Production and uptake of dissolved carbon, nitrogen, and phosphorus in overlying water of aquaculture shrimp ponds in subtropical estuaries, China

Substantial increase in P release following conversion of coastal wetlands to aquaculture ponds from altered kinetic exchange and resupply capacity

The reclamation of wetlands and its subsequent conversion to aquaculture may alter regional nutrient (im)mobilization and cycling, although direct assessments of phosphorus (P) cycling and its budget balance following wetland conversion are currently scarce. Here, parallel field experiments were conducted to investigate and compare the availability and mobilization mechanisms of P from natural coastal wetlands and the adjacent converted aquaculture ponds based on high-resolution diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) techniques and the DGT-induced fluxes in sediments (DIFS) model. The study found that the conversion of wetland to pond strongly reduced the sediment P pool by changing its forms and distribution. High-resolution data showed that concentrations of labile P and soluble reactive P across the sediment-water profiles were markedly enhanced by the converted aquaculture pond, although they exhibited large spatiotemporal heterogeneity. Moreover, the synchronous distribution of labile P, iron (Fe) and sulfur (S) across profiles in coastal wetlands indicated that the dissolution of Fe (III) oxyhydroxide-phosphate complexes coupled with sulfate reduction were the main mechanisms regulating sediment P mobilization in coastal areas. However, the converted aquaculture pond weakened or even reversed this dependence by decoupling the Fe-S-P reactions by changing the sediment structure and nutrient balance. Substantial increases in labile P, Fe and S fluxes in the pond suggested the conversion of wetland to aquaculture facilitated the internal release of P, Fe and S from sediment into water. The high resupply parameter (R) and desorption rate (k_-1) combined with low response time (T_c) in the pond, as fitted by DIFS model, indicated the strong resupply capacity and fast kinetic exchange of sediment P across the sediment-water interface, which is consistent with the high P diffusion fluxes recorded in the pond. It was concluded that converted aquaculture ponds act as an important source of P release in coastal areas, potentially exacerbating water quality degradation and eutrophication. Specific initiatives and actions are therefore urgently needed to alleviate the internal P-loading during aquaculture.

Large variations in indirect N2O emission factors (EF5) from coastal aquaculture systems in China from plot to regional scales

Aquaculture ponds are important anthropogenic sources of nitrous oxide (N₂O). Direct N₂O emissions arising from feed application to ponds have been widely investigated, but indirect emissions from N₂O production from residual feeds in pond water are much less understood and characterized to refine the IPCC emission factor. In this study, we determined the concentrations and spatiotemporal variations of dissolved N₂O and NO₃^--N in situ in three aquaculture ponds at the Min River Estuary in southeastern China during the culture period over two years, and calculated the indirect N₂O emission factor (EF₅) for aquaculture ponds using the N₂O-N/NO₃^--N mass ratio methodology. Our results indicated that the EF₅ values in the ponds over the culture period ranged between 0.0007 and 0.0543, with a clear seasonal pattern which closely followed that of the DOC:NO₃^-N ratio. We also observed significant spatial variations in EF₅ among the three ponds, which could be attributed to the difference in feed conversion rate. In addition, we assessed the EF₅ values from aquaculture ponds in five regions of the Chinese coastline across the latitudinal gradient from the tropical to the temperate zones. The average EF₅ value from aquaculture ponds across the five coastal regions was 0.0093±0.0024, which was approximately 3.7 times of the IPCC default value for rivers and estuaries (0.0025). Moreover, the EF₅ values demonstrated considerable spatial variations across these coastal regions with a coefficient of variation of 59%, which were largely related to the difference in water salinity. Our findings filled a key knowledge gap about the indirect N₂O emission factor from aquaculture ponds, and provided field evidence for the refinement of EF₅ value currently adopted by IPCC in the national greenhouse gas inventory.

Reclamation-induced tidal restriction increases dissolved carbon and greenhouse gases diffusive fluxes in salt marsh creeks

Intertidal creeks play an important role in transporting nutrients between coastal ecosystems and ocean. Reclamation is a predominant anthropogenic disturbance in coastal regions; however, the influence of reclamation on carbon and nitrogen species and greenhouse gas (GHG) fluxes in creek remains unclear. In a subtropical salt marsh of eastern China, the seasonal patterns of dissolved carbon (DOC, DIC, CO₂, and CH₄) and inorganic nitrogen (NH₄⁺-N, NO₂^--N, and NO₃^--N and N₂O) species, and the diffusive fluxes of CO₂, CH₄, and N₂O, were compared between the natural tidal creeks and the reclaimed creeks. Due to notably changed hydrological and biological conditions in the reclaimed creeks, concentrations of all dissolved carbon species, NH₄⁺-N and NO₂^--N increased significantly by 60.2-288.2%, while NO₃^--N and N₂O decreased slightly, compared to the natural tidal creeks. DIC and NO₃^--N were the primary components of the total dissolved carbon and inorganic nitrogen in both creek types; however, their proportions decreased as a result of elevated DOC, CO₂, CH₄, NH₄⁺-N, and NO₂^--N following reclamation. Significantly higher global warming potential (0.58 ± 0.15 g CO₂-eq m^-2 d^-1) was found in the reclaimed creeks, making them hotspot of greenhouse effects, compared to the natural tidal creeks. Our results indicated that changes in flow velocity, salinity, Chlorophyll a, and pH were the main factors controlling the dissolved carbon and nitrogen and consequent GHG emissions, due to reclamation. This study is helpful in understanding of carbon and nitrogen sink-source shifts resulting from land use changes in coastal wetlands.

Nitrogen Loss in Vegetable Field under the Simulated Rainfall Experiments in Hebei, China

Agricultural non-point source pollution is one of the main factors contaminating the environment. However, the impact of rainfall on loss of non-point nitrogen is far from well understood. Based on the artificial rainfall simulation experiments to monitor the loss of dissolved nitrogen (DN) in surface runoff and interflow of vegetable field, this study analyzed the effects of rainfall intensity and fertilization scheme on nitrogen (N) loss. The results indicated that fertilizer usage is the main factor affecting the nitrogen loss in surface runoff, while runoff and rainfall intensity play important roles in interflow nitrogen loss. The proportion of DN lost through the surface runoff was more than 91%, and it decreased with increasing rainfall intensity. There was a clear linear trend (r2 > 0.96) between the amount of DN loss and runoff. Over 95% of DN was lost as nitrate nitrogen (NN), which was the major component of nitrogen loss. Compared with the conventional fertilization treatment (CF), the amount of nitrogen fertilizer applied in the optimized fertilization treatment (OF) decreased by 38.9%, and the loss of DN decreased by 28.4%, but root length, plant height and yield of pak choi increased by 6.3%, 2.7% and 5.6%, respectively. Our findings suggest that properly reducing the amount of nitrogen fertilizer can improve the utilization rate of nitrogen fertilizer but will not reduce the yield of pak choi. Controlling fertilizer usage and reducing runoff generation are important methods to reduce the DN loss in vegetable fields.

Spatiotemporal Analysis of Water Quality Using Multivariate Statistical Techniques and the Water Quality Identification Index for the Qinhuai River Basin, East China

Monitoring water quality is indispensable for the identification of threats to water environment and later management of water resources. Accurate monitoring and assessment of water quality have been long-term challenges. In this study, multivariate statistical techniques (MST) and water quality identification index (WQII) were applied to analyze spatiotemporal variation in water quality and determine the major pollution sources in the Qinhuai River, East China. A rotated principal component analysis (PCA) identified three potential pollution sources during the wet season (mixed pollution, physicochemical, and nonpoint sources of nutrients) and the dry season (nutrient, primary environmental, and organic sources) and they explained 81.14% of the total variances in the wet season and 78.42% of total variances in the dry season. The result of redundancy analysis (RDA) showed that population density, urbanization, and wastewater discharge are the main sources of organic pollution, while agricultural fertilizer consumption and industrial wastewater discharge are the main sources of nutrients such as nitrogen and phosphorus. The water quality of the Qinhuai River basin was determined to be mainly Class III (slightly polluted) and Class IV (moderately polluted) based on WQII. Temporally, the change trend of WQII showed that water quality gradually deteriorated between 1990 and 2005, improved between 2006 and 2010, and then deteriorated again. Spatially, the WQII distribution map showed that areas with more developed urbanization were relatively more polluted. Our results show that MST and WQII are useful tools to help the public and decision makers to evaluate the water quality of aquatic environment.

Spatial Variation in Aragonite Saturation State and the Influencing Factors in Jiaozhou Bay, China

Both natural processes and human activities affect seawater calcium carbonate saturation state (Ωarag), while the mechanisms are still far from being clearly understood. This study analysed the seawater surface Ωarag during summer and winter in Jiaozhou Bay (JZB), China, based on two cruises observations performed in January and June 2017. The ranges of Ωarag values were 1.55~2.92 in summer and 1.62~2.15 in winter. Regression analyses were conducted to identify the drivers of the change of Ωarag distribution, and then the relative contributions of temperature, mixing processes and biological processes to the spatial differences in Ωarag were evaluated by introducing the difference between total alkalinity (TA) and dissolved inorganic carbon (DIC) as a proxy for Ωarag. The results showed that biological processes were the main factor affecting the spatial differences in Ωarag, with relative contributions of 70% in summer and 50% in winter. The contributions of temperature (25% in summer and 20% in winter) and the mixing processes (5% in summer and 30% in winter) were lower. The increasing urbanization in offshore areas can further worsen acidification, therefore environmental protection in both offshore and onshore is needed.