Catchment Processes Research Articles

Evidence-based identification of integrated water quality systems

Identification of integrated models is still hindered by submodels’ uncertainty propagation. In this article, a novel identifiability and identification framework is applied to screen and establish reasonable hypotheses of an integrated instream (WASP) and catchment water quality (VENSIM) model. Using the framework, the models were linked, and critical parameters and processes identified. First, an ensemble of catchment nutrient loads was simulated with randomized parameter settings of the catchment processes (e.g. nutrient decay rates). A second Monte Carlo analysis was then staged with randomized loadings and parameter values mimicking insteam processes (e.g. algae growth). The most significant parameters and their processes were identified. This coupling of models for a two-step global sensitivity analysis is a novel approach to integrated catchment-scale water quality model identification. Catchment processes were, overall, more significant to the river’s water quality than the instream processes of this Prairie river system investigated (Qu’Appelle River).

The Main Features of Phosphorus Transport in World Rivers

Data on the geochemistry of phosphorus in the continental runoff of dissolved and solid substances were systematized and generalized, with a separate consideration of the processes of runoff transformation in river mouth areas. It has been established that atmospheric deposition, which many authors consider to be an important source of phosphorus in river runoff and not associated with mobilization processes in catchments, actually contains phosphorus from soil-plant recycling. This is confirmed by the fact that the input of phosphorus from the atmosphere into catchments exceeds its removal via water runoff. An analysis of the mass ratio of phosphorus in the adsorbed form and in the form of its own minerals was carried out. It was shown that the maximum mass of adsorbed phosphorus is limited by the solubility of its most stable minerals. The minimum concentrations of dissolved mineral and total phosphorus were observed in the rivers of the Arctic and subarctic belts; the maximum concentrations were confined to the most densely populated temperate zone and the zone of dry tropics and subtropics. In the waters of the primary hydrographic network, the phosphorus concentration exhibited direct relationships with the population density in the catchments and the mineralization of the river water and was closely correlated with the nitrogen content. This strongly suggests that economic activity is one of the main factors in the formation of river phosphorus runoff. The generalization of the authors’ and the literature’s data on the behavior of phosphorus at the river–sea mixing zone made it possible to draw a conclusion about the nonconservative distribution of phosphorus, in most cases associated with biological production and destruction processes. The conservative behavior of phosphorus was observed only in heavily polluted river mouths with abnormally high concentrations of this element.

Investigating flood processes in karst catchments by combining concentration-discharge relationship analysis and lateral flow simulation

Concentration-Discharge (C-Q) relationship analyses allow better understanding catchment water origin and hydrological process variability across time (from storm-event to pluriannual scale). Though, those approaches mostly include integrative methods that consider catchments as a whole. The aim of this article is to assess the potential of a combined approach to investigate the spatio-temporal variability of flood processes within catchments (across river reaches and seasons), focusing on karst areas. The methodology consists in combining C-Q relationship analysis in nested catchments and simulation of lateral exchanges at the reach scale. For that, we analyze high-frequency records of discharge and electrical conductivity (EC) in karst catchments, in which EC informs on water residence time. Contributions of pre-event water (PEW) and event water (EW) during storm events are investigated through hysteresis loop analysis at each gauging station. Combined with lateral EC simulation at the reach scale, they make it possible to infer runoff processes during storms. A unary karst catchment in a temperate/mountainous climate (Loue River) and a binary catchment in a Mediterranean climate (Cèze River) were selected in France, and their conceptual models of water origin and hydrological process variability were drawn. In the Loue catchment, summer and fall storm events are characterized by contributions of PEW through piston-type flows, whereas decreasing EC values in winter and spring storm events indicate the major contribution of EW through surface runoff and fast infiltration. Within the catchment, EW contribution increases downstream, due to the canyon river morphology grading into open valleys. Regarding the Cèze catchment, contributions of EW are higher, indicating that fast infiltration and surface runoff are the dominant processes, associated with a PEW signature in summer and fall. Within the catchment, PEW contribution is found higher in karstified areas. These results show that flood process seasonality is mainly related to karst aquifer saturation rate, while intra-site variability is linked to karst area extension and river morphology. Results are encouraging to extend this approach to a variety of sites, notably those affected by significant surface water-groundwater interaction and groundwater flooding. Such approach, by providing a discretized information on flood processes, could help refining lumped hydrological models, or facilitate the use of semi-distributed ones.

Turbidity‐discharge hysteresis in a meso‐scale catchment: The importance of intermediate scale events

AbstractIn‐situ sensors for riverine water quality monitoring are a powerful tool to describe temporal variations when efficient and informative analyses are applied to the large quantities of data collected. Concentration‐discharge hysteresis patterns observed during storm events give insights into headwater catchment processes. However, the applicability of this approach to larger catchments is less well known. Here, we evaluate the potential for high‐frequency turbidity‐discharge (Q) hysteresis patterns to give insights into processes operating in a meso‐scale (722 km2) northern mixed land use catchment. As existing event identification methods did not work, we developed a new, objective method based on hydrograph characteristics and identified 76 events for further analysis. Qualitative event analysis identified three recurring patterns. Events with low mean Q (≤ 2 m3/s) often showed short‐term, quasi‐periodic turbidity variation, to a large extent disconnected from Q variation. High max Q events (≥15 m3/s) were often associated with spring flood or snowmelt, and showed a disconnection between turbidity and Q. Intermediate Q events (mean Q: 2–11 m3/s) were the most informative when applying hysteresis indexes, since changes in turbidity and Q were actually connected. Hysteresis indexes could be calculated on a subset of 60 events, which showed heterogeneous responses: 38% had a clockwise response, 12% anticlockwise, 12% figure eight (clockwise–anticlockwise), 10% reverse figure eight (anticlockwise–clockwise) and 28% showed a complex response. Clockwise hysteresis responses were associated with the wetter winter and spring seasons. Generally, changes in Q and turbidity were small during anticlockwise hysteresis events. Precipitation often influenced figure‐eight patterns, while complex patterns often occurred during summer low flows. Analysis of intermediate Q events can improve process understanding of meso‐scale catchments and possibly aid in choosing appropriate management actions for targeting a specific observed pattern.

Sediment Yield and Reservoir Sedimentation in Highly Dynamic Watersheds: The Case of Koga Reservoir, Ethiopia

Soil erosion is exacerbated by unsustainable land-use activities and poor management practices, undermining reservoir storage capacity. To this effect, appropriate estimation of sediment would help to adopt sustainable land-use activities and best management practices that lead to efficient reservoir operations. This paper aims to investigate the spatial variability of sediment yield, amount of sediment delivery into the reservoir, and reservoir sedimentation in the Koga Reservoir using the Soil and Water Assessment Tool (SWAT). Sediment yield and the amount entered into the reservoir were also estimated using a rating curve, providing an alternative approach to spatially referenced SWAT generated suspended sediment load. SWAT was calibrated from 1991 to 2000 and validated from 2002 to 2007 using monthly observations. Model performance indicators showed acceptable values using Nash-Sutcliffe efficiency (NSE) correlation coefficient (R2), and percent bias (PBIAS) for flow (NSE = 0.75, R2 = 0.78, and PBIAS = 11.83%). There was also good agreement between measured and simulated sediment yields, with NSE, R2, and PBIAS validation values of 0.80, 0.79, and 6.4%, respectively. The measured rating curve and SWAT predictions showed comparable mean annual sediment values of 62,610.08 ton/yr and 58,012.87 ton/yr, respectively. This study provides an implication for the extent of management interventions required to meet sediment load targets to a receiving reservoir, providing a better understanding of catchment processes and responses to anthropogenic and natural stressors in mixed land use temperate climate catchments. Findings would benefit policymakers towards land and water management decisions and serve as a prototype for other catchments where management interventions may be implemented. Specifically, validating SWAT for the Koga Reservoir is a first step to support policymakers, who are faced with implementing land and water management decisions.

Use of stable Mg isotope ratios in identifying the base cation sources of stream water in the boreal Krycklan catchment (Sweden)

The knowledge of the sources of base cations in stream water is a prerequisite to assess potential effects of changing environmental conditions such as changing rainfall, weathering or groundwater flows on cation export with stream water. This study use stable Mg isotopes to identify potential sources in the well-studied catchment of Krycklan located on gneissic bedrock covered by quaternary sediments in Sweden. Samples were collected from open filed rain, throughfall, stream, soil, rock and litterfall. The δ26Mg values of these samples was determined and the contributions of different sources to Mg fluxes in the stream were determined from the variation of the Mg isotope and Sr / Mg ratios.The results show an overall variation of 1.10‰ between all samples. In addition, Magnesium isotope ratios varied little in the streamwater and in soil solution, except during snowmelt periods during which a large portion of the annual runoff occurs. Magnesium in the streamwater is explained as a mixture of three pools (open field rain, soil solution and groundwater) with the latter two influenced by catchment processes. Outside the snowmelt period, Mg in streamwater mainly derived from the groundwater, assumed to be mineral weathering signature in this catchment, with a contribution ranging from 12 to 63% to Mg fluxes. Open field rain dominates Mg fluxes in streamwater during spring flood (0 to 78%) and may contribute significantly during larger summer and autumn rainfall events. Soil solution input to streamwater range from 16 to 59% of Mg fluxes in streamwater. Our results demonstrate that δ26Mg values together with Mg concentrations and Sr/Mg ratios can be used to constrain the Mg sources of stream water and quantify weathering release rates.

The influence of land cover changes on landscape hydric potential and river flows: Upper Vistula, Western Carpathians

This paper presents an approach for mitigating water-related problems, that relates the retention of precipitation and use of ecosystems, as a tool for improving the quality, quantity and availability of water resources throughout the region. One approach is the determination of the landscape’s hydric potential (LHP). Land cover changes can influence ecosystems in different ways depending on crucial factors that depend on water resources. Thus-far, many studies describe the influence of land cover changes on individual features of catchments and hydrological regime, but few have made an attempt to analysis how changes in land cover can holistically influence processes in catchments. In this context the innovative aspect of this work presents the consequence of land cover changes on water retention expressed by LHP — a general descriptor that is linked with many composite influence factors on water retention. A study was carried out on 33 catchments located in central Europe. Results show increased forest cover having a positive trend in the context of LHP. Conversely, increased urban and industrial areas negatively impact LHP. The presented analysis reveals that a few land cover changes had direct and clearly positive effects on LHP. The LHP had decreased in 21 catchments and increased in 12 catchments. Decreased LHP was generally observed in catchments located on the left side of the Vistula basin where there are more advantageous conditions for agriculture, rising industry and urban areas. The study showed that during the analyzed period, obtained changes of LHP values did not strongly affect hydrological regimes in studied catchments. Our results show that LHP is a very good and useful descriptor that includes many characteristics of catchment and could be used for river catchment management purposes. The landscape’s hydric potential can be used in any catchment in the world.

A multi-proxy record of environmental change through the last 53,000 years recorded in the sediments of Lake Kanono, Northland, New Zealand

ABSTRACTLake sediment archives from Southern Hemisphere mid-latitude regions provide invaluable records of late Quaternary environmental change. Here, changes in depositional environment over the past ca. 53,000 years were reconstructed using a range of physical, sedimentological, geochemical, and μ-XRF elemental proxy datasets analyzed from lake sediment cores obtained from Lake Kanono, Northland, New Zealand. The Lake Kanono stratigraphy displays a terrestrial peat environment (ca. 53,700–6,670 cal yr BP), followed by a trend of increased influx of detrital sediment during the Late Glacial–Interglacial Transition (LGIT) at ca. 14,000 cal yr BP with a peak from ca. 12,000 to 9,000 cal yr BP driven by increasingly dry conditions. The increase in sediment influx continued during the early to mid-Holocene, leading to dune reactivation which altered the catchment dynamics of the region, leading to the inception of a shallow lake basin at ca. 6,670 cal yr BP. The timing of the formation of this lake basin can be associated with changes in intensity of the Southern Westerly Winds (SWW) and the appearance and increase in intensity of the El Niño Southern Oscillation (ENSO) after ca. 7,500–7,000 cal yr BP (Moy et al. 2002; Moreno et al. 2018). Drier conditions peaked from ca. 4,000 to 2,400 cal yr BP, possibly culminating in decreased lake levels that persisted from ca. 2,400 to 2,210 cal yr BP, renewed dune accumulation, and blocked stream outlets, resulting in a deep lake basin with thermal stratification that persisted to the present. Cluster analysis of the μ-XRF data demonstrates that the most prominent change in chemistry is near the onset of the Last Glacial Maximum (LGM) at ca. 26,700 cal yr BP associated with a transition to a drier, windier climate. The second most prominent change in the μ-XRF data is during the Polynesian phase of human settlement at ca. 612–575 cal yr BP (1338– 1375 CE). Hence, we can demonstrate the utility and power of a multi-proxy approach coupled with μ-XRF element data to interpret changing sediment sources to a lake basin. Such an approach allows rapid and reliable evaluation of catchment processes influenced by climate events and land-use changes at a resolution not available using other approaches.

Process‐Based Flood Risk Assessment for Germany

AbstractLarge‐scale flood risk assessments are crucial for decision making, especially with respect to new flood defense schemes, adaptation planning and estimating insurance premiums. We apply the process‐based Regional Flood Model (RFM) to simulate a 5000‐year flood event catalog for all major catchments in Germany and derive risk curves based on the losses per economic sector. The RFM uses a continuous process simulation including a multisite, multivariate weather generator, a hydrological model considering heterogeneous catchment processes, a coupled 1D–2D hydrodynamic model considering dike overtopping and hinterland storage, spatially explicit sector‐wise exposure data and empirical multi‐variable loss models calibrated for Germany. For all components, uncertainties in the data and models are estimated. We estimate the median Expected Annual Damage (EAD) and Value at Risk at 99.5% confidence for Germany to be €0.529 bn and €8.865 bn, respectively. The commercial sector dominates by making about 60% of the total risk, followed by the residential sector. The agriculture sector gets affected by small return period floods and only contributes to less than 3% to the total risk. The overall EAD is comparable to other large‐scale estimates. However, the estimation of losses for specific return periods is substantially improved. The spatial consistency of the risk estimates avoids the large overestimation of losses for rare events that is common in other large‐scale assessments with homogeneous return periods. Thus, the process‐based, spatially consistent flood risk assessment by RFM is an important step forward and will serve as a benchmark for future German‐wide flood risk assessments.

Shallow landslides and vegetation at the catchment scale: A perspective

Shallow, rainfall-triggered landslides are an important catchment process that affect the rate and calibre of sediment within river networks and create a significant hazard, particularly when shallow landslides transform into rapidly moving debris flows. Forests and trees modify the magnitude and rate of shallow landsliding and have been used by land managers for centuries to mitigate their effects. We understand that at the tree and slope scale root reinforcement provides a significant role in stabilising slopes, but at the catchment scale root reinforcement models only partially explain where shallow landslides are likely to occur due to the complexity of subsurface material properties and hydrology. The challenge of scaling from slopes to catchments (from 1-D to 2-D) reflects the scale gap between geomorphic process understanding and modelling, and temporal evolution of material properties. Hence, our understanding does not, as yet, provide the necessary tools to allow vegetation to be targeted most effectively for landslide reduction. This paper aims to provide a perspective on the science underpinning the challenges land and catchment managers face in trying to reduce shallow landslide hazard, manage catchment sediment budgets, and develop tools for catchment targeting of vegetation. We use our understanding of rainfall-triggered shallow landslides in New Zealand and how vegetation has been used as a tool to reduce their incidence to demonstrate key points.

Territories of Faith: 1000 Years of Landscape Multifunctionality in Santa Mariña de Augas Santas (NW Spain)

Landscape multifunctionality is increasingly recognized as an important aspect in sustainability and developmental debates. Yet, how and why a multifunctional landscape configuration develops over time has not been sufficiently studied. Here we present the geoarchaeological investigation of the Santa Mariña de Augas Santas site, in northwestern Spain. We focus on the role of religious practice, and of its interplay with productive strategies, in landscape transformation. A geochemical, mineralogical, and geochronological characterization of the pedo-sedimentary record (including XRF, EA-IRMS, XRD, OSL and 14C measurements) allowed to characterize catchment scale sedimentation processes in relation to agricultural activities. The geographical and chronological coincidence of production functions with documented religious activities demonstrate that both aspects shared geographical spaces during the last millennium. Current landscape multifunctionality at Santa Mariña is thus not the final outcome of a specific evolution, but an essential aspect of traditional land use strategies through history and a driver of change. This work highlights the need of a long-term study of the processes of landscape configuration when assessing the sustainability of traditional productive systems.

Hydrological Modeling in Agricultural Intensive Watershed: The Case of Upper East Fork White River, USA

Identifying the core hydrological processes of catchments is a critical step for operative hydrological modeling. This study attempts to assess the long-term alterations in streamflow in three adjacent catchments of Upper East Fork White River, Indiana USA, by employing the SWAT hydrological model. The model simulations are spanning from 1980 up to 2015 and distributed in three configurations periods to identify monthly alterations in streamflow. For this purpose, water abstraction, land use, tillage, and agricultural field drainage practices have been incorporated in the model to provide accurate data input. The model setup also integrates spatially disaggregated sectorial water use data from surface and groundwater resources integrating the significant increases of water abstractions mainly for agricultural and public water supply purposes. The land cover of the study area is governed by rotating crops, while agricultural practices and tile drainage are crucial model parameters affecting the regional hydrological balance. Streamflow prediction is based on the SUFI-2 algorithm and the SWAT-CUP interface has been used for the monthly calibration and validation phases of the model. The evaluation of model simulations indicate a progressively sufficient hydrological model setup for all configuration periods with NSE (0.87, 0.88, and 0.88) and PBIAS (14%, −7%, and −2.8%) model evaluation values at the Seymour outlet. Surface runoff/precipitation as well as percolation/precipitation ratios have been used as indicators to identify trends to wetter conditions. Model outputs for the upstream areas, are successful predictions for streamflow assessment studies to test future implications of land cover and climate change.

Response of Zooplankton Size Structure to Multiple Stressors in Urban Lakes

Urban lakes are important environmental assets that contribute significant ecosystem services in urbanised areas around the world. Consequently, urban lakes are more exposed to anthropogenic pressures. Zooplankton communities play a central role in lake processes and, as such, are very sensitive to the impacts of human activities both through in-lake and catchment processes. Understanding their ecological function in urban lakes and how they respond to urbanisation is essential for environmental sustainability. In this study, we investigated the reliability of zooplankton size structure as indicators of anthropogenic stressors in urban lakes. We examined the relationship between environmental variables and zooplankton community size spectra derived as mean body size, density, and biomass. Our study showed that the overall mean body size was within the small size group ranged from 416 to 735 µm equivalent spherical diameter (ESD). Despite no significant difference in total zooplankton density between lakes, there was variability in the total density of the five different size classes. Total biomass was characterised by a significant proportion of size >750 µm. As the specific parameter of normalised biomass size spectra (NBSS), the slopes of the NBSS varied from moderate (−0.83 to −1.04) for a community with higher biomass of the larger size zooplankton to steeper slopes (from −1.15 to −1.49) for a community with higher biomass of smaller size. The environmental variables, represented by total phosphorus (TP) and chlorophyll a (chl-a), had a strong effect on zooplankton biomass and NBSS, where TP and chl-a were significantly correlated with the increase of total biomass and corresponded well with a less negative slope. Our results indicated that the community metric was sensitive to nutrient input and that size-based metrics have the potential to serve as key indicators for the management of urban lakes.

Rock magnetic properties of Grand Lake sediments as evidence of environmental changes during the last 60 000 years in North‐East Russia

Dansgaard–Oeschger events of the last glacial cycle are widely observed in the North Atlantic sector. In far northeast Asia, however, a lack of data means the presence or absence of these events is uncertain and, if present, the mechanisms that connect this region with the North Atlantic remain unresolved. A sediment record from Grand Lake, North‐East Russia (latitude 60°44′N, longitude 151°53′E), spans much of the last glacial cycle and shows strong variations in magnetic parameters. These variations are related to sedimentation changes that were likely influenced by the palaeoclimate. Rock magnetic properties of a 1020‐cm‐long sediment core were determined for four lithological units. Additionally, samples taken from across the modern catchment aided the interpretation of the magnetic properties of the down‐core sediments. Magnetic minerals in all sediment units consist of detrital, oxidized Ti‐poor titanomagnetites. Sediments of Holocene age (Unit 1) display the lowest values of magnetic susceptibility (MS), saturation magnetization (Js) and saturation remanence (Jrs), and reflect dilution by biogenic silica and organic matter. Variations of magnetic properties in Units 2, 3 and 4, which correspond to Marine Isotope Stages (MIS) 2, 3 and 4, respectively, parallel granulometric changes. Massive sediment structure, which characterizes Unit 3, has a coarser grain size and higher MS, Jrs and Js as compared with the laminated sediments of Unit 2 and Unit 4, suggesting higher‐energy catchment processes. Nine distinct peaks in MS, Js and Jrs within MIS 3 coincide with TiO2 minima and with high pollen percentages of trees and shrubs, supporting the conclusion that lake and catchment sediment processes are associated with fluctuating regional climate. While the dating is not resolved enough to be certain, the data are suggestive that these cycles correlate with Dansgaard–Oeschger events.

Catchment processes can amplify the effect of increasing rainfall variability

By filtering the incoming climate signal when producing streamflow, river basins can attenuate—or amplify—projected increases in rainfall variability. A common perception is that river systems dampen rainfall variability by averaging spatial and temporal variations in their watersheds. However, by analyzing 671 watersheds throughout the United States, we find that many catchments actually amplify the coefficient of variation of rainfall, and that these catchments also likely amplify changes in rainfall variability. Based on catchment-scale water balance principles, we relate that faculty to the interplay between two fundamental hydrological processes: water uptake by vegetation and the storage and subsequent release of water as discharge. By increasing plant water uptake, warmer temperatures might exacerbate the amplifying effect of catchments. More variable precipitations associated with a warmer climate are therefore expected to lead to even more variable river flows—a significant potential challenge for river transportation, ecosystem sustainability and water supply reliability.

Climate-driven soil erosion processes in alpine environments over the last century: Evidence from the Taibai Mountain (central China)

Over the last century, climate warming has increased vegetation coverage in alpine regions, having major implications for associated terrestrial and aquatic ecosystems. Due to the remoteness of mountainous regions, there is a paucity of monitoring data regarding the response of catchment processes to climate change. Particle size distribution, magnetic properties, and elemental contents were investigated in 210Pb-dated sediment cores (34 cm in length) collected from two hydrologically-connected cirque lakes in the Taibai Mountain (the highest mountain in east mainland China). Both lakes have experienced directional environment changes since the 1980s, suggested by concurrent increases in magnetic susceptibility and organic matter. Prolonged growth season under a scenario of warmer climate has promoted vegetation development and soil conservation, subsequently increasing the contribution of terrestrial detritus derived from top soils that are rich in magnetic minerals and organic matter. Contents of clay and silt remained high throughout the two sediment cores, suggesting that depositional environment was relatively stable in both lakes. However, few peaks in coarse particles probably resulted from the cold and wet climate between the 1940s and the 1970s. During the relatively cold and wet episodes, prolonged snow-packs and enhanced melt-water discharge would strengthen bedrock weathering and erosion intensity, subsequently increasing the influxes of coarse particles. This is the first study to reveal decadal-scale changes in catchment processes of the Taibai Mountain, and the results highlight positive effects of climate warming on environmental protection of our study site and other similar alpine mountains through promoting water and soil conservation.

A commentary on ecohydrology as a science-policy interface in implementing the UN Sustainable Development Goals

Ecohydrology, as an integrating discipline, can contribute strongly to achieving the Sustainable Development Goals (SDGs) especially in the context of Integrated Water Resources Management (IWRM). Using ecohydrology principles in applying IWRM affords a better understanding of the links between water, biodiversity, and human wellbeing in the catchments. One problem with the SDGs is that they are presented in a “silo” style, which means there are few if any connections between SDG6 “clean water and sanitation” and SDG 15 “life on land”, which deals with wetlands and catchments. Ecohydrology principles, drawing on the need to better understand green and blue infrastructure, can help bridge between the SDGs in an IWRM context, allowing thus for their more successful achievement.The Convention on Wetlands (Ramsar, 1971) has degrees of overlap and redundancy with the UNECE Water Convention (1992). And both rely on better science, policy, and management of catchments for effective implementation. Ecohydrology can help promote and develop the science base of these Multilateral Environment Agreements allowing for interpretation of catchment processes and dynamics, and thus better application of IWRM, and ultimately better policies for management of water resources, and thus alleviation of water scarcity.Ecohydrology can also play a role in fostering social science research in catchments, heritage management and conservation. So far in UNESCO, ecohydrology has been largely dealt with by the International Hydrological Programme, in the Division of Water Sciences. There is a future role for ecohydrology as a cross-cutting theme across all UNESCO Sectors, led by the Natural Sciences Sector.

Cladocera responses to climate changes and treeline shifts in an alpine lake-catchment since the Last Glacial Maximum

The study of alpine lake ecosystem ontogeny provides an understanding of how climate dynamics and the associated catchment changes impact on lake functioning. Here, a high-resolution subfossil Cladocera sedimentary record, together with geochemistry proxies, from an alpine, treeline lake (Cuoqia Lake) in SW China were analyzed to understand the lake ecosystem response to changes in the regional Indian summer monsoon (ISM) climate and the local mountain environment since the Last Glacial Maximum (LGM, ca. 19.5 cal kyr BP to present). The results show that the cladoceran record from Cuoqia Lake clearly responds to long-term climate change and significant climate events. The lake was depauperate in cladoceran species throughout the late glacial period (ca. 16–11 cal kyr BP) but Daphnia longispina-group and Chydorus sphaericus were the pioneer taxa when the lake was formed. The rapid proliferation of open-water and littoral cladoceran taxa began with increasing water temperature. The Younger Dryas (YD) cold event was recorded by high abundance of the cold-tolerant taxon C. sphaericus during ca. 12–11 cal kyr BP. At the onset of the Holocene (ca. 11 cal kyr BP), Alona spp. and macrophyte-associated taxa became dominant while numbers of C. sphaericus decreased dramatically. In the mid- to late-Holocene, the productivity of Cuoqia Lake seemed to have increased as reflected by higher Cladocera concentration and the appearance of Ceriodaphnia spp. and raptorial P. pediculus which are common in high nutrient waters. Partial Redundancy Analysis (partial-RDA) results highlight the strong effect of indirect climate change (mediated via catchment processes) on cladoceran assemblages during the Holocene. Abrupt shifts in sediment geochemical proxies and regional catchment vegetation at ca. 11 cal kyr BP indicate that substantial nutrient inputs, treeline advancement, and soil development in catchment resulted from the intensified ISM. Therefore, the cladoceran community changes suggest a long-term direct response of the alpine lake ecosystem to climate change since the LGM, but one that was strongly mediated by catchment-vegetation processes, such as the treeline shifts.

Basin-specific records of lake oligotrophication during the middle-to-late Holocene in boreal northeast Ontario, Canada

Descriptions of regional climate expression require data from multiple lakes, yet little is known of how variation in records within morphometrically complex lakes may affect interpretations. In northeast Ontario (Canada), this issue was addressed using records of pollen, pigments, and diatoms in three sediment cores from two small boreal lakes spanning the last ~6000 years. Pollen analysis suggested warm conditions between ~6000 and ~4000 cal yr BP, coherent with previous assessments from boreal eastern Ontario and western Quebec. Analysis of phototrophic communities from fossil pigments and diatom valves suggested relatively eutrophic conditions with lower lake-levels during this interval. Generalized additive model trends identified significant regional changes in pollen assemblages and declines in pigment concentrations after ~4000 cal yr BP consistent with cooler and wetter climate conditions that resulted in regional lake oligotrophication and increased lake levels during the late-Holocene. Despite contemporaneous changes in pollen and pigment biomarkers across lakes, cores collected from adjacent basins of the same lake (Green Lake) did not show similar trends in fossil pigments likely reflecting preferential deposition of clay-rich allochthonous material in the deeper central basin and suggesting that regional signals in climate may be complicated by lake- or basin-specific catchment processes.

Holocene lake phosphorus species and primary producers reflect catchment processes in a small, temperate lake

AbstractThis paleolimnological study aims to investigate how natural processes and anthropogenic land‐use changes have affected sedimentary phosphorus (P) forms and primary producers in a small, temperate lake (Lake Fuglsø, Denmark) throughout the Holocene. Our multi‐proxy approach uses pollen, X‐ray fluorescence scanning, carbon (C) and nitrogen contents and stable isotopes, sequential P extraction, 31P nuclear magnetic resonance spectroscopy, pigments, diatoms, and plant macrofossils from a 14C‐dated sediment record. We found three periods of human impact: (1) low disturbance from domestic grazing during the early/mid Neolithic (~3600 to ~2600 BC), (2) higher disturbance because of animal husbandry and some grain cultivation during the Late Bronze and Pre‐Roman Iron Age (~800 BC to AD ~100), and (3) strong disturbance caused by domestic grazing, intensified crop cultivation and, in particular, by retting of fiber plants during the Middle Ages and Renaissance (AD ~1000 to ~1700). Cultural eutrophication during the latter phase caused unprecedented changes in the lake, including altered species composition, high production, and strongly accelerated sediment accumulation rates. Generally, catchment deforestation was related to elevated proportions of metal (iron, aluminum, calcium)‐bound P forms in the sediment, while high tree cover correlated with elevated proportions of P forms associated with organic material (“organic” P, humic‐bound P, refractory organic P) and loosely bound P. During phases with forest in the catchment, silicon (Si) inputs to the lake were insufficient and diatom frustules were mostly absent in the sediments. In contrast, diatoms thrived in the lake when the landscape was open and erosional Si influx was high. This study is the first to show long‐term (~eight millennia) and recurring Si limitation of diatoms, a finding that may explain the absence of diatoms in sediment records of other sites too. In summary, human land‐use with preceding deforestation accelerated the transport of nutrients and elements from the terrestrial to the aquatic environment, leading to substantial and irreversible changes in Lake Fuglsø. Our study is a good example of the tight links between catchment processes and lake status, indicating that catchment dynamics should be considered in lake restoration projects, particularly for lowland lakes with high catchment : lake area ratios.