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Summary: Microbial communities in natural ecosystems are subject to strong ecological rules. The study of local communities along a regional metacommunity can reveal patterns of community assembly, and disentangle the underlying ecological processes. In particular, we seek drivers of community assembly at the regional scale using a large lacustrine dataset (>300 lakes) along the geographical, limnological and physico‐chemical gradients in the Pyrenees. By using high throughput amplicon sequencing of the 16S rRNA gene, and inferring environmental sources of bacterial immigrants, we showed that surface aquatic bacterial assemblages were strongly influenced by terrestrial populations from soil, biofilms or sediments, and primarily selected by a pH‐alkalinity gradient. Indeed, source proportions explained 27% of the community variation, and chemistry 15% of the total variation, half of it shared with the sources. Major taxonomic groups such as Verrucomicrobia, Actinobacteria and Bacteroidetes showed higher aquatic affinities than Parcubacteria, Gammaproteobacteria, Alphaproteobacteria or Betaproteobacteria, which may be recruited and selected through different hydrographic habitats. A regional fingerprint was observed with lower alpha diversity and higher beta diversity in the central Pyrenees than in both ends. We suggest an ecological succession process, likely influenced by complex interactions of environmental source dispersal and environmental filtering along the mountain range geography. [ABSTRACT FROM AUTHOR]

In this study, we explored the impact of eutrophication and stratification on hypoxia in deep waters of moderately warm Croatian marine lakes. Although the Mljet Lakes (MLs) are predominantly oligotrophic, mesotrophic conditions are present at depths below 20 m in the Small Lake (SL) and below 30 m in the Big Lake (BL), along with higher apparent oxygen utilization (AOU). Hypoxia at depths ≥ 25 m in SL and and ≥ 40 m in BL was observed between October 2009 and January 2010, and in SL in summer (July and September 2010). Significant differences (p < 0.05) in several physical, biological and chemical parameters were detected between the lakes, while AOU, derived oxygen utilization rate (OUR) and organic carbon remineralization rate (OCRR) were not significantly different (p >0.05) between the lakes. An intense and persistent pycnocline throughout the year, comparatively high water temperature, extended water renewal time and summer phytoplankton bloom were identified as physical and biological parameters which might have significantly contributed to increased frequency of hypoxic events in a shallow SL. Significantly (p < 0.05) higher ammonium concentration in SL, especially in its deep water, seems to be a long-term chemical feature related to the poor ventilation and higher sediment oxygen demand. At the current level of eutrophication and the present climate change trends, the MLs and similar systems may experience more persistent and intense stratification, which could further prevent mixing between upper and deep waters, likely leading to increasing duration of hypoxia and its negative impacts on the biodiversity of benthic communities.

Globally, there has been a decrease in dissolved oxygen in the oceans, that is more pronounced in coastal waters, resulting in more frequent hypoxia exposure for many marine animals. Managing hypoxia requires an understanding of the dynamics of dissolved oxygen (DO) where it occurs. The French coast facing the Bay of Biscay (N-E Atlantic Ocean) hosts at least a dozen tidal and turbid estuaries, but only the large estuaries of the Gironde and the Loire, are subject to a continuous monitoring. Here, we compared the DO dynamics in these two systems, in order to evaluate to what extent it is possible to predict the sensitivity to hypoxia of a tidal and turbid estuary based on its most common and easily accessible characteristics (the estuary surface area and length, liquid and solid discharge, suspended particulate load, water renewal time, presence of cities). Whereas the hyperturbid Gironde estuary is the longest (170 km), implying longer water and particle residence times, and is bordered by a large metropole (>750,000 inhabitants), only episodic summer hypoxia events (DO < 2 mg L−1) were recorded in its urban Garonne subestuary. In contrast, the turbid Loire estuary, smaller in terms of surface area and length, experiences permanent summer hypoxia. This demonstrates the inability to evaluate the susceptibility of a tidal and turbid estuary to hypoxia only by considering its general characteristics. We urge that there is a need for a pertinent DO survey strategy based on an initial assessment of DO in a specific estuary based on longitudinal investigations, in particular during the warm season (as higher water temperatures decrease DO solubility), for detecting the potential zones of minimum DO.

Identifying zones of stagnation and deposition of terrigenous matter or contaminants induced by human activity is a key issue in coastal areas. In this paper, circulation processes and potential contaminant stagnation and deposition zones were assessed using hydrodynamic time parameters forced with meteorology. The study focused on an Eulerian time parameter, the local e-flushing time (eFT). The hydrodynamic modeling of coastal zones was applied to two bays of New Caledonia, located downstream of open mining sites. Numerous simulations were performed to classify the influence of forcing conditions on the eFT variability. The need to consider meteorological forcings rather than average weather conditions for the calculation of eFTs was demonstrated. In coastal zones, high wind velocity was the major forcing influencing eFTs, but below a threshold wind velocity, tidal range and river inputs became significant parameters. Spatial variations of eFT values, depending on meteorological conditions, induced varying risks of stagnation zones. General responses of the bays' hydrodynamics and the exposure of zones to potential contaminants were defined under various forcings. Our findings demonstrate that strong turbulence zones are not always characterized by short eFTs because of antagonistic forcing effects. Diurnal tidal alternations were also proven to have less influence on eFT variations than tidal range changes over a lunar cycle. [ABSTRACT FROM AUTHOR]

The oceanographic forecast capability in coastal seas is often limited by the capacity of the numerical models in correctly reproducing the complex morphology of the coastline and the exchange processes between the shelf and the open seas. In the marginal Adriatic Sea this task is of uppermost importance due to the presence of several coastal water bodies and rivers. We present here a new operational oceanographic system, called Tiresias, based on the unstructured grid model SHYFEM and representing the whole Adriatic Sea together with the lagoons of Marano-Grado, Venice and Po Delta. The novelty of this oceanographic system resides in the very high-resolution, up to 10 m, of the numerical mesh, and in the high spatial and temporal resolution of the forcing and boundary conditions that drive the forecasts. The forecast results are evaluated against sea temperature and salinity profiles, mean circulation fields derived from a regional ocean model, tide gauges and drifter trajectory. The presented results highlighted the capacity of Tiresias in forecasting the general circulation in the Adriatic Sea, as well as several relevant coastal dynamics, such as saltwater intrusion, storm surge and riverine waters dispersion. [ABSTRACT FROM AUTHOR]

This study aimed to compare the toxicity for fish of two active ingredients (lambda-cyhalothrin-20 g L−1, a pyrethroid, and acetamiprid-15 g L−1, a neonicotinoid) which are components of a commercial insecticide (Acer 35 EC) used in cotton crop in many West African countries. The juveniles of Oreochromis niloticus (4.01 ± 0.34 g, mean body weight) were exposed for 96 h to increasing concentrations of active ingredients (lambda-cyhalothrin and acetamiprid) or a mixture similar to Acer 35 EC (composed by 20 g of chemical compound lambda-cyhalothrin and 15 g of acetamiprid dissolved in 1 L of acetone). The experiments were carried out under controlled conditions in aquaria according to OECD Guidelines. During the experiments, the behavioral responses (loss of balance, color change, hyperactivity, etc.) that usually precede death were observed in exposed fish. Mortalities were recorded in each aquarium and the LC50-96h of each chemical was determined. The LC50-96h obtained were respectively 0.1268, 0.0029, 182.9 and 0.5685 ppm for Acer 35 EC, lambda-cyhalothrin, acetamiprid and mixture. All insecticides used in this study had profound impact on Nile tilapia behavior which may confirm the neurotoxicity of each single active compound as well as of their mixture. [ABSTRACT FROM AUTHOR]

Purpose: Our goal was to understand how differences in source material (allochthonous vs. autochthonous) and phytoplankton communities (diatoms vs. cyanobacteria) influence composition and settling properties of suspended particulate matter. Materials and methods: We characterized the composition and settling properties of suspended particulate matter in two systems—one which has a high hydrologic loading factor (watershed to surface area ratio), and a diatom-dominated phytoplankton community (James River Estuary, USA), and a second, where hydrologic inputs are proportionally smaller, and the summer phytoplankton community is dominated by cyanobacteria (Curonian Lagoon, Lithuania). Results and discussion: In the James, we found that TSS concentrations were positively related to discharge, whereas POC concentrations were negatively correlated with discharge and positively correlated with Chl-a. We infer that suspended particulate matter in this system was largely derived from allochthonous inputs, but that the organic matter fraction was derived from autochthonous production. In the Curonian Lagoon, TSS concentrations were correlated with Chl-a, but not discharge, indicating that suspended particulate matter was largely of autochthonous origin. In the James, the proportion of settleable materials was greater during high TSS concentrations, indicating that high discharge events delivered heavier particulates. In the Curonian Lagoon, we observed a seasonal decline in settling rates associated with the shift from mixed (diatoms and chlorophytes) to cyanobacteria-dominated phytoplankton, which we attribute to the presence of positively buoyant cyanobacteria. Conclusions: We found that a comparative approach yielded useful insights regarding sources, composition, and settling properties of suspended particulate matter in two estuaries that differ in hydrologic loading and phytoplankton community composition. Our findings suggest that the presence of positively buoyant cyanobacteria favors export losses of particulate C, N, and P to marine waters over retention through sedimentation in transitional zones. [ABSTRACT FROM AUTHOR]

This study investigates and compares the seasonal variations during winter and summer of living benthic foraminiferal assemblages in the Saquarema Lagoon System (SLS) located in a tropical coastal region of SE Brazil. The physicochemical parameters in the SLS presented strong differences between both seasons; however, they remained quite homogeneous in winter and exhibited great oscillations in summer. In the study area, 29 and 19 living foraminiferal species were identified in summer and winter, respectively. Ammonia tepida was the dominant species in both seasons followed by Ammonia parkinsoniana and Cribroelphidium excavatum. The detrended correspondence analysis showed that in the SLS: the calcareous species, such as Adelosina carinatastriata, Bulimina patagonica, and Miliolinella antartica were mostly influenced by high dissolved oxygen contents and low temperature, whereas the agglutinated foraminiferal species were indicators of the most confined lagoonal areas, under the lowest pH and salinity values. The results of this work evidence that the seasonal variation in the SLS might play a more important role in the benthic foraminiferal assemblages' distribution than the anthropogenic impacts and shows the importance of seasonal studies for coastal biomonitoring programs. [ABSTRACT FROM AUTHOR]

Monitoring environmental contaminants in edible animal species is important for the assessment of environmental quality and for food safety as well. In this study, the occurrence of 2,3,7,8 dioxins, and furans, dioxin-like polychlorinated biphenyls, six non-dioxin-like polychlorinated biphenyl indicators (ndl-PCBs), seven polybrominated diphenylethers (PBDEs), four polycyclic aromatic hydrocarbons (PAHs) and four metals (As, Hg, Ni, Pb) was evaluated in 90 European eels (Anguilla anguilla) from Lake Garda. The results on eel samples showed a frequent contamination of dl-PCBs, found at concentrations higher than the maximum limit (ML) provided by European Union in 41 % of the eels. Two PBDE congeners (BDE 47 and BDE 99) were detected in almost all the samples. The contamination by PAHs was found negligible; only one sample had a high concentration of benzo[a]pyrene. Concerning metals, Pb showed concentrations higher than the maximum level (ML) in 77% of the samples, while Ni content could be a cause of acute toxicity due to dermatitis. Extreme consumers (95th percentile) are subjected to an actual risk attributable to dioxins, furans, and mainly dl-PCBs and PBDEs. The contamination of Lake Garda eels is therefore a matter of concern and a constant monitoring of the pollution levels of its freshwater is mandatory. [ABSTRACT FROM AUTHOR]

Ciliates represent central nodes in freshwater planktonic food webs, and many species show pronounced seasonality, with short-lived maxima of a few dominant taxa while many being rare or ephemeral. These observations are primarily based on morphospecies counting methods, which, however, have limitations concerning the amount and volume of samples that can be processed. For high sampling frequencies at large scales, high throughput sequencing (HTS) of freshwater ciliates seems to be a promising tool. However, several studies reported large discrepancy between species abundance determinations by molecular compared to morphological means. Therefore, we compared ciliate DNA metabarcodes (V9 regions of the 18S rRNA gene) with morphospecies counts for a 3-year study (Lake Zurich, Switzerland; biweekly sampling, n = 74). In addition, we isolated, cultivated and sequenced the 18S rRNA gene of twelve selected ciliate species that served as seeds for HTS analyses. This workflow allowed for a detailed comparison of V9 data with microscopic analyses by quantitative protargol staining (QPS). The dynamics of V9 read abundances over the seasonal cycle corroborated well with morphospecies population patterns. Annual successions of rare and ephemeral species were more adequately characterized by V9 reads than by QPS. However, numbers of species specific sequence reads only partly reflected rank orders seen by counts. In contrast, biomass-based assemblage compositions showed higher similarity to V9 read numbers, probably indicating a relation between cell sizes and numbers / sizes of macronuclei (or 18S rRNA operons). Full-length 18S rRNA sequences of ciliates assigned to certain morphospecies are urgently needed for barcoding approaches as planktonic taxa are still poorly represented in public databases and the interpretation of HTS data depends on profound reference sequences. Through linking operational taxonomic units (OTUs) with known morphospecies, we can use the deep knowledge about the autecology of these species. [ABSTRACT FROM AUTHOR]

Land-based activities are, undoubtedly, the main source of marine litter, particularly in a highly populated closed sea basin, such as the Mediterranean Sea. Rivers, consequently, act as a pathway of mismanaged waste to the sea. While quantification of inputs is a difficult task, the assessment of abundance, composition trends and baselines, and the identification of sources and main sectors producing marine litter are of crucial importance to support the ability of policy makers to improve waste reduction measures. For this reason, the Joint Research Centre (JRC/RIMMEL) coordinated a network of several research bodies that monitored floating litter (> 2.5 cm) from fixed observation points located on rivers near the sea using the same systematic research protocol. In Italy, one of the surveyed rivers was the Tiber, the third longest river on the peninsula, which after running through the city of Rome divides into two branches before flowing into the Tyrrhenian Sea. Results of 1 year of monitoring, September 2016-August 2017, highlight that 82% of the floating items were plastic and belong to the food and cosmetic sector, and it was estimated that 85.4% (± 9.4) of litter items get into the sea each hour from the Tiber river canal in Fiumicino, of which approximately 30% were already fragmented. [ABSTRACT FROM AUTHOR]

Assembly rules of fish communities in Tuamotu Archipelago lagoons have been previously investigated but without considering lagoons where shallow habitats are dominated by giant clam populations. We tested if assembly rules for 14 atolls were challenged by such an atoll (Fangatau), and in doing so investigated the robustness of inter-island population patterns to specialized habitats. Fangatau had significantly higher proportions of Pomacentridae and Scaridae, but lower proportions of Labridae, Acanthuridae, Balistidae and Serranidae. Functionally, herbivores, omnivores, species <7 cm or between 50 and 80 cm, and species forming schools were found in higher proportions, while sessile invertebrate feeders, medium-size species (30-50 cm) and solitary species were lower. These differences were rather related to Fangatau isolation, as no other physical or ecological feature could explain them. Total richness, functional richness, and functional redundancy remained correlated mainly to atoll size, without outliers related to high clam abundance and presence of peculiar habitats. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the variability of the water exchanges in the Curonian Lagoon based on the hydraulic regime and the atmospheric forcings. A finite element hydrodynamic model has been applied to the Curonian Lagoon to simulate the circulation patterns for 10 years. With the help of a transport–diffusion model, the salinity distribution and the renewal times of the Curonian Lagoon have been investigated when forced by river runoff, wind, and Baltic Sea level fluctuations. The hydrodynamic model has been validated using in situ salinity measurements. Model results show that the variability depends mainly on seasonal changes in hydrographic forcing and on the dominant wind regimes that prevail over the Curonian Lagoon. Exchanges between the southern and the northern part of the lagoon mostly depend on the wind forcing and are much less influenced by the river discharge. However, when looking at the water renewal time, the most important factor is the river discharge into the lagoon. Other physical forcings only marginally determine the renewal time, and not even ice cover is able to influence it. Even if ice cover strongly inhibits the exchanges between the southern and northern lagoon, it is basically not able to change the absolute value of the renewal times.

We investigated the effects of seasonal air temperature on the north basin of Lake Lugano (Switzerland and Italy), a deep peri-Alpine lake that is recovering from eutrophication. A priori ideas concerning the effects of temperature on key ecosystem responses were formalized in a conceptual model, which was tested against observed responses (from 28 years of monitoring data) using structural equation modeling. The results broadly supported our model, and indicated that air temperature had pervasive effects on the lake’s ecosystem. Warmer-than-usual winters restricted the depth of vertical mixing during turnovers and, as a result, reduced the associated replenishment of phosphorus (P) to surface waters. Moreover, although the reduction of P increased the potential for nutrient limitation, warmer winters and summers led to increases in summer chlorophyll a content (an index of phytoplankton biomass) through complex direct and indirect (food-web) effects. This apparently paradoxical effect on chlorophyll mimics a symptom of eutrophication, and suggests that climate warming might impede the recovery of the lake from eutrophication or increase the management interventions that will be required to meet the restoration targets. [ABSTRACT FROM AUTHOR]

We combined a mass balance approach with measurements of air-water and sediment-water nitrogen (N) exchange to better understand the mechanisms attenuating N throughputs in a eutrophic coastal lagoon. We were particularly interested in how seasonal shifts in external versus internal N fluxes and the transition from diatom- to cyanobacteria- dominated phytoplankton communities influence N storage and loss to the atmosphere. We found that on an annual basis almost all of the N removed by the lagoon was due to sediment storage following the spring diatom bloom. This period was characterized by high riverine inputs of dissolved inorganic nitrogen, high rates of assimilatory conversion to particulate nitrogen (PN), and net accrual of N in sediments. By contrast, the larger summer bloom was associated with low sediment N storage, which we attribute in part to the presence of positively-buoyant cyanobacteria. Low settling rates during cyanobacteria blooms favored export of PN to the Baltic Sea over sediment accrual in the lagoon. In addition, summer dinitrogen (N2) fixation by cyanobacteria largely offset annual N2 losses via denitrification. These findings show that cyanobacteria blooms diminish N attenuation within the lagoon by altering the balance of N exchange with the atmosphere and by promoting export of particulate N over sediment burial. [ABSTRACT FROM AUTHOR]

Abstract: Extreme weather events may be just as important as gradual trends for the long‐term trajectories of ecosystems. For alpine lakes, which are exposed to both exacerbated atmospheric warming and intense episodic weather events, future conditions might not be appropriately forecast by only climate change trends, i.e. warming, if extreme events have the potential to deflect their thermal and metabolic states from their seasonal ranges. We used high‐frequency monitoring data over three open‐water seasons with a one‐dimensional hydrodynamic model of the high‐altitude Lake Muzelle (France) to show that rainstorms or windstorms, notwithstanding their intensity, did not trigger long‐lasting consequences to the lake characteristics when light penetration into the lake was not modified. In contrast, storms associated with high turbidity input from the watershed (“turbid storms”) strongly modified the lacustrine hydrodynamics and metabolism for the rest of the open‐water season through reduced light penetration. The long‐lasting effects of turbid storms were related to the inputs and in‐lake persistence of very light glacial suspensoids from the watershed. The occurrence of the observed turbid storms was not related to the wind or rain intensities during the events. Instead, the turbid storms occurred after dry and atypically warm spells, i.e. meteorological conditions expected to be more frequent in this alpine region in the upcoming decades. Consequently, storm events, notwithstanding their intensity, are expected to strongly imprint the future ecological status of alpine lakes under climate warming. [ABSTRACT FROM AUTHOR]

The hydrodynamics of a tidally forced semi‐enclosed coral reef atoll (North Scott) at the edge of the continental shelf of northwestern Australia were investigated by combining field observations and numerical modeling. The observations revealed that the spring tidal range outside the atoll reaches 4 m, and as the water level drops below mean sea level, the reef rim surrounding the shallow (~10–15 m) lagoon becomes exposed. During this time, the lagoon can only exchange with the open ocean through two narrow channels, resulting in highly asymmetric water levels and velocities that were most pronounced during spring tide. On average, the ebb tide duration was ~2 hr longer than the flood, with rapid flood velocities in the channel reaching 2 m/s. We applied an unstructured grid model Delft3D‐Flexible Mesh to simulate the atoll hydrodynamics and were able to replicate the asymmetric water levels and complex velocities in the lagoon. The results revealed that at higher tidal stages, a dominant momentum balance exists between the pressure gradient (established by the propagation of the tide on the shelf) and the local flow acceleration of water throughout the interior of the atoll. At lower tidal stages, which coincided with a reversal of the offshore tidal pressure gradient, the lagoon became isolated from offshore dynamics and all momentum terms were negligible. This resulted in a tidally averaged residual westward flow within the lagoon that drove an asymmetric flushing pattern within the atoll, which we propose would be a common flushing mechanism within other tide‐dominated atolls worldwide. Key Points: The hydrodynamics of a tide‐dominated atoll reef system were assessed with field observations and numerical modelingThe semi‐enclosed atoll morphology and large tidal range resulted in highly asymmetric water levels, velocities, and residual flowsA residual east‐west flow, caused by interactions between the phasing of the ocean tide and lagoon water levels, dominated flushing [ABSTRACT FROM AUTHOR]

Two 3-year time series of hourly measurements of the fugacity of CO2 (fCO2) in the upper 10m of the surface layer of the northwestern Mediterranean Sea have been recorded by CARIOCA sensors almost two decades apart, in 1995-1997 and 2013-2015. By combining them with the alkalinity derived from measured temperature and salinity, we calculate changes in pH and dissolved inorganic carbon (DIC). DIC increased in surface seawater by ∼ 25 μmol kg-1 and fCO2 by 40 μatm, whereas seawater pH decreased by ∼ 0:04 (0.0022 yr-1). The DIC increase is about 15% larger than expected from the equilibrium with atmospheric CO2. This could result from natural variability, e.g. the increase between the two periods in the frequency and intensity of winter convection events. Likewise, it could be the signature of the contribution of the Atlantic Ocean as a source of anthropogenic carbon to the Mediterranean Sea through the Strait of Gibraltar. We then estimate that the part of DIC accumulated over the last 18 years represents∼ 30% of the total inventory of anthropogenic carbon in the Mediterranean Sea. [ABSTRACT FROM AUTHOR]

Ongoing socio-economic and climatic changes can differentially affect the biogeochemistry of the key nutrients nitrogen (N), silica (Si), and phosphorus (P) by altering their soil budgets, their transfer to aquatic environments and their ecological stoichiometry. This may lead to cascade consequences for aquatic communities and biogeochemical processes. Soil budgets, river export, and N, Si, and P ecological stoichiometry were assessed in a heavy impacted basin (Mincio River, Italy) in two decades (1991-2000; 2001-2010). The main aim was to analyse element-specific mechanisms of terrestrial-aquatic transport and retention within aquatic habitats. Budget results suggest a net accumulation (inputs exceeding outputs) of all nutrients in agricultural lands, mainly due to livestock manure, with a reduction for N (196 kg N ha−1 year−1 in 2000, and 132 kg N ha−1 year−1 in 2010), and constant values for Si (up to 3 kg Si ha−1 year−1) and P (43 kg P ha−1 year−1) along the study period. River export of N and P accounted for 3-27% and ~ 2% of N and P soil net accumulation, respectively, while Si export was significantly greater (25 kg Si ha−1 year−1) than Si net accumulation on farmlands. The stoichiometry of net nutrient accumulation in soils was not reflected by the stoichiometry of nutrient riverine export, due to element-specific mechanisms. We speculate that N and Si vertical and horizontal mobilization is increased by the irrigation loop, while P retention is favored by limited erosion due to limited slopes in the Mincio River basin. The simultaneous analysis of N, Si and P allows us to better understand the different paths, transformation and retention mechanisms at the watershed scale. [ABSTRACT FROM AUTHOR]

In this study we investigated the hydrodynamics and the interaction between riverine and marine waters in the Po River‐Delta‐Sea (RDS) system (Italy). Through the application of an unstructured 3D numerical model to a domain comprising the Po river branches, seven coastal lagoons and the shelf sea, many of the hydrodispersive phenomena evolving from the interaction of the different water bodies of the RDS system were described. The model was successfully calibrated and validated for water levels, fluxes at the inlets and at the river branches, and water temperature and salinity observations. The use of such a comprehensive approach allows the characterization of the general hydrodynamics of all the components of the Po RDS system, of the water exchange between different water bodies as well as the Po river discharge distribution among all branches. The analysis of calculated coastal current patterns in the prodelta of two different years and during a flood event confirms that the Po River is the main driver of the baroclinic coastal sea circulation. However, when strong winds occur (Sirocco and Bora), the surficial circulation of the shelf area is significantly modified. Generally, the lagoons of the Po RDS system can be considered hydrodynamically active since their tidal prisms represent a relevant fraction of each lagoon's volume. The natural consequence is that the lagoons show an active flushing with water renewal times in the order of few days. The hydrodynamics is mainly forced by the tide and is locally enhanced by riverine inputs and wind. Where freshwater inputs are significant, they are responsible of the lagoons salinity patterns that show high spatial gradients (not all the lagoons are well mixed) and temporal variability. The variability of the freshwater discharges and the complex morphology of the Po RDS allows the presence of both marine and riverine environments. Key Points: The application of a 3‐D shallow water equation hydrodynamic model simulated the circulation, freshwater mixing, and residence time in the Po River‐Delta‐Sea systemThe hydrodynamic characterization of the different water bodies, their mutual interaction, and mixing properties were studiedThe spatial and temporal variability of the renewal times and flushing properties of the lagoons were investigated, as like the main natural factors driving them [ABSTRACT FROM AUTHOR]

Abstract: Lakes can store water for long periods of time, which influences the transport of water and hydrologic tracers and changes catchment transit times downstream. However, the impact that the transit time of lakes has on catchment transit times has received little attention to date. We derived water and isotope mass balances for two lakes and examine the use of time‐variant transit time solutions with StorAge Selection functions to estimate the water ages of evaporation and lake outflows. We used the convolution of the StorAge Selection function transit time estimations with inflow transit times to estimate the transit times downstream of each lake. The lakes exhibited contrasting storage effects for discharge; with direct storage effects (newest water exiting during low flow) for a larger lake fed by a small catchment, and inverse storage effects (newest water exiting during high flow) for a smaller lake with a large catchment. The water and isotope mass balance yielded estimates of daily and annual evaporation fluxes, which were similar between the two lakes. The proposed framework is an effective tool to identify the effects of precipitation, surface inflow, and evaporation on the transit times for relatively small, shallow lakes, using a combination of water and isotope mass balance methods. [ABSTRACT FROM AUTHOR]

High-throughput sequencing (HTS) was used to analyze the seasonal variations in the bacterioplankton community composition (BCC) in the euphotic layer of a large and deep lake south of the Alps (Lake Garda). The BCC was analyzed throughout two annual cycles by monthly samplings using the amplification and sequencing of the V3-V4 hypervariable region of the 16S rRNA gene by the MiSeq Illumina platform. The dominant and most diverse bacterioplankton phyla were among the more frequently reported in freshwater ecosystems, including the Proteobacteria, Cyanobacteria, Bacteroidetes, Verrucomicrobia, Actinobacteria, and Planctomycetes. As a distinctive feature, the development of the BCC showed a cyclical temporal pattern in the two analyzed years and throughout the euphotic layer. The recurring temporal development was controlled by the strong seasonality in water temperature and thermal stratification, and by cyclical temporal changes in nutrients and, possibly, by the remarkable annual cyclical development of cyanobacteria and eukaryotic phytoplankton hosting bacterioplankton that characterizes Lake Garda. Further downstream analyses of operational taxonomic units associated to cyanobacteria allowed confirming the presence of the most abundant taxa previously identified by microscopy and/or phylogenetic analyses, as well as the presence of other small Synechococcales/Chroococcales and rare Nostocales never identified so far in the deep lakes south of the Alps. The implications of the high diversity and strong seasonality are relevant, opening perspectives for the definition of common and discriminating patterns characterizing the temporal and spatial distribution in the BCC, and for the application of the new sequencing technologies in the monitoring of water quality in large and deep lakes. [ABSTRACT FROM AUTHOR]

Abstract: Hysteresis linked to alternative stable states may explain delays in water quality recovery despite reduced nutrient loadings in shallow lakes. Because deep lakes are assumed to be less prone to critical transitions, similar delays are attributed to the confounding effects of additional environmental disturbances, such as climate warming. Herein, we hypothesised that the lack of evidence of nutrient‐driven alternative stable states in a deep lake arises from the fact that the nutrient threshold that causes the critical transition is lower than the nutrient threshold in shallow lakes. Thereby, it might have been crossed much earlier in the lake history. To test this hypothesis, we focused on the palaeo‐ecological trajectory of Lake Varese, which is a deep, hypereutrophicated peri‐alpine lake undergoing restoration. Proxies for drivers of ecological state (i.e. total phosphorus—TP—through diatoms and pigments) and ecological responses (Cladocera), as well as a repeatable analysis, were used to identify transitions and to distinguish hysteretic delays from those of the ecosystems responding to additional constraints over the past century. Our results suggest spatial heterogeneity in the ecological response. The littoral habitats changed abruptly and prematurely for a low TP threshold, causing a shift that met many criteria of a flickering‐type critical transition. Soon after the littoral shift, a striking increase in the lake phosphorous concentration was recorded and drove the pelagic assemblage towards a new state. This transition was abrupt, and the pelagic communities exhibited limited evidence of recovery; however, we found no evidence of hysteresis. Therefore, the modern ecological trajectory of the pelagic communities is currently driven by climate warming. This detailed analysis allowed us to go beyond the general pattern that links ecological responses to drivers and suggest that a nonlinear transition following eutrophication can take place in a deep lake synchronously with linear transitions. Instead of triggering a new regime shift, climate warming, to which pelagic habitats are more sensitive than littoral ones, has driven the lake further from its safe operating space. [ABSTRACT FROM AUTHOR]

Exposure time is an important characteristic for hydrodynamics that has simultaneous impacts on the biochemical processes in tidal systems. To eliminate man-made errors, decrease computational effort, and increase simulation efficiency, exposure time was evaluated under different hydrodynamic conditions for a bay to investigate the impact of the external domain on the accuracy of the computational results for exposure time. The exposure time was explicitly defined and computed using a hydrodynamic model and tracer experiments for a set of ten external domain sizes (EDS), five external domain lengths (EDL), and three special hydrodynamic conditions. The results indicated that the external domain had a significant influence on the exposure time, and the intensity of this influence was related to hydrodynamic conditions. The sensitivity of the exposure time to the external domain increased with increasing tidal range, while freshwater inflows decreased this sensitivity. However, the variation trends for exposure time with different EDS and EDL were independent of the hydrodynamic conditions. Considering the computational efficiency (maximum), the calculated error (minimum) of the exposure time, and the impact of the boundary conditions (minimum), the recommended EDS and EDL range from 9 to 13 times the initial domain size and 1.30 to 1.45 times the length in the bay, respectively. The research regarding exposure time and external domains not only helps to eliminate the errors caused by man-made factors and reduce the computational effort but also provides a reference for understanding the interrelationship between coastal waters, reciprocating flow, and the water environment. [ABSTRACT FROM AUTHOR]

Howard K, Noble P. 2017. Hydrological perturbations drive rapid shifts in phytoplankton biodiversity and population dynamics in Butte Lake (Lassen Volcanic National Park, California). Lake Reserv Manage. 34:21-41.Extreme interannual fluctuations in precipitation affect the hydrologic balance of montane lake systems and impact phytoplankton community composition and biodiversity. The 2013 water year (WY) was the driest period on record for California and caused drought conditions that reduced the total volume of Butte Lake (Lassen Volcanic National Park) by ∼40% at the end of WY 2014. These conditions resulted in changing lake hydrology from an open system (surface outflow) in 2012 to a prolonged period of being a closed system (no surface outflow) in WYs 2013 and 2014. Hydrologic perturbations experienced by Butte Lake in WYs 2013 and 2014 resulted in degraded water quality and shifts in phytoplankton biovolume composition and diatom biodiversity. Chlorophyll a, NH4+, TP, DP, Ca2+, Mg2+, Na+ concentrations and specific conductivity (>10 µS/cm) increased in Butte Lake from WY 2012 to WY 2014, while dissolved oxygen concentrations decreased, and the hypolimnion became anoxic by August 2014. Declining water quality corresponded with increasing lake trophic status from oligo-mesotrophic in 2012 to borderline eutrophic by end of 2014, where DIN>DP (from increased concentration of NH4+). Total surface phytoplankton biovolume shifted from being diatom-dominated in August 2012 to being cyanobacteria dominated by August and September 2014. Diatom biodiversity was significantly different in August 2014 compared with August 2012, and demonstrated a trend of increasing species richness and evenness from WY 2012 to WY 2014. These observed shifts imply the need for an increased focus on water resource and aquatic ecosystem management in Lassen Volcanic National Park lakes. [ABSTRACT FROM AUTHOR]

Recent anthropogenic climate change and the exponential increase over the past few decades of Saharan dust deposition, containing ecologically important inputs of phosphorus (P) and calcium (Ca), are potentially affecting remote aquatic ecosystems. In this study, we examine changes in cladoceran assemblage composition and chlorophyll- a concentrations over the past ~150 years from high-resolution, well-dated sediment cores retrieved from six remote high mountain lakes in the Sierra Nevada Mountains of Southern Spain, a region affected by Saharan dust deposition. In each lake, marked shifts in cladoceran assemblages and chlorophyll- a concentrations in recent decades indicate a regional-scale response to climate and Saharan dust deposition. Chlorophyll- a concentrations have increased since the 1970s, consistent with a response to rising air temperatures and the intensification of atmospheric deposition of Saharan P. Similar shifts in cladoceran taxa across lakes began over a century ago, but have intensified over the past ~50 years, concurrent with trends in regional air temperature, precipitation, and increased Saharan dust deposition. An abrupt increase in the relative abundance of the benthic cladoceran Alona quadrangularis at the expense of Chydorus sphaericus, and a significant increase in Daphnia pulex gr. was a common trend in these softwater lakes. Differences in the magnitude and timing of these changes are likely due to catchment and lake-specific differences. In contrast with other alpine lakes that are often affected by acid deposition, atmospheric Ca deposition appears to be a significant explanatory factor, among others, for the changes in the lake biota of Sierra Nevada that has not been previously considered. The effects observed in Sierra Nevada are likely occurring in other Mediterranean lake districts, especially in softwater, oligotrophic lakes. The predicted increases in global temperature and Saharan dust deposition in the future will further impact the ecological condition of these ecosystems. [ABSTRACT FROM AUTHOR]

The effects of global change on biological systems and functioning are already measurable, but how ecological interactions are being altered is poorly understood. Ecosystem resilience is strengthened by ecological functionality, which depends on trophic interactions between key species and resilience generated through biogenic buffering. Climate-driven alterations to coral reef metabolism, structural complexity and biodiversity are well documented, but the feedbacks between ocean change and trophic interactions of non-coral invertebrates are understudied. Sea cucumbers, some of the largest benthic inhabitants of tropical lagoon systems, can influence diel changes in reef carbonate dynamics. Whether they have the potential to exacerbate or buffer ocean acidification over diel cycles depends on their relative production of total alkalinity ( AT) through the dissolution of ingested calcium carbonate (Ca CO3) sediments and release of dissolved inorganic carbon ( CT) through respiration and trophic interactions. In this study, the potential for the sea cucumber, Stichopus herrmanni, a bêche-de-mer (fished) species listed as vulnerable to extinction, to buffer the impacts of ocean acidification on reef carbonate chemistry was investigated in lagoon sediment mesocosms across diel cycles. Stichopus herrmanni directly reduced the abundance of meiofauna and benthic primary producers through its deposit-feeding activity under present-day and near-future pCO2. These changes in benthic community structure, as well as AT (sediment dissolution) and CT (respiration) production by S. herrmanni, played a significant role in modifying seawater carbonate dynamics night and day. This previously unappreciated role of tropical sea cucumbers, in support of ecosystem resilience in the face of global change, is an important consideration with respect to the bêche-de-mer trade to ensure sea cucumber populations are sustained in a future ocean. [ABSTRACT FROM AUTHOR]

We investigate the carbon dynamics in Guanabara Bay, an eutrophic tropical coastal embayment surrounded by the megacity of Rio de Janeiro (southeast coast of Brazil). Nine sampling campaigns were conducted for dissolved, particulate and total organic carbon (DOC, POC and TOC), dissolved inorganic carbon (DIC), partial pressure of CO ( pCO), chlorophyll a (Chl a), pheo-pigments and ancillary parameters. Highest DOC, POC and Chl a concentrations were found in confined-shallow regions of the bay during the summer period with strong pCO undersaturation, and DOC reached 82 mg L, POC 152 mg L, and Chl a 800 μg L. Spatially and temporally, POC and DOC concentrations varied positively with total pigments, and negatively with DIC. Strong linear correlations between these parameters indicate that the production of TOC translates to an equivalent uptake in DIC, with 85% of the POC and about 50% of the DOC being of phytoplanktonic origin. Despite the shallow depths of the bay, surface waters were enriched in POC and DOC relative to bottom waters in periods of high thermohaline stratification. The seasonal accumulation of phytoplankton-derived TOC in the surface waters reached about 105 g C m year, representing between 8 and 40% of the net primary production. The calculated turnover time of organic carbon was 117 and 34 days during winter and summer, respectively. Our results indicate that eutrophication of coastal bays in the tropics can generate large stocks of planktonic biomass and detrital organic carbon which are permanently being produced and partially degraded and buried in sediments. [ABSTRACT FROM AUTHOR]

In Canada, climate change and forest harvesting may both threaten the ecological integrity of boreal lakes. To disentangle the effects of natural variation in climate and lake environments from those of logging, we evaluated long-term variation (1991-2003) in zooplankton communities of six boreal lakes in Ontario. We monitored concomitantly changes in zooplankton abundance and composition in three undisturbed and three harvested lakes, five years prior and eight years after watershed clearcut logging. We tested the hypothesis that long-term natural variation in climate and lake environments will be more important drivers of zooplankton community changes than short-term impacts of logging. We used space/time interaction tests and asymmetric eigenvector maps to model zooplankton responses to environmental changes and logging. Year-to-year variation in zooplankton abundance and composition were almost an order of magnitude whereas among-lake variation was stable through time. Breakpoints in time series of zooplankton in each lake were not directly related to logging. Climatic and limnological features were the most important drivers of long-term variation in the zooplankton community, shading the effect of logging. These results highlight the need to better understand the pressures exerted by climate change on boreal lake ecosystems in the context of anthropogenic pressure, such as logging. [ABSTRACT FROM AUTHOR]

Albeit subfossil Cladocera remains are considered as a reliable proxy for tracking historical lake development, they have been scarcely studied in large subalpine lakes south of the Alps. In this study, subfossil Cladocera remains from Lake Garda in northern Italy were analyzed to track the lake's environmental changes since the Middle Ages. One core was retrieved from the largest sub-basin of Lake Garda (Brenzone, 350 m deep) and two cores were retrieved from the profundal and littoral zone of the smaller lake sub-basin (Bardolino, 80 and 40 m deep, respectively). The species distribution of Cladocera remains in recent sediment layers was similar to that observed in contemporary water samples. The deepest sections of the three cores were characterized by species sensitive to water temperature, which suggest a key role of major climatic events in driving the lake response during the late Holocene. The most evident change in Cladocera assemblages in the studied cores was observed during the 1960s, when planktonic taxa such as Daphnia spp. and Bosmina spp. became dominant at the expense of littoral taxa. Despite the highly coherent general pattern of subfossil Cladocera, the cores showed a set of minor differences, which were attributed to different morphological and hydrological features of the two basins forming Lake Garda. Multivariate analysis revealed a clear relation of Cladocera to climate variability during periods of low lake nutrient levels (i.e. before the 1960s). This provides additional information on past ecological responses of Lake Garda, as contrast previous data from subfossil diatom and pigment analyses did not fully capture effects of climate change during the same period. Since the 1960s shifts in assemblage composition of Cladocera and diatoms were highly coherent, in response to nutrient increase and the following, less pronounced, decrease in nutrient concentrations. During the last five decades, the response of the Cladocera to climate variability appeared to be masked by nutrient change. This work points up the potential of the multi-proxy approach for disentangling the multifaceted biological responses to multiple environmental stressors in large and deep lakes. [ABSTRACT FROM AUTHOR]

Nutrient loadings are an important component of aquaculture impacts as they can lead to cascade effects at the ecosystem level. An evaluation of these effects on foodweb functioning is presented and discussed for the case study of Lake Burullus in the Nile Delta, Egypt, where semi-intensive aquaculture in earthen ponds has grown remarkably in recent decades. Estimations of nutrient loads into the lake's brackish water, from agriculture and fish ponds, were assessed. A biogeochemical model was set up and calibrated to simulate the loads from nitrogen, phosphorus and phytoplankton, with and without fish ponds. The biogeochemical model was coupled offline to a foodweb model to test the effects of additional nutrient loads from aquaculture on the lake's foodweb functioning. The results obtained by comparing the present conditions of aquaculture development with a no-aquaculture scenario allowed us to identify a set of indicators of foodweb functioning which are sensitive to changes in nutrient loads from aquaculture. The limited changes in phytoplankton concentration with respect to changes in nutrient loads suggested that primary production in Lake Burullus is presently controlled largely by factors other than nutrient influxes from aquaculture. This study emphasises the potential benefits of using a combination of biogeochemical and foodweb models to assess the cumulative effects of nutrient loading from aquaculture on a receiving water body. [ABSTRACT FROM PUBLISHER]

In this study, the optimum design of the entrance of a fishpond laterally to the main flow of an open channel was investigated numerically and experimentally. The flow characteristic measurements were realized with the PIV (particle image velocimetry) method. The mathematical simulations were based on the development of a two dimensional -mean in depthhydrodynamic model and a quasi three dimensional sediment transport model which includes processes of advection, diffusion, and settling of conservative suspended matter. The study was completed with the comparison of the final results of the mathematical models with the findings of the physical model revealing the hydrodynamic interaction and coupling between the main flow of the channel and the lateral reservoir-fishpond and leading to the optimum technical design of the system. [ABSTRACT FROM AUTHOR]

White spot disease is one of the most serious problems in the shrimp-farming industry in Mexico. An assessment of risk of an outbreak of the disease could be useful for farmers making decisions concerning early harvest to minimize economic losses. A group of researchers identified the most important risk factors, classified in four broad categories: management variables at the regional level, environmental variables at the regional level, management variables at the farm and population variables at the farm. For each variable, categories were defined and a risk score assigned. To validate these scores, a survey was carried out with 65 stakeholders. A semi-quantitative risk assessment model was proposed and implemented as a questionnaire containing 21 risk variables with weighted scores. The most important risk factors were the period of fallowing within farm's region and health quality of larvae. Also important were estimated level of viral load and virus detection in wild hosts in the surrounding area, pond water temperature, length of fallowing period of pond, vector control at water intake and stocking density. The overall estimated risk was calculated as the sum of all scores with a further adjustment, according to maximum and minimum total scores. Three risk levels for white spot disease outbreak at ponds are based on estimated risk: low (≤0.2), moderate (0.2-0.4) and high (≥0.4). [ABSTRACT FROM AUTHOR]

Several studies have demonstrated that the most effective way to control eutrophication is to reduce phosphorus input at the scale. Water quality monitoring programs need to separately evaluate the different polluting sources to provide a suitable estimate of their relative contributions and thus accurately prioritize possible restoration actions. Urban area cases, where a portion of untreated wastewater is often discharged directly into receiving surface waters by combined sewer overflows (CSOs) during rain events, remain unsolved. In this context, an urban watershed located in Northern Italy with 60 CSOs has been chosen as a case study, and four rainy events have been hourly monitored. The proposed monitoring program consists of the combined use of caffeine and turbidity to estimate the volume and phosphorus load spilled into the river from the CSOs, respectively. Caffeine proved to be a suitable molecule to quantify the volume of wastewater discharged into water bodies, based on a per capita caffeine load of 10.8 mg inhab d, estimated in the present work. This research showed that, on average, more than half of the total phosphorus loads transported by the river is due to the CSO discharges (56.5%). The knowledge of the prevailing responsibility of the CSO discharges for the Lambro River quality allows prioritizing effective restoration actions. [ABSTRACT FROM AUTHOR]

Freshwater aquaculture increases dissolved and particulate nutrients near fish cages, but the degree to which they are incorporated into tissues of native animals is uncertain. At the Experimental Lakes Area in northwestern Ontario, Canada, a cage culture of Rainbow Trout was operated seasonally for 5 y, and invertebrates and fishes were collected before, during, and after culturing from the experimental lake and a reference lake to assess changes in stable isotopes of C and N in their tissues. The feed contained marine fishmeal and was higher in C and N isotope values (δ13C and δ15N) than lake biota by ≥4‰(all taxa) and 3‰(all invertebrates), respectively. During the aquaculture operation, δ15N of littoral and pelagic invertebrates, profundal chironomids, minnows, and Lake Trout (Salvelinus namaycush) increased by 2 to 5‰ relative to before aquaculture values. In the 1st and 2nd years after aquaculture, δ15N of several invertebrate taxa and all fishes continued to increase 1 to 2‰/y. In contrast, during aquaculture, only minnows and trout had significant increases (up to 3‰) in δ13C. In the period after aquaculture, the δ13C of fishes, plankton, and profundal chironomids declined to below values measured before or during aquaculture. Isotopic analysis of native biota can be used to monitor assimilation of cage culture wastes in freshwater ecosystems. [ABSTRACT FROM AUTHOR]

The rising atmospheric carbon dioxide (CO2) concentration is increasing the solubility of limestone, calcium silicate, and feldspars, resulting in greater total alkalinity concentration in inland waters. This phenomenon will result in inland waters having slightly greater alkalinity concentration (and buffering capacity), higher pH when at equilibrium with atmospheric CO2, and more available carbon for photosynthesis. However, the changes in water quality will be small. Fluctuations in CO2 concentration resulting from CO2 use in photosynthesis by aquatic plants and release of CO2 by respiration, acidity resulting from nitrification of ammonia nitrogen from feeding waste and fertilizer, and application of liming materials to ponds will continue to be the dominant factors affecting pH and alkalinity in waters of inland aquaculture systems. [ABSTRACT FROM AUTHOR]

The representation of spatial wind distribution is recognized as a serious difficulty when modeling the hydrodynamics of lakes surrounded by a complex topography. To address this issue, we propose to force a 3-D lake model with the wind field simulated by a high-resolution atmospheric model, considering as a case study a 61 km2 prealpine lake surrounded by mountain ranges that reach 1800 m above the lake's surface, where a comprehensive data set was available in the stratified season. The improved distributed description of the wind stress over the lake surface led to a significant enhancement in the representation of the main basin-scale internal wave motions, and hence provided a reference solution to test the use of simplified approaches. Moreover, the analysis of the power exerted by the computed wind field enabled us to identify measuring stations that provide suitable wind data to be applied uniformly on the lake surface in long-term simulations. Accordingly, the proposed methodology can contribute to reducing the uncertainties associated with the definition of wind forcing for modeling purposes and can provide a rational criterion for installing representative measurement locations in prealpine lakes. [ABSTRACT FROM AUTHOR]

European whitefish ( Coregonus lavaretus) embryos and larvae were exposed to 6 different manganese sulfate (MnSO4) concentrations from fertilization to the 3-d-old larvae. The fertilization success, offspring survival, larval growth, yolk consumption, embryonic and larval Mn tissue concentrations, and transcript levels of detoxification-related genes were measured in the long-term incubation. A full factorial breeding design (4 females × 2 males) allowed examination of the significance of both female and male effects, as well as female-male interactions in conjunction with the MnSO4 exposure in terms of the observed endpoints. The MnSO4 exposure reduced the survival of the whitefish early life stages. The offspring MnSO4 tolerance also was affected by the female parent, and the female-specific mean lethal concentrations (LC50s) varied from 42.0 mg MnSO4/L to 84.6 mg MnSO4/L. The larval yolk consumption seemed slightly inhibited at the exposure concentration of 41.8 mg MnSO4/L. The MnSO4 exposure caused a significant induction of metallothionein-A ( mt-a) and metallothionein-B ( mt-b) in the 3-d-old larvae, and at the exposure concentration of 41.8 mg MnSO4/L the mean larval mt-a and mt-b expressions were 47.5% and 56.6% higher, respectively, than at the control treatment. These results illustrate that whitefish reproduction can be impaired in waterbodies that receive Mn and SO4 in concentrations substantially above the typical levels in boreal freshwaters, but the offspring tolerance can be significantly affected by the parents and in particular the female parent. Environ Toxicol Chem 2017;36:1343-1353. © 2016 SETAC [ABSTRACT FROM AUTHOR]

We studied changes in sea water pH, temperature and salinity with focus on two depth layers, along the Gulf of Finland (the Baltic Sea) using long-term monitoring data from 1979 to 2015. Data from the most frequently sampled monitoring stations between western and eastern Gulf of Finland were used. The main result of the study reveals that pH has decreased both in surface and deep-water in the western Gulf of Finland with values ranging between −0.005 and −0.008 units year. We also demonstrate a rise in temperature (~2 °C) and decrease in salinity (~−0.7 g kg) at several stations over the last 36 years. In general, the changes are shown to be more pronounced in the western part of the gulf. This paper also stresses the importance of improving the sampling frequency and quality of monitoring measurements. [ABSTRACT FROM AUTHOR]

The stable oxygen isotopic signature δ18O of water has been proved to be a useful marker in hydrological lake studies. However, it is applied only sporadically to determine the extent of mixis and stratification, which is vital for all hydrological and limnological studies. We present a study of the changes in δ18O, temperature and Schmidt stability in the water column of three adjacent lakes of very different mixis types (polymictic to meromictic) over almost one year (May 2008 to April 2009). The response of δ18O to important hydrologic processes (entrainment, stratification, circulation, groundwater inflow) and weather influences (cooling period, ice cover and melt water inflow) is discussed. The lakes are part of the Osterseen chain of lakes south of Munich, Germany. Although hydrologically connected, these lakes show various mixis types (polymictic to meromictic) due to large differences in size, groundwater inflow and water renewal time. Polymixis and the strong subsurface inflow of groundwater in Lake Waschsee (25.6 × 103 m3) were indicated by the same trends in the δ18O signature throughout all water layers and by the mean overall signature (-9.94‰) being very close to δ18O of local groundwater (-10.01‰ ± 0.06). δ18O signatures of the larger dimictic Lake Fohnsee (2298.3 × 103 m3) revealed a highly significant trend towards lower values of δ18O in its hypolimnion, indicating inflow of groundwater. A cooling period during the summer stratification characterised by high wind speeds resulted in a considerable drop of lake surface temperatures and Schmidt stability (up to 25%) in lakes Fohnsee and Eishaussee and was followed by a deepening of the mixed upper water layer and entrainment of hypolimnetic water layers. This was clearly shown by a signal change in deeper water layers formerly constant in δ18O. The permanent meromixis present in Lake Eishaussee (297.0 × 103 m3) could also be confirmed by isotopic signatures, as bottom water layers remained significantly isolated in δ18O from the remaining water column over the whole study period. We summarize that the oxygen isotopic signature of water is an easy to interpret, excellent indicator of important hydrologic processes in a lake and can readily be integrated into routine sampling. The present findings will further contribute to the analysis of hydrological data as well as to the interpretation of paleoclimatic reconstructions using proxies of lake water δ18O.

Low concentrations of dissolved oxygen remain a global concern regarding the ecological health of lakes and reservoirs. In addition to high nutrient loads, climate-induced changes in lake stratification and mixing represent additional anthropogenic menace resulting in decreased deepwater oxygen levels. The analysis of 43 years of monitoring data from Lake Geneva shows no decreasing trend neither in the areal hypolimnetic mineralization rate nor in the extent of hypoxia. Instead, hypoxic conditions are predominantly controlled by deep mixing in winter and much less by the trophic variations over the past decades. To reproduce winter mixing, the one-dimensional hydrodynamic model SIMSTRAT was specially adapted to deep lakes and run for several climate scenarios. The simulations predicted a decrease in the maximum winter mixing depth from an average of ∼172 m for 1981-2012 to ∼136 m and ∼127 m in response to predicted atmospheric temperatures between 2045-2076 and 2070-2101 according to Intergovernmental Panel on Climate Change scenarios. Concurrently, events with complete homogenization of temperature and oxygen in winter will decrease by ∼50%. Consequently, the hypolimnetic oxygen concentrations will significantly decrease. These results demonstrate that changes in deep mixing can have stronger impact than eutrophication on the deepwater oxygen levels of oligomictic lakes. [ABSTRACT FROM AUTHOR]