Nutrient dynamics and microbial community response in macrophyte-dominated lakes: Implications for improved restoration strategies.

Although lakes dominated by macrophytes are conducive to ecological balance, this balance is easily disrupted by excessive nutrients flowing into the lake. However, knowledge of whether excessive nutrients lead to different microbial environmental vulnerabilities in the lake sediment between macrophyte-dominated areas and macrophyte-free areas is a prerequisite for the implementation of targeted protection measures. In this study, we investigated bacterial communities in sediments using high-throughput sequencing of 16S rRNA genes. Our results showed that the sources of total nitrogen (TN) and organic matter (OM) were related to the macrophytes. The structure, drivers, and interspecific associations of bacterial community, which were more susceptible to increased changes in TN and OM, differed significantly between macrophyte-dominated areas and macrophyte-free areas. More precisely, the lake edge, where was occupied by macrophytes, had a higher proportion of deterministic phylogenetic turnover (88.89%) than other sites, as well as a wider ecological niche and a tighter network structure. Further, as the difference in TN increased, the main assembly processes in surface sediments changed from stochastic to deterministic. However, the majority of phyla from the lake edge showed a greater correlation with excessive nutrients, and the selection of the community by excessive nutrients was more obvious at the edge of the lake. In addition, our results demonstrated that the stability of the bacterial community in macrophyte-free areas is greater than in macrophyte-dominated areas, while an excessively high deterministic process ratio and nutrient (TN and OM) concentration significantly reduced bacterial community stability at macrophyte-dominated areas. Taken together, these results provide a better understanding of the effects of excessive nutrients derived from macrophytes on bacterial community patterns, and highlight the importance of avoiding the accumulation of TN and OM in macrophyte-dominated areas to enhance the sustainability of the ecosystem after restoration of lakes with macrophytes.

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.

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

Water quality deterioration can adversely affect the long-term sustainability of aquaculture industry. Understanding the processes of nutrient regeneration and uptake is important for improving water quality and the overall ecosystem health of aquaculture system. In spite of the importance of dissolved nutrients (DOC, DIC, N-NOx-, N-NH4+, and P-PO43-) in governing water quality and ecosystem functioning, the spatiotemporal variations in the production and uptake of dissolved nutrients in aquaculture ponds is still poorly understood. In this study, the nutrient production and uptake rates in the overlying water were quantified among different shrimp growth stages in the aquaculture ponds in the Min River Estuary (MRE) and Jiulong River Estuary (JRE), southeast China. Significant differences in the nutrient production and uptake rates in the overlying water were observed among the three growth stages and two estuaries. The temporal variations of DOC and DIC production rates in both estuarine ponds closely followed the seasonal cycle of temperature, while the difference in DOC and DIC production rates between the two estuaries was likely caused by differences in water salinity. The changes in the production and uptake rates of dissolved inorganic nitrogen (N-NOx- and N-NH4+) and P-PO43- in the water column over time were partly related to the interactions between thermal conditions and phytoplankton biomass (e.g., chlorophyll a concentrations) in the ponds. Our results demonstrate the complex dynamics and environmental risk of dissolved nutrients in subtropical shrimp ponds, and call for a more effective management of nutrient-laden wastewater in safeguarding the long-term sustainability of aquaculture production.

Fluxes of carbon dioxide and methane across the water-atmosphere interface of aquaculture shrimp ponds in two subtropical estuaries: The effect of temperature, substrate, salinity and nitrate.

While aquaculture pond is a dominant land use/cover type and a distinct aquatic ecosystem in the coastal zones of China and southeast Asia, their contributions to the fluxes of greenhouse gases (GHGs) have only been poorly quantified. Fluxes of CO2 and CH4 in the shrimp ponds with different salinities were simultaneously measured in situ using the floating chamber technique in two different subtropical estuaries, namely, the Min River Estuary (MRE) and Jiulong River Estuary (JRE). The average CO2 and CH4 fluxes in the shrimp ponds over the observation periods varied from -2.09 to 3.37mmol CO2 m-2h-1 and from 0.28 to 16.28mmol CH4 m-2h-1, respectively, with higher fluxes being detected during the middle stage of aquaculture. The temporal variation of CO2 and CH4 fluxes in both estuaries ponds closely followed the seasonal cycle of temperature. Higher CH4 emissions were observed in MRE ponds than in JRE ponds because of the lower water salinity and N-NO3- concentrations as well as a greater supply of carbon substrates. Our findings suggested that shrimp ponds were CH4 emission "hotspots" in the subtropical estuaries of China. Based on a new global warming potential model, we conservatively estimated an anuual GHG emission rate of approximately 63.68Tg CO2-eq during the culture period from aquaculture ponds across the subtropical estuaries of China. Our results demonstrate the importance of aquaculture ponds as a major GHG source and a contributor to climate warming in the subtropical estuarine regions of China, and call for effective regulation of GHG emissions from these ponds for climate mitigation in future.

Dynamics of dissolved nutrients in the aquaculture shrimp ponds of the Min River estuary, China: Concentrations, fluxes and environmental loads.

Dissolved inorganic nutrients (NO2--N, NO3--N, NH4+-N, and PO43--P) play a critical role in the effective management of water quality and prevention of fish and shrimp diseases in aquaculture systems. In this study, dissolved inorganic nutrient concentrations in the water column and sediment porewater, and the fluxes across the sediment-water interface (SWI) were investigated in three intensive shrimp ponds with zero water exchange to examine nutrient cycling during the different growth stages of shrimps. Distinct changes in the dissolved inorganic nutrient concentrations in both the water column and sediment porewater were observed among the three growth stages. Average NO2--N, NO3--N, NH4+-N, and PO43--P concentrations in the sediment porewater were 3.53, 2.81, 29.68, and 6.44 times higher, respectively, than those in the water column over the study period, indicating that the pond sediment acted as a net source of nutrients to the water column. This was further supported by the net release of nutrients from the sediments to the water column observed during the incubation experiment. Nutrient fluxes were dominated by NH4+-N, while NOx--N (NO2--N and NO3--N) and PO43--P fluxes remained low. The high rates of NH4+-N release from the sediment highlight the need of taking into account the biogeochemical role of sediments in mitigating the problem of water quality degradation in coastal shrimp ponds. Based on a total water surface area of mariculture ponds and a total mariculture production of 2.57×106ha and 2.30×109kg, respectively, we estimated conservatively that approximately 4.77×104tons of total nitrogen and 3.75×103tons of total phosphorus are being discharged annually from the mariculture ponds into the adjacent coastal zones across China. Results demonstrated the importance of aquaculture pond effluent as a major contributor of water pollution in the coastal areas of China, and called for actions to properly treat these effluents in alleviating the eutrophication problem in the Chinese coastal zones.