Catchment Vegetation Research Articles

Biomarkers in sedimentary sequences: Indicators to track sediment sources over decadal timescales

Long-term sedimentary sequence research can reveal how human activities and climate interact to affect catchment vegetation, flooding, soil erosion, and sediment sources. In this study, a biomarker sediment fingerprinting technique based on n-alkanes was used to identify long timescale (decadal) sediment sources in a small agricultural catchment. However, the highly saline carbonate environment and bacterial and algal activities elevated the levels of even-chain n-alkanes in the sediments, leading to an obvious even-over-odd predominance of short and middle components (C15-C26). Therefore, by analyzing three odd, long-chain n-alkanes (C27, C29 and C31) in 27 source samples from cropland, gully, and steep slope areas and one sediment sequence (one cultivated horizon and 47 flood couplets), a composite fingerprinting method and genetic algorithm optimization were applied to find the optimal source contributions to sediments. The biomarker fingerprinting results demonstrated that the primary sediment source is gullies, followed by cropland and steep slope areas. The average median source contributions associated with 47 flood couples collected from sediment core samples ranged from 0±0.1% to 91.9±0.4% with an average of 45.0% for gullies, 0±0.4% to 95.6±1.6% with an average of 38.2% for cropland, and 0±2.1% to 60.7±0.4% with an average of 16.8% for steep slopes. However, because farmers were highly motivated to manage the cropland after the 1980s, over half the sediments were derived from cropland in the 1980s. Biomarkers have significant advantages in the identification of sediments derived from different landscape units (e.g., gully and steep slope areas), and n-alkanes have considerable potential in high-resolution research of environmental change based on soil erosion in the hilly Loess Plateau region.

Diatom assemblages reveal regional-scale differences in lake responses to recent climate change at the boreal-tundra ecotone, Manitoba, Canada

The direction of pan-Arctic ecosystem shifts in response to climate warming is relatively well understood; however, landscape-level linkages among terrestrial, wetland, and lake ecosystems significantly influence the dynamics of each, making it difficult to generalize about lake responses to warming across the Arctic, and at times difficult to interpret paleoclimate records from lake sediments. To investigate differences in lake responses to recent climate change at the boreal-tundra ecotone, we conducted a 2-year survey of diatom assemblages from lakes with varying catchment characteristics in northern Manitoba, Canada. We investigated whether catchment geomorphology and landscape, including slope and vegetative cover, result in characteristic water chemistries and hence diatom assemblage signatures, which could then be used in paleolimnological studies to infer past changes in the catchment. Forty-four lakes were sampled for water chemistry and catchment vegetation was characterized using Landsat Imagery. Lake catchments were generally small (median 702 ha) and dominated by peat (Sphagnum) with or without lowland forest (Picea-Larix), or open tundra, with different amounts of exposed rock/till, upland forest/woodlands, and burn recovery area. Lakes were generally nutrient-poor, with lower nutrient and DOC concentrations in tundra-dominated catchments, and higher nutrients and DOC in catchments with greater forest cover. A diatom-based transfer function for pH (R2 = 0.72, \( {\text{R}}_{\text{boot}}^{2} \) = 0.54) was developed and compared with diatom assemblage turnover and sediment geochemistry in cores from eight lakes to reconstruct limnologic conditions over the past ~200 years. Most cores showed similar increases in biogenic silica and carbon burial, beginning around AD 1880 in the tundra lakes and about 1920 in the more forested catchments, likely in response to regional warming. In contrast to lakes in other Arctic regions, our lakes showed only minor pH changes in recent decades. The shift, however, was more pronounced in higher-latitude lakes with less forest cover, suggesting small-scale watershed influence on lake response to climate, even on short time scales. Diatom assemblages did not follow previously published models of climate-linked community change seen in circum-Arctic and sub-Arctic lakes. Translating local changes detected in the paleolimnological record to the regional level requires an understanding of how different catchment properties mediate the response of lakes, and their diatom assemblages, to climate change.

From greening to browning: Catchment vegetation development and reduced S-deposition promote organic carbon load on decadal time scales in Nordic lakes.

Increased concentrations of dissolved organic carbon (DOC), often labelled “browning”, is a current trend in northern, particularly boreal, freshwaters. The browning has been attributed to the recent reduction in sulphate (S) deposition during the last 2 to 3 decades. Over the last century, climate and land use change have also caused an increasing trend in vegetation cover (“greening”), and this terrestrially fixed carbon represents another potential source for export of organic carbon to lakes and rivers. The impact of this greening on the observed browning of lakes and rivers on decadal time scales remains poorly investigated, however. Here, we explore time-series both on water chemistry and catchment vegetation cover (using NDVI as proxy) from 70 Norwegian lakes and catchments over a 30-year period. We show that the increase in terrestrial vegetation as well as temperature and runoff significantly adds to the reduced SO4-deposition as a driver of freshwater DOC concentration. Over extended periods (centuries), climate mediated changes in vegetation cover may cause major browning of northern surface waters, with severe impact on ecosystem productivity and functioning.

Terrestrial carbohydrates support freshwater zooplankton during phytoplankton deficiency.

Freshwater food webs can be partly supported by terrestrial primary production, often deriving from plant litter of surrounding catchment vegetation. Although consisting mainly of poorly bioavailable lignin, with low protein and lipid content, the carbohydrates from fallen tree leaves and shoreline vegetation may be utilized by aquatic consumers. Here we show that during phytoplankton deficiency, zooplankton (Daphnia magna) can benefit from terrestrial particulate organic matter by using terrestrial-origin carbohydrates for energy and sparing essential fatty acids and amino acids for somatic growth and reproduction. Assimilated terrestrial-origin fatty acids from shoreline reed particles exceeded available diet, indicating that Daphnia may convert a part of their dietary carbohydrates to saturated fatty acids. This conversion was not observed with birch leaf diets, which had lower carbohydrate content. Subsequent analysis of 21 boreal and subarctic lakes showed that diet of herbivorous zooplankton is mainly based on high-quality phytoplankton rich in essential polyunsaturated fatty acids. The proportion of low-quality diets (bacteria and terrestrial particulate organic matter) was <28% of the assimilated carbon. Taken collectively, the incorporation of terrestrial carbon into zooplankton was not directly related to the concentration of terrestrial organic matter in experiments or lakes, but rather to the low availability of phytoplankton.

Varve microfacies and varve preservation record of climate change and human impact for the last 6000 years at Lake Tiefer See (NE Germany)

The Holocene sediment record of Lake Tiefer See exhibits striking alternations between well-varved and non-varved intervals. Here, we present a high-resolution multi-proxy record for the past ~6000 years and discuss possible causes for the observed sediment variability. This approach comprises microfacies, geochemical and microfossil analyses and a multiple dating concept including varve counting, tephrochronology and radiocarbon dating. Four periods of predominantly well-varved sediment were identified at 6000–3950, 3100–2850 and 2100–750 cal. a BP and AD 1924–present. Except of sub-recent varve formation, these periods are considered to reflect reduced lake circulation and consequently, stronger anoxic bottom water conditions. In contrast, intercalated intervals of poor varve preservation or even extensively mixed non-varved sediments indicate strengthened lake circulation. Sub-recent varve formation since AD 1924 is, in addition to natural forcing, influenced by enhanced lake productivity due to modern anthropogenic eutrophication. The general increase in periods of intensified lake circulation in Lake Tiefer See since ~4000 cal. a BP presumably is caused by gradual changes in the northern hemisphere orbital forcing, leading to cooler and windier conditions in Central Europe. Superimposed decadal- to centennial-scale variability of the lake circulation regime is likely the result of additional human-induced changes of the catchment vegetation. The coincidence of major non-varved periods at Lake Tiefer See and intervals of bioturbated sediments in the Baltic Sea implies a broader regional significance of our findings.

Does stream flow structure woody riparian vegetation in subtropical catchments?

The primary objective of this study was to test the relevance of hydrological classification and class differences to the characteristics of woody riparian vegetation in a subtropical landscape in Queensland, Australia. We followed classification procedures of the environmental flow framework ELOHA – Ecological Limits of Hydrologic Alteration. Riparian surveys at 44 sites distributed across five flow classes recorded 191 woody riparian species and 15, 500 individuals. There were differences among flow classes for riparian species richness, total abundance, and abundance of regenerating native trees and shrubs. There were also significant class differences in the occurrence of three common tree species, and 21 indicator species (mostly native taxa) further distinguished the vegetation characteristics of each flow class. We investigated the influence of key drivers of riparian vegetation structure (climate, depth to water table, stream‐specific power, substrate type, degree of hydrologic alteration, and land use) on riparian vegetation. Patterns were explained largely by climate, particularly annual rainfall and temperature. Strong covarying drivers (hydrology and climate) prevented us from isolating the independent influences of these drivers on riparian assemblage structure. The prevalence of species considered typically rheophytic in some flow classes implies a more substantial role for flow in these classes but needs further testing. No relationships were found between land use and riparian vegetation composition and structure. This study demonstrates the relevance of flow classification to the structure of riparian vegetation in a subtropical landscape, and the influence of covarying drivers on riparian patterns. Management of environmental flows to influence riparian vegetation assemblages would likely have most potential in sites dominated by rheophytic species where hydrological influences override other controls. In contrast, where vegetation assemblages are dominated by a diverse array of typical rainforest species, and other factors including broad‐scale climatic gradients and topographic variables have greater influence than hydrology, riparian vegetation is likely to be less responsive to environmental flow management.

Interpreting vegetation change in tropical arid ecosystems from sediment molecular fossils and their stable isotope compositions: A baseline study from the Pilbara region of northwest Australia

Detection of source diagnostic molecular fossils (biomarkers) within sediments can provide valuable insights into the vegetation and climates of past environments. However, hot and arid regions offer particularly challenging interpretive frameworks for reconstructions because baseline data are scarce, organic matter is generally very low and in the inland tropics in particular, sediments are also often subject to flooding and drought. Here, we investigated whether biomarkers and compound-specific δ13C values could be extracted from a late Holocene sediment record from the Fortescue Marsh (Pilbara, northwest Australia) to allow interpretation of past catchment vegetation and hydroclimate. The low total carbon (TC) content (<1.4%) was a major challenge for the molecular analyses over the ~2000years old sequence. Nevertheless, they revealed that the dominant hydrocarbon features (e.g., long chain n-alkanes) indicative of terrestrial plants (e.g., C4 grasses; riparian and other C3 plants) encompassed the last ~1300yrs and that low abundance of products from aquatic sources (e.g., n-C17) were detected in the uppermost sediments only when permanently inundated conditions prevailed (recent decades). Similarly, the lower δ13C values (i.e., a difference of −2.3‰) of long chain n-alkanes in upper sediments reflected a vegetation response to the emergence of wetter conditions through the late Holocene in the region. Based on the diverging dominant source contributions obtained from the molecular distributions and arid based Bayesian mixing model (δ13C of n-C27–33 alkanes) results, less arid conditions may have favoured the input of 13C depleted n-alkanes from the Eucalyptus (C3) dominant riparian vegetation. The deepest sediments (<700CE) however, had a TC content of <0.4%, and no organic compounds were detected, consistent with local and regional records of hyperarid conditions. These results demonstrate that n-alkanes can provide a molecular and stable isotopic fingerprint of important - and perhaps underappreciated - ecological processes in modern tropical arid environments for future paleoclimate investigations.

High elevation watersheds in the southern Appalachians: Indicators of sensitivity to acidic deposition and the potential for restoration through liming

Southern Appalachian high elevation watersheds have deep rocky soils with high organic matter content, different vegetation communities, and receive greater inputs of acidic deposition compared to low elevation sites within the region. Since the implementation of the Clean Air Act Amendment in the 1990s, concentrations of acidic anions in rainfall have declined. However, some high elevation streams continue to show signs of chronic to episodic acidity, where acid neutralizing capacity (ANC) ranges from 0 to 20μeqL−1. We studied three 3rd order watersheds (North River in Cherokee National Forest, Santeetlah Creek in Nantahala National Forest, and North Fork of the French Broad in Pisgah National Forest) and selected four to six 1st order catchments within each watershed to represent a gradient in elevation (849–1526m) and a range in acidic stream ANC values (11–50μeqL−1). Our objectives were to (1) identify biotic, physical and chemical catchment parameters that could be used as indices of stream ANC, pH and Ca:Al molar ratios and (2) estimate the lime required to restore catchments from the effects of excess acidity and increase base cation availability. We quantified each catchment’s biotic, physical, and chemical characteristics and collected stream, O-horizon, and mineral soil samples for chemical analysis seasonally for one year. Using repeated measures analysis, we examined variability in stream chemistry and catchment characteristics; we used a nested split-plot design to identify catchment characteristics that were correlated with stream chemistry. Watersheds differed significantly and the catchments sampled provided a wide range of stream chemical, biotic, physical and chemical characteristics. Variability in stream ANC, pH, and Ca:Al molar ratio were significantly correlated with catchment vegetation characteristics (basal area, tree height, and tree diameter) as well as O-horizon nitrogen and aluminum concentrations. Total soil carbon and calcium (an indicator of parent material), were significant covariates for stream ANC, pH and Ca:Al molar ratios. Lime requirement estimates did not differ among watersheds but this data will help select catchments for future restoration and lime application studies. Not surprisingly, this work found many vegetation and chemical characteristics that were useful indicators of stream acidity. However, some expected relationships such as concentrations of mineral soil extractable Ca and SO4 were not significant. This suggests that an extensive test of these indicators across the southern Appalachians will be required to identify high elevation forested catchments that would benefit from restoration activities.

Main drivers affecting the Holocene sedimentary record – implications from small lake in Latvia

A multi-indicator paleolimnological study of sediments from Lake Ķūži (central Latvia) was used to obtain a comprehensive record of environmental changes in the Holocene. Periodicity in the changes and the main drivers (lake basin development, catchment properties, climate, human activities) of the sedimentary record was studied. In order to comprehend the whole-lake sedimentation during the Holocene, a detailed multiindicator record from the central part of the lake was integrated with records of sediment mass accumulation rates from four cores from different parts of the lake and a GPR (ground-penetrating radar) survey of the lake basin. The observed changes in the sedimentation regime that took place during the early Holocene are strongly linked with variations in climatic conditions, but the catchment and lake basin also played an important role. In the middle Holocene the water level was stable and the lake was influenced by climate mediated through changes in the catchment vegetation. Around 5 000 cal. BP sedimentation pattern changed, and three of the four main drivers made a comparably strong impact on the sedimentary signal. Since 2 000 cal. BP multiple indicators point to a major disturbance clearly related to human activity, which conceals the impact of the natural drivers.

Direct versus indirect climate controls on Holocene diatom assemblages in a sub-tropical deep, alpine lake (Lugu Hu, Yunnan, SW China)

The reconstruction of Holocene environmental changes in lakes on the plateau region of southwest China provides an understanding of how these ecosystems may respond to climate change. Fossil diatom assemblages were investigated from an 11,000-year lake sediment core from a deep, alpine lake (Lugu Hu) in southwest China, an area strongly influenced by the southwest (or the Indian) summer monsoon. Changes in diatom assemblage composition, notably the abundance of the two dominant planktonic species, Cyclotella rhomboideo-elliptica and Cyclostephanos dubius, reflect the effects of climate variability on nutrient dynamics, mediated via thermal stratification (internal nutrient cycling) and catchment-vegetation processes. Statistical analyses of the climate–diatom interactions highlight the strong effect of changing orbitally-induced solar radiation during the Holocene, presumably via its effect on the lake's thermal budget. In a partial redundancy analysis, climate (solar insolation) and proxies reflecting catchment process (pollen percentages, C/N ratio) were the most important drivers of diatom ecological change, showing the strong effects of climate–catchment–vegetation interactions on lake functioning. This diatom record reflects long-term ontogeny of the lake-catchment ecosystem and suggests that climatic changes (both temperature and precipitation) impact lake ecology indirectly through shifts in thermal stratification and catchment nutrient exports.

Driving Factors for Torrential Mass-Movements Occurrence in the Western Alps

To understand the behaviour of torrential processes in the alpine environment, the conditions mainly responsiblefor the occurrence of these phenomena have to be identified and distinguished(classified) aspredisposing and triggering factors. In this regard, this study is aimed to understanding which factors lead to the occurrence of a given torrential processes in alpine catchments in the Western Alps, where information on past events are exhaustive and characterized by a long historical series. More than 769 documented torrential eventsoccurred from 1728 to 2015 within 78 catchments. Datasets concerning climate, geology and morphology, land use and the presence of historical landslide activity have been elaborated as input for multivariate statistical analysis to characterize the behaviour of the catchments. The results pinpoint the factors that mainly drive the type of torrential dominant process occurring in a given catchment, its occurrence probability, and its frequency. This study has demonstrated that catchments characterized by a significant percentage of outcropping rocks show a greater occurrence of torrential processes, especially hyperconcentrated flows and debris flows; on the contrary highly vegetated catchments are typically subject to water flows. This result can be a useful tool for the evaluation of hazards related to this specific phenomenon, making it possible to predict the most likely torrential processes that can be generated in a specific basin, given the characteristics of outcropping rock and vegetation cover.

Phosphorus and nitrogen loading restraints are essential for successful eutrophication control of Lake Rotorua, New Zealand

Anthropogenic activity has greatly enhanced the inputs of nitrogen (N) and phosphorus (P) to lakes, causing widespread eutrophication. Algal or cyanobacterial blooms are among the most severe consequences of eutrophication, impacting aquatic food webs and humans that rely on lakes for ecosystem services. In New Zealand, recent debate on the relative importance of N versus P control for limiting occurrences of algal blooms has centered on the iconic Lake Rotorua (North Island). Water quality in Lake Rotorua has declined since the late 1800s following catchment vegetation clearing and subsequent land-use intensification, as well as from sewage inputs. A multimillion dollar restoration programme began in the early 2000s, with key mitigation actions including nutrient load targets for the entire catchment and alum dosing in 2 tributaries. In this manuscript we analyse 2 water quality datasets (>10 yr) from Lake Rotorua and compare these with a global lake dataset. Generalised additive models predicted highly significant (p < 0.001) declines in total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl- a) in surface waters between 2001 and 2015. Alum dosing had a negative (i.e., reducing) and highly significant effect on TP and Chl- a (p < 0.001). Correlations of Chl- a on TP and TN were highly significant, but the difference between the 2 correlation coefficients was not, indicating a need to control both nutrients to reduce algal productivity. This conclusion is reinforced by recent bioassay studies which show co-limitation by N and P. Collectively, our data and previous studies provide strong support for the current strategy of limiting both N and P loads to Lake Rotorua for effective eutrophication control.

Climate and tectonic effects on Holocene development of an alpine lake (Muge Co, SE margin of Tibet)

South-west China is an important area for determining variability of the SW Asian Monsoon; it is, however, also tectonically very active. Palaeolimnological studies in this region generally focus on climate as a direct driver of ecological change, rarely considering the effects of tectonic activity and catchment ontogeny. We present diatom and organic matter data from a Holocene sediment record from Muge Co, an alpine lake located on an active fault zone in the southeast margin of Tibet, and explore the possible drivers of ecological change. A major change occurred at 9000 cal. BP, when the diatom assemblage switched from benthic to predominantly planktonic forms, and organic matter content decreased rapidly. Comparison of the Holocene diatom record with a regional data set of lake surface sediments using detrended canonical correspondence analysis (DCCA) suggests that this change in diatom development was mainly associated with changing water depth, most likely linked to a seismically triggered landslide (which deepened the lake) rather than climate-driven hydrological change. However, diatom changes associated with principal components analysis (PCA) axis 2 agree well with inferred Holocene temperature changes. From the mid-Holocene, shifts in planktonic diatom species were probably related to indirect climate forcing, notably water column mixing depth and catchment vegetation–driven changes in dissolved organic carbon (DOC) and nutrient export. The diatom stratigraphy at Muge Co reflects a complex range of indirect climate- and catchment-controlled processes that were superimposed over a change in lake morphology resulting from tectonic activity.

Effects of ecological engineering on water balance under two different vegetation scenarios in the Qilian Mountain, northwestern China

Abstract Study region The Qilian Mountain, northwestern China. Study focus Land degradation is a global eco-environmental issue. To minimize soil erosion and land degradation, China has implemented several ecological engineering such as “Grain for Green” program (GFG) since 1999. Relationship between vegetation and water budgets in catchments has been widely studied, however very few studies addressed the effects of eco-environmental restoration on water balance in mountain areas, especially with a focus on soil moisture content. Therefore SWAT model was used to quantify the effects of ecological engineering actions (taken place in 2005) on water balance in Qilian Mountain. New hydrological insights for the region After the ecological engineering, water yield and soil water content experienced an increment of 32%, and 46%. The opposite trend was monitored in runoff and evapotranspiration, which decreased by 48% and 4%, respectively. Therefore ecosystem restoration have increased soil water retention capacity, a greater proportion of precipitation reaching the catchment is absorbed by the soil rather than flowing out of the region as runoff. Therefore trade-offs between environmental sustainability and water resources security should be carefully addressed in arid region that experienced severe water shortages.

Treatability of organic matter derived from surface and subsurface waters of drinking water catchments

The treatability of NOM present in runoff and subsurface waters from discrete zero-order catchments (ZOCs) with three land management practices (Australian native vegetation, pine plantation, grasslands) on varying soil textures of a closed drinking water reservoir-catchment was investigated. Subsurface water samples were collected by lysimeters and shallow piezometers and surface waters by installation of barriers that diverted waters to collection devices. For small sample volumes collected, a 'micro' jar testing procedure was developed to assess the treatability of organics by enhanced coagulation using alum, under standardised conditions. DOM present in water samples was quantified by measurement of DOC and UV absorbance (at 254 nm) and characterized using these and F-EEM. The mean alum dose rate (mg alum per mg DOC removed or Al/DOC) was found to be lower for DOM from sandy soil ZOCs (21.1 ± 11.0 Al/DOC) than from clayey soil ZOCs (38.6 ± 27.7 Al/DOC). ZOCs with Pinus radiata had prominent litter layers (6.3 ± 2.6 cm), and despite differences in soil textures showed similarity in DOM character in subsurface waters, and in alum dose rates (22.2 ± 5.5 Al/DOC). For sandy soil ZOCs, the lowest alum dose rates (16.5 ± 10.6 Al/DOC) were for waters from native vegetation catchment while, for clayey soil ZOCs, waters from pine vegetation had the lowest alum dose rates (23.0 ± 5.0 Al/DOC). Where ZOCs have a prominent O horizon, soil minerals had no apparent influence on the treatability of DOM.

Influence of catchment vegetation on mercury accumulation in lake sediments from a long-term perspective

Organic matter (OM) cycling has a large impact on the cycling of mercury (Hg) in the environment. Hence, it is important to have a thorough understanding on how changes in, e.g., catchment vegetation – through its effect on OM cycling – affect the behavior of Hg. To test whether shifts in vegetation had an effect on Hg-transport to lakes we investigated a sediment record from Herrenwieser See (Southern Germany). This lake has a well-defined Holocene vegetation history: at ~8700years BP Corylus avellana (hazel) was replaced by Quercus robur (oak), which was replaced by Abies alba (fir) and Fagus sylvatica (beech) ~5700years BP). We were particularly interested in testing if coniferous vegetation leads to a larger export of Hg to aquatic systems than deciduous vegetation. When hazel was replaced by oak, reduced soil erosion and increased transport of DOM-bound mercury from the catchment resulted in increases in both Hg-concentrations and accumulation rates (61ngg−1 and 5.5ngcm−2yr.−1 to 118ngg−1 and 8.5ngcm−2yr.−1). However, even if Hg-concentrations increased also in association with the introduction of fir and beech (173ngg−1), as a result of higher Hg:C, there was no increase in Hg-accumulation rates (7.6ngcm−2yr.−1), because of a decreased input of OM. At around 2500years BP Hg-accumulation rates and Hg-concentration indicated an additional input of Hg to the sediment (316ngg−1 and 10.3ngcm−2yr.−1), which might be due to increased human activities in the area, e.g., forest burning or mining. Our results contrast those of several paired-catchment studies that suggest a higher release of Hg from coniferous than deciduous forest, and there is a need for studies with a long-term perspective to increase our understanding of the effects of slow and gradual processes on mercury cycling.

Role of terrestrial carbon in aquatic UV exposure and photoprotective pigmentation of meiofauna in subarctic lakes

Summary Aquatic organisms are adversely influenced by ultraviolet radiation (UV) and utilise photoprotective strategies, including pigmentation. We examined UV‐protective melanin pigmentation of aquatic meiofauna (Cladocera) in relation to the UV exposure across 25 tree line lakes in Finland to address the potential effects of increased UV and altered input of UV‐screening terrestrial dissolved organic carbon (DOC) on aquatic organisms. Bio‐optical parameters, including concentration of DOC, the coloured dissolved organic matter (CDOM) fraction, a suite of carbon quality indices and chlorophyll a, were analysed from lake water, and their role in controlling underwater UV environment (measured as diffuse UV attenuation coefficient Kd at 305 and 340 nm) was examined. Cladoceran (Alona affinis) carapaces were extracted from the surface sediments, and their melanisation was assessed with spectroscopic UV‐visible light absorbance measurements. DOC, CDOM and specific UV absorbance (SUVA) had strong positive relationships with the attenuation of UV in the lakes, suggesting that terrestrial organic carbon controls underwater UV exposure in the examined lakes. The absorbance measurements indicated the presence of melanin in the cladoceran carapaces, the degree of melanisation varying strongly among the lakes. Melanisation had significant relationships with SUVA and fluorescence index (FI). It was higher in lakes with low SUVA and high FI, indicating that cladocerans exhibit strong melanisation in lakes with low contribution of UV‐attenuating allochthonous DOC (i.e. high UV exposure). The results suggest that cladoceran meiofauna respond to UV by utilising photoprotective pigmentation and that the degree of pigmentation is affected by site‐specific underwater UV exposure, which is ultimately controlled by UV‐attenuating DOC of terrestrial origin. Although cladoceran meiobenthos are able to adapt to varying underwater UV doses, climate change with its multiple consequences on hydrology, limnology and catchment vegetation in the tree line zone may cause major changes in underwater UV environment for the organisms to adapt.

Simulation and classification of the impacts of projected climate change on flow regimes in the arid Hexi Corridor of Northwest China

AbstractTraditional assessment approaches of climate change on flow regimes usually focus on flow magnitude and cannot capture the overall variation of flow regimes (e.g., variability, frequency, duration, timing, and rating). The Hexi Corridor, which is a typical arid and semiarid region in Northwest China, was selected as the study area. Streamflows were simulated by an integrated water system model in a historical period (1985–2005) and three future periods (2030s, 2050s, and 2070s). Twenty‐nine global climate models were picked from the Coupled Model Intercomparison Project Phase 5 based on projection performance, and the high (RCP85) and intermediate (RCP45) ranges of Representative Concentration Pathways were selected for future scenario analysis. All the streamflows were characterized in detail by flow regime metrics, including the magnitude, variability, and frequency of the overall flow events. The regional impacts of climate change were assessed and clustered into several similar spatial patterns. Results showed that the projected climate change would sensibly increase the magnitudes of average and low flows (75th percentile), as well as the frequencies of low and high flows (25th percentile), but decrease the flow variability. By contrast, it would not sensibly change the magnitude of high‐flow events. The flow regime variations of all scenarios and periods were clustered into three robust classes (highly, moderately, and slightly impacted classes) in the entire region. The flow regimes would be highly and moderately impacted in the middle stream and downstream with large‐sized and semidesert catchments but slightly impacted in the upper and middle streams with small‐sized and montane vegetation catchments. The impacted classes will sensibly vary at most stations in different future periods because of the spatial differences of climate change. This study would provide scientific support to implement the integrated adaptive water resource management for climate change at regional scales in the arid Hexi Corridor.

A model of the 4000-year paleohydrology (δ18O) record from Lake Salpetén, Guatemala

A simple mass-balance model provides insights into the influence of catchment vegetation changes and climate variability on the hydrologic and stable oxygen isotope (δ18O) evolution of Lake Salpetén, in the Maya Lowlands of northern Guatemala. Model simulations for the last 4000years incorporate pollen-inferred changes in vegetation cover and account for 75% of the variance observed in the biogenic carbonate δ18O record from a long lake sediment core. Vegetation-driven hydrologic changes, however, failed to capture the full range of late Holocene sediment core δ18O variability. The model requires incorporation of additional shifts in catchment vegetation cover, inclusion of regional precipitation changes, or likely both, to explain the fluctuations observed in the lake core oxygen isotope record. Climatic interpretation of the model results suggests that there was relatively greater moisture availability between about 2400 and 1800years ago, but increased δ18O values centered at ~3300, 2900, 500, and 200 calendar years before present (calyr BP) indicate abrupt precipitation decreases. There is evidence for protracted aridity between 1500 and 800calyrBP.

Stream ecosystem responses to an extreme rainfall event across multiple catchments in southeast Alaska

Summary Floods are a key component of the flow regime of many rivers and a major structuring force of stream communities. Climate change is predicted to increase the frequency of extreme rainfall (i.e. return intervals &gt;100 years) leading to extensive flooding, but the ecological effects of such events are not well understood. Comparative studies of flood impacts are scarce, despite the clear need to understand the potentially contingent responses of multiple independent stream systems to extreme weather occurring at meso‐ and synoptic spatial scales. We describe the effect of an extreme rainfall event affecting an area &gt;100 000 km2 that caused extensive flooding in SE Alaska. Responses of channel morphology and three key biological groups (meiofauna, macroinvertebrates and fish) were assessed in four separate and recently deglaciated stream catchments of contrasting age (38–180 years) by comparing samples taken before and after the event. Ecological responses to the rainfall and subsequent flooding differed markedly across the four catchments in response to variations in rainfall intensity and to factors such as channel morphology, stream sediment composition and catchment vegetation type and cover, which were themselves related to stream age. Our study demonstrates the value of considering multiple response variables when assessing the effects of extreme events, and highlights the potential for contrasting biological responses to extreme events across catchments. We advocate more comparative studies to understand how extreme rainfall and flooding affects ecosystem responses across multiple catchments.