High Surface Water Temperatures Research Articles

Lake and Atmospheric Heatwaves Caused by Extreme Dust Intrusion in Freshwater Lake Kinneret in the Eastern Mediterranean

The role of dust intrusions in the formation of lake heatwaves has not yet been discussed in previous publications. We investigated a lake heatwave (LHW) and an atmospheric heatwave (AHW) in the freshwater Lake Kinneret in the Eastern Mediterranean: these were caused by an extreme dust intrusion that lasted for a 10-day period (7–17 September 2015). The AHW and LHW were defined as periods of abnormally high air temperature (Tair) and lake surface water temperature (SWT) compared to their 90th percentile thresholds in September. In the daytime, the maximal intensities of AHW and LHW reached 3 °C and 2 °C, respectively. This was despite the pronounced drop in solar radiation due to the dust radiative effect. The satellite SWT retrievals were incapable of representing the abnormally high SWT in the presence of the extreme dust intrusion. Both METEOSAT and MODIS-Terra showed a sharp decrease in the SWT compared to the actual SWT: up to 10 °C in the daytime and up to 15 °C in the nighttime. Such a significant underestimation of the actual SWT in the presence of a dust intrusion should be considered when using satellite data to analyze heatwaves. In the absence of moisture advection, the AHW and LHW were accompanied by an increase of up to 30% in absolute humidity (ρv) over the lake. Being a powerful greenhouse gas, water vapor (characterized by an increased ρv) absorbed most of both the upwelling and downwelling longwave thermal radiation, heating the near-ground atmospheric layer (which is in direct contact with the lake water surface), in the daytime and nighttime. In the nighttime, the maximal intensity of the AHW and LHW reached 4 °C and 3 °C, respectively. Because of the observed steadily increasing dust pollution over the Eastern Mediterranean during the past several decades, we anticipate that dust-related lake heatwaves will intensify adverse effects on aquatic ecosystems such as reducing fishery resources and increasing harmful cyanobacteria blooms.

Performance analysis and multi-objective optimization of the offshore renewable energy powered integrated energy supply system

Offshore renewable energy technologies offer a promising prospect for energy supply on offshore islands. However, traditional wind and solar energy sources exhibit strong variability and cannot fully meet the energy demands of users. At the same time, the energy demand of users also has great volatility, so the supply side and demand side of energy cannot match well. Existing renewable energy systems often utilize thermal power generation as a stabilizer for the system, however, this approach is difficult to apply in offshore conditions. This study introduces an innovative multi-energy complementary system that integrates seawater freezing desalination, refrigeration and power-hydrogen-ammonia co-generation, driven by ocean thermal energy, wind energy, and solar energy. The system incorporates the use of a new developed small temperature difference ejector refrigeration cycle to stabilize the entire system and produce low-temperature refrigeration. By utilizing the seawater freezing desalination process to minimize energy conversion losses and balancing fluctuations in multiple energy inputs and user demands through ammonia-hydrogen co-production, this approach effectively minimizes energy waste, enhances energy utilization efficiency, and reduces excess power handling. The purpose is to optimize the configuration of the energy conversion subsystem within the multi-energy complementary multi-effect co-generation system, thus improving the economic feasibility and reliability of the system in providing energy and freshwater supply, ultimately achieving the goals of reducing energy waste, achieving high conversion efficiency, and minimizing equipment investment. This study conducts an economic analysis and optimization of a multi-energy complementary system, utilizing a 2 MW-level OTEC subsystem as a stabilizer. Both equipment investment and operating costs are considered in the economy analysis. The carbon emission reduction performance of this system is evaluated. The performance optimization results show that the system achieves an energy storage efficiency of 65.96 %, and the curtailment rate of this system is only 3.99 %, significantly lower than the 10 % curtailment rate in traditional system. The proposed system can meet the annual electricity demand of 33.09 million kWh, as well as a cooling demand of 546.81 million kWh. Under typical conditions, the ocean thermal energy conversion cycle exhibits a power efficiency of 0.85 % and a refrigeration efficiency of 29.10 %. Meanwhile, it annually reduces CO2 emissions by 2.54 million tons, demonstrating significant environmental benefits. This proposed system is mainly applicable to tropical waters with high surface water temperatures and depths of no less than 600 m worldwide. This study offers a practical and economically beneficial solution for energy and freshwater supply on offshore islands.

Exploring the trade-offs among hydropower benefits, environmental flow, and surface water temperature in a large reservoir under deep uncertainty

The negative environmental impacts of large hydroelectric reservoirs have attracted wide attention. Reservoir operation alters seasonal flow patterns and reduces downstream flow during fish spawning seasons, which may lead to the loss of fish biodiversity. Meanwhile, reservoir impoundment reduces the flow velocity and raises the reservoir surface water temperature (SWT), potentially increasing the risk of algal blooms. While previous studies have designed various operation rules to balance the hydropower benefits and environmental flow or water temperature through multi-objective optimization approaches, a comprehensive analysis of trade-offs among the hydropower benefits, environmental flow, and SWT under an uncertain future remains lacking. This study contributes the many-objective robust decision-making (MORDM) framework to the multi-objective reservoir ecological operation for the first time. We quantify the trade-offs among maximizing the hydropower benefits, maximizing the reliability of environmental flow, and minimizing the number of days with high SWT; and evaluate the robustness of the operation rules under deep uncertainties from the climate conditions and electricity market. We apply MORDM to a large and deep reservoir, Nuozhadu, located in southwest China. Our results indicate that trade-offs exist between environmental flow and SWT, emphasizing the necessity to consider multiple environmental objectives in reservoir operation. Based on MORDM, we identify an operation scheme that can not only balance hydropower benefits and two environmental objectives, but also have robust performance against the uncertain future states. Scenario discovery analysis shows that air temperature is the most important uncertain factor that influences the robustness of operation rules. Our study provides a new perspective on reservoir operation planning to achieve balanced and sustainable developments in hydropower and environments under uncertain climate and electricity market conditions.

Impacts of Reservoir Operation on Hydrodynamic and Thermal Conditions in a Large Deep Reservoir and the Potential Ecological Impacts

Many large-deep reservoirs have been built, are under construction, or are planned in southwestern China, to meet increasingly high energy demands. The construction of dams can significantly alter the natural hydrodynamic and thermal conditions of rivers, thereby, threatening aquatic ecosystems, e.g., fish and algae communities. However, the impacts of large-deep reservoir operations on the hydrodynamic and thermal conditions and their potential influence on aquatic ecosystems have seldom been addressed. In this study, a quasi-3D hydrodynamic model is developed to describe the hydrodynamic and thermal conditions of a large deep reservoir, Nuozhadu, the largest reservoir along the Lancang River. The water temperature and flow velocity after the impoundment of the reservoir are examined. To explore how reservoir operation changes the hydrodynamic and thermal conditions, five groups of reservoir operational scenarios based on different water levels and discharge rates are conducted. Our results indicate that a lower water level combined with a higher discharge rate can increase the flow velocity and decrease the surface water temperature in the reservoir, which may contribute to diminishing the growth of Cyanobacteria. Meanwhile low water levels can also increase the outflow water temperature to protect the downstream fish. Further, the LowLevel and MaxPower schemes were designed to provide ecologically favorable hydrodynamic conditions, and to maximize power generation, respectively. The LowLevel scheme was found to reduce the duration of high surface water temperature (>25°C) by 18 days annually and reduce the outflow temperature by 0.8°C in the fish spawning period, at the expense of a 10% reduction in power generation. This study contributes to a better knowledge of the linkage among hydrodynamic, thermal conditions, and ecological impacts due to reservoir operation, and provides a novel perspective regarding the ecological safety of a large deep reservoir operation.

Eocene-Oligocene southwest Pacific Ocean paleoceanography new insights from foraminifera chemistry (DSDP site 277, Campbell Plateau)

Despite its major role in the Earth’s climate regulation, the evolution of high-latitude ocean dynamics through geological time remains unclear. Around Antarctica, changes in the Southern Ocean (SO) circulation are inferred to be responsible for cooling from the late Eocene and glaciation in the early Oligocene. Here, we present a geochemical study of foraminifera from DSDP Site 277 (Campbell Plateau), to better constrain thermal and redox evolution of the high latitude southwest Pacific Ocean during this time interval. From 56 to 48 Ma, Mg/Ca- and δ18O-paleothermometers indicate high surface and bottom water temperatures (24–26°C and 12–14°C, respectively), while weak negative Ce anomalies indicate poorly oxygenated bottom waters. This is followed by a cooling of ∼4° between 48 and 42 Ma, possibly resulting from a weakening of a proto-EAC (East Australian Current) and concomitant strengthening of a proto-Ross gyre. This paleoceanographic change is associated with better ventilation at Site 277, recorded by an increasing negative Ce anomaly. Once this proto-Ross gyre was fully active, increasing biogenic sedimentation rates and decreasing Subbotina sp. δ13C values indicate enhanced productivity. This resulted in a shoaling of the oxygen penetration in the sediment pile recorded by increasing the foraminiferal U/Ca ratio. The negative Ce anomaly sharply increased two times at ∼35 and ∼31 Ma, indicating enhanced seawater ventilation synchronously with the opening of the Tasmanian and Drake Passage gateways, respectively. The Oligocene glaciation is recorded by a major increase of bottom seawater δ18O during the EOT (Eocene-Oligocene Transition) while Mg/Ca-temperatures remain rather constant. This indicates a significant ice control on the δ18O record.

Warm surface waters increase Antarctic ice shelf melt and delay dense water formation

Melting ice shelves around Antarctica control the massive input of freshwater into the ocean and play an intricate role in global heat redistribution. The Amery Ice Shelf regulates wintertime sea-ice growth and dense shelf water formation. We investigated the role of warm Antarctic Surface Water in ice shelf melting and its impact on dense shelf water. Here we show that the coastal ocean in summer 2016/17 was almost sea-ice free, leading to higher surface water temperatures. The glacial meltwater fraction in surface water was the highest on record, hypothesised to be attributable to anomalous ice shelf melting. The excess heat and freshwater in early 2017 delayed the seasonal evolution of dense shelf water. Focused on ice shelf melting at depth, the importance and impacts of warming surface waters has been overlooked. In a warming climate, increased surface water heating will reduce coastal sea-ice production and potentially Antarctic Bottom Water formation.

Feasibility of United Arab Emirates Native Seaweed Ulva intestinalis as a Food Source: Study of Nutritional and Mineral Compositions

Food resources are limited in arid countries such as the United Arab Emirates (UAE); the salinity of the groundwater, together with a lack of natural fresh water sources and arable land, force the country to import most of its food. However, seaweed could play an important role in providing a locally available food resource, as it does not require fresh water and arable land to grow. The traditional use of several seaweed species as food sources has been documented in Asia and the Americas, where their nutritional composition has been well reported. Although the UAE’s aquatic environment is quite harsh due to high water salinity (over 40 g/L) and high surface water temperatures (over 35 °C), its native seaweed species could play a role as a food source in this arid region, thereby bolstering the country’s level of food security. To evaluate its potential in this context, fresh samples of the native Ulva intestinalis seaweed were collected in the shallow waters of Abu Dhabi Emirate, UAE. These samples were calculated to contain 34.38 ± 0.24 kcal, with a biomass composition of 5.185 ± 0.04% carbohydrate, 3.32 ± 0.14% protein, and 0.04 ± 0.01% fat (by dry matter). Of all the minerals present in the biomass, potassium had the highest concentration (7947 ± 319.5 ppm), followed by magnesium (3075.9 ± 1357 ppm) and sodium (756.3 ± 478 ppm). The water-soluble vitamins B1, B2, B3, B6, and C were below the detection limit in the samples. The rich concentration of essential minerals such as potassium, magnesium, iron and zinc in Ulva intestinalis makes it a promising novel food source. To the best of our knowledge, this is the first experimental study to examine the feasibility of using seaweed that is native to the UAE as a nutritional and sustainable food source in order to address the challenge of food security currently being faced by the country.

Drivers of Atmosphere-Ocean CO2 Flux in Northern Norwegian Fjords

High-latitude fjords and continental shelves are shown to be sinks for atmospheric CO2, yet large spatial-temporal variability and poor regional coverage of sea-air CO2 flux data, especially from fjord systems, makes it difficult to scale our knowledge on how they contribute to atmospheric carbon regulation. The magnitude and seasonal variability of atmosphere-sea CO2 flux was investigated in high-latitude northern Norwegian coastal areas over 2018 and 2019, including four fjords and one coastal bay. The aim was to assess the physical and biogeochemical factors controlling CO2 flux and partial pressure of CO2 in surface water via correlation to physical oceanographic and biological measurements. The results show that the study region acts as an overall atmospheric CO2 sink throughout the year, largely due to the strong undersaturation of CO2 relative to atmospheric concentrations. Wind speed exerted the strongest influence on the instantaneous rate of sea-air CO2 exchange, while exhibiting high variability. We concluded that the northernmost fjords (Altafjord and Porsangerfjord) showed stronger potential for instantaneous CO2 uptake due to higher wind speeds. We also found that fixation of CO2 was likely a significant factor controlling ΔpCO2 from April to June, which followed phenology of spring phytoplankton blooms at each location. Decreased ΔpCO2 and the resulting sea-air CO2 flux was observed in autumn due to a combined reduction of the mixed layer with entrain of high CO2 subsurface water, damped biological activity and higher surface water temperatures. This study provides the first measurements of atmospheric CO2 flux in these fjord systems and therefore an important new baseline for gaining a better understanding on how the northern Norwegian coast and characteristic fjord systems participate in atmosphere carbon regulation.

Moderate Seasonal Dynamics Indicate an Important Role for Lysogeny in the Red Sea.

Viruses are the most abundant microorganisms in marine environments and viral infections can be either lytic (virulent) or lysogenic (temperate phage) within the host cell. The aim of this study was to quantify viral dynamics (abundance and infection) in the coastal Red Sea, a narrow oligotrophic basin with high surface water temperatures (22–32 °C degrees), high salinity (37.5–41) and continuous high insolation, thus making it a stable and relatively unexplored environment. We quantified viral and environmental changes in the Red Sea (two years) and the occurrence of lysogenic bacteria (induced by mitomycin C) on the second year. Water temperatures ranged from 24.0 to 32.5 °C, and total viral and bacterial abundances ranged from 1.5 to 8.7 × 106 viruses mL−1 and 1.9 to 3.2 × 105 bacteria mL−1, respectively. On average, 12.24% ± 4.8 (SE) of the prophage bacteria could be induced by mitomycin C, with the highest percentage of 55.8% observed in January 2018 when bacterial abundances were low; whereas no induction was measurable in spring when bacterial abundances were highest. Thus, despite the fact that the Red Sea might be perceived as stable, warm and saline, relatively modest changes in seasonal conditions were associated with large swings in the prevalence of lysogeny.

Environmental Factors Causing Stress in Avicennia marina Mangrove in Rabigh Lagoon Along the Red Sea: Based on a Multi-Approach Study

Lagoons along the Red Sea are socio-economically important but most vulnerable ecosystems. Strong evaporation, strengthened due to global warming, leads to high salinity and high temperature in surface water, which exacerbate physiological stress in mangroves. Rabigh Lagoon is characterized by unusually large spatial differences in nutrient dynamics and physicochemical parameters. We conducted a multi-approach study to demonstrate how environmental factors cause growth retardation and stress in mangroves. There were significant variations (P < 0.05) in water salinities and temperatures, nitrogen and phosphorus abundance in sediments, and antioxidants in mangroves among 8 sites across the lagoon. Salinity and water temperature rapidly increased from the northern where Red Sea seawater enters to the southern end, but dissolved oxygen declined. High salinity (44.9), high surface water temperature (28.8°C) and relatively high nutrients at the southern end of the lagoon (S8) corresponded to high concentrations of antioxidants. High δ13C (−12.4‰) and δ15N (4.9‰) in the sediments at S8 also reveal relatively high nutrient level due to stagnant water, camel grazing, and runoff bringing in fertilizer from agriculture activities in the catchments during seasonal flooding events. Principal component analyses showed that N and P limitation at the lagoon entrance is the leading cause for mangrove stress, while high salinity, temperature, and low dissolved oxygen are the predominant factors for high antioxidants concentrations at the southern end of the lagoon.

Water Temperature Simulation in a Tropical Lake in South China

To study the vertical water temperature structure and thermodynamic characteristics of tropical lake-like reservoirs, a water temperature model was developed by a vertical one-dimensional numerical model for Songtao Reservoir in Hainan Province, China. The model was verified by the measured water temperature data, and sensitivity analysis of key model parameters was carried out. The results show that water temperature simulated by the model in Songtao Reservoir agreed with the observations quite well, and the model is feasible for water temperature simulations in large reservoirs in tropical zones. The sensitivity of vertical water temperature structure to different model parameters varied. For example, the extinction coefficient greatly affected surface water temperature, which is important for the formation and development of the surface water temperature hybrid layer. The vertical mixing coefficient significantly influenced the inflection point position and thickness of the thermocline. The vertical water temperature structure in Songtao Reservoir was stratified. Reservoir surface water temperature varied from 19.4 °C to 33.8 °C throughout a year. The hypolimnion mainly appeared in elevation below 150 m, where the water temperature is basically maintained at 19 °C throughout the year. This study also found that the surface water temperature of Songtao Reservoir in the tropical zone was higher than the air temperature throughout a year, with an annual average of 3.5 °C higher than that of air temperature. The preliminary analysis found out that the higher surface water temperature may be caused by the strong air temperature and solar radiation in tropical zones, in addition to the enhanced capacity of heat absorption and heat storage due to the slow water flow in the reservoir.

Future Projections of Water Temperature and Thermal Stratification in Connecticut Reservoirs and Possible Implications for Cyanobacteria

AbstractFuture climate warming may increase water temperature, stratification, and the occurrence of cyanobacteria blooms in drinking water reservoirs. Herein, past relationships between air temperature, water temperature, thermal stratification, and cyanobacteria prevalence are quantified in six Connecticut, U.S.A., reservoirs. Lake‐specific empirical models were developed and used to project historical (1971–2000) and future (mid‐century, 2041–2070) water temperature and thermal stratification in the study lakes. Projections are driven with downscaled air temperature projections from three general circulation models (i.e., HadGEM2‐CC365, CCSM4, and GFDL‐ESM 2M) under Representative Concentration Pathway 8.5. Results suggest that reservoirs in Connecticut are warming relatively quickly over time and that surface, average, and bottom water temperatures may continue to increase by 0.44°C, 0.30°C, and 0.16°C per decade, respectively. Future projections of thermal stratification indicate that significant increases may occur in July–September and that stratification is likely to begin 2–4 weeks earlier and last 2–4 weeks longer by mid‐century. High‐risk cyanobacteria blooms, defined as those exceeding 70,000 cells ml−1, historically occurred in three of the study reservoirs and are correlated with the occurrence of high surface water temperatures, cool bottom water temperatures, and high total Relative Thermal Resistance to Mixing (RTRM). Days per year with extreme high water temperatures and total RTRM are projected to increase significantly in the future. This shift may favor increased dominance of certain cyanobacteria species that tend to grow best in these extreme conditions, especially in reservoirs where cyanobacteria blooms are already a concern.

Changes in the vertical distribution of hyporheic and benthic fauna associated with low flow conditions in the headwaters of the Tafna river (northwest Algeria)

This study investigated temporal changes in the vertical distribution of invertebrates during periods of low flows in the Tafna wadi, a semi-arid Mediterranean river characterised by a general decline in average flows. The hyporheos was sampled along a 130-m stretch at depths of 0.3, 0.6 and 1 m in a pool-riffle sequence and a lateral gravel bar. Benthic fauna was collected on each sampling occasion from different locations in the studied sections. Sampling was conducted in two consecutive years (2014 and 2015) at the end of the spring (high waters) and over a 2-month period during the low summer flow. The results indicated that high surface water temperatures may initiate the movement of some taxa into the interstitial habitat, thus supporting the hyporheic refuge hypothesis. In the hyporheic zone, the low oxygen rates recorded deeper in the sediment and in the last low-flow months caused significant declines in taxonomic richness, and lower pH values resulted in a reduction in invertebrate abundances. In the benthic zone, high flow velocities increased richness but decreased invertebrate abundances. A large number of taxa were found in the gravel bar section reflecting the more suitable living conditions in this habitat compared with the river reach (high DO rates).

Investigating the potential impact of wind farms on Lake Erie

Power generation is a leading cause of air pollution and major source of global warming emissions. Renewable energy resources, like wind and solar power, generate electricity with little to no global warming emissions and are reliable, affordable and beneficial for health, economy and climate. There are two sectors in wind energy: offshore and onshore. Offshore wind speeds are faster and steadier and coastal areas often exhibit a high energy demand. Offshore wind farms are coming to the Great Lakes. We therefore investigate the potential impact of wind farms on Lake Erie’s dynamic and thermal structure using the COHERENS (a Coupled Hydrodynamical-Ecological model for Regional and Shelf Seas) and simulate a large wind farm with 432 offshore turbines located in the shallow southern waters of the central basin. The simulation is run twice to compare physical parameters such as temperature and circulation pattern and velocity results in the absence and presence of a large wind farm. In the case of no wind farm, model results are validated with data from buoys located in Lake Erie, while with the wind farm, the results show that the central basin is impacted by the wind turbines. This occurs because the reduced wind speed and stress leads to less mixing, lower current speeds and higher surface water temperature. There is no significant impact, however, in the eastern and western basins. This research examines Lake Erie, since this lake has a high potential for offshore wind turbine installation due to its proximity to population centers and its shallow depth.

Occurrence of potentially toxic cyanobacteriaMicrocystis aeruginosain aquatic ecosystems of central Kerala (south India)

Microcystis aeruginosais a potentially toxic bloom-forming freshwater cyanobacterium, usually found in eutrophic water bodies worldwide. The present study reports the occurrence ofMicrocystis aeruginosaand its bloom in freshwater ponds along central Kerala (south India). Monitoring of cyanobacterial blooms was conducted from May 2019 to February 2020 along the aquatic ecosystems of central Kerala and theM. aeruginosablooms were recorded from two freshwater ponds of Kochi. Massive blooms ofM. aeruginosawas observed during the period prior to summer monsoon (May) with an abundance of 1.17 × 106cells L‑1(Station 1) and during early summer (February) latter being more thick scum (2 × 108cells L‑1) with high chlorophylla. Dense aggregates ofM. aeruginosascums were more prevalent during the periods characterised by higher Surface Water Temperature (SWT). The nutrient characteristic pattern of the study area showed the abundance ofM. aeruginosacorrelated very well with higher dissolved nitrate (96.7 μmol L‑1) and phosphate (19.88 μmol L‑1) concentrations. Thus in the stable freshwater ponds with higher SWT and nutrients were the major factors influencing the growth and abundance of the cyanobacteriaM.aeruginosa.Toxicological studies conducted revealed that theMicrocystisbloom was hepatotoxic, inflicting fish mortality.

Do thermal infrared (TIR) remote sensing and direct hyporheic measurements (DHM) similarly detect river-groundwater exchanges? Study along a 40 km-section of the Ain River (France)

Water exchanges through the hyporheic zone are crucial to many ecological processes in streams. One major challenge for river managers is to find a practical method for localizing these exchanges using rapid data acquisition techniques. This work compares spatially continuous data, acquired by Thermal Infrared (TIR) techniques, with discrete data collected in surface water and in the hyporheic zone (DHM), at sites of expected water exchanges (gravel bars). Forty gravel bars, distributed along a 40 km-sector of the Ain River were sampled at upstream- and downstream-bar positions (80 sites) in order to reveal hyporheic exchanges. At each site, 4 physico-chemical parameters were measured at 0, −20 and –50 cm beneath the sediment surface. The field collections of TIR high-resolution images were conducted concomitantly, at low flow and high surface-water temperatures. Among the 80 sites selected for field measurements, 14 were identified as upwellings (groundwater inputs) and 66 as downwelling sites. From those 14 upwellings, 13 were also identified with TIR. The 44 additional sites identified with TIR corresponded to small-sized cold-water patches situated along the gravel bars or to groundwater discharge sites located between the bars (19 lateral seeps). Nevertheless, the DHM method documented on downwelling exchanges (infiltration of surface water), which were not captured by TIR images, and may represent hyporheic hotspots especially for benthic invertebrates. Along the studied sector of the Ain River, these downwelling zones were much more numerous than upwelling ones. Both methods in combination provide a rather complete picture of water exchange along rivers and are needed to evaluate the potential as refuges zones during critical dry periods.

Ice cover and thermal regime in a dimictic seepage lake under climate change

ABSTRACTChanges in water temperature and ice cover are important for controlling the biogeochemical and food-web processes of dimictic lakes in temperate regions. To investigate these changes, we applied a 1-dimensional hydrodynamic lake-ice model to Crystal Lake, a 20 m deep, dimictic seepage lake in northern Wisconsin, USA, to a period of 23 years (1989–2011). The model, DYRESM-WQ-I, incorporates the evolution of blue ice, white ice, and snow cover; time-varying ice and snow albedo; and 2-way coupling across the ice–water interface by incorporating components of previous ice models embedded in the hydrodynamic component of the DYRESM-WQ model. A new element of the current model is time-varying sediment heat fluxes, important to under-ice water temperatures in shallow lakes. Simulations closely reproduce the observed annual stratification regime and ice phenology with standard errors in temperature of 0.7 °C; total difference of 10–20% in ice thickness compared with measured values; and standard errors of 0.9 d for ice-on and 1.55 d for ice-off. Under a likely future climate scenario involving changes in air temperature forecasted to occur by the middle of this century (2055), results suggest an average reduction in mean ice cover thickness of about 0.18 m and an increase in surface mixed-layer temperature of up to 3.5 °C over summer. In addition, water levels decreased by 5.1 m over the simulation period, mostly in response to greater evaporation from higher surface water temperature in summer. The results have important implications for seepage lakes in temperate regions, where projected changes in precipitation with climate warming will not compensate for increased evaporation from the lake surface and the likely alteration in groundwater flows.

The spatial and interannual dynamics of the surface water carbonate system and air–sea CO<sub>2</sub> fluxes in the outer shelf and slope of the Eurasian Arctic Ocean

Abstract. The Arctic is undergoing dramatic changes which cover the entire range of natural processes, from extreme increases in the temperatures of air, soil, and water, to changes in the cryosphere, the biodiversity of Arctic waters, and land vegetation. Small changes in the largest marine carbon pool, the dissolved inorganic carbon pool, can have a profound impact on the carbon dioxide (CO2) flux between the ocean and the atmosphere, and the feedback of this flux to climate. Knowledge of relevant processes in the Arctic seas improves the evaluation and projection of carbon cycle dynamics under current conditions of rapid climate change. Investigation of the CO2 system in the outer shelf and continental slope waters of the Eurasian Arctic seas (the Barents, Kara, Laptev, and East Siberian seas) during 2006, 2007, and 2009 revealed a general trend in the surface water partial pressure of CO2 (pCO2) distribution, which manifested as an increase in pCO2 values eastward. The existence of this trend was defined by different oceanographic and biogeochemical regimes in the western and eastern parts of the study area; the trend is likely increasing due to a combination of factors determined by contemporary change in the Arctic climate, each change in turn evoking a series of synergistic effects. A high-resolution in situ investigation of the carbonate system parameters of the four Arctic seas was carried out in the warm season of 2007; this year was characterized by the next-to-lowest historic sea-ice extent in the Arctic Ocean, on satellite record, to that date. The study showed the different responses of the seawater carbonate system to the environment changes in the western vs. the eastern Eurasian Arctic seas. The large, open, highly productive water area in the northern Barents Sea enhances atmospheric CO2 uptake. In contrast, the uptake of CO2 was strongly weakened in the outer shelf and slope waters of the East Siberian Arctic seas under the 2007 environmental conditions. The surface seawater appears in equilibrium or slightly supersaturated by CO2 relative to atmosphere because of the increasing influence of river runoff and its input of terrestrial organic matter that mineralizes, in combination with the high surface water temperature during sea-ice-free conditions. This investigation shows the importance of processes that vary on small scales, both in time and space, for estimating the air–sea exchange of CO2. It stresses the need for high-resolution coverage of ocean observations as well as time series. Furthermore, time series must include multi-year studies in the dynamic regions of the Arctic Ocean during these times of environmental change.

Distribution and evolution of sterols and aliphatic hydrocarbons in dated marine sediment cores from the Cabo Frio upwelling region, SW Atlantic, Brazil.

We report the distribution of selected lipid biomarkers specifically sterols and aliphatic hydrocarbons in sediment cores from Cabo Frio, SW Atlantic continental shelf, Brazil, corresponding approximately to the last 700years. In the Cabo Frio region, a costal upwelling occurs as a quasi-seasonal phenomenon characterized by nutrient-rich bottom waters that intrude on the continental shelf and promote relatively high biological productivity compared to other Brazilian continental shelf areas. The results for sterols indicate the predominance of organic matter (OM) inputs related to marine organisms, mainly plankton, in all of the cores along the time scale studied. Principal component analyses show three different groups of variables, which may be associated with (i) the more effective intrusion of the nutrient-rich South Atlantic Central Water, resulting in the increase of marine lipid biomarkers such as sterols and short-chain n-alkanes; (ii) the influence of the Coastal Water with higher surface water temperature and subsequently lower primary productivity; and (iii) OM characterized by high total organic carbon and long-chain n-alkanes related to an allochthonous source. Relatively high concentrations of sterols and n-alkanes between 1450 and 1700AD, chronologically associated with the Little Ice Age, suggest a period associated with changes in the local input of specific sources of these compounds. The concentrations of lipid biomarkers vary over core depth, but this does not suggest a notably high or low intensity of upwelling processes. It is possible that the climatic and sea surface temperature changes reported in previous studies did not affect the input of the sedimentary lipid biomarkers analyzed here.

Effects of reservoir operation and climate change on thermal stratification of a canyon-shaped reservoir, in northwest China

Abstract Thermal stratification has a significant impact on water quality and ecological characteristics. Reservoir operation and climate change have an effect on the thermal regime. The Jinpen Reservoir is a large canyon-shaped reservoir located in Shaanxi Province with a strong thermal stratification, which resulted in an anaerobic condition in the hypolimnion. We used a hydrodynamic module based on MIKE 3 to simulate the thermal structure of the Jinpen Reservoir and study the relationship between the thermal regime, reservoir operation and climate change. Based on the daily hydrological and climatic data from 2004 to 2013, we made 13 hypothetical simulated conditions that included extreme change of inflow volume, water level, air temperature, radiation, inflow water temperature and selective withdrawal to explore the effect of different factors on the thermal regime. The results showed that the period of thermal stratification, water column stability and surface water temperature were influenced by these factors. With the increase of air temperature, the simulation results indicated a stronger thermal stratification and a higher surface water temperature, which could cause water safety problems. Deep withdrawal could decrease water column stability and prompt water column mixing early, which could be used by reservoir managers to optimize the reservoir operation.