Water Temperature Trends Research Articles

Regional Response to Global Warming: Water Temperature Trends in Semi-Natural Mountain River Systems

River water temperature (TW) is a key environmental factor that determines the quality of the fluvial environment and its suitability for aquatic organisms. Atmospheric warming, accompanied by more frequent extreme weather phenomena, especially heat waves and prolonged drought, may pose a serious threat to the river environment and native river ecosystems. Therefore, reliable and up-to-date information on current and anticipated changes in river flow and thermal conditions is necessary for adaptive water resource management and planning. This study focuses on semi-natural mountain river systems to reliably assess the magnitude of water temperature change in the Polish Carpathians in response to climatic warming. The Mann–Kendall test was used to detect trends in water temperature series covering the last 35 years (1984–2018). Significant, rising trends in annual water temperature were found for all studied sites, with differences in intensity (0.33–0.92 °C per decade). Trends in TW were strongest in summer and autumn (0.75–1.17 and 0.51–1.08 °C per decade), strong trends were found in spring (0.82–0.95 °C per decade), and weaker in winter (0.25–0.29 °C per decade). Simultaneous air temperature trends were broadly consistent with water temperature trends. This indicates the urgent need for adaptive management strategies to counteract thermal degradation of the fluvial environment under study.

Stream temperature and discharge evolution in Switzerland over the last 50 years: annual and seasonal behaviour

Abstract. Stream temperature and discharge are key hydrological variables for ecosystem and water resource management and are particularly sensitive to climate warming. Despite the wealth of meteorological and hydrological data, few studies have quantified observed stream temperature trends in the Alps. This study presents a detailed analysis of stream temperature and discharge in 52 catchments in Switzerland, a country covering a wide range of alpine and lowland hydrological regimes. The influence of discharge, precipitation, air temperature, and upstream lakes on stream temperatures and their temporal trends is analysed from multi-decadal to seasonal timescales. Stream temperature has significantly increased over the past 5 decades, with positive trends for all four seasons. The mean trends for the last 20 years are +0.37±0.11 ∘C per decade for water temperature, resulting from the joint effects of trends in air temperature (+0.39±0.14 ∘C per decade), discharge (-10.1±4.6 % per decade), and precipitation (-9.3±3.4 % per decade). For a longer time period (1979–2018), the trends are +0.33±0.03 ∘C per decade for water temperature, +0.46±0.03°C per decade for air temperature, -3.0±0.5 % per decade for discharge, and -1.3±0.5 % per decade for precipitation. Furthermore, we show that snow and glacier melt compensates for air temperature warming trends in a transient way in alpine streams. Lakes, on the contrary, have a strengthening effect on downstream water temperature trends at all elevations. Moreover, the identified stream temperature trends are shown to have critical impacts on ecological and economical temperature thresholds (the spread of fish diseases and the usage of water for industrial cooling), especially in lowland rivers, suggesting that these waterways are becoming more vulnerable to the increasing air temperature forcing. Resilient alpine rivers are expected to become more vulnerable to warming in the near future due to the expected reductions in snow- and glacier-melt inputs. A detailed mathematical framework along with the necessary source code are provided with this paper.

Assessing effects of temporal changes in River water temperature on stratification in the Ariake Sea

The upward trend of river water temperature due to climate change has recently been confirmed. However, its effects on thermal stratification in coastal waters are not clear. Therefore, targeting the Ariake Sea – a semi-enclosed bay in the island of Kyushu, Japan – we continuously monitored river water temperature during and after August 2015 at the discharge observation stations of Class-A rivers that feed the sea, located in the non-tidal areas of the rivers and closest to the respective river mouths. Numerical simulations were performed on the density stratification in the Ariake Sea using the obtained hourly river water discharge and temperature data, to assess the effects of temporal changes in river water temperature on thermal stratification. It was shown that during a summer flood, river water temperature could influence the reproducibility of the development of thermal stratification depending on the river water temperature used, and the reproducibility of the base water temperature differed during the transition to the mixing period. Effects of river water temperature on the water temperature structure of the sea were indicated.

Historical modelling of changes in Lake Erken thermal conditions

Abstract. Historical lake water temperature records are a valuable source of information to assess the influence of climate change on lake thermal structure. However, in most cases such records span a short period of time and/or are incomplete, providing a less credible assessment of change. In this study, the hydrodynamic GOTM (General Ocean Turbulence Model, a hydrodynamic model configured in lake mode) was used to reconstruct daily profiles of water temperature in Lake Erken (Sweden) over the period 1961–2017 using seven climatic parameters as forcing data: wind speed (WS), air temperature (Air T), atmospheric pressure (Air P), relative humidity (RH), cloud cover (CC), precipitation (DP), and shortwave radiation (SWR). The model was calibrated against observed water temperature data collected during the study interval, and the calibrated model revealed a good match between modelled and observed temperature (RMSE =1.089 ∘C). From the long-term simulations of water temperature, this study focused on detecting possible trends in water temperature over the entire study interval 1961–2017 and in the sub-intervals 1961–1988 and 1989–2017, since an abrupt change in air temperature was detected in 1988. The analysis of the simulated temperature showed that epilimnetic temperature increased on average by 0.444 and 0.792 ∘C per decade in spring and autumn in the sub-interval 1989–2017. Summer epilimnetic temperature increased by 0.351 ∘C per decade over the entire interval 1961–2017. Hypolimnetic temperature increased significantly in spring over the entire interval 1961–2017, by 0.148 and by 0.816 ∘C per decade in autumn in the sub-interval 1989–2016. Whole-lake temperature showed a significant increasing trend in the sub-interval 1989–2017 during spring (0.404 ∘C per decade) and autumn (0.789 ∘C per decade, interval 1989–2016), while a significant trend was detected in summer over the entire study interval 1961–2017 (0.239 ∘C per decade). Moreover, this study showed that changes in the phenology of thermal stratification have occurred over the 57-year period of study. Since 1961, the stability of stratification (Schmidt stability) has increased by 5.365 J m−2 per decade. The duration of thermal stratification has increased by 7.297 d per decade, corresponding to an earlier onset of stratification of ∼16 d and to a delay of stratification termination of ∼26 d. The average thermocline depth during stratification became shallower by ∼1.345 m, and surface-bottom temperature difference increased over time by 0.249 ∘C per decade. The creation of a daily time step water temperature dataset not only provided evidence of changes in Erken thermal structure over the last decades, but is also a valuable resource of information that can help in future research on the ecology of Lake Erken. The use of readily available meteorological data to reconstruct Lake Erken's past water temperature is shown to be a useful method to evaluate long-term changes in lake thermal structure, and it is a method that can be extended to other lakes.

Sixty years since the creation of Lake Kariba: Thermal and oxygen dynamics in the riverine and lacustrine sub-basins.

The current boom of dam construction at low latitudes endangers the integrity and function of major tropical river systems. A deeper understanding of the physical and chemical functioning of tropical reservoirs is essential to mitigate dam-related impacts. However, the development of predictive tools is hampered by a lack of consistent data on physical mixing and biogeochemistry of tropical reservoirs. In this study, we focus on Lake Kariba (Southern Africa), the largest artificial lake in the world by volume. Kariba Dam forms a transboundary reservoir between Zambia and Zimbabwe, and therefore its management represents a socio-politically sensitive issue because the Kariba Dam operation completely changed the downstream hydrological regime. Although Lake Kariba represents a unique and scientifically interesting case study, there is no consistent dataset documenting its physical and chemical behaviour over time. This limits the scope for quantitative studies of this reservoir and its downstream impacts. To address this research gap, we aggregated a consistent database of in situ measurements of temperature and oxygen depth profiles for the entire 60 years of Lake Kariba’s lifetime and performed a detailed statistical analysis of the thermal and oxygen regime of the artificial lake to classify the different behaviours of the lake’s sub-basins. We demonstrate that the seasonal stratification strongly depends on the depth of the water column and on the distance from the lake inflow. Satellite data confirm these spatiotemporal variations in surface temperature, and reveal a consistent longitudinal warming trend of the lake surface water temperature of about 1.5°C from the inflow to the dam. Finally, our results suggest that the stratification dynamics of the lacustrine sub-basins have the potential to alter the downstream Zambezi water quality. Future research should focus on assessing such alterations and developing strategies to mitigate them.

Distributions of 224, 223Ra and 137Cs in the Luzon Strait and its adjacent waters and their response to Typhoon Rainbow

This paper reports the distribution of natural radionuclides of 224, 223Ra in the surface water and stratified waters of the Luzon Strait and its adjacent waters during the cruises of September 2015 and May 2016. To understand the impact of the Fukushima nuclear accident, the artificial radionuclide 137Cs in the waters was also analyzed. The results showed that the activities of 224, 223Ra and 137Cs were all within the natural radioactive background levels of the marine environment in the South China Sea. 224Ra had a higher activity level in the water of the north South China Sea to the west of the Luzon Strait, and a lower activity level in the oceanic Philippine Sea to the east. The 137Cs activity had no obvious spatial trends. Based on the vertical trends of 224Ra, 137Cs, and water temperature and salinity at three stations (LS3, LS5 and LS8), the distinct characteristics of the activity levels and gradients of 224Ra and 137Cs among the tropical surface water, subsurface water and mid-deep water were revealed. Typhoon Rainbow event reversed the overall circulation of the Luzon Strait and its adjacent area. A huge amount of western Pacific water characterized by low 224Ra activities flooded into the South China Sea, reducing the activity level of 224Ra in the waters. However, there were no significant differences of 137Cs activity between the West Pacific and the north South China Sea, and ocean current changes had no effect on the 137Cs activity levels of the water bodies.

Assessment of Full-Scale N2O Emission Characteristics and Testing of Control Concepts in an Activated Sludge Wastewater Treatment Plant with Alternating Aerobic and Anoxic Phases.

This work aims to obtain full-scale N2O emission characteristics translatable into viable N2O control strategies and conduct full-scale testing of the proposed N2O control concepts. Data of a long-term monitoring campaign was first used to quantify full-scale N2O emission and probe into the seasonal pattern. Then trends between N2O production/emission and process variables/conditions during typical operating cycles were revealed to explore the dynamic N2O emission behavior. A multivariate statistical analysis was performed to find the dependency of N2O emission on relevant process variables. The results show for the first time that relatively low/high N2O emission took place in seasons with a decreasing/increasing trend of water temperature, respectively. Aerobic phase contributed to N2O production/emission probably mainly through the hydroxylamine pathway. Comparatively, heterotrophic bacteria had a dual role in the anoxic phase and could be responsible for both net N2O production and consumption. Incomplete denitrification might contribute mainly to the N2O production/emission in the anoxic phase and the accumulation of N2O to be significantly emitted in the following cycle due to the competition between different denitrification steps for electron donors. Therefore, properly extending the length of anoxic phase could serve as a potential control means to regulate N2O accumulation in the anoxic phase. The full-scale testing not only verified the efficacy of reduced dissolved oxygen set-point in reducing N2O emission by 60%, but also confirmed the proposed concepts of control over the aerobic and anoxic phases collectively.

The Rising and Decreasing Heat of Waters

According to the investigation data of Jiaozhou Bay in May, August and October of 1979, the vertical distribution and seasonal variation of water temperature at surface and bottom in inside and outside waters were studied, and the seasonal distribution, variation range and horizontal distribution trend were determined. The results showed that, in inside waters, bay mouth and outside waters, from May to August and to October, the seasonal variation was spring, autumn and spring, respectively, from high to low. It indicated that the water temperature was transported to everywhere in the ocean after the current transport in horizontal and vortex motion mixing in vertical direction. In terms of variation range, it was basically the same at surface and bottom in May, August and October. When it was low at surface, it was low at corresponding bottom; otherwise, it was opposite. From inside waters to bay mouth, the horizontal distribution of water temperature at surface was inconsistent with that at bottom in May, contrary in August and consistent in October. From bay mouth to outside waters, it was contrary in May, consistent in August and inconsistent in October. The horizontal distribution trend of water temperature disclosed that the source of high water temperature was the rising and decreasing heat.

ESTIMATES OF THERMAL DIFFUSIVITY IN DIMICTIC LAKES

Based on the results of long-term measurements of water temperature in three dimictic lakes, we obtained estimates of the average effective coefficients of vertical turbulent exchange for the period of formation of stable thermal stratification (July-August). Deep lakes were chosen as objects of this study: Onega (Russia), Kallavesi (Finland), Inari (Finland). For seasonal water temperature trends averaged over a multi-annual period the effects of horizontal heat transfer can be neglected, so the estimates were based on the analysis of the one-dimensional version of the heat transfer differential equation. The effective thermal diffusivity was calculated in three ways: based on changes in the heat content of the water column, local changes in water temperature, and also through the analysis of the amplitude and phase response of the deep layers temperature on quasi-periodic changes in the average temperature of the epilimnion. These estimates are two to four orders of magnitude greater than the value of the coefficient of molecular thermal diffusivity in water.

Temperature Variability of the Baltic Sea Since 1850 and Attribution to Atmospheric Forcing Variables

AbstractThe Baltic Sea is highly impacted by global warming and other anthropogenic changes and is one of the fastest‐warming marginal seas in the world. To detect trends in water temperature and to attribute them to atmospheric parameters, the results of two different ocean circulation models driven by reconstructed atmospheric forcing fields for the period 1850–2008 were analyzed. The model simulations were analyzed at temporal and spatial scales from seasonal to centennial and from intrabasin to basin, respectively. The strongest 150‐year trends were found in the annual mean bottom temperature of the Bornholm Deep (0.15 K/decade) and in summer mean sea surface temperature (SST) in Bothnian Bay (0.09–0.12 K/decade). A comparison of the time periods 1856–2005 and 1978–2007 revealed that the SST trends strengthened tenfold. An attribution analysis showed that most of the SST variability could be explained by the surface air temperature (i.e., sensible heat flux) and the latent heat flux. Wind parallel to the coast and cloudiness additionally explained SST variability in the coastal zone affected by the variations in upwelling and in offshore areas affected by the variations in solar radiation, respectively. In contrast, the high variability in stratification caused by freshwater and saltwater inflows does not impact the long‐term variability in the SST averaged over the Baltic Sea. The strongest SST trends since the 1980s can be explained by the superposition of global warming and a shift from the cold to the warm phase of the Atlantic Multidecadal Oscillation.

Large Fine‐Scale Spatiotemporal Variations of CH4 Diffusive Fluxes From Shrimp Aquaculture Ponds Affected by Organic Matter Supply and Aeration in Southeast China

AbstractMariculture shrimp ponds are important CH4 sources to the atmosphere. However, the spatiotemporal variations of CH4 concentration and flux at fine spatial scales in mariculture ponds are poorly known, particularly in China, worlds largest aquaculture producer. In this study, the plot‐scale spatiotemporal variations of water CH4 concentration and flux, both within and among ponds, were researched in shrimp ponds in Shanyutan wetland, Min River Estuary, Southeast China. The average water CH4 concentration and diffusion flux across the water‐air interface in the shrimp ponds over the shrimp aquaculture period varied from 2.29 ± 0.29 to 50.48 ± 20.91 μM and from 0.09 ± 0.01 to 2.32 ± 0.95 mmol·m−2·hr−1, respectively. The CH4 emissions from the estuarine ponds varied greatly between seasons, with peaks in August and September, which was similar to the trend of water temperature and dissolved oxygen concentrations. There was no remarkable difference in CH4 concentration and flux between shrimp ponds but significantly spatiotemporal differences in CH4 concentration and flux within the ponds. Significantly higher emissions occurred in the feeding zone, accounting for approximately 60% of total CH4 emission flux, while much lower CH4 emissions appeared in aeration zone, contributing 14% to total flux. This study suggests the importance of considering spatiotemporal variation in the whole‐pond estimates of CH4 concentration and flux. In light of such high spatial variation within ponds, improving aeration and feed utilization efficiency would help to mitigate CH4 emissions from mariculture ponds.

Long-term water temperature trends of the Warta River in the years 1960–2009

In the context of climate changes, detailed information concerning the response of different components of the environment to the observed situation is of high importance. Due to strong correlations of air and water temperatures, water ecosystems have been subject to a considerable transformation for several decades. The paper presents the analysis of long-term (1960-2009) water temperature fluctuations in the Warta River, third longest river in Poland. Based on six stations located along the bank of the Warta River, its thermal parameters and tendencies and rate of changes were determined. Moreover, based on data concerning air temperature, correlations between those two media were analysed. Water temperature in the Warta River over the last fifty years was characterised by low variability, ranging in the analysed stations from 9.5°C in Sieradz to 10.3°C in Poznan and Skwierzyna. The Mann-Kendall trend tests, Sen's slope estimator and the Pettitt test were applied to long-term time series of air and water temperature analysis. Long-term fluctuations of mean annual water temperatures were characterised by an increase (statistically significant, depending on the station at a level of p=0.05 and p=0.001) from 0.096 to 0.281°C.dec-1. Water temperature in the case of the analysed stations appears to be particularly determined by climatic factors, as suggested by very strong correlations with air temperature (from 0.82 to 0.94). Evident changes in the thermal regime of the Warta River in the context of functioning of the entire ecosystem should be considered unfavourable. The observed as well as future increase in water temperature will modify current processes occurring in the river, referring to among others water quality, composition of ichthyofauna, or hydrological conditions. Further temperature changes should be mitigated, by among others appropriate management of the riverbank zone (planting trees) and by increasing the share of forest areas in the catchment.

Effect of climate change on thermal-ice regime of shallow lakes compared to deep lakes: Case study of lakes in the temperate zone (Northern Poland)

The objective of this study is to determine the usefulness of results of hydrological observations (1961-2015) carried out in a deep lake for the assessment changes in the ice-thermal regime of a shallow lake. The paper is based on results of daily values of surface water temperature and ice phenomena in shallow Lake Łebsko and deep Lake Charzykowskie. Similar, statistically significant, positive trends of water temperature were determined in both lakes (0.26°C 10 year-1 in Lake Łebsko and 0.25°C 10 year-1 in Lake Charzykowskie). The fastest increase in mean monthly water temperature in the analysed period (1961-2015) occurred in May (3.37°C in Lake Charzykowskie) and April (2.76°C in Lake Łebsko). The majority of elements of the thermal regime of surface waters in shallow lakes were generally determined to be similar to the same elements in deep lakes, and the effect of climatic changes on the thermal regime of both lakes was found to be similar. Less similarity is observed between elements of the ice regime, particularly in date of decline of the ice cover.

A Long-Term Fine-Resolution Record of AVHRR Surface Temperatures for the Laurentian Great Lakes

Inland waters are warming at highly variable rates that often differ from regional air temperature trends. This variable warming is partially attributable to an individual lake’s geographical and morphological characteristics. In very large lakes, significant intralake variability in long-term warming trends has also been observed. In light of this intralake and interlake heterogeneity of lake surface water temperature (LSWT) and LSWT trends, we revisit the 1.1 km Advanced Very High Resolution Radiometer (AVHRR) record for the Laurentian Great Lakes. In this work, we have assembled a long-term (1986–2016) and high-spatial-resolution (0.018°) daily LSWT dataset using AVHRR record. Subtracting an empirically-determined mean diurnal cycle mitigates the effects of varying observation times. Adjustments in the georegistration of the images are made to reduce the impact of AVHRR navigational errors on the earlier platforms. Both the original daily composites, and a gap-filled product using locally weighted interpolation methods will be made available to support fine-scale physical and environmental research in the region.

The rise and fall of the Cretaceous Hot Greenhouse climate

Abstract A compilation of foraminiferal stable isotope measurements from southern high latitude (SHL) deep-sea sites provides a novel perspective important for understanding Earth's paleotemperature and paleoceanographic changes across the rise and fall of the Cretaceous Hot Greenhouse climate and the subsequent Paleogene climatic optimum. Both new and previously published results are placed within an improved chronostratigraphic framework for southern South Atlantic and southern Indian Ocean sites. Sites studied were located between 58° and 65°S paleolatitude and were deposited at middle to upper bathyal paleodepths. Oxygen isotope records suggest similar trends in both bottom and surface water temperatures in the southern sectors of the South Atlantic and in the Indian Ocean basins. Warm conditions were present throughout the Albian, extreme warmth existed during the Cretaceous Thermal Maximum (early-mid-Turonian) through late Santonian, and long-term cooling began in the Campanian and culminated in Cretaceous temperature minima during the Maastrichtian. Gradients between surface and seafloor δ18O and δ13C values were unusually high throughout the 11.5 m.y. of extreme warmth during the Turonian-early Campanian, but these vertical gradients nearly disappeared by the early Maastrichtian. In absolute terms, paleotemperature estimates that use standard assumptions for pre-glacial seawater suggest sub-Antarctic bottom waters were ≥21 °C and sub-Antarctic surface waters were ≥27 °C during the Turonian, values warmer than published climate models support. Alternatively, estimated temperatures can be reduced to the upper limits of model results through freshening of high latitude waters but only if there were enhanced precipitation of water with quite low δ18O values. Regardless, Turonian planktonic δ18O values are ~1.5‰ lower than minimum values reported for the Paleocene-Eocene Thermal Maximum (PETM) from the same region, a difference which corresponds to Turonian surface temperatures ~6 °C warmer than peak PETM temperatures if Turonian and Paleocene temperatures are estimated using the same assumptions. It is likely that warm oceans surrounding and penetrating interior Antarctica (given higher relative sea level) prevented growth of Antarctic ice sheets at all but the highest elevations from the late Aptian through late Campanian; however, Maastrichtian temperatures may have been cool enough to allow growth of small, ephemeral ice sheets. The standard explanation for the sustained warmth during Cretaceous Hot Greenhouse climate invokes higher atmospheric CO2 levels from volcanic outgassing, but correlation among temperature estimates, proxy estimates of pCO2, and intervals of high fluxes of both mafic and silicic volcanism are mostly poor. This comparison demonstrates that the relative timing between events and their putative consequences need to be better constrained to test and more fully understand relationships among volcanism, pCO2, temperature ocean circulation, Earth's biota and the carbon cycle.

Seasonal drought effects on the water quality of the Biobío River, Central Chile.

Quantifying the effect of droughts on ecosystem functions is essential to the development of coastal zone and river management under a changing climate. It is widely acknowledged that climate change is increasing the frequency and intensity of droughts, which can affect important ecosystem services, such as the regional supply of clean water. Very little is understood about how droughts affect the water quality of Chilean high flow rivers. This paper intends to investigate the effect of an, recently identified, unprecedented drought in Chile (2010-2015), on the Biobío River water quality, (36°45'-38°49' S and 71°00'-73°20' W), Central Chile. This river is one of the largest Chilean rivers and it provides abundant freshwater. Water quality (water temperature, pH, dissolved oxygen, electrical conductivity, biological oxygen demand, total suspended solids, chloride, sodium, nutrients, and trace metals), during the drought (2010-2015), was compared with a pre-drought period (2000-2009) over two reaches (upstream and downstream) of the river. Multivariate analysis and seasonal Mann-Kendall trend analyses and a Theil-Sen estimator were employed to analyze trends and slopes of the reaches. Results indicated a significant decreased trend in total suspended solids and a slightly increasing trend in water temperature and EC, major ions, and trace metals (chrome, lead, iron, and cobalt), mainly in summer and autumn during the drought. The reduced variability upstream suggested that nutrient and metal concentrations were more constant than downstream. The results evidenced, due to the close relationship between river discharge and water quality, a slightly decline of the water quality downstream of the Biobío River during drought period, which could be attenuated in a post-drought period. These results displayed that water quality is vulnerable to reductions in flow, through historical and emerging solutes/contaminants and induced pH mobilization. Consequently, seasonal changes and a progressive reduction of river flow affect the ecosystem functionality in this key Chilean river. The outcomes from this research can be used to improve how low flow conditions and the effects of a reduction in the river volume and discharge are assessed, which is the case under the scenario of more frequent drought periods.

Driver detection of water quality trends in three large European river basins

This study analyses how indicators of water quality (thirteen physico-chemical variables) and drivers of change (i.e., monthly aggregated air temperature and streamflow, population density, and percentage of agricultural land use) coevolve in three large European river basins (i.e., Adige, Ebro, Sava) with different climatic, soil and water use conditions. Spearman rank correlation, Principal Component Analysis, and Mann-Kendall trend tests were applied to long-term time series of water quality data during the period 1990–2015 in order to investigate the relationships between water quality parameters and the main factors controlling them. Results show that air temperature, considered as a proxy of climatic change, has a significant impact, in particular in the Adige and Ebro: positive trends of water temperature and negative of dissolved oxygen are correlated with upward trends of air temperatures. The aquatic ecosystems of these rivers are, therefore, experiencing a reduction in oxygen, which may exacerbate in the future given the projected further increase in temperature. Furthermore, monthly streamflow has been shown to reduce in the Ebro, thereby reducing the beneficial effect of dilution, which appears evident from the observed upward patterns of chloride concentrations and electrical conductivity. Upward trends of chloride and biological oxygen demand in the Adige and Sava, and of phosphate in the Adige appears to be related to increasing human population density, whereas phosphates in the Sava and biological oxygen demand in the Ebro are highly correlated with agricultural land use, considered as a proxy of the impact of agricultural practises.The present study shows the complex relationships between drivers and observed changes in water quality parameters. Such analysis can represent, complementary to a deep knowledge of the investigated systems, a reliable tool for decision makers in river basin planning by providing an overview of the potential impacts on the aquatic ecosystem of the three basins.

Atmospheric hydrological cycles in the Arctic and Antarctic during the past four decades

Atmospheric hydrological cycles over the Arctic and Antarctic have been investigated in the previous studies and there are some similarity and dissimilarity in the two polar regions. The Arctic and Antarctic are areas of moisture flux convergence through the year. So the precipitation (P) exceeds the evaporation (E) and the net precipitation (P-E) is positive. Therefore, the atmospheric moisture transport is a primary input of water into the polar regions. Meanwhile the climatological seasonal cycles of P-E over these regions are dominated by transient moisture flux associated with cyclone activities, the interannual variations are governed by the stationary flux associated with the Arctic Oscillation and the Antarctic Oscillation (AAO). In addition, recent climate changes influence the polar hydrological cycles. Our analyses using an atmospheric reanalysis up to recent years indicated that there were no significant long-term changes in the poleward moisture transport into both the Arctic and Antarctic during 1979-2016. On the other hand, the water vapor (precipitable water) were clearly increasing over the Arctic and gradually decreasing over the Antarctic during the same period. As expected, the increasing trend of water vapor was due to the large warming over the Arctic. There were two reasons for the gradually decreasing trend of water vapor over the Antarctic. The first one was the positive trend of AAO in summer and the second was deepening trend of the Amundsen low in autumn. The trends in water vapor and temperature during the past 38 years further suggest that both polar regions were getting dryer in several seasons. The trend, however, needs to be confirmed by follow-up climatological analyses.

Seasonality of change: Summer warming rates do not fully represent effects of climate change on lake temperatures

AbstractResponses in lake temperatures to climate warming have primarily been characterized using seasonal metrics of surface‐water temperatures such as summertime or stratified period average temperatures. However, climate warming may not affect water temperatures equally across seasons or depths. We analyzed a long‐term dataset (1981–2015) of biweekly water temperature data in six temperate lakes in Wisconsin, U.S.A. to understand (1) variability in monthly rates of surface‐ and deep‐water warming, (2) how those rates compared to summertime average trends, and (3) if monthly heterogeneity in water temperature trends can be predicted by heterogeneity in air temperature trends. Monthly surface‐water temperature warming rates varied across the open‐water season, ranging from 0.013 in August to 0.073°C yr−1 in September (standard deviation [SD]: 0.025°C yr−1). Deep‐water trends during summer varied less among months (SD: 0.006°C yr−1), but varied broadly among lakes (–0.056°C yr−1 to 0.035°C yr−1, SD: 0.034°C yr−1). Trends in monthly surface‐water temperatures were well correlated with air temperature trends, suggesting monthly air temperature trends, for which data exist at broad scales, may be a proxy for seasonal patterns in surface‐water temperature trends during the open water season in lakes similar to those studied here. Seasonally variable warming has broad implications for how ecological processes respond to climate change, because phenological events such as fish spawning and phytoplankton succession respond to specific, seasonal temperature cues.

A Climate Change Assessment via Trend Estimation of Certain Climate Parameters with In Situ Measurement at the Coasts and Islands of Viet Nam

This study presents results on an assessment of climate change in the nearshore and coastal areas of Viet Nam through an evaluation of trends of certain climatic parameters (air temperature, sea water temperature, sea level, and number of typhoons landed at the Vietnamese coast by year) using time series data of hydro-meteorological records at the coasts and islands of Viet Nam. The method used for the trend evaluation is the Mann–Kendall test ran at the 5% significance level and Spearman rank correlation coefficient. It was found that there is an extremely likely increasing trend of air temperature for almost all observation stations at the coasts and islands of Viet Nam. However, it was unable to confirm a general trend for sea surface water temperature; except for very few stations in semi-closed waters, there is no clear trend in annual average sea water temperature at a majority of stations. Additionally, there is an extremely likely rising trend of sea level at a majority of stations with reliable data, but the rates of increase are very different for different stations. The reasons for discrepancies in the trend of annual average sea water temperature and sea level at different stations are still not understood, but it seems that an assessment of the vertical movement of the ground surface at the stations is necessary to have an accurate assessment of the rate of sea level rise due to climate change and of the influence of general circulation in the East Viet Nam Sea on the trend of sea water temperature in that location. It is also found that there is a likely decreasing trend in the frequency of typhoons landed at the Vietnamese coast; however, this trend might not be due to climate change, but to climate variability.