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Reservoirs can emit substantial amounts of the greenhouse gas methane (CH4) via different emission pathways. In some reservoirs, reservoir flushing is employed as a sediment management strategy to counteract growing sediment deposits that threaten reservoir capacity. Reservoir flushing utilizes the eroding force of water currents during water level drawdown to mobilize and transport sediment deposits through the dam outlet into the downstream river. During this process, CH4 that is stored in the sediment can be released into the water and degas to the atmosphere resulting in CH4 emissions. Here, we assess the significance of this CH4 emission pathway and compare it to other CH4 emission pathways from reservoirs. We measured seasonal and spatial CH4 concentrations in the sediment of Schwarzenbach Reservoir, providing one of the largest datasets on CH4 pore water concentrations in freshwater systems. Based on this dataset we determined CH4 fluxes from the sediment and estimated potential CH4 emissions due to reservoir flushing. CH4 emissions due to one flushing operation can constitute 7–14 % of the typical annual CH4 emissions from Schwarzenbach Reservoir, whereby the amount of released CH4 depends on the timing of the flushing operation within the season. The larger the thickness of the sediment layer mobilized during the flushing operation the larger the average CH4 concentration per unit volume of flushed sediment. This suggests that regular flushing of smaller sediment layers releases less CH4 than removal of the same sediment volume in fewer flushing events of thicker sediment layers. In other reservoirs with higher sediment loadings than Schwarzenbach Reservoir, reservoir flushing could cause substantial CH4 emissions, especially when flushing operations are conducted frequently. Therefore, CH4 emissions due to reservoir flushing must be included in estimates of annual overall greenhouse gas emissions from reservoirs that are subject to regular flushing operations.

Objectives To compare the diagnostic accuracy of a single T2 Dixon sequence to the combination T1+STIR as anatomical sequences used for detecting tumoral bone marrow lesions in whole-body MRI (WB-MRI) examinations. Methods Between January 2019 and January 2020, seventy-two consecutive patients (55 men, 17 women, median age = 66 years) with solid (prostate, breast, neuroendocrine) cancers at high risk of metastasis or proven multiple myeloma (MM) prospectively underwent a WB-MRI examination including coronal T1, STIR, T2 Dixon and axial diffusion-weighted imaging sequences. Two radiologists independently assessed the combination of T1+STIR sequences and the fat+water reconstructions from the T2 Dixon sequence. The reference standard was established by consensus reading of WB-MRI and concurrent imaging available at baseline and at 6 months. Repeatability and reproducibility of MRI scores (presence and semi-quantitative count of lesions), image quality (SNR: signal-to-noise, CNR: contrast-to-noise, CRR: contrast-to-reference ratios), and diagnostic characteristics (Se: sensitivity, Sp: specificity, Acc: accuracy) were assessed per-skeletal region and per-patient. Results Repeatability and reproducibility were at least good regardless of the score, region, and protocol (0.67 ≤ AC1 ≤ 0.98). CRR was higher on T2 Dixon fat compared to T1 (p < 0.0001) and on T2 Dixon water compared to STIR (p = 0.0128). In the per-patient analysis, Acc of the T2 Dixon fat+water was higher than that of T1+STIR for the senior reader (Acc = +0.027 [+0.025; +0.029], p < 0.0001) and lower for the junior reader (Acc = −0.029 [−0.031; −0.027], p < 0.0001). Conclusions A single T2 Dixon sequence with fat+water reconstructions offers similar reproducibility and diagnostic accuracy as the recommended combination of T1+STIR sequences and can be used for skeletal screening in oncology, allowing significant time-saving. Key Points • Replacement of the standard anatomic T1 + STIR WB-MRI protocol by a single T2 Dixon sequence drastically shortens the examination time without loss of diagnostic accuracy. • A protocol based on fat + water reconstructions from a single T2 Dixon sequence offers similar inter-reader agreement and a higher contrast-to-reference ratio for detecting lesions compared to the standard T1 + STIR protocol. • Differences in the accuracy between the two protocols are marginal (+ 3% in favor of the T2 Dixon with the senior reader; −3% against the T2 Dixon with the junior reader).

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There is a current understanding of type 1 diabetes (T1D) as being the result of a heterogeneous disease of whole pancreas. So far, descriptions of pancreas alterations suggested a global involvement of inflammation throughout the organ, characterized by abnormal age-specific patterns (i.e. > or 0.05) decrease of PI (0.6±0.2 vs 1.1±0.3 mL/kg, p0.05). PVTAIL positively correlated with CPEPEST and CPEPBASAL at Δ+3 months (R=0.55, p=0.002; R=0.47, p0.05). PVBODY did not correlate with any pancreatic function. Finally, we identified multivariate models that included PV and predicted pancreatic endocrine function at Δ+6 and +12 months (i.e. IDAA1C, HbA1C). MRI performed early after T1D onset helped us define specific pancreas alterations and demonstrate for the first time: (1) the increased pancreas atrophy in prepubertal children; (2) correlations between pancreatic subregion volumes and their respective residual functions early after diagnosis and (3) the improvement of endocrine function prediction models by adding PV estimates as predictors. (1) Wentworth et al. 2019;62(1): 33–40 Presentation: Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m., Sunday, June 12, 2022 1:00 p.m. - 1:05 p.m.

Climate warming alters the seasonal timing of biological events. This raises concerns that species-specific responses to warming may de-synchronize co-evolved consumer-resource phenologies, resulting in trophic mismatch and altered ecosystem dynamics. Here we explore effects of warming on the temporal coherence of two key phenological events in lakes across Europe: The onset of the phytoplankton spring bloom and the spring/summer maximum of the grazer Daphnia. Simulation of 1,891,744 lake years revealed that, under the current climate, the phenological delay between the two events varies greatly (20-190 days) across lake types and geographic locations. Warming moves both phenological events forward in time and can predictably lengthen or shorten the delay between them by up to 60 days. Our findings expose large extant variation in phenological synchrony of planktonic organisms, provide quantitative predictions of its dependence on physical lake properties and geographic location, and highlight research needs concerning its ecological consequences.

Methane emissions from freshwater systems, and especially from small lakes source a significant proportion of naturally produced atmospheric methane. In small temperate lakes, storage flux, i.e. the diffusive emission of methane that was stored in anoxic waters during the seasonal overturn of the water column, can contribute a large fraction of annual methane emissions. Here we use an extensive field dataset to quantify methane storage in anoxic deep water and identify as well as quantify the sources and losses of this methane. The comparison of measurements from four years in a small temperate lake (Illmensee) shows that methane storage can differ substantially between years. In 2012 and 2018, the annual maximum of the methane stored in the entire water column was around twice as high (5350 kg and 5822 kg) as in 2013 and 2017 (2722 kg and 2295 kg). A methane mass balance approach suggests that the variability of methane storage in the anoxic water between the years was not caused by the methane flux at the anoxic-oxic water interface, but rather was related to changes in the methane source from the anoxic sediments. The interannual differences in sediment flux could not be explained by sediment temperature, but rather by the differing supply of organic matter. Our findings suggest that phytoplankton blooms promote methane storage within the same year and thus cause interannual variability in emissions during autumn overturn. published

The formation of ice cover on lakes alters heat and energy transfer with the water column. The fraction of surface area covered by ice and the timing of ice‐on and ice‐off therefore affects hydrodynamics and the seasonal development of stratification and related ecosystem processes. Multi‐year model simulations of temperate lake ecosystems that freeze partially or completely therefore require simulation of the formation and duration of ice cover. Here we present a multi‐year hydrodynamic simulation of an alpine lake with complex morphology (Lower Lake Constance, LLC) using the three‐dimensional (3D) model Aquatic Ecosystem Model (AEM3D) over a period of 9 years. LLC is subdivided into three basins (Gnadensee, Zeller See and Rheinsee) which differ in depth, morphological features, hydrodynamic conditions and ice cover phenology and thickness. Model results were validated with field observations and additional information on ice cover derived from a citizen science approach using information from social media. The model reproduced the occurrence of thin ice as well as its inter‐annual variability and differentiated the frequency and extent of ice cover between the three sub‐basins. It captured that full ice cover occurs almost each winter in Gnadensee, but only rarely in Zeller See and Rheinsee. The results indicate that the 3D model AEM3D is suitable for simulating long‐term dynamics of thin ice cover in lakes with complex morphology and inter‐annual changes in spatially heterogeneous ice cover.

Model skills of two 3D hydrodynamic models, ELCOM and Delft3D, were assessed by simulating internal seiches and surface currents and comparing the results with field data in Upper Lake Constance, Germany. Both models were set up using the same horizontal and vertical discretisation and forced by a horizontally resolved wind field. Model simulations were carried out during a stable stratification period in summer. Oscillations of the simulated and the measured isotherms in Lake Überlingen (north basin of Upper Lake Constance) indicated the existence of a Kelvin-type wave with a period of 84 h. The major wave pattern in period and amplitude was consistent between the simulations and the measurements, whereas discrepancies appeared in the phase match. The measured and the simulated vertical velocity structure in the middle of the main basin of Upper Lake Constance showed occurrence of a Poincaré-type wave with a 14 h period. Major oscillations of the velocity structure were captured by both models, and the agreement between the simulations and the measurements was higher in deep layers than in near-surface layers. Model prediction of the near-surface velocities as well as the surface circulation pattern was more accurate under high current velocities during strong winds than under weak winds. This study concluded that both ELCOM and Delft3D were significantly skilled at simulating internal wave motions and surface currents, and there was no significant difference in reproducing the measured lake dynamics between ELCOM and Delft3D. published

Introduction The British Antarctic Survey Medical Unit works in a very remote area of the world, with several Antarctic bases receiving only a single annual resupply of consumable goods. Pharmaceuticals supplied in this manner will often be approaching or past the end of their nominal shelf life before the following year's resupply. Drugs are transported from the UK via ship; the hold is not temperature controlled, and the ship crosses through the tropics (air temperature 25–30°C for approximately 3 wk). The drugs then must be transported from the ship to the base, often in temperatures substantially below freezing. This study assessed the stability of 5 expired drugs (atropine, nifedipine, flucloxacillin, naproxen, and bendroflumethiazide) returned from Antarctic bases. Methods Drugs were opportunistically obtained and tested using stability-indicating assays. Results All tested drugs were stable. Conclusions The results suggest that the studied drugs may be stable beyond expiry, even when not maintained in strictly temperature-controlled conditions.

The translation of the original seawater signal (i.e. ambient temperature and δ18Osw) into distinct chambers of a single shell of a foraminifer during calcification can influence our interpretation of surface ocean conditions of the past, when based upon oxygen and carbon stable isotope geochemistry. In this study three different hypotheses were tested to gain more insight into biological and ecological processes that influence the resultant composition of stable isotopes of oxygen (δ18O) in the shells of planktonic foraminifera. These hypotheses were related to the shell size; the differences in isotopic composition between the final chamber and the remaining shell; and the differences between different species. Shells of Trilobatus sacculifer, Globigerinoides ruber white and Neogloboquadrina dutertrei were picked from the top of multi-core GS07-150-24, of modern age, offshore of north-eastern Brazil (3∘46.474′ S, 37∘03.849′ W) and analysed for single-shell and single-chamber stable isotope analysis. We show that the mean value of δ18O of the final chambers (δ18OF) is 0.2 ‰ ± 0.4 ‰ (1σ) higher than the mean value δ18O of the test minus the final chamber (δ18O

Reservoirs are a common way to store and retain water serving for a multitude of purposes like storage of drinking and irrigation water, recreation, flood protection, navigation, and hydropower production, and have been built since centuries. Today, few reservoirs serve only one purpose, which requires management of present demands and interests. Since each reservoir project will cause negative impacts alongside desired advantages both on a local, regional and global scale, it is even more urgent to develop a common management framework in an attempt to mitigate negative impacts, incorporate different demands and make them visible within the discourse in order to avoid conflicts from early on. The scientific publications on reservoirs are manifold, yet a comprehensive and integrative holistic tool about management of this infrastructure is not available. Therefore, a comprehensive and integrated conceptual tool was developed and proposed by the authors of this paper that can contribute to the sustainable management of existing reservoirs. The tool presented herein is based on the results from the interdisciplinary CHARM (CHAllenges of Reservoir Management) project as well as the condensed outcome of relevant literature to aid and enhance knowledge of reservoir management. The incorporated results are based on field, laboratory and empirical social research. The project CHARM focused on five different aspects related to existing reservoirs in southern Germany (Schwarzenbachtalsperre, Franconian Lake District), namely: sedimentation of reservoirs, biostabilisation of fine sediments, toxic cyanobacteria(l) (blooms), greenhouse gas emissions from reservoirs and social contestation, respectively consent. These five research foci contributed to the topics and setup of a conceptual tool, put together by the research consortium via delphi questioning, which can be found alongside this publication to provide insights for experts and laymen. Conceptualising and analysing the management in combination with quantitative and qualitative data in one descriptive tool presents a novelty for the case studies and area of research. The distribution within the scientific community and interested public will possibly make a positive contribution to the goal of sustainable water resources management in the future.

A key phenological event in the annual cycle of many pelagic ecosystems is the onset of the spring algal bloom (OAB). Descriptions of the factors controlling the OAB in temperate to polar lakes have been limited to isolated studies of single systems and conceptual models. Here we present a validated modelling approach that, for the first time, enables a quantitative prediction of the OAB and a systematic assessment of the processes controlling its timing on a continental scale. We used a weather-driven, 1-dimensional lake model to simulate the seasonal dynamics of the underwater light climate in 16 lake types characterized by the factorial combination of 4 lake depths with 4 levels of water transparency. We did so at 1,962 locations across western Europe and over 31 years (1979-2009). Assuming that phytoplankton production is light-limited in winter, we identified four patterns of OAB control across lake types and climate zones. OAB timing is controlled by (i) the timing of ice-off in ice-covered clear or shallow lakes, (ii) the onset of thermal stratification in sufficiently deep and turbid lakes, and (iii) the seasonal increase in incident radiation in all other lakes, except for (iv) ice-free, shallow and clear lakes in the south, where phytoplankton is not light-limited. The model predicts that OAB timing should respond to two pervasive environmental changes, global warming and browning, in opposite ways. OAB timing should be highly sensitive to warming in lakes where it is controlled by either ice-off or the onset of stratification, but resilient to warming in lakes where it is controlled by incident radiation. Conversely, OAB timing should be most sensitive to browning where it is controlled by incident radiation, but resilient to browning where it is controlled by ice-off or the onset of stratification. Available lake data are consistent with our findings. published

Temperature is a key environmental factor controlling rates of litter decomposition in streams and other ecosystems. Normalizing decomposition rates for temperature not only enables assessments of the importance of temperature effects but also facilitates comparisons of potential other controlling factors. This chapter describes procedures to achieve such normalization. In streams, temperature normalization of decomposition rates has almost invariably assumed a linear relationship. Accordingly, litter mass remaining at various time points after deployment of litter in the field is regressed against thermal sums (degree days) obtained from continuous temperature records. The same approach is taken when an exponential temperature dependency is assumed, commonly by using a Q10 model, with Q10 being assumed to take a given value, such as 2, or being fitted along with the decomposition rate coefficient. Here the thermal sums of the degree-day model are replaced by an integral that captures the exponential relationship. Worked examples of the calculations are provided in a supplementary spreadsheet and as computer code. Similar approaches to temperature normalization can be adopted based on dependencies encapsulated in the Arrhenius law used in the metabolic theory of ecology. published

Comprehensive analysis of food webs requires identifying dietary sources that support the production of all major organisms within the food web/food chain. Here, we use stable isotope ratios (δ13C, δ34S and δ15N) to assess the relative contribution of different basal carbon sources to the diet of Nile tilapia (Oreochromis niloticus L.) in three tropical lakes Hawassa (also Awasa in literature), Ziway and Koka (Ethiopia). Computations were carried out with Stable Isotope Analysis in R (SIAR) model to quantify the dietary proportion of each prey for the tilapia fish. Basal food sources were distinguishable based on their δ13C, δ34S and δ15N values. In Lake Ziway, macrophytes (64%) were the dominant assimilated diet of tilapia while particulate organic matter (POM) and zooplankton contributed only 20% and 16%, respectively. In parallel, Nile tilapia in Lake Hawassa assimilated macrophytes (35%), POM (33%) and zooplankton (32%) at comparatively equal proportion. The dietary sources of the fish in Lake Koka were POM (49%) and zooplankton (51%). In contrast with earlier studies based on gut content analysis, the present results reveal that macrophytes contributed more and phytoplankton less than previously reported especially in macrophyte-dominated lakes Ziway and Hawassa. The ecological condition of the lakes might have been predominantly accountable for the diet change of the tilapia. As dietary data are prerequisite for food web/food chain analysis and aquaculture industry, re-evaluating the diet of aquatic organisms appear relevant.

Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies., Design Type(s)modeling and simulation objective • data collection and processing objective • source-based data analysis objectiveMeasurement Type(s)age-depth modelTechnology Type(s)computational modeling techniqueFactor Type(s)geographic location • depthSample Characteristic(s)Atlantic Ocean • marine sediment Machine-accessible metadata file describing the reported data (ISA-Tab format)

The West Antarctic Peninsula (WAP) exhibits strong spatial and temporal oceanographic variability, resulting in highly heterogeneous biological productivity. Calcifying organisms that live in the waters off the WAP respond to temporal and spatial variations in ocean temperature and chemistry. These marine calcifiers are potentially threatened by regional climate change with waters already naturally close to carbonate undersaturation. Future projections of carbonate production in the Southern Ocean are challenging due to the lack of historical data collection and complex, decadal climate variability. Here we present a six-year long record of the shell fluxes, morphology, and stable isotope variability of the polar planktic foraminifera Neogloboquadrina pachyderma (sensu stricto) from near Palmer Station, Antarctica. This species is fundamental to Southern Ocean planktic carbonate production as it is one of the very few planktic foraminifer species adapted to the marine polar environments. We use these new data to obtain insights into its ecology and to derive a robust assessment of the response of this polar species to environmental change. Morphology and stable isotope composition reveal the presence of different growth stages within this tightly defined species. Inter- and intra-annual variability of foraminiferal flux and size is evident and driven by a combination of environmental forcing parameters, most importantly food availability, temperature, and sea-ice duration and extent. Foraminiferal growth occurs throughout the austral year and is influenced by environmental change, a large portion of which is driven by the Southern Annular Mode and El Niño Southern Oscillation. A distinct seasonal production is observed, with highest shell fluxes during the warmest and most productive months of the year. The sensitivity of calcifying foraminifera to environmental variability in this region, from weeks to decades, has implications both for their response to future climatic change, and for their use as palaeoclimate indicators. A longer ice-free season could increase carbonate production in this region at least while carbonate saturation is still high enough to allow for thick tests to grow.

Metabolic transformations have a major impact on the development of primary producers in aquatic systems and thus affect the dynamics of the entire aquatic food web. Furthermore, metabolic transformations contribute to the carbon budget and thereby influence CO

Estimates of global methane (CH4) emissions from lakes and the contributions of different pathways are currently under debate. In situ methanogenesis linked to algae growth was recently suggested to be the major source of CH4 fluxes from aquatic systems. However, based on our very large dataset on CH4 distributions within lakes, we demonstrate here that methane-enriched water from shallow-water zones is the most likely source of the basin-wide mean CH4 concentrations in the surface water of lakes. Consistently, the mean surface CH4 concentrations are significantly correlated with the ratio between the surface area of the shallow-water zone and the entire lake, fA,s/t, but not with the total surface area. The categorization of CH4 fluxes according to fA,s/t may therefore improve global estimates of CH4 emissions from lakes. Furthermore, CH4 concentrations increase substantially with water temperature, indicating that seasonally resolved data are required to accurately estimate annual CH4 emissions.

Many Neolithic and Bronze Age cultural layers and pile fields embedded in sediments at the shorelines of pre-alpine lakes in France, Switzerland, Germany, Austria, and Italy (in part UNESCO World Heritage Sites), are threatened by littoral erosion which is partly caused by human agency. Based on measurements of the wave field at four prehistoric sites on Lake Constance (Germany, Switzerland) we discuss the role of passenger ship navigation. We also review the stability and ecological compatibility of archaeological preservation control fills on the littoral platform of Lake Constance, and we make some recommendations for future preservation measures. published

Methane emissions from lakes and reservoirs are a major natural source in the global budget of atmospheric CH4. A large fraction of these emissions are due to diffusive transport of CH4 from surface waters to the atmosphere. It was suggested recently that CH4 production in the oxic surface waters is required to compensate for diffusive CH4 emissions from lakes. In contrast, we demonstrate here that typical diffusive CH4-fluxes from sediments in shallow water zones, Fsed,S, suffice to explain CH4 emissions to the atmosphere. Our analysis is based on the combination of an exceptional data set on surface concentrations of CH4 with a mass balance model of CH4 that is focused on the surface mixed layer and considers CH4-fluxes from sediments, lateral transport, gas exchange with the atmosphere, and includes temperature dependencies of sediment fluxes and gas exchange. Fsed,S not only explains observed surface CH4 concentrations but also concentration differences between shallow and open water zones, and the seasonal variability of emissions and lateral concentration distributions. Hence, our results support the hypothesis that diffusive fluxes from shallow sediments and not oxic methanogenesis are the main source of the CH4 in the surface waters and the CH4 emitted from lakes and reservoirs. published

Horizontal dispersion in the surface waters of a medium-sized lake was investigated based on four experiments with ensembles of 14 to 17 drifters deployed in Lake Constance during the winter season. The experiments cover length scales between ∼30 and ∼3000 m which are typical for the grid length scales employed in 3-D models. Horizontal dispersion coefficients Kdisp were estimated to range from ∼0.01 to ∼0.03 m2 s−1 at 100 m and from ∼0.1 to ∼0.7 m2 s−1 at 1000 m length scale L of the drifter distributions. In all experiments Kdisp increased with L and in three of the four experiments this increase was about linear in L. However, although a linear increase of Kdisp with L is consistent with dispersion by shear diffusion in flow fields with constant current shear, numerical modelling of the paths of the drifters, and the resulting spreading of the drifter ensembles indicates that horizontal shear diffusion is not sufficient to explain the increase of Kdisp with L. The model results suggest that not shear diffusion due to large-scale shear across the entire drifter distribution, but scale-dependent turbulent diffusion is dominating the scale-dependence of Kdisp at length scales between 100 and 1000 m. According to the model results, effects due to horizontal shear contribute less than 40% to the overall dispersion coefficient at length scales between 100 and 1000 m. However, horizontal shear is required to explain shape and orientation of the drifter distributions.

Here we investigate the diel vertical migration (DVM) of the different larval stages of Chaoborus flavicans between spring and summer in two different lakes and three different years. Specific attention is given to the influence of the vertical distribution of dissolved oxygen (DO) on the DVM of the different larval instars. To our knowledge, this study is the first that combines continuous observations of DVM of C. flavicans with continuous measurements of DO distributions over several months, allowing the assessment of changes in DVM due to the development of hyperoxic conditions in the deep water of lakes. With ontogenetic development, C. flavicans larvae increase their sensitivity to changes in light intensity and their tolerance to low oxygen conditions. Our results suggest that the physiological changes of C. flavicans larvae are adaptations to seasonal changes in DO, improving migration abilities to enable utilization of hypoxic and anoxic waters to avoid predation. Interannual change in the abundance and vertical distribution of phytoplankton affecting DO concentrations was sufficient to alter DVM patterns of C. flavicans larvae between years.

Changes in the budget of dissolved methane measured in a small temperate lake over 1 year indicate thatanoxic conditions in the hypolimnion and the autumnoverturn period represent key factors for the overall annual methane emissions from lakes. During periods of stable stratification, large amounts of methane accumulate in anoxic deep waters. Approximately 46% of the stored methane wasemitted during the autumn overturn, contributing ∼80% of the annual diffusive methane emissions to the atmosphere. After the overturn period, the entire water column was oxic, and only 1% of the original quantity of methane remained in the water column. Current estimates of global methane emissions assume that all of the stored methane is released, whereas several studies of individual lakes have suggested that a major fraction of the stored methane is oxidized during overturns. Our results provide evidence that not all of the stored methane is released to the atmosphere during the overturn period. However, the fraction of stored methane emitted to the atmosphere during overturn may be substantially larger and the fraction of stored methane oxidized may be smaller than in the previous studies suggesting high oxidation losses of methane. The development or change in the vertical extent and duration of the anoxic hypolimnion, which can represent the main source of annual methane emissions from small lakes, may be an important aspect to consider for impact assessments of climate warming on the methane emissions from lakes.

The implications of climatic change on Lake Constance are studied by long-term hydrodynamic model simulations. In contrast to earlier studies that mostly have applied one-dimensional models, this investigation utilizes a three-dimensional hydrodynamic model and thus profits from the advantages of a spatial representation of lake bathymetry and hydrophysical processes. Model adaptation and validation are based on half a century of vertically resolved temperature recordings (1961 to 2011). Three different horizontal grid layouts are used to test the sensitivity of the thermal stratification and effective vertical turbulent diffusivities (D V ) determined with the heat-budget method to grid resolution. D V calculated from observations and from simulations with different grid resolutions agree rather well. However, in the deep water simulated D V are overestimated if the basin is resolved only by few grid cells. The investigation of the effect of climatic changes on Lake Constance is focused on the effects of altered air temperatures and wind velocities on deep-water renewal. Numerical tracers are used as indicators of the winter vertical transport and mixing. Effects on stratification and mixing strongly depend on the seasonal course of the climatic changes. Warmer winter temperatures result in reduced deep-water exchange and more frequent years with incomplete mixing. Increased air temperatures in summer have almost no effect on deepwater renewal in winter but increase water-column stability during summer and autumn. Increased wind speeds influence vertical mixing and surface heat fluxes and cause higher deep-water temperatures if wind speeds are higher during the summer. The application of numerical models to investigate the consequences of changes in physical forcing for dynamic processes in lakes has become an established methodology in limnology. Models are chosen according to the questions addressed but also with respect to the limitations of

The upper branch of the Atlantic Meridional Overturning Circulation predominantly enters the Atlantic Ocean through the southeast, where the subtropical gyre is exposed to the influence of the Agulhas leakage (AL). To understand how the transfer of Indian Ocean waters via the AL affected the upper water column of this region, we have generated new proxy records of planktic foraminifera from a core on the central Walvis Ridge, on the eastern flank of the South Atlantic Gyre (SAG). We analyzed the isotopic composition of subsurface dweller Globigerinoides ruber sensu lato, and thermocline Globorotalia truncatulinoides sinistral, spanning the last five Pleistocene glacial-interglacial (G-IG) cycles. The former displays a response to obliquity, suggesting connection with high latitude forcing, and a warming tendency during each glacial termination, in response to the interhemispheric seesaw. The δ18O difference between the two species, interpreted as a proxy for upper ocean stratification, reveals a remarkably regular sawtooth pattern, bound to G-IG cyclicity. It rises from interglacials until glacial terminations, with fast subsequent decrease, appearing to promptly respond to deglacial peaks of AL. Stratification, however, bears a different structure during the last cycle, being minimal at Last Glacial Maximum, and peaking at Termination I. We suggest this to be the result of the intensified glacial wind field over the SAG and/or of the invasion of the South Atlantic thermocline by Glacial North Atlantic Intermediate Waters. The δ13C time series of the two species have similar G-IG pattern, whereas their difference is higher during interglacials. We propose that this may be the result of the alternation of intermediate water masses in different circulation modes, and of a regionally more efficient biological pump at times of high pCO2.

[1] Past water column stratification can be assessed through comparison of theδ18O of different planktonic foraminiferal species. The underlying assumption is that different species form their shellssimultaneously, but at different depths in the water column. We evaluate thisassumption using a sediment trap time-series of Neogloboquadrinapachyderma(s) and Globigerina bulloides from the NWNorth Atlantic. We determined fluxes, δ18O and δ13C of shells from two size fractionsto assess size-related effects on shell chemistry and to better constrain the underlying causes of isotopic differences between foraminifera in deep-sea sediments. Our data indicate that in the subpolar North Atlantic differences in the seasonality of the shell flux, and not in depth habitat or test size, determine the interspecies Δδ18O. N. pachyderma(s) preferentially forms from early spring to late summer, whereas the flux ofG. bulloides peaks later in the season and is sustained until autumn.Likewise, seasonality influences large and small specimens differently, with large shells settling earlier in the season. [2] The similarity of the seasonal δ18O patterns between the two species indicatesthat theycalcify in an overlapping depth zone close to the surface. However, their δ13C patterns are markedly different (>1 ‰). Bothspecies have a seasonally variable offset from δ13CDIC that appears to be governed primarily by temperature, with larger offsets associated with higher temperatures. The variable offset from δ13CDIC implies that seasonality of the flux affects the fossil δ13C signal, which has implications for reconstruction of the past oceanic carbon cycle.

The inter-ocean exchange of warm and salt-enriched waters around South Africa (Agulhas leakage), may have played an important role in the mechanism of deglaciations. Paleoceanographic reconstructions from the Agulhas leakage corridor show that leakage maxima occurred during glacial terminations. Therefore enhanced leakage has been suggested as a forcing mechanism to shift the Atlantic Meridional Overturning Circulation into the interglacial mode of circulation. At present, studies have not considered that upstream changes in the properties of the Agulhas Current itself may, in part, explain the observed variability in the Agulhas leakage and play an important role in defining the upper ocean hydrography of the South Atlantic. Here, we present a multi-proxy record from a marine sediment core (CD154 17-17K) located in the main trajectory of the Agulhas Current that spans the past 100 kyr. The record shows considerable variability in reconstructed upper ocean temperatures and salinity. We also find that the relative abundance of tropical and sub-tropical planktic foraminifera, previously used as a proxy for Agulhas leakage fauna, shows considerable upstream variability, likely reflecting changes in the hydrography of the southwest Indian Ocean sub-gyre (SWIOSG) and upper ocean temperatures. Idealised numerical model simulations demonstrate that both a shifting and an intensification of the Southern Hemisphere westerlies modify the vigour of the SWIOSG. These changes also drive increased kinetic and eddy variability in the Agulhas Return Current that potentially enhances cross frontal mixing of southern sourced waters into the SWIOSG system. Our results suggest that variability in the upstream Agulhas Current hydrography is strongly linked to the dynamics of the Agulhas Return Current and strength of the SWIOSG and that downstream variability in the leakage area (Atlantic sector) at least partly reflects regional variations of the Agulhas Current as a whole. © 2013 Elsevier B.V.

Pacific Ocean sea surface temperatures (SST) influence rainfall variability on multidecadal and interdecadal timescales in concert with the Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscillation (IPO). Rainfall variations in locations such as Australia and North America are therefore linked to phase changes in the PDO. Furthermore, studies have suggested teleconnections exist between the western Indian Ocean and Pacific Decadal Variability (PDV), similar to those observed on interannual timescales related to the El Niño Southern Oscillation (ENSO). However, as instrumental records of rainfall are too short and sparse to confidently assess multidecadal climatic teleconnections, here we present four coral climate archives from Madagascar spanning up to the past 300 yr (1708–2008) to assess such decadal variability. Using spectral luminescence scanning to reconstruct past changes in river runoff, we identify significant multidecadal and interdecadal frequencies in the coral records, which before 1900 are coherent with Asian-based PDO reconstructions. This multidecadal relationship with the Asian-based PDO reconstructions points to an unidentified teleconnection mechanism that affects Madagascar rainfall/runoff, most likely triggered by multidecadal changes in North Pacific SST, influencing the Asian Monsoon circulation. In the 20th century we decouple human deforestation effects from rainfall-induced soil erosion by pairing luminescence with coral geochemistry. Positive PDO phases are associated with increased Indian Ocean temperatures and runoff/rainfall in eastern Madagascar, while precipitation in southern Africa and eastern Australia declines. Consequently, the negative PDO phase that started in 1998 may contribute to reduced rainfall over eastern Madagascar and increased precipitation in southern Africa and eastern Australia. We conclude that multidecadal rainfall variability in Madagascar and the western Indian Ocean needs to be taken into account when considering water resource management under a future warming climate.

We determine wind-wave properties and estimate the wave exposure along the entire shore of Lake U¨ berlingen, a subbasin of Lake Constance, using a third-generation spectral wave model (SWAN), and compare results to field data on surface waves at three different sites and to predictions from a simple fetch-based model (FETCH). Forcing the models with local wind data measured at a meteorological station in the center of Lake U¨ berlingen provides better simulation results than a spatially resolved wind field obtained from the numerical weather system of the Consortium for Small Scale Modeling (COSMO). Because wave diffraction is considered in SWAN but not in FETCH, SWAN provides on average higher wave heights than FETCH and agrees better with observations for waves with heights above 0.15 m. Wave exposure, that is, the frequency of occurrence of wind waves with heights above 0.15 m, varied substantially between different sites along the shore even at small spatial scales. Although the general pattern of wave exposure was similar for simulations with SWAN and FETCH, the model results differed at specific sites, especially at bays and headlands, and also with respect to the absolute values for wave exposure and its variation between sites. Simple fetch models may be insufficient to reliably quantify the spatial variability of wave exposure at small spatial scales, and complex wave models such as SWAN may be required even in medium-size lakes.

Environmental change strongly affects primary productivity of ecosystems via modifying bottom–up and top-down regulation of primary producers. Here we present a novel approach to quantify the relative importance of regulating factors in natural systems over various time scales: we calculated daily effect sizes of major factors affecting phytoplankton growth during the spring bloom period during almost three decades of lake oligotrophication using numerical experiments with a data based simulation model. We show that with oligotrophication the regulation of spring phytoplankton shifts from primarily top–down to bottom–up, and that the changes in regulation are non-linearly related to the nutrient (phosphorus) concentrations. Our findings indicate that long-term changes in top–down regulation cannot be understood without considering multiple herbivore taxa, here, microzooplankton (ciliates) and mesozooplankton (daphnids). We further demonstrate that bottom-up and top–down regulation are not independent from each other and that their interaction is time-scale dependent.

Okely et al. (2010a,b) are concerned with horizontal dispersion and mixing in lakes and employ numerical modeling to estimate horizontal dispersion coefficients. In both papers, the dispersal of four numerical Lagrangian particles is employed to estimate dispersion coefficients, and the authors explicitly claim that ‘‘...this method measures horizontal dispersion due to large-scale horizontal shear only, and the influence of other processes, such as turbulent diffusion, is not accounted for’’ (Okely et al. 2010b, p. 591). However, we demonstrate here that without a diffusive process, e.g., molecular or turbulent diffusion, the dispersal rate estimated from Lagrangian particles as in Okely et al. (2010a,b) should be zero in principal for largescale nondivergent horizontal flow fields, independent of the horizontal shear. The particle dispersal presented by Okely et al. (2010a,b) is not a measure of horizontal dispersion due to horizontal shear but depends on the initial position of the particles and the divergence of the simulated mean flow field and may be affected by undersampling of the flow field, since only four particles were considered. Whereas the study of Okely et al. (2010b) was entirely based on this Lagrangian particle technique, Okely et al. (2010a) compared horizontal dispersion coefficients estimated from this technique with coefficients determined from the spread of numerically simulated tracer distributions. According to Okely et al. (2010a), their horizontal dispersion coefficients agree reasonably well with an estimate based on the assumption that the effects of vertical shear and vertical turbulent mixing (Kz) determine horizontal dispersion (see Table 1 and Eq. 11 in Okely et al. 2010a). Okely et al. (2010a, p. 1875) state that ‘‘The vertical shear dispersion scaling was a good approximation for the magnitude and spatial variation of the average horizontal dispersion rates.’’ However, the horizontal dispersion coefficients obtained from Eq. 11 using the observed vertical diffusivities are an order of magnitude smaller than the values presented in Table 1 of Okely et al. (2010a) and thus are substantially smaller than the horizontal dispersion coefficients obtained from the tracer simulations. Hence, the combined effect of vertical shear and vertical diffusion cannot explain the large horizontal dispersion in the simulation of Okely et al. (2010a). Finally, the Lagrangian particle technique and also the dispersion of the simulated tracer clouds appear to depend mainly on the horizontal divergence of the simulated flow field. Comparison of simulated and observed particle tracks and horizontal currents at a single location indicates that the quality of the simulated flow field is not sufficient to provide the divergence of the true flow field in the lakes studied. Hence, Okely et al. (2010a,b) may provide estimates of the spread of particles and tracers in their simulated flow field but cannot contribute information on horizontal particle dispersal or horizontal dispersion and mixing under field conditions in lakes.

Temporal variability of vertical zooplankton distributions is analyzed on timescales from minutes to days based on 7 months of acoustic backscatter strength data in Lake Constance. A comparison with net samples reveals that most of the observed variability in volume backscatter strength is associated with variations in the abundance of large Daphnia. Diel vertical migration (DVM) of zooplankton was a persistent feature throughout the entire period of observation, while amplitude, daily migration timing, and migration speed varied with season. This active motion of zooplankton was affected by internal waves on a broad range of timescales. In spring, the maximum depth of DVM follows variations in isothermal depths associated with long-wavelength, basin-scale internal waves. Propagating, short-wavelength, internal waves with comparable amplitudes affect vertical zooplankton distributions on timescales of minutes. Spectral analyses of zooplankton abundance, temperature,and current velocity reveal close correspondence of spectral peaks and slopes, indicating that up to timescales of several days, the only temporal scale at which biological processes dominate over passive transport is associated with DVM. Whereas horizontal transport dominates at long periods, vertical transport occurs only at short timescales.

The oxygen isotopic composition of planktonic foraminifera tests is one of the widest used geochemical tools to reconstruct past changes of physical parameters of the upper ocean. It is common practice to analyze multiple individuals from a mono-specific population and assume that the outcome reflects a mean value of the environmental conditions during calcification of the analyzed individuals. Here we present the oxygen isotope composition of individual specimens of the surface-dwelling species Globigerinoides ruber and Globigerina bulloides from sediment cores in the Western Arabian Sea off Somalia, inferred as indicators of past seasonal ranges in temperature. Combining the δ18O measurements of individual specimens to obtain temperature ranges with Mg/Ca based mean calcification temperatures allows us to reconstruct temperature extrema. Our results indicate that over the past 20 kyr the seasonal temperature range has fluctuated from its present value of 16 °C to mean values of 13 °C and 11 °C for the Holocene and LGM, respectively. The data for the LGM suggest that the maximum temperature was lower, whilst minimum temperature remained approximately constant. The rather minor variability in lowest summer temperatures during the LGM suggests roughly constant summer monsoon intensity, while upwelling-induced productivity was lowered.

High-resolution thermistor chain data collected between 5 and 20 m in a large stratified freshwater lake (Lake Constance) at a water depth of 60 m reveal the frequent occurrence of large-amplitude (2: I-m) vertical density inversions that indicate overturns in the pelagic thermoc1ine. Velocity data collected simultaneously with the temperature measurements indicate that the density inversions are mainly produced by shear instabilities. A comparison between the timing of the passage of the basin-scale internal Kelvin wave and the density inversions demonstrates a pronounced phase relationship, implying that the processes leading to the occurrence of turbulence and mixing are connected to the passage of the Kelvin wave. Two different processes are identified during which density inversions were particularly common. An increased number of density inversions and especially high dissipation rates of turbulent kinetic energy were observed when Kelvin wave-induced critical shear supported the generation of large Kelvin-Helmholtz billows. A particularly large number of density inversions was also associated with the passage of nonlinear high­ frequency waves of large amplitudes. The density inversions typically occurred at the wave troughs, which indicates breaking of these waves. These observations indicate that self-induced shear generated by the basin­ scale seiche and by high-frequency internal waves leads to a significant amount of turbulence and mixing in the pelagic thermoc1ine.

Analyses of the effects of extreme climate periods have been used as a tool to predict ecosystem functioning and processes in a warmer world. The winter half-year 2006/2007 (w06/07) has been extremely warm and was estimated to be a half-a-millennium event in central Europe. Here we analyse the consequences of w06/07 for the temperatures, mixing dynamics, phenologies and population developments of algae and daphnids (thereafter w06/07 limnology) in a deep central European lake and investigate to what extent analysis of w06/07 limnology can really be used as a predictive tool regarding future warming. Different approaches were used to put the observations during w06/07 into context: (1) a comparison of w06/07 limnology with long-term data, (2) a comparison of w06/07 limnology with that of the preceding year, and (3) modelling of temperature and mixing dynamics using numerical experiments. These analyses revealed that w06/07 limnology in Lake Constance was indeed very special as the lake did not mix below 60 m depth throughout winter. Because of this, anomalies of variables associated strongly with mixing behaviour, e.g., Schmidt stability and a measure for phosphorus upward mixing during winter exceeded several standard deviations the long-term mean of these variables. However, our modelling results suggest that this extreme hydrodynamical behaviour was only partially due to w06/07 meteorology per se, but depended also strongly on the large difference in air temperature to the previous cold winter which resulted in complete mixing and considerable cooling of the water column. Furthermore, modelling results demonstrated that with respect to absolute water temperatures, the model ‘w06/07’ most likely underestimates the increase in water temperature in a warmer world as one warm winter is not sufficient to rise water temperatures in a deep lake up to those expected under a future climate.

This study investigates the role of surface waves and the associated disturbance of littoral sediments for the release and later distribution of dissolved methane in lakes. Surface wave field, wave-induced currents, acoustic backscatter strength, and the concentration and distribution of dissolved methane were measured simultaneously in Lake Constance, Germany. The data indicate that surface waves enhance the release of dissolved methane in the shallow littoral zone via burst-like releases of methane during the passage of wave groups. The amount of released methane depends on the surface wave field and the water temperature that controls the methane production in the sediments. The dissolved methane concentrations in the shallow littoral zone were always higher than concentrations in the deep water and open water, while methane concentrations in the epilimnion were typically higher than methane concentrations in the metalimnion and upper hypolimnion. The relatively high epilimnetic methane concentrations in the pelagial can be explained by lateral transport of methane from the littoral zone to the pelagic zone. Littoral zones can thus be an important source of methane in lakes. Methane released by surface waves from littoral sediments may cause elevated near-surface methane concentrations in large areas that enhance the flux of methane at the air water interface and, thus, the overall methane emissions from lakes to the atmosphere.

Modeling studies examining the effect of lakes on regional and global climate, as well as studies on the influence of climate variability and change on aquatic ecosystems, are surveyed. Fully coupled atmosphere–land surface–lake climate models that could be used for both of these types of study simultaneously do not presently exist, though there are many applications that would benefit from such models. It is argued here that current understanding of physical and biogeochemical processes in freshwater systems is sufficient to begin to construct such models, and a path forward is proposed. The largest impediment to fully representing lakes in the climate system lies in the handling of lakes that are too small to be explicitly resolved by the climate model, and that make up the majority of the lake-covered area at the resolutions currently used by global and regional climate models. Ongoing development within the hydrological sciences community and continual improvements in model resolution should help ameliorate this issue.

In the present study, a numerical model of the hydrodynamic and thermal structure of artificial shallow lakes in eastern India has been developed as a tool to assess the ecological water quality, driven by the meteorological forcings. It allows quantification of the vertical mixing processes that govern not only the thermal structure but also nutrient exchanges and the distribution of dissolved and particulate matter among water layers. Vertical temperature profiles were calculated by solving coupled partial differential equations for temperature (heat energy balance equation), one-dimensional momentum equation and a second order closure scheme for small-scale turbulence effects, i.e. turbulent kinetic energy and turbulent dissipation rates numerically using an implicit time integration method. The effect of advection due to the inflow and outflow is not taken into consideration as these shallow waterbodies are assumed to behave as a closed lake. The oxygen level measured at different layers during the simulation period in these waterbodies reflects hypolimnetic oxygen depletion due to the thermal stratification in the aquatic environment. The changes in the stratification regime in these waterbodies are expected to affect the water quality and the health of the ecosystem, primarily based on temperature and dissolved oxygen parameter and in particular, the certain features of the oxygen resources of the hypolimnion.