Yongdong Zhang

Sedimentary lipid biomarker record of human-induced environmental change during the past century in Lake Changdang, Lake Taihu basin, Eastern China

During the past hundred years, the Lake Taihu basin has been greatly impacted by human interventions. The undesirable changes in water quality of lakes, presumably caused by the human activities, remain relatively undescribed in this area. In order to investigate these anthropogenic effects, a 210Pb dated sediment core from a relatively small lake in the upper reaches of Lake Taihu known as Lake Changdang was subject to a detailed lipid biomarker study and other geochemical analyses including quantification of biogenic silica (BSi), nutrients and heavy metals. Based on the results, the recent environmental history of Lake Changdang can be divided into three periods. The first period from approximately 1906 to 1950, represents a natural state, with minimal anthropogenic impact on the lake. Human induced environmental change is recorded in the following stage, ca. 1950-1982, during which the trophic status of the lake increased slightly in response to inputs of agricultural waste and nitrogen (N) and phosphorus (P) based fertilizers. In particular, the signs of eutrophication during this phase accelerated from ca. 1973, perhaps owing to large-scale using phosphate based chemical fertilizer around the lake at the time. A second phase of nutrient input in the most recent stage, ca. from 1982 to 2016, initiated by wastewater discharge from rapid urbanization and industrialization of the catchment, greatly enhanced the nutrient level in the lake. However, the central zone of the lake has yet to reach a phytoplankton-dominated stable state, with both algae and aquatic macrophytes tracking an increased trend in productivity driven by intermediate nutrient levels in the water of this zone.

A sediment record of environmental change in and around Lake Lugu, SW China, during the past two centuries

AbstractnSediment lipid biomarkers and major elements record three periods of environmental conditions in and around Lake Lugu during the past ~190xa0years. The first period, ca. 1820–1946, represents a time when there was minimal anthropogenic impact in the watershed. The period from 1946 to 1987 was characterized by major land-use change in the catchment, including removal of the native forest. In response, allogenic clastic mineral inputs to the lake exhibited a minor increase, whereas terrestrial plant inputs decreased to their lowest level. Aquatic algal productivity (diatoms and dinoflagellates) and the proportion of diatoms to dinoflagellates displayed a decreasing trend, indicating progressive nutrient depletion. The last stage, from 1987 to 2012, was caused mainly by development of tourism near Lake Lugu. Rock and soil erosion around the lake reached maximum values during this period, in part a consequence of earlier logging, which resulted in a rapid increase of clastic mineral input to the lake. Terrestrial plant input to the lake showed a minor increase in response to ecological restoration and reforestation activities. The lake is now at risk of becoming more nutrient-rich as a consequence of increasing sewage discharge from tourism, and the highest algal productivities and diatom proportions were recorded in this period. Nevertheless, Lake Lugu remains relatively unproductive.n

Fish-mediated plankton responses to increased temperature in subtropical aquatic mesocosm ecosystems: Implications for lake management

Although it is well established that climate warming can reinforce eutrophication in shallow lakes by altering top-down and bottom-up processes in the food web and biogeochemical cycling, recent studies in temperate zones have also shown that adverse effects of rising temperature are diminished in fishless systems. Whereas the removal of zooplanktivorous fish may be useful in attempts to mitigate eutrophication in temperate shallow lakes, it is uncertain whether similar mitigation might be achieved in warmer climates. We compared the responses of zooplankton and phytoplankton communities to climate warming in the presence and absence of fish (Aristichthys nobilis) in a 4-month mesocosm experiment at subtropical temperatures. We hypothesized that 1) fish and phytoplankton would benefit from warming, while zooplankton would suffer in fish-present mesocosms and 2) warming would favor zooplankton growth but reduce phytoplankton biomass in fish-absent mesocosms. Our results showed significant interacting effects of warming and fish presence on both phytoplankton and zooplankton. In mesocosms with fish, biomasses of fish and phytoplankton increased in heated treatments, while biomasses of Daphnia and total zooplankton declined. Warming reduced the proportion of large Daphnia in total zooplankton biomass, and reduced the zooplankton to phytoplankton biomass ratio, but increased the ratio of chlorophyll a to total phosphorus, indicating a relaxation of zooplankton grazing pressure on phytoplankton. Meanwhile, warming resulted in a 3-fold increase in TP concentrations in the mesocosms with fish present. The results suggest that climate warming has the potential to boost eutrophication in shallow lakes via both top-down (loss of herbivores) and bottom-up (elevated nutrient) effects. However, in the mesocosms without fish, there was no decline in large Daphnia or in total zooplankton biomass, supporting the conclusion that fish predation is the major driver of low large Daphnia abundance in warm lakes. In the fishless mesocosms, phytoplankton biomass and nutrient levels were not affected by temperature. Our study suggests that removing fish to mitigate warming effects on eutrophication may be potentially beneficial in subtropical lakes, though the rapid recruitment of fish in such lakes may present a challenge to success in the long-term.

Comparison of bacterial growth in response to photodegraded terrestrial chromophoric dissolved organic matter in two lakes

Terrestrial chromophoric dissolved organic matter (CDOM) could subsidize lake food webs. Trophic state and altitude have a pronounced influence on the CDOM concentration and composition of a lake. The impact of future changes in solar radiation on high-altitude lakes is particularly alarming because these aquatic ecosystems experience the most pronounced radiation variation worldwide. Photodegradation experiments were conducted on terrestrial CDOM samples from oligotrophic alpine Lake Tiancai and low-altitude eutrophic Lake Xiaohu to investigate the response of bacterial growth to photodegraded CDOM. During the photo-irradiation process, the fluorescent CDOM intensity evidently decreased in an inflowing stream of Lake Tiancai, with the predominance of humic-like fluorescence. By contrast, minimal changes were observed in the riverine CDOM of Lake Xiaohu, with the predominance of protein-like fluorescence. The kinetic constants of photodegradation indicated that the degradation rate of terrestrial (soil) humic acid in Lake Tiancai was significantly higher than that in Lake Xiaohu (p<0.001). Soil humic and fulvic acids irradiated in the simulated experiment were applied to incubated bacteria. The specific growth rate of bacteria incubated with soil humic substances was significantly higher in Lake Tiancai than in Lake Xiaohu (p<0.05). Furthermore, the utilizing rate of dissolved oxygen (DO) confirmed that the DO consumption by bacteria incubated with terrestrial CDOM in Lake Tiancai was significantly greater than that in Lake Xiaohu (p<0.05). In summary, the exposure of terrestrial CDOM to light significantly enhances its availability to heterotrophic bacteria in Lake Tiancai, an oligotrophic alpine lake, which is of importance in understanding bacterial growth in response to photodegraded terrestrial CDOM for different types of lakes.

Geochemical records of anoxic water mass expansion in an oligotrophic alpine lake (Yunnan Province, SW China) in response to climate warming since the 1980s

In order to elucidate the effect of recent warming, we studied lipid biomarkers and trace elements in a dated sediment core from Lake Heihai, a small, deep, and ultraoligotrophic alpine lake in Yunnan Province (SW China), being only marginally affected by anthropogenic activities. The variation in lipid biomarkers (such as 10-methyl-C16:0 fatty acid (FA), iso-branched C15 (i-C15) and anteiso-branched C15 (ai-C15) FA, and tetrahymanol) suggests a rapid productivity increase in sulfate reducing bacteria and ciliates since 1980, likely reflecting expansion of the hypolimnion anoxia and a prolonged duration of an oxic–anoxic chemocline in the water column. The concentrations of element molybdenum (Mo) in pre-1980 sediments approach the values in average crust. After 1980, the concentration increased, reaching levels approximately sixfold higher than the initial abundances. This likely reflects a high authigenic Mo deposition when the bottom water was more anoxic and enrichment in H2S. The suggested spatial and temporal expansion of the anoxic bottom water since 1980 was probably a response to the regional climate warming, resulting in stronger water column stratification and terrestrial grass inputs to the lake, and thus higher dissolved oxygen (DO) loss in hypolimnion.

Lipid Evidence for Oil Depletion by Sulfate-Reducing Bacteria during U Mineralization in the Dongsheng Deposit

Fatty acids were extracted from fluid inclusions, and analyzed for distribution and individual δ13C values to determine if there exists sulfate-reducing bacteria (SRB) and what substrates SRB depleted during U mineralization in the Dongsheng deposit. The 10-methylhexadecanoic acid (10Me16:0) has been detected from most of the samples using GC-MS based on relative retention time and co-injection of standard 10Me16:0 compound. Samples with higher 10Me16:0 concentrations, show higher U contents. The presence of 10Me16:0 along with iso-ω7-cis-heptadecenoic acid (i17:1ω7c) and ω7-cis-octadecenoic acid (18:1ω7c) in the fluid inclusions may indicate the occurrence of SRB during the U mineralization. Saturated fatty-acids and unsaturated fatty-acids detected in this study have individual δ13C values from -30.3‰ to -28.5‰ and -30.5‰ to -27.9‰, respectively. These values are close to those of n-alkanes of the associated oils and the bulk oils, but significantly distinct from methane, thus the oils are concluded to have been used as substrate for the microorganisms to survive on. This proposal is supported by oil biodegradation and ore-stage calcite cement with δ13C values from -1.4‰ to -17.2‰.

Sediment lipid biomarkers record phytoplankton dynamics of Lake Heihai (Yunnan Province, SW China) driven by climate warming since the 1980s

Increased phosphorus (P) export from sediments to the overlying water column is a significant factor driving the variation of phytoplankton in productivity and community structure in lakes. However, the lack of long-term instrumental data often impeded analyses attempting to associate dynamics of phytoplankton with variation of internal P loading. Here, elements and lipid biomarkers were analyzed in a sediment core from Lake Heihai, a small, deep, and ultraoligotrophic alpine lake in Haba Mountain, Yunnan Province, SW China. The data document incredible enrichment of element iron (Fe) in the sediment, whose concentrations are much higher than those of other common major elements including titanium (Ti), aluminum (Al), calcium (Ca), and magnesium (Mg). This finding, together with the abundance correlation between P and Fe (nxa0=xa030, R2xa0=xa00.783) suggested that P was probably retained in sediments through sorption with micro-layer of FeOOH at the sediment-water interface. The P/Ti ratios, P/Fe ratios, and P/total organic carbon (TOC) ratios all declined in the sediment since 1980, perhaps indicating increased P release from sediments to the overlying water column initiated by hypolimnion anoxia and sulfidic, which is presumably triggered by regional climate warming since the 1980s. The P-rich bottom water can be injected into photic zone during wind-driven mixing and overturn of the water column, although its frequency and intensity might decline due to stronger water column stratification in warming climate. In response, diatoms exhibited a rapid increase of productivity at this time, because diatoms have a storage vacuole and thereby nutrients such as P can be concentrated and used for cell division long after they are depleted in the bulk fluid. Elevated diatom biomass produced shading of light penetration, allowing for a low productivity for dinoflagellates. This study deepens our understanding of the impact of climate warming on lake systems and highlights the element biogeochemical cycle contributing to the variation of nutrients in the lake water column.