Composition, mineralization potential and release risk of nitrogen in the sediments of Keluke Lake, a Tibetan Plateau freshwater lake in China

Fate of heavy metals in ecosystems of dam reservoirs: Transport, distribution and significance of the origin of organic matter

In this article, a multivariate analysis of the parameters determining the transport and fate of selected heavy metals in the water - bottom sediment interface was carried out. The studies were carried out in the summer season of 2019 at Nielisz Reservoir (southeastern Poland, Lublin Voivodeship). Finally, a previously unknown factor related to the quality of organic matter was identified. Autochthonous organic matter was shown to promote the accumulation of the studied heavy metals. To date, the significance of the origin of organic matter in the context of the transport and fate of heavy metals in retention reservoirs has rarely been reported in the scientific literature. More than that, this factor was not considered an important component in the process of heavy metal deposition in bottom sediments. However, it turns out that not only the quantity of organic matter, but also its quality plays an important role in the circulation of heavy metals in retention reservoir ecosystems. It was found that autochthonous organic matter promotes the accumulation of the studied heavy metals. It can be assumed that, in a sense, it plays the role of a catenary ("hub") controlling the fate of heavy metals in the water-sediment system. It has also been conjectured that, in a sense, OMS may reflect the potential for heavy metal assimilation by aquatic vascular plants (mainly of the C3 group). Plants with a photosynthetic pathway similar to the C3 group generally have a much lower enrichment in the 13C isotope (δ13C from -38‰ to -22‰). In our case, the lowest δ13C-TOCS value was -24.05‰, and the average for the whole reservoir was -21.53‰. In addition, it was observed that quantitative changes in the isotopic composition of total organic carbon δ13C-TOCS, corresponded with changes in the content of the heavy metals studied in entrapped sediments.

15N-DNA stable isotope probing reveals niche differentiation of ammonia oxidizers in paddy soils

Chemoautotrophic canonical ammonia oxidizers (ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB)) and complete ammonia oxidizers (comammox Nitrospira) are accountable for ammonia oxidation, which is a fundamental process of nitrification in terrestrial ecosystems. However, the relationship between autotrophic nitrification and the active nitrifying populations during 15N-urea incubation has not been totally clarified. The 15N-labeled DNA stable isotope probing (DNA-SIP) technique was utilized in order to study the response from the soil nitrification process and the active nitrifying populations, in both acidic and neutral paddy soils, to the application of urea. The presence of C2H2 almost completely inhibited NO3--N production, indicating that autotrophic ammonia oxidation was dominant in both paddy soils. 15N-DNA-SIP technology could effectively distinguish active nitrifying populations in both soils. The active ammonia oxidation groups in both soils were significantly different, AOA (NS (Nitrososphaerales)-Alpha, NS-Gamma, NS-Beta, NS-Delta, NS-Zeta and NT (Ca. Nitrosotaleales)-Alpha), and AOB (Nitrosospira) were functionally active in the acidic paddy soil, whereas comammox Nitrospira clade A and Nitrosospira AOB were functionally active in the neutral paddy soil. This study highlights the effective discriminative effect of 15N-DNA-SIP and niche differentiation of nitrifying populations in these paddy soils. KEY POINTS: • 15N-DNA-SIP technology could effectively distinguish active ammonia oxidizers. • Comammox Nitrospira clade A plays a lesser role than canonical ammonia oxidizers. • The active groups in the acidic and neutral paddy soils were significantly different.

Response of phytoplankton composition to environmental stressors under humidification in three alpine lakes on the Qinghai-Tibet Plateau, China

Owning to their extreme environmental conditions, lakes on the Qinghai-Tibet Plateau have typically displayed a simplistic food web structure, rendering them more vulnerable to climate change compared to lakes in plains. Phytoplankton, undergoing a changing aquatic environment, play a crucial role in the material cycle and energy flow of the food chain, particularly important for the unique fish species of the Tibetan Plateau. To identify the changing environment indexes and determine the response of phytoplankton composition to the environment change in alpine lakes, three lakes-Lake Qinghai, Lake Keluke and Lake Tuosu-were selected as study areas. Seasonal sampling surveys were conducted in spring and summer annually from 2018 to 2020. Our findings revealed there were significant changes in physicochemical parameters and phytoplankton in the three lakes. Bacillariophyta was the predominant phytoplankton in Lake Qinghai from 2018 to 2020, with the genera Synedra sp., Navicula sp., Cymbella sp. and Achnanthidium sp. predominated alternately. Lake Keluke alternated between being dominated by Bacillariophyta and cyanobacteria during the same period. Dolichospermum sp., a cyanobacteria, was prevalent in the summer of 2018 and 2019 and in the spring of 2020. In Lake Tuosu, Bacillariophyta was the predominant phytoplankton from 2018 to 2020, except in the summer of 2019, which was dominated by cyanobacteria. Synedra sp., Oscillatoria sp., Pseudoanabaena sp., Chromulina sp. and Achnanthidium sp. appeared successively as the dominant genera. Analysis revealed that all three lakes exhibited higher phytoplankton abundance in 2018 that in 2019 and 2020. Concurrently, they experienced higher average temperatures in 2018 than in the subsequent years. The cyanobacteria, Bacillariophyta, Chlorophyta and overall phytoplankton increased with temperature and decreased with salinity and NH4-N. Besides, the ratios of cyanobacteria, and the ratios of Bacillariophyta accounted in total phytoplankton increased with temperature. These findings suggest that cyanobacteria and phytoplankton abundance, especially Bacillariophyta, may have an increase tendency in the three alpine lakes under warm and wet climate.

Trophic state resilience to hurricane disturbance of Lake Yojoa, Honduras

Cyclones are a poorly described disturbance in tropical lakes, with the potential to alter ecosystems and compromise the services they provide. In November 2020, Hurricanes Eta and Iota made landfall near the Nicaragua-Honduras border, inundating the region with a large amount of late-season precipitation. To understand the impact of these storms on Lake Yojoa, Honduras, we compared 2020 and 2021 conditions using continuous (every 16 days) data collected from five pelagic locations. The storms resulted in increased Secchi depth and decreased algal abundance in December 2020, and January and February 2021, and lower-than-average accumulation of hypolimnetic nutrients from the onset of stratification (April 2021) until mixus in November 2021. Despite the reduced hypolimnetic nutrient concentrations, epilimnetic nutrient concentrations returned to (and in some cases exceeded) pre-hurricane levels following annual water column turnover in 2021. This response suggests that Lake Yojoa's trophic state had only an ephemeral response to the disturbance imposed by the two hurricanes, likely due to internal input of sediment derived nutrients. These aseasonal storms acted as a large-scale experiment that resulted in nutrient dilution and demonstrated the resilience of Lake Yojoa's trophic state to temporary nutrient reductions.

Importance of ammonia nitrogen potentially released from sediments to the development of eutrophication in a plateau lake

Sedimentary nitrogen (N) in lakes significantly influenced by eutrophication plays a detrimental role on the ecological sustainability of aquatic ecosystems. Here, we conducted a thorough analysis of the importance of N potentially released from sediments during the shift of "grass-algae" ecosystem in plateau lakes. From 1964 to 2013, the average total amount of sedimentary potential mineralizable organic nitrogen (PMON) and exchangeable N in whole Lake Dianchi were 5.50 × 10³ t and 3.44 × 10³ t, respectively. NH₄⁺-N was the main product (>90%) of sedimentary PMON mineralization. The PMON in sediments had great release potential, which tended to regulate the distribution of aquatic plants and phytoplankton in Lake Dianchi and facilitated the replacement of dominant populations. Moreover, NH₄⁺-N produced by sedimentary PMON mineralization and exchangeable NH₄⁺-N have increased the difficulty and complexity of ecological restoration in Lake Dianchi to a certain extent. This study highlights the importance of sedimentary N in lake ecosystem degradation, showing the urgent need to reduce the continuous eutrophication of lakes and restore the water ecology.

Nitrogen Removal of Water and Sediment in Grass Carp Aquaculture Ponds by Mixed Nitrifying and Denitrifying Bacteria and Its Effects on Bacterial Community

Nitrification and denitrification are important for nitrogen (N) cycling in fish ponds culture, but the effects of nitrifying and denitrifying bacteria concentrations on pond water and sediments remain largely unknown. Here, we used 0, 0.15, 0.30, 0.60 mg/L different concentrations of mixed nitrifying and denitrifying bacteria to repair the pond substrate through an enclosure experiment lasting 15 days. The results showed that the purification effect of nitrifying and denitrifying bacteria was most obvious on pond nitrogen from day 4 to day 7. The optimal relative concentration was 0.60 mg/L for nitrifying and denitrifying bacteria; NH4+-N (ammonia nitrogen) decreased by 75.83%, NO2−-N (nitrite) by 93.09%, NO3−-N (nitrate) by 38.02%, and TN (total nitrogen) by 45.16% in this concentration group on pond water. In one cycle, C/N (carbon/nitrogen) ratio of both water body and bottom sediment significantly increased, but C/N ratio of water body increased more significantly than that of sediment. Water C/N ratio increased by 76.00%, and sediment C/N ratio increased by 51.96% in the 0.60 mg/L concentration group. Amplicon sequencing of pond sediment showed that the change in nitrifying and denitrifying bacterium diversity was consistent with that in water quality index. Dominant nitrifying bacteria had a relatively high percentage, with significant differences in dominant bacterium percentage across different bacterial addition groups, while dominant denitrifying bacterium percentage was not high without significant differences among different groups. The dominant species of nitrifying bacteria were, respectively, Nitrosomonas, Nitrosovibrio, Nitrosospira, and Aeromonas, and the dominant species of denitrifying bacteria were Thauera, Azoarcus, Magnetospirillum, Azospira, and Idiomarina. The correlation analyses showed an aerobic nitrification and facultative anaerobic denitrification in pond sediments. Research shows that the addition of exogenous nitrifying and denitrifying bacteria can effectively reduce the nitrogen load of pond water and sediment. At the concentration of 0.6 mg/L, the nitrogen load of pond water and sediment decreased most obviously, which had the best effect on pond purification.

Occurrence and driving forces of different nitrogen forms in the sediments of the grass and algae-type zones of Taihu Lake

Excessive nitrogen (N) load in sediments is at risk of release resulting in the degradation of grass-type lake ecosystems. At present, the occurrence characteristics of N forms and the driving forces of organic N (ON) hydrolysis in the sediments of Taihu Lake were still unclear. Here, 52 sampling sites in 7 lake areas in Taihu Lake were investigated to compare the spatial occurrence characteristics of the sedimentary free N (FN), exchangeable N (EN), acid hydrolyzable N (HN), and residual N (RN) and their associated driving forces. The results showed that the total N contents in the dry sediment ranged from 1811.56 to 5594.06 mg kg^-1, and the contribution was in the order of RN > HN > EN > FN. Spatially, RN and total organic carbon were significantly consistently influenced by dam construction and deposition algal residue. The HN concentration was high in the estuaries affected by N inputs from the rivers. The coupling relationship of spatial distribution between ON and N forms was revealed. The factors, i.e., algal residue deposition and terrigenous N inputs, were considered as the main driving forces stimulating the ON hydrolysis in the algae-type lake zones. It can be deduced that controlling terrigenous N inputs and sediment suspension may be the key to inhibiting the transformation from grass-type to algae-type lake ecosystem.

Evaluation of the Potential Release Risk of Internal N and P from Sediments—A Preliminary Study in Two Freshwater Reservoirs in South China

Growing evidence has demonstrated the influence of internal nitrogen (N) and phosphorus (P) on harmful algae blooms in eutrophic freshwater ecosystems. However, the main controlling factors for internal N and P release risks, and whether these factors vary as environmental conditions change, remains poorly understood. We evaluated potential release risks of N and P from sediments in two freshwater reservoirs in Beihai City, southern China, by evaluating apparent nutrient fluxes during simulated static incubation experiments at two temperatures (15 °C and 25 °C). Sediments were analyzed to determine their basic properties as well as N and P fractions. Results showed that the main controlling factors of the apparent fluxes in dissolved total P, soluble reactive P, total N, and ammonium were related to sediment adsorption properties, redox properties, and microbial-mediated properties (e.g., water-extractable P, total inorganic N, redox-sensitive P, total organic carbon, organic P). The primary controlling factors for apparent N and P fluxes were dependent on the form of N and P and changed with temperature. The results suggest that care should be taken when simply using total N and P contents in sediments to evaluate their internal nutrient release risks.

Driving mechanisms of gross primary productivity geographical patterns for Qinghai-Tibet Plateau lake systems

Being a fundamental property of aquatic systems, gross primary productivity (GPP) is affected by complex environmental factors, such as salinity, nutrients, pH, and sunlight. Under conditions of intensified anthropogenic activity and climate change, it is critical to understand the driving mechanisms of GPP in alpine lakes. In this study, we investigated GPP and associated environmental factors of 23 lake systems in the Qinghai-Tibet Plateau (QTP) along an altitudinal range (from 2500 m to 4500 m). Results showed an increase in chlorophyll a (Chl a) content as altitude increased and a corresponding decrease as salinity increased. Furthermore, geographical patterns of GPP were higher at the mid-gradient and lower at the extreme gradient. Higher solar radiation and water temperatures, stronger evaporation and higher salinity levels, and lower pH and higher nutrient content were all driving mechanisms of GPP in low altitudinal lake systems within high latitudinal regions. Such conditions have collectively resulted in the current GPP pattern via the promotion or inhibition of phytoplankton growth and photosynthesis. Specifically, geographical features and climate change jointly drive algal growth and GPP of alpine lake systems via internal circulation processes; however, anthropogenic activities interfere with external circulation processes for most of lower-middle altitudinal lake systems, thus playing a certain role in regulating environmental factors and GPP alongside climate change.

Seasonal and temporal factors leading to urea-nitrogen accumulation in surface waters of agricultural drainage ditches

Urea-nitrogen (N) is commonly applied to crop fields, yet it is not routinely monitored despite its association with reduced water quality and its ability to increase toxicity of certain phytoplankton species. The purpose of this work was to characterize temporal fluctuations in urea-N concentrations and associated environmental conditions to infer sources of urea-N in agricultural drainage ditches. Physicochemical properties and N forms in ditch waters were measured weekly during the growing seasons of 2015-2018. Fertilizer application was only associated with spring peaks of urea-N concentrations in ditches next to cornfields, whereas summer peaks in ditches adjacent to corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] fields were not associated with fertilizer applications. Environmental conditions of warmer temperatures, lower dissolved oxygen concentrations, and lower redox potentials were correlated with higher urea-N concentrations. In 2018, peaks of urea-N and ammonium-N during the summer co-occurred with peaks of dissolved organic N and total dissolved N, suggesting they might be associated with the breakdown of organic matter and with the turnover of the organic N pool. Although the highest urea-N concentrations occurred when ditch surface waters were hydrologically disconnected from nearby streams, heavy rainfalls can potentially flush accumulated urea-N into coastal waters, where it may affect algal bloom toxicity. Therefore, implementation of available drainage ditch management practices is recommended, but these strategies need to be optimized for targeting periods with high rainfall that coincide with fertilizer additions as well as for periods with low rainfall that promote stagnant water conditions.