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A bs tract: Climate change and anthropogenic activities mutually degraded global lake ecosystems during the Anthropocene. Nutrient mitigation via catchment management has been a constant priority for lake ecological restoration; however, the outcomes after mitigation remain uncertain. In particular, there is limited knowledge on the effects of large‐scale forest restoration and soil erosion reduction in catchments on the ecological dynamics of downstream aquatic systems.Here, we study a strategically chosen alpine lake from the Loess Plateau in northern China, which has been extensively afforested in the past decades. We present high‐resolution, multi‐proxy records to reconstruct environmental and ecological dynamics since 1960. To quantify the potential mechanisms, we combined evidence from palaeolimnological records, contemporary field observations and process‐based ecosystem modelling. We developed palaeorecord‐based proxies for water transparency, nutrient concentrations and algal composition, and applied these proxies to ecosystem modelling.Our results show that despite the concurrent decline in nutrient levels, the lake ecosystem has experienced large‐magnitude rapid shifts since 2000, characterised by unambiguously increased algae proliferation and community changes. The modelling results revealed that the reduction in soil erosion acts as a "double‐edged sword"; that is, lower external nutrient loading via soil erosion is ideal for in‐lake algae inhibition, whereas increasing light availability counteracts the reduced yet still sufficient nutrient concentrations and, ultimately, promotes temporary algal proliferation via extended habitats and intensive photosynthesis. Intriguingly, lake warming plays a secondary role in driving these ecological dynamics.Our results emphasised the unexpected algal proliferation when the nutrient level declined in lake ecosystems; thus, other compound factors, including water transparency and climate warming, need to be incorporated for reliable prediction and effective restoration. A multi‐disciplinary approach integrating palaeorecords as ecological indicators and process‐based ecosystem modelling holds great potential for investigating long‐term lake ecological dynamics in the context of rapid global changes. [ABSTRACT FROM AUTHOR]

Regime shifts in ecology are characterised by major, often abrupt changes in ecosystem structure and functioning in response to one or more driving variables, or pressures. Changes in the provision of ecosystem services are a potential outcome. Despite the current combination of rapidly increasing pressures on what are often highly important socio‐ecological systems, the resilience of lakes in the warm tropics to human perturbation is far less well understood than those at higher latitudes. This paper focuses on evidence of aquatic ecosystem change from a cluster of three deep, freshwater, volcanic crater lakes (Yambo, Mohicap, and Sampaloc) at low altitude on the island of Luzon, Philippines. The lakes support different intensities of aquaculture, an important livelihood but also a driver of poor water quality throughout tropical Asia.Measured and monitored climate and water quality data, in addition to sedimentary evidence from sediment cores collected from the three study lakes, were used to determine the magnitude and trajectory of changes in lake water quality. Sediment cores were radiometrically dated and analysed for organic matter, spheroidal carbonaceous particles, and diatom remains. Diatom data were zoned numerically using cluster analysis. Diatom remains were also used to infer past variations in pH and possible relationships between potential driving climatic variables (temperature and rainfall). Diatom data sets were explored using detrended component analysis and principle component analysis.Despite differences in intensity of aquaculture, a common trajectory and timing of a potential regime shift, characterised by a replacement of benthic with planktonic diatoms and an increase in diatom accumulation rates from the early to mid‐1980s, is evident, and attests a low threshold for disturbance effects. A predominantly planktonic diatom flora has persisted even after recent improvements in environmental quality.The potential new regime may be less resilient and more susceptible to harmful algal blooms, abrupt expansions of anoxic conditions, and periodic mass fish kills when compared with its former state. The research further highlights the sensitivity of freshwater ecosystems in the warm tropics to disturbance pressures, and the risks to livelihoods, ecosystem services, and sustainable development. [ABSTRACT FROM AUTHOR]

• C/N ratios and δ13C suggest regional similarities in floodplain lake carbon cycling one century ago. • Timings of shifts in sedimentary proxies indicate larger responses to eutrophication than dam installation in the 20th century. • Eutrophication influences production and burial of autochthonous carbon by stimulating aquatic plant or algal growth. Carbon cycling in shallow floodplain lakes is complex due to variability in delivery of flood-derived allochthonous organic matter (OM). Human activities have potential to significantly modify the carbon balance of lakes by damming which restricts external OM inputs and via eutrophication which can increase the in-lake production of algae and/or aquatic plants. In order to understand how these human activities influence carbon cycling in shallow floodplain lakes over decadal-centennial timescales, we analysed C/N ratios and δ13C from terrestrial plants, catchment soils, aquatic plants and dated sediment cores from six heavily modified lakes in the middle Yangtze floodplain. Submerged macrophytes (−21.4 ± 4.6 ‰) had higher δ13C than C 3 plants from the catchment (−26.6 ± 0.6 ‰) and emergent and floating plants (−26.6 ± 4.0 ‰). Increases in sedimentary chlorophyll a (from primary producers) were associated with a decline in sedimentary δ13C in the severely eutrophic Dongting, Luhu, Wanghu and Poyang Lakes after the 1980s. In contrast, sedimentary δ13C increased in Honghu and Futou Lakes which have abundant submerged macrophytes. The timing and scale of sedimentary δ13C changes indicated stronger responses to eutrophication than damming, with eutrophication responses ranging from a macrophyte proliferation to the dominance of phytoplankton. [ABSTRACT FROM AUTHOR]

Freshwater aquaculture is an important source of protein worldwide. Over‐exploitation of fisheries can, however, add severely to pressures on ecosystem functioning and services. In Southeast Asia, aquaculture in freshwater lakes contributes significantly to the economy and to reductions in poverty and nutritional insecurity. However, overstocking and excessive feeding of fish can lead to a degradation of affected water bodies, manifest as eutrophication, toxic algal blooms, losses of biodiversity and amenity, anoxia and, in extreme cases, collapse of fisheries. Projected increased warming and storminess associated with global climate change are likely to magnify existing problems. Matching levels of aquaculture production with ecological carrying capacity is therefore likely to become increasingly challenging, requiring levels of data and understanding that are rarely available, a problem that is impossible to rectify in the short term using standard limnological approaches. This paper reviews the development of freshwater aquaculture in the Philippines, associated environmental impacts, and relevant environmental regulations and regulatory bodies. The potential role of palaeolimnology, a science that is relatively under‐utilised in the tropics generally and in tropical Asia in particular, in complementing extant datasets, including monitoring records, is highlighted through reference to a preliminary study at Lake Mohicap. Lake Mohicap currently supports aquaculture and is one of a cluster of seven volcanic crater lakes on Luzon, the largest of the archipelago of islands forming the Philippines. [ABSTRACT FROM AUTHOR]

Highly calcareous (marl) lakes are infrequent but important freshwater ecosystems, protected under the EU Habitats and Species Directive. Chara lakes have been considered resistant to eutrophication owing to the self-stabilising properties of charophyte meadows. However, the opposite is suggested by the large-scale biodiversity declines in marl lake taxa in Europe, and evidence of charophyte sensitivity to eutrophication. We combined contemporary, palaeolimnological and archival methods to investigate the eutrophication of Hawes Water, a shallow marl lake in north-west England ( U.K.)., Changes in aquatic macrophyte and invertebrate communities were reconstructed through the analysis of historical macrophyte surveys and sedimentary plant and animal macrofossils in two dated sediment cores from the littoral and deep zones of the lake. In addition, chlorophyll and carotenoid pigments were analysed to track changes in primary production from benthic and pelagic areas. Substantial changes in macrophyte communities were detected over centennial timescales, suggesting high ecosystem sensitivity considering the presently moderate phosphorus concentrations in Hawes Water (mean annual total phosphorus 20 μg L−1)., Two apparent periods of threshold-like change were identified from the sediment record: (i) changes in cyanobacteria (aphanizophyll + myxoxanthophyll to canthaxanthin + zeaxanthin) and potentially in nutrient stoichiometry, reductions in the maximum macrophyte colonisation depth and water clarity, reduced charophyte and Potamogeton diversity, and increases in Nymphaeaceae; and (ii) severe reductions in light availability inferred from subdecadal doubling in phytoplankton abundance, substantial increases in Daphnia abundance and the extinction of charophytes from higher water depths., Further, change in both the littoral and deeper water has confined key marl lake taxa to smaller niches. In the littoral, increasing siltation and reed and Nymphaeaceae densities caused extinction of Littorella uniflora in the early 1900s and have reduced the evenness of Characeae with suspected imminent extinction of two highly localised Chara spp. In the deeper water, upslope creep of maximum colonisation depth has reduced habitat for intermediate-depth marl lake taxa leading to the loss of four Potamogeton and one Chara species, and replacement of these taxa by Nuphar lutea., The large changes in macrophyte community composition and increased incidences of turbid water have reduced the distinctive and valued marl lake features of Hawes Water, indicating that marl lakes can, as a habitat type, be highly sensitive to eutrophication. The persistence of abundant generalist macrophyte species at considerable water depth may be a feature of high-alkalinity lakes in clearwater, macrophyte-dominated states, but is a distinct eutrophication response in marl lakes rather than an indication of resistance to eutrophication. [ABSTRACT FROM AUTHOR]

Disentangling the role of nutrient pollution and climate change on lake ecosystem functioning is paramount to protect water quality in lake catchments worldwide. For more effective management, however, we need to determine whether these two forcing factors interact at different spatial and temporal scales., This study compares centennial-scale archival data and lake sediment records of eutrophication from Blelham Tarn and previously published data from Lake Windermere's North Basin in the English Lake District. We aimed to quantify how lake morphometry, catchment characteristics and landscape position influence the relationship between climate, local land use and algal community change., Redundancy analysis revealed that increases in cyanobacterial pigments and stable isotopes of nitrogen and carbon in sediments of Blelham Tarn from the 1970s onwards correlate strongly with rising densities of sheep and cattle in the catchment. Concomitant installation of piped water and sewage processing facilities appeared to lead to the expansion of filamentous cyanobacteria. In contrast, elevated fossil pigments from siliceous algae after 1990 were related inversely to winter precipitation, suggesting seasonal changes in hydraulic flushing also influenced the algal community response to centennial-scale fertilisation., Abundance of vernal algae increased synchronously in Blelham Tarn and Lake Windermere's North Basin after regional agricultural intensification in the mid-nineteenth century. In contrast, differences in timing of wastewater disposal and treatment at each site led to asynchronous changes in summer taxa such as filamentous cyanobacteria., This study highlights that lake catchments can act as local filters to regional climate change, both due to differences in localised land-use and intrinsic hydrological features (e.g. catchment:lake area, flushing rate). Further, this paper highlights the ability of palaeolimnology to aid identification of significant nutrient sources over different spatial scales for effective catchment water management. [ABSTRACT FROM AUTHOR]

The south-east margin of Tibet is highly sensitive to global environmental change pressures, in particular, high contemporary reactive nitrogen (Nr) deposition rates (ca. 40 kg ha−1 yr−1), but the extent and timescale of recent ecological change is not well prescribed. Multiproxy analyses (diatoms, pigments and geochemistry) of 210Pb-dated sediment cores from two alpine lakes in Sichuan were used to assess whether they have undergone ecological change comparable to those in Europe and North America over the last two centuries. The study lakes have contrasting catchment-to-lake ratios and vegetation cover: Shade Co has a relatively larger catchment and denser alpine shrub than Moon Lake. Both lakes exhibited unambiguous increasing production since the late 19th to early 20th. Principle component analysis was used to summarize the trends of diatom and pigment data after the little ice age ( LIA). There was strong linear change in biological proxies at both lakes, which were not consistent with regional temperature, suggesting that climate is not the primary driver of ecological change. The multiproxy analysis indicated an indirect ecological response to Nr deposition at Shade Co mediated through catchment processes since ca. 1930, while ecological change at Moon Lake started earlier (ca. 1880) and was more directly related to Nr deposition (depleted δ15N). The only pronounced climate effect was evidenced by changes during the LIA when photoautotrophic groups shifted dramatically at Shade Co (a 4-fold increase in lutein concentration) and planktonic diatom abundance declined at both sites because of longer ice cover. The substantial increases in aquatic production over the last ca. 100 years required a substantial nutrient subsidy and the geochemical data point to a major role for Nr deposition although dust cannot be excluded. The study also highlights the importance of lake and catchment morphology for determining the response of alpine lakes to recent global environmental forcing. [ABSTRACT FROM AUTHOR]

Holocene changes in effective precipitation (precipitation-evaporation; P-E) were reconstructed for the Søndre Strømfjord region of southwest Greenland using the sediment records of two neighbouring closed-basin 'saline' lakes. Past lakewater conductivities (a proxy for P-E balance) were estimated using a diatom-inferred conductivity model. Broadly similar changes in both lake records corroborate the use of DI conductivity as a regional climate proxy. An increasing DI conductivity trend through the Holocene indicated net negative or balanced P-E ratios in this region, even during most of the Neoglacial period (post-4000 yr BP). This contrasts with other regions of West Greenland, where conditions became more humid in the Neoglacial period. The reconstructions presented here therefore suggest a high degree of spatial variability in the P-E balance, likely caused by region-specific orographic features. While interpretation of the mid-Holocene P-E balance was limited by dissolution of the diatom record, other sediment indicators suggest a period of extremely negative effective precipitation (~7000-5600 yr BP), followed by a period of positive effective precipitation lasting until 4700 yr BP. This contrasts markedly with the later Holocene, after c. 4000 yr BP, when high-frequency oscillations in DI conductivity probably reflect short-term climatic variations (amplified by in-lake processes connected with meromixis) but no long-term trends in the P-E balance. [ABSTRACT FROM AUTHOR]

Abstract Multi-proxy analyses of an 8 m sediment core from Lake Hayk, a closed, freshwater lake in the north-central highlands of Ethiopia, provide a record of changing lake level and inferred regional climatic change for the last 15.6 cal ka years. Between ca. 15.6–15.2 cal ka BP, a lowstand was synchronous with Heinrich Event 1 and an intense drought across Eastern Africa. At ca. 15.2 cal ka BP a lake began to develop at the core site in response to wetter conditions, at the onset of the African Humid Period (AHP). However, in contrast to other lakes in eastern Africa, Hayk lake level fell around ca. 14.8 cal ka BP, indicating a climate shift towards aridity. The lake began filling again at ca. 12.3 cal ka BP and reached maximum water depth between ca. 12.0–10.0 cal ka BP. Lake level declined slowly during the Holocene, culminating in the termination of the AHP at Hayk between ca. 5.2–4.6 cal ka BP. In the late Holocene, ca. 2.2–1.3 cal ka BP, Lake Hayk was again deep and fresh with some evidence of short-term lake level variability. The palaeo-record from Lake Hayk indicates that while it experienced, to a broad degree, the same glacial-interglacial dynamics and sub-millennial shifts in climate found in other palaeolimnological records from eastern Africa, there are offsets in timing and rate of response. These differences reflect chronological discrepancies between records, as well as the varying climate sensitivities and site-specific factors of individual lake basins. This record highlights the different responses by lakes in a climatically vulnerable area of Ethiopia. Highlights • Discrepancies in the timing of the Younger Dryas Stadial proper. • No evidence for the catastrophic shift in climate centred at 8.2 ka. • Diatom evidence suggesting relatively rapid AHP termination. • High-resolution evidence of millennial to multi-decadal variability at Lake Hayk. • Regional discrepancies in the timing and expression of climatic events. [ABSTRACT FROM AUTHOR]