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The dissolved organic matter (DOM) composition in river ecosystems could reflect the human impacts on the river ecosystem, and plays an important role in the carbon cycling process. We collected water and phytoplankton samples at 107 sites in the Dongjiang River in two seasons to assess the impact of the sub-catchments land use structure on the DOM composition. The results showed that (1) the forested subcatchments had higher humic-like C1 (16.45%) and C2 (25.04%) and lower proteinlike C3 (22.57%) and C4 (35.95%) than urbanized and mixed forest-agriculture subcatchments, while the urbanized sub-catchments showed an inverse trend (4.54%, 15.51%, 33.97% and 45.98%, respectively). (2) The significant variation in the proportion of C1 and C4 between the dry and rainy seasons was recorded in both the forested and the mixed forest-agriculture sub-catchments (p<0.01), but only C4 showed an obvious seasonal variation in the urbanized sub-catchments (p<0.01). While the DOM composition was mainly related to the proportion of urbanized land and forested land year-round (p<0.01), it had stronger correlation with forested land in the dry season and urbanized land in the rainy season.(3) No significant correlation between the DOM composition and the agricultural land proportion was found in either season (p>0.05).Our findings indicated that the DOM composition was strongly dependent on the land use structure of the sub-catchments and varied seasonally, but the seasonal variation pattern could be disturbed by human activities in the extensively urbanized catchments.Notably, the higher C4 proportion compared with those in temperate rivers implied a larger amount of CO2 released from the subtropical rivers into the atmosphere when considering bioavailability.

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Abstract:The phytoplankton community in the river is closely related to the location of the river and the impact of human activities. To summarize the patterns of phytoplankton community changes in rivers and to analyze the reasons for these patterns and differences, we sampled the three tributaries of the Dongjiang River at different latitudes in the dry and rainy season for three years. The results showed that the three rivers were mesotrophic, lightly eutrophic and moderately eutrophic respectively. From the south to the north, the water temperature and nutrition showed an increasing trend. In two different seasons, the differences in the water temperature and dissolved oxygen were clear. In the dry season, results of the multidimensional scaling (MDS) analysis indicated that the phytoplankton community structures in the Li River and Qiuxiang River were similar. Regardless of the number of species, the cell abundance or the dominance index, Bacillariophyta were found to be dominant. Chlorophyta was dominant in the Danshui River. In the rainy season, Bacillariophyta, Bacillariophyta-Chlorophyta and Chlorophyta-Cyanophyta became the dominant types in the Li River, Qiuxiang River and Danshui River, respectively. These different patterns in phytoplankton community variation were affected by both the water quality and temperature.

High-speed rail (HSR) is an important form of transportation that affects the economic development of the regional spatial structure. However, there is less discussion about the impact of economically underdeveloped regions and the rapid construction of HSR on the region. This study uses a spatial econometric model to explore whether a rapidly formed high-speed rail network with changes in the network structure can bring economic effects based on the spatio-temporal panel data on high-speed rail construction and economic development in western China from 2015 to 2020. First, data of the daily departures between high-speed rail cities were used to analyze the western high-speed rail network’s spatial and temporal evolution characteristics. Second, we analyzed the changes in the centrality, external and internal connectivity, and transfer potential of the economic gap of the western HSR network. Finally, we analyzed the different economic effects of the HSR network structure by combining the Cobb–Douglas production function with the spatial econometric model. The conclusions are as follows: (1) The HSR network in western China is dense at the intra-provincial HSR network; then it expands along the cross-provincial region; and is gradually embedded in the national HSR network, forming a figure-8-shaped spatial structure. (2) In the rapid expansion and densification of the HSR network in western China, connectivity takes precedence, and dominance and control are then increased. The external connectivity of the western HSR city network develops first and shows fluctuating growth, while the internal connectivity improves relatively slowly. (3) The connectivity, convenience of transit, transshipment capacity, and internal and external connection structure of the HSR network all contribute to the economic development of western cities. The transfer potential of economic gaps is detrimental to their economic development but has a positive effect on adjacent cities.