Developing a salinity-based approach for the evaluation of DIN removal rate in estuarine ecosystems

Nutrients transport behavior in inlet river in the Yellow River Delta in winter

The nutrients such as dissolved inorganic nitrogen (DIN, NH4+-N, NO2--N, and NO3--N), dissolved inorganic phosphorus (DIP, PO43-) and dissolved SiO2 (DSi) funneled by the inlet river are the dominant factors to coastal eutrophication. This study investigated nutrient transport process in typical inlet rivers in the Yellow River Delta. The indicator of coastal eutrophication potential and concentration ratio between upstream and downstream stations were used to evaluate the influence of different sources to the nutrient risks. It showed that urban areas are the most important source of the nutrients in studied rivers. The harbor and mariculture would have greater risk because of their proximity close to the coastal area. Wetland was a vital conversion to eliminate the river nutrients, and the retention could reach 80 %. It is imperative to protect and construct wetlands to reduce the nutrient pollution in the inlet river.

Effects of Aquatic Acidification on Microbially Mediated Nitrogen Removal in Estuarine and Coastal Environments

Acidification of estuarine and coastal waters is anticipated to influence nitrogen (N) removal processes, which are critical pathways for eliminating excess N from these ecosystems. We found that denitrification rates decreased significantly under acidified conditions (P < 0.05), which reduced by 41-53% in estuarine and coastal sediments under an approximately 0.3 pH reduction of the overlying water. However, the N removal rates through the anaerobic ammonium oxidation (anammox) process were concomitantly promoted under the same acidification conditions (increased by 47-109%, P < 0.05), whereas the total rates of N loss were significantly inhibited by aquatic acidification (P < 0.05), as denitrification remained the dominant N removal pathway. More importantly, the emission of nitrous oxide (N₂O) from estuarine and coastal sediments was greatly stimulated by aquatic acidification (P < 0.05). Molecular analyses further demonstrated that aquatic acidification also altered the functional microbial communities in estuarine and coastal sediments; and the abundance of denitrifiers was significantly reduced (P < 0.05), while the abundance of anammox bacteria remained relatively stable. Collectively, this study reveals the effects of acidification on N removal processes and the underlying mechanisms and suggests that the intensifying acidification in estuarine and coastal waters might reduce the N removal function of these ecosystems, exacerbate eutrophication, and accelerate global climate change.

Shift of DNRA bacterial community composition in sediment cores of the Pearl River Estuary and the impact of environmental factors

Dissimilatory nitrate reduction to ammonia (DNRA) process, competing with denitrification and anaerobic ammonia oxidation (anammox) for nitrate, is an important nitrogen retention pathway in the environment. Previous studies on DNRA bacterial diversity and composition focused on the surface sediments in estuaries, but studies on the deep sediments are limited, and the linkage between DNRA community structure and complex estuarine environment remains unclear. In this study, through high-throughput sequencing of nrfA gene followed by high-resolution sample inference, we examined spatially and temporally the composition and diversity of DNRA bacteria along a salinity gradient in five sediment cores of the Pearl River Estuary (PRE). We found a higher diversity and richness of DNRA bacteria in sediments with lower organic carbon, where sea water intersects fresh water. Moreover, the DNRA bacterial communities had the specific spatially distribution coupling with their metabolic difference along the salinity gradient of the Pearl River Estuary, but no obvious difference along the sediment depth. The distribution of DNRA bacteria in the PRE was largely driven by various environmental factors, including salinity, Oxidation-Reduction Potential (ORP), ammonium, nitrate and C_org/NO₃^-. Furthermore, dominant DNRA bacteria were found to be the key populations of DNRA communities in the PRE sediments by network analysis. Collectively, our results showed that niche difference of DNRA bacteria indeed occurs in the Pearl River Estuary.

Nitrogen removal in the sediments of the Pearl River Estuary, China: Evidence from the distribution and forms of nitrogen in the sediment cores

In this study, the spatial distribution and forms of nitrogen in sediment cores collected from the Pearl River Estuary were analyzed. Exchangeable nitrogen (N_ex) comprised only a small proportion of total nitrogen (N_tot), with a mean of 3.54% in the sediment cores. NH₄⁺ was the main form of N_ex. No obvious change was observed in the vertical content of fixed ammonia (N_fix) in the sediments, and the mean N_fix in all five sediment cores was 141.23 mg·kg^-1. The organic nitrogen (N_org), strongly related to organic carbon (C_org), was the main form in N_tot. The dissolved inorganic nitrogen in sediment pore water was much lower than that in estuarine water and no significant variation was observed from upstream to downstream. Our results indicated that most nitrogen deposited on surface sediments from overlying water was rapidly removed by a series of microbial processes, reducing the extent of nitrogen returning to overlying waters.

Evolution of the sink and source of dissolved inorganic nitrogen with salinity as a tracer during summer in the Pearl River Estuary

In this study, we evaluated the sink and source of the surface water along the PRE using a mixing model method with salinity as tracer. The observational data showed that the decreasing of dissolved inorganic nitrogen (DIN) did not closely follow the physical mixing lines of freshwater and modified seawater. In the western part, DIN consumption by phytoplankton and bacteria uptake (ΔDIN)varied from 15.81 μmol L⁻¹ to 88.53 μmol L⁻¹. On the contrary, in the eastern part, ΔDIN varied from −63.66 μmol L⁻¹ to −10.45 μmol L⁻¹. DIN source in the eastern part may be mainly caused by organic matter decomposition, while DIN remove is strongly associated with phytoplankton growth and bacteria consumption. These differential behaviors of the estuary with respect to DIN are largely due to varying degrees of hydrodynamics due to different topography in the two areas. Sensitivity analysis indicated reduction strategies of DIN inputs to coastal waters may improve environment quality in the PRE, due to DIN changes in the freshwater end-member having a determined influence on biological activities (R). Our results indicate that the model may be a valuable way to address the sources and sink of DIN in the river-dominated estuaries.