An optical mechanism-based deep learning approach for deriving water trophic state of China's lakes from Landsat images.

Widespread eutrophication has been considered as the most serious environment problems in the world. Given the critical roles of lakes in human society and serious negative effects of water eutrophication on lake ecosystems, it is thus fundamentally important to monitor and assess water trophic status of lakes. However, a reliable model for accurately estimating the trophic state index (TSI) of lakes across a large-scale region is still lacking due to their high complexity. Here, we proposed an optical mechanism-based deep learning approach to remotely estimate TSI of lakes based on Landsat images. The approach consists of two steps: (1) determining the optical indicators of TSI and modeling the relationship between them, and (2) developing an approach for remotely deriving the determined optical indicator from Landsat images. With a large number of in situ datasets measured from lakes (2804 samples from 88 lakes) across China with various optical properties, we trained and validated three machine learning methods including deep neural network (DNN), k-nearest neighbors (KNN) and random forest (RF) to model TSI with the optical indicators and TSI and derive the determined optical indicator from Landsat images. The results showed that (1) the total absorption coefficients of optically active constituents at 440 nm (at-w(440)) performs best in characterizing TSI, and (2) DNN outperforms other models in the inversion of both TSI and at-w(440). Overall, our proposed optical mechanism-based deep learning approach demonstrated a robust and satisfactory performance in assessing TSI using Landsat images (root mean squared error (RMSE) = 5.95, mean absolute error (MAE) = 4.81). This highlights its merit as a nationally-adopted method in lake water TSI estimation, enabling the convenience of the acquisition of water eutrophic information in large scale, thereby assisting us in managing lake ecology. Therefore, we assessed water TSI of 961 lakes (>10 km2) across China using the proposed approach. The resulting at-w(440) and TSI ranged from 0.01 m-1 to 31.42 m-1 and from 6 to 96, respectively. Of all these studied lakes, 96 lakes (11.40 %) were oligotrophic, 338 lakes were mesotrophic (40.14 %), 360 lakes were eutrophic (42.76 %), and 48 were hypertrophic (5.70 %) in 2020.

Construction and assessment of an indicator system of stream conditions in a typical mountain landscape in Northeast China.

Stream ecosystem health assessments are crucial for the effective management of aquatic ecosystem service functions and the assessment of anthropogenic and climatic impacts on aquatic ecosystems. However, there is still a lack of comprehensive assessment methods based on complete ecosystem structures. This study improved an ecological health assessment system, which consists of 13 indicators based on ecosystem structure. The assessment system was applied to 30 streams on Changbai Mountain. The results showed that the comprehensive index of stream health (ISH) in Changbai Mountain was relatively high, and the ISH increased with the elevation gradient. The dominant factors controlling the ecosystem health level were water pH, water flow, vegetation coverage, total phosphorus (TP), and vegetation diversity index. All these results indicated that the damage caused by excessive deforestation and farmland occupation in recent years has not been fully reversed and that human disturbance from tourism may be the main reason for the low stream health at lower elevations. Our results suggest that the key to future work is to increase quantitative research on disturbance sensitivity and to develop economically viable restoration measures.

Six decades of field observations reveal how anthropogenic pressure changes the coverage and community of submerged aquatic vegetation in a eutrophic lake.

Six decades field observation data series on submerged aquatic vegetation (SAV), water level and water quality from Lake Taihu were compiled to reveal the dynamics in coverage and species composition of SAV and their anthropogenic drivers. We found that both SAV species composition and coverage area declined significantly in Lake Taihu during the period, and the increasing nutrient levels and water level as well as decreasing water clarity were responsible for these change trends. Specifically, the decrease in species richness could be particularly well predicted by total nitrogen (TN) and the ratio of water clarity (i.e., Secchi disk depth (SDD)) to water level (WL), contributing 47.3 % and 32.3 %, respectively, while the coverage of macrophytes was most strongly related to the water level, accounting for 70.1 % of the variation. A classification tree analysis revealed a threshold of TN of 3.2 mg/L and SDD/WL of 0.14 that caused a shift to a eutrophic low-macrophyte dominated state. Our results highlight that SDD/WL must be improved for SAV recolonization, rather than merely reducing nutrient input and regulating water level. Our findings provide scientific information for lake managers to prevent plant degradation in macrophyte-dominant lakes and facilitate a shift to a macrophyte-dominant state in eutrophic lakes.

Response of community composition and biomass of submerged macrophytes to variation in underwater light, wind and trophic status in a large eutrophic shallow lake.

Light climate is of key importance for the growth, community composition of submerged macrophytes in lakes and, they, in turn, are affected by lake depth and the degree of eutrophication. To test the relationships between submerged macrophyte presence and the ratio of Secchi disk depth (SDD) to water depth, i.e. SDD/depth, nutrients and wind, we conducted an extensive sampling campaign in a macrophyte-dominated area of the eastern region (n = 36) in 2016 in Lake Taihu, China, and combined the data gathered with results from extensive physico-chemical monitoring data from the entire lake. We confirmed that SDD/Depth is the primary factor controlling the community composition of macrophytes and showed that plant abundance increased with increasing SDD/Depth ratio (p < 0.01), but that only SDD/Depth > 0.4 ensured growth of submerged macrophytes. Total phosphorus and total nitrogen also influenced the growth and community composition of macrophytes (p < 0.01), while Chla was an indirectly affecting factor by reducing underwater light penetration. Wave height significantly influenced plant abundance (p < 0.01), whereas it had little effect on the biomass (p > 0.05). The key to restore the macrophyte beds in the lake is to reduce the nutrient loading. A decrease of the water level may contribute as well in the shallow bays but will not bring plants back in the main part of the lake. As the tolerance of shade and nutrients varied among the species studied, this should be taken into account in the restoration of lakes by addition of plants.

The response of epiphytic microbes to habitat and growth status of Potamogeton malaianus Miq. in Lake Taihu.

To investigate the effects of different habitats and plant growth status on abundance, biomass and community structure of epiphytic microbes, Potamogeton malaianus Miq. at two different habitats (Gonghu Bay and East Taihu) in Lake Taihu were collected in June, August and November (corresponding to the period of development of submerged macrophytes). The relative abundance of major epiphytic algae groups was determined with high performance liquid chromatography (HPLC), and the structures and dynamics of epiphytic bacteria were assessed by terminal restriction fragment length polymorphism (T-RFLP) analysis. Results showed that the biomass of epiphytic microbes was not significant difference between the two sites, and the analysis of similarity found no significant intra-lake heterogeneity in community structure, but the temporal heterogeneity of epiphytic microbes was significant, which linked to the growth state of submerged macrophytes and water temperature. The difference in community structure between June and August was larger than that between August and November at each site, indicating that the growth status of submerged macrophytes has a greater impact on the community structure of epiphytic microbes than the seasonal variation of environmental conditions.

[Effects of exogenous nitric oxide on drought-resistance of poplar].

This paper studied the effects of NO donor sodium nitroprusside (SNP) on the photosynthesis and active oxygen-scavenging enzyme activities of poplar leaves under drought stress. The results showed that SNP had a remarkable positive effect on leaf water content. When the SNP level was > 500 micromol.L(-1), no significant difference in leaf water content was found between SNP treatments. SNP could increase photosynthesis rate, photochemical efficiency of PSII (F(v)/F(m)), and ratios of F(m)/F(o) and F(v)/F(o), but the effects decreased with the prolongation of drought stress. The SOD and POD activities of poplar leaves were higher in short time (1 h) drought stress than in longer time (3 h) one. The activity of POD was increased by SNP, while that of SOD was changed little. With increasing SNP level, the POD and SOD activities first increased and then decreased. The NO donor could retard the accumulation of active oxygen through inducing POD and SOD activities, alleviate the effects of drought stress on photosynthesis, and increase the drought-resistance of poplar.

[Effects of exogenous nitric oxide on photosynthetic characteristics of poplar leaves under water stress].

Nitric oxide (NO) is an active molecule involved in many biological pathways, but its effects on photosynthesis of tree leaves have not been established yet. This paper studied the effects of exogenous NO, sodium nitroprusside (SNP), on the water status, photosynthesis and scavenging enzyme activities in poplar leaves. Different levels of SNP treatments had remarkable effects on the water content of leaves, which increased with increasing SNP levels. When the SNP level exceeded 500 micromol x L(-1), differences in leaf water content were no longer significant between different SNP treatments. Exogenous NO increased the photosynthesis rate, photochemical efficiency of PSII Fv/Fm, and Fm/Fo and Fv/Fo ratios, and the effects decreased with increasing duration of water stress. The SOD and POD activities in poplar leaves were higher in 1 hour water stress treatment than in 3 h treatment. Treating with SNP could markedly increase POD activity, but SOD activity did not change much. POD and SOD activities increased initially, and then decreased with increasing SNP levels. The results indicated that exogenous NO delayed the accumulation of active oxygen by increasing POD and SOD activities, and thereby, alleviated the effects of water stress on photosynthetic organization of poplar leaves.

[Effects of soil water regimes on the growth of Quercus mongolica seedlings in Changbai Mountains].

This paper studied the response of the seedlings of Quercus mongolica, one of the dominant tree species in Changbai Mountains, to the artificially controlled three soil water gradients, including their morphology, biomass and photosynthetic characteristics. The results indicated that various water regimes significantly affected the biomass and its distribution pattern in the leaves, branches and roots, as well as the leaf gas exchange. Under soil water stress, the crown structure changed, and the tree height, groundline diameter, single leaf area, and aboveground and belowground biomass were inhibited. As soil water content decreased, the ratio of belowground and aboveground biomass dry weight significantly increased. Water stress had a negative effect on net photosynthetic rate, CO2 use efficiency and carbon use efficiency. The responses of stomatal conductance, transpiration rate and water use efficiency to water stress were complicated. Only at low soil water content, the stomatal conductance and transpiration rate significantly decreased, while water use efficiency increased. It was demonstrated that Quercus mongolica was the tree species with variable resistance to drought, and the resistance could be improved by long-term soil water stress.

[Nitric oxide, a signaling component of ABA-induced stomatal closure].

The present study provided the evidence that exogenous NO and ABA regulated the stomatal movement in poplar leaves. It showed that different levels of SNP and ABA induced the stomatal closure in detached leaves. NO enhanced stomatal closure that induced by ABA. The effect of NO and ABA on stomatal closure was suppressed by c-PTIO, a specific NO scavenger. It was confirmed that NO involved in ABA-mediated stomatal movement. The activities of POD were dramatically inhibited, but those of SOD hardly decreased in different levels of SNP and ABA treatment. However, in vitro experiments indicated that the activities of POD were dramatically inhibited by NO, but not ABA. The results demonstrated that NO mediated ABA-induced stomatal closure of trees, and was a signaling component of ABA-induced stomatal closure.

[Insect species diversity in Korean pine broad-leaved mixed forest in Changbai Mountains].

The study on the insect species diversity in Changbai Mountains showed that there were 1960 insect species belonging to 26 orders, 131 families and 1162 genera, among which, 881 species were pests belonging to 11 orders and 105 families, and 638 were serious ones. In forest insect communities, defoliator insect community was more divers than natural enemy community, and predatory insects were more divers than other natural enemies. The indices of annual species evenness of defoliator insect community, parasitic insect community and predatory insect community were 0.884, 0.830, and 0.806, respectively, and the diversity changed with season was in the order of predatory insects > parasitic insects > defoliator insects.