediments, aiming to assess their potential role in the carbon cycling of these ecosystems. Besides studying direct effects of different factors on decomposition and turnover processes, some studies particularly focused on relevant to combine biogeochemical process analysis with microbiological information to confirm. The thesis is structured in 5 chapters, including a general introduction to the topic (Chapter 1), three manuscripts as the main part (Chapters 2-4), and a synthesis discussing the links between the single chapters, their implications, and resulting future research perspectives (Chapter 5).

In Chapter 2, to better understand how spatial and temporal variation of invasive Spartina anglica affect biogeochemical processes in intertidal sediments, we examined the seasonal dynamics of rates and pathways of organic carbon oxidation and biogeochemical properties from 2017 to 2018 in intertidal sediments vegetated by invasive Spartina anglica (SA), and indigenous marsh plant Suaeda japonica (SJ) and unvegetated mud flats (UMF). The above- and belowground biomass at SA patch were 13.6- and 30.6- fold, respectively, greater than that of the SJ patch. In addition, Spartina spp. have a relatively longer growing season and higher primary productivity compared with native plant species, indicating that releasing a substantial amount of labile dissolved organic matter and creating relatively oxidized conditions at SA site. Consequently, presence of invasive S. anglica have consistently high rates of anaerobic organic carbon (Corg) oxidation, FeR, and SR to compared to native SJ and UMF over season, except in April. The role of SR for the Corg oxidation at SA and SJ was generally highest in growing stage (August to October; 52.2 to 65.3%) and lowest in senescence stage (January; 28.0 to 33.9%), but there were significant changes in the relative importance of FeR in growing season, comprising 49–78% of anaerobic Corg oxidation. The continual and rapid regeneration of Fe(III) oxides in the rhizosphere may be supported relatively high rates of FeR earlier in the growing season. Interestingly, despite the high FeRR and SRR of SA, accumulation of reduced products of anaerobic metabolic products (i.e., NH4+ and Fe2+) in the pore water was lower at the SA site than at the SJ site, and H2S in the pore water of SA was depleted. This discrepancy is due to abiotic reduction of iron oxides coupled to H2S oxidation. Our findings reflect that the invasion of S. anglica and its subsequent displacement of native S. japonica would greatly alter the biogeochemical C-Fe-S cycles, which contributes to fundamental scientific information for decision-makers tasked with maintaining sustainable coastal environments where exotic plants start to expand.

In Chapter 3, biogeochemical process studies and molecular microbiological analyses were applied to assess the effect of invasive S. anglica (SA) on Corg oxidation pathways and microbial community structures in intertidal sediments vegetated by the indigenous marsh plant S. japonica (SJ) and unvegetated mud flats (UMF). Invasive S. anglica possessed 10 times the below-ground biomass of native S. japonica, which was responsible for releasing a substantial amount of labile dissolved organic matter and creating relatively oxidized conditions at SA site. As a result, microbial metabolic activities measured by rates of anaerobic Corg oxidation, iron reduction (FeR) and sulfate reduction (SR) appeared to be greater at SA site compared with the SJ and UMF sites. SR was the dominant anaerobic respiration pathway at a depth of 0–10 cm for vegetated sediments, but the contribution of FeR to Corg oxidation was exceptionally high in the rhizosphere of the vegetated sites, comprising 60% and 70% of anaerobic Corg oxidation of SA and SJ, respectively. The iron turnover rate at the rhizosphere was 3 times higher at SA site (0.063 d-1) compared with the SJ site (0.023 d-1), indicating that the denser root system of invasive S. anglica greatly accelerates iron cycling. Bacterial communities based on 16S rRNA genes analysis revealed that members in Desulfuromonadaceae related to the reduction of FeOOH and S0 were highly abundant at the relatively oxidized SA site, whereas Desulfobulbaceae, which are known as sulfate reducers, were more dominant at the relatively reduced SJ site. Similarly, two sulfur-oxidizing bacteria groups with different eco-physiological strategies thrived in each of the two vegetated sites. Thioprofundaceae in Gammaproteobacteria were the predominant S-oxidizers at the less-reduced SA site, whereas Sulfurovum in Epsilonproteobacteria dominated at the relatively reduced SJ site. Our results suggest that an invasion of tall S. anglica and its subsequent displacement of native S. japonica would greatly alter the biogeochemical C-Fe-S cycles and associated microbial communities, which ultimately generate multidirectional variations in ecological and biogeochemical processes in coastal ecosystems.

In Chapter 4, we investigated sediment geochemistry, partitioning of organic carbon (Corg) oxidation by iron reduction (FeR) and sulfate reduction (SR), and benthic phosphorus (P) release, together with the P speciation in the sediments to elucidate the P dynamics at two contrasting sediments (i.e., estuarine vs. limnetic) separated by a large dyke in the Yeongsan River estuary of the Yellow Sea. In the sediments of the Yeongsan River estuary (St. YE), SR dominated Corg oxidation pathway, accounting for 81.7% of total anaerobic Corg oxidation. Under the SR-dominated condition, H2S derived from SR reacts quickly with iron oxides to form iron sulfides, which ultimately release the P bound to Fe(III) into the pore water. The enhanced benthic P flux (0.24 mmol m-2 d-1) at the YE site accounted for 80% of the P required for primary production in the water column. In contrast, in the limnetic sediments of the Yeongsan Lake (St. YL), where high levels of CH4 accumulated, most P was bound to Fe and Al, which resulted in a low benthic P flux (0.03 mmol m-2 d-1). The results suggest that the frequent discharge of relatively P-depleted freshwater into the estuary via the artificial dyke may result in relatively P-limiting conditions in estuarine ecosystem. As a result, benthic P release from the SR-dominated estuarine sediment plays a significant internal source of P in the coastal ecosystem. Our results indicate that the construction of a large dyke at a river mouth greatly alters Corg oxidation pathways and P dynamics in coastal ecosystems.