Effects Of High Summer Temperatures Research Articles

Positive effects of warming do not compensate growth reduction due to increased aridity in Mediterranean mixed forests

AbstractIncreased winter and early spring temperatures due to climate change can enhance forest productivity due to earlier growth onset in temperate regions. However, drought‐prone forests can be highly vulnerable to the combined effect of high summer temperatures and water stress. Understanding how water stress and rising temperatures along the growing season control tree growth in co‐occurring species with contrasting drought tolerances is key to project climate change effects on forest dynamics. Here, we evaluated the interactive effect of seasonal temperature and water availability on annual radial growth and climate–growth instability over six decades (1951–2015) in three pine species with contrasting drought tolerances (Pinus pinaster, Pinus nigra, and Pinus sylvestris). For this, we retrospectively observed radial tree growth using dendroecological methods and evaluated seasonal temperature and water availability effects by using linear mixed models. Early growing season temperature and water availability had a positive effect on tree growth, but the positive effect of late season temperature was modulated by water availability. Moving time‐window analyses revealed temporal instability in climate–growth relationships. Since the 1980s, pine species showed a higher growth sensitivity to both seasonal temperatures and annual water availability. Furthermore, growth reductions were more pronounced due to increased summer temperatures and reduced precipitation. Our results were similar for the three studied pine species despite their contrasting tolerance to drought. Overall, climate warming effects on pine growth are contingent upon water availability in Mediterranean continental forests. Synchronization among species, climate–growth instability, and negative growth trends suggests an increased vulnerability to drought of Mediterranean pine species in response to ongoing climate change.

Dynamics of climate changes and its effect on the performance of cereals according to satellite data

Aim. To study the effect of climate changes on the performance of cereals. Methods. Remote, statistical, and analytic methods. Climate changes were determined by the sum of radiation temperatures of the earth’s surface, calculated using the data of the infrared range (10.3–11.3; 11.4–12.4 μm) of the high precision radiometer AVHRR of the meteorological satellites of the Earth, NOAA. The effect of climate changes on the state of vegetation, including the phenological parameters, such as the beginning, the end, and the length of the vegetation peri- od, was determined by the Normalized Difference Vegetation Index (NDVI) obtained using the data of infrared (0.72–1.1 μm) and red (0.58–0.68 μm) ranges of the abovementioned radiometer. The satellite data are openly accessed on the website of STAR NESDIS NOAA – Satellite Applications and Research of NOAA’s National En- vironmental Satellite Data Information Services of the National Ocean and Atmosphere Research Department of the USA – http://www.star.nesdis.noaa.gov/smcd/emb/vci/VH/. The dynamics of СО2 concentration in the air was determined using the satellite data of AIRS/Aqua (https://disc.gsfc.nasa.gov/datasets/AIRS3C2M_005/summary), the precipitation dynamics – using the data of ERA5 ECMWF/Copernicus Climate Change Service (https://devel- opers.google.com/earth-engine/datasets/catalog/ECMWF_ERA5_MONTHLY#description). The association be- tween the performance of cereals and satellite data was found by the method of correlation and regression analysis. Results. Using the satellite data, it was found that in 1982–2021, the sum of radiation temperatures of the earth’s surface reliably increased in the vegetation period (April–September), and the amount of precipitation tended to decrease in the zones of Polissia, Forest-Steppe, and Steppe. The highest tempo of warming was observed in the territory of the Steppe. In general, the warming had a positive effect on the state of vegetation, according to NDVI, and on the performance of cereals. However, in the recent decade (2010–2020), the positive effect of the warming on the vegetation state decreased, and according to the NDVI trend, there has been a tendency towards the negative effect of the further increase in the temperature, which is especially remarkable for the Steppe and Forest-Steppe. On the contrary, the warming in the Polissia zone created the conditions for the increase in the performance of agri- cultural cereals. Conclusions. From 1982 till 2021, there was a strong tendency towards the increase in the sum of radiation temperatures of the earth’s surface in the vegetation period and the decrease in the precipitation amount in Polissia, Forest-Steppe, and Steppe. According to the NDVI data, due to the warming, the length of the vegeta- tion period increased by 14–21 days on average, which had a positive effect on the vegetation. Since 2010, the acceleration in the warming tempo has been observed. The average annual increase in the sum of the radiation tem- peratures of the earth’s surface in the vegetation period is 15–16 °С – for Polissia, 14–18 °С for the Forest-Steppe, and 18–20 °С for the Steppe. According to this tendency, if the current regularity in the temperature increase in 1982–2021 is preserved in the nearest future, the temperature of the vegetation period will additionally increase, and the average sum of temperatures of the earth’s surface will reach 4000 °С in Polissia, which was notable for the Forest-Steppe in 1982–2021, 4440–4600 °С – in the Forest-Steppe, and up to 5800–5900 °С in the Steppe. In 1982–2000, there was a strong regularity of the positive effect of the warming during the vegetation period both on the vegetation state according to the NDVI index, and the performance of cereals in all three natural climatic zones. However, after 2000, due to the negative effect of high summer temperatures on the crops, the effect of the warming on vegetation considerably decreased, which was especially notable for the Steppe where the increase in NDVI compared to the previous period was on average 9.41 % in 1992–2001, and in 2012–2021 – 1.78 %.

Childhood cognitive and behavioral effects of high summer temperatures

Childhood cognitive and behavioral effects of high summer temperatures

Spatial and temporal dynamics of microbial community composition and factors influencing the surface water and sediments of urban rivers

The temporal and spatial characteristics of urban river bacterial communities help us understand the feedback mechanism of bacteria to changes in the aquatic environment. The Fuhe River plays an important role in determining the water ecological environment of Baiyangdian Lake. 16S rRNA gene sequencing was used to study the microbial distribution characteristics in the Fuhe River in different seasons. The results showed that some environmental factors of the surface water (ammonia nitrogen (NH3-N), total nitrogen (TN), and total phosphorus (TP)) were different on the spatial and temporal scales. Moreover, there were no seasonal differences in the contents of TN, TP, total organic carbon (TOC), or heavy metals in the sediments. The distributions of Cyanobacteria, Actinomycetes and Firmicutes in the water and Actinomycetes and Planctomycetes in the sediments differed significantly among seasons (P < 0.05). There were significant spatial differences in bacteria in the surface water, with the highest abundance of Proteobacteria recorded in the river along with the highest nutrient concentration, while the abundance of Bacteroidetes was higher in the upstream than the downstream. Microbial communities in the water were most sensitive to temperature (T) and the TP concentration (P < 0.01). Moreover, differences in the bacterial community were better explained by the content of heavy metals in the sediments than by the chemical characteristics. A PICRUSt metabolic inference analysis showed that the effect of high summer temperatures on the enzyme action led to an increase in the abundances of the metabolic-related genes of the river microorganisms.

Изучение морфобиологических особенностей некоторых сортов Gladiolus × hybridus hort. в условиях Саратовского Поволжья и Пермского края

The paper describes the results of a three-year study of biological characteristics of ‘Malika’, ‘Professor Parolek’, ‘Dolgozhdannyy Debut’ gladiolus cultivars on the territory of the Education and Research Center «Botanical Garden» in Saratov State University (Saratov) and the Genkel Botanical Garden in Perm State University (Perm). Plants in Saratov region experienced a detrimental effect of high summer temperatures and insufficient humidity: the height of the plant and the size of the inflorescence, the number of flowers in the inflorescence decreased. Nevertheless, the gladiolus development rate matched the biological characteristics of the cultivars in the Saratov Botanical Garden, they happened to go through the growing season before the frost and formed a mature replacement corm. In the Perm Botanical Garden, the studied cultivars also went through all the stages of growth and development, but the flowering period of the studied cultivars started later, which may be associated with a decrease in summer temperature below the optimum temperature in some years. This may have impeded the maturation of replacement corms and increased the susceptibility to fungal diseases. At the same time, the studied gladiolus cultivars retained most fully their decorative features in the Perm Botanical Garden.

Microclimate modeling in naturally ventilated dairy barns during the hot season: Checking the accuracy of forecasts.

Monitoring and predicting the microclimate in naturally ventilated barns (NVB) is important given the adverse effects of high summer temperatures on dairy cows in the context of global climate change. The aim of the work was to verify the accuracy of the microclimate forecast in a NVB using linear regression (LR). Our working hypothesis suggested that multiple periodic measurements of air temperature and relative humidity outside and inside the barns at the same time will allow us to build LR models for predicting the temperature-humidity index (THI). This was done not only for a specific dairy barn based on this indicator outside, but also in other dairy barns with a similar design, located in similar weather conditions. The results of the research indicate that the use of LR had a high accuracy of forecasting (93-96%) the THI in NVB of various designs during the summer heat. At the same time, differences were found between traits (air temperature, relative humidity as well as resulting THI) provided by meteorological weather stations and these data measured simultaneously next to the dairy barns. The proposed LR models can be used to predict THI in NVBs of different designs.

The Effect of High Summer Temperatures on Reproduction in Holstein and Czech Fleckvieh

The aim of this study was to evaluate the effect of the hot summer season on the length of service period (number of days from calving to the next conception). For the analysis was created two databases of twins. Twins had the same sire and dam and the first and second lactations took place on the same farm. In the first database (105 pairs) one of the twins was calved (and had the following service period) during the summer months and the second of the twin was calved in the cooler months. To compare the difference in service period between twins in the hot and cool seasons, it was made also second group of twin pairs with reproduction only during the cool period of the year (cool vs. cool season; 58 pairs). The data were analysed using PROC GLM of Statistica®. In addition to season, the effect of breed (Holstein, Czech Fleckvieh), milk production at 1st lactation and calving year were also taken into consideration. The results showed a significantly longer service period in the summer (133.8 days) than in the cool months (114.7 days). In the case of twins with reproduction in the cool months of the year, the difference between length of service period was shorter and insignificant (113.6 vs. 119.4 days).

Kaolin treatments on Tonda Giffoni hazelnut (Corylus avellana L.) for the control of heat stress damages

The aim of the study was to verify if a new kaolin formulation, which contains 100% aluminium silicate, can reduce the negative effects of high summer temperatures and heat stress on canopy physiological processes, yield and fruit quality in hazelnut (Corylus avellana L.). Kaolin was applied on young Tonda Giffoni plants three times in 2018 summer at 3% concentration: sprays were carried out when the maximum air temperature exceeded 35 °C for 2–3 consecutive days. Kaolin coating increased light albedo, reducing leaf temperature from 2.9 °C up to 6.9 °C and the consequent heat stress, without significant negative effects on photosynthetic activity (Pn), stomatal conductance (gs) and water use efficiency (WUE). Occasionally kaolin significantly reduced leaf transpiration (E). Under water shortage stomatal resistance increased, independent of leaf temperature, therefore in kaolin treated plants (KL plants) the Pn, gs, E and WUE were less than those in control, whereas in post-harvest period Pn and WUE were significantly higher. At re - watering the KL plants have greater Pn and WUE values and less water loss due to transpiration. Moreover, summer rainfall is not able to wash out the whole kaolin covering, allowing an average foliar coverage of the kaolin sufficient to protect the photochemical efficiency of PSII (Fv/Fm). At harvest, the KL plants showed a higher kernel yield and oil content. Kaolin application is a viable strategy to contrast the increase of anomalous high temperature due to actual global warming and can allow to extend hazelnut growing in areas where summer heat stress are limiting.

Passive strategies used in Southern Portugal vernacular rammed earth buildings and their influence in thermal performance

In Portugal, there is a lack of quantitative studies on the thermal performance of earthen buildings. This paper aims at contributing to this research context by studying site-specific strategies, and the thermal performance and comfort conditions of a rammed earth building located in southern Portugal. The study is based on objective and subjective assessments and consists of assessing the hygrothermal conditions, thermal comfort levels and analysing the occupants' perception regarding thermal sensation. The results showed that the strategies used are closely related to local conditions, mitigating the effects of high summer temperatures and ensuring a good summer thermal performance by passive means alone. During the summer monitoring, results showed that the building stayed most of the time (80%) in Category I (high level of expectation) and the remaining time in Category II, according to the classification method defined by the standard EN15251. During the winter period, the building had the worst performance, being necessary a heating system to guarantee comfort conditions. Additionally, the results showed that the good thermal performance of the case study depended more on the high thermal inertia than on the U-value of the envelope. Limitations and advantages of the use of earthen construction elements are discussed.

The Effect of High Summer Temperatures on Emergency Hospital Admissions in Switzerland: A Comparison to Heat-Related Mortality

Background. While previous studies showed a clear association between high temperatures and mortality in Switzerland, the effect of heat on morbidity is less well understood. To evaluate how hot weather affects morbidity in Switzerland and how it compares to heat-related mortality, we assessed a) cause-specific morbidity during the hot summer 2015, and b) the relationship between heat and emergency hospital admissions (EHA) between 2006 and 2015.Methods. Daily EHA during the summer 2015 (June-August) were analysed in relation to comparison periods. For aim b) daily relationships between day-time maximum (Tmax) temperature and EHA during the warm season (May-September) 2006 to 2015 were assessed for different diagnoses and age groups by applying conditional Poisson regression models combined with Non-Linear Distributed lag models. Results were compared to our similar analyses on heat-related mortality.Results. Overall, we estimated an increase in EHA (non-external causes) by 2.6% (95%CI: 0.2-5.0) during the summer 2015. For the same period, an excess mortality of 5.4% (3.0-7.9%) was estimated. Between 2006 and 2015 an increase in Tmax from the median (22&amp;#176;C) to the 98th percentile (33&amp;#176;C) was associated with an overall 3% increase in EHA over the following four days (relative risk (RR) of 1.03 (95% CI: 1.02-1.05)). Larger increases were observed for respiratory (1.20 (1.09-1.31)) and renal diseases (1.21 (1.04-1.40)) in people aged &amp;#8805;74 years, and for infection-related EHA, mainly in children &amp;lt;15 years old (1.24 (1.11-1.37)). In contrast to heat-related mortality, no significant association between Tmax and cardiovascular EHA was found.Conclusions. Our results indicate that increases in ambient temperature during summer months have important impacts on morbidity. In general, the impact was smaller than what was observed for heat-related mortality. Nevertheless, current public health strategies can greatly be improved when targeting measures to both groups of outcomes.

In this issue of Weather

We begin this issue on p. 207 with an examination of heatwaves and their occurrence in Poland since 1950. In ‘The August 2015 mega-heatwave in Poland in the context of past events’ by Agniewska Krzyźewska and Jamie Dyer, the effects of high summer temperatures and the occurrence of heatwaves in this central European country with a high degree of continentality are discussed. On p. 215, Rodney Hale gives us a fascinating brief overview of unusual electrical phenomena in ‘Rare, anomalous non-transient lights associated with cumulonimbus Clouds’. Through much of the history of meteorological observation, recording instruments produced graphical charts (and this is still the case from some instruments). Tilting syphon rain gauges have been an important method of recording rainfall rates and quantities, but they use daily, weekly or monthly charts which have to be changed at these intervals and that do not provide a digital record that can be used and stored readily. ‘Open-source tool for interactive digitisation of pluviograph strip charts’ by Nejc Sušin and Peter Peer provides a description of freely available software that can be used to extract both rainfall amounts and their rates as digital records from these charts on p. 222. Although there is a well-established observations system for hurricanes in the Atlantic and eastern Pacific Oceans, this a developing science for typhoons in the North-West Pacific, even though this area is globally most active for these storms. Recently, the Chinese government has provided an aircraft to carry out typhoon observation from Hong Kong, as described in ‘The first complete dropsonde observation of a tropical cyclone over the South China Sea by the Hong Kong Observatory’ by Pak-Wai Chan, NG Wu, CZ Zhang, WJ Deng and KK Hon on p. 227. This final paper in our July 2018 issue describes the insights they gained into Tropical Storm Aere and its structure.

Wet feed and cold water as heat stress modulators in growing Muscovy ducklings

In an attempt to alleviate the deleterious effects of high summer temperatures, the present study investigated the effects of wet feed and cold water on the growth performance, carcass and meat quality, leg problems, physiological responses, and blood parameters of growing Muscovy ducklings. A total of 180 4-week-old ducklings was randomly divided into 6 experimental groups in a 3 × 2 factorial design that included 3 feed systems (AD: ad libitum dry; DW: diurnal wet; and AW: ad libitum wet) and 2 systems of water (TW: tap water; and CW: cold water). Access to wet feed and cold water affected the growth performance, dressed carcass, gizzard, meat quality (tenderness, juiciness, and susceptibility), tonic immobility, body temperature, and blood parameters [albumin: globulin (A: G) ratio and levels of glucose, alanine transferase (ALT), total antioxidant capacity (T-AOC), and malondialdehyde (MDA)] of the ducklings but had no significant effect on plumage condition, shank length, keel bone length, leg problems, or breast blisters. The body weight (BW) of the DW group was 1.97 and 3.12% greater than that of the AD and AW groups, respectively, and the BWG of the DW group was 6.91 and 10.72% greater than that of the AD and AW groups, respectively. Therefore, providing access to wet feed and cold water is highly recommended when raising Muscovy ducks in open houses under high-temperature conditions.

Forage and grain yield of common buckwheat in Mediterranean conditions: response to sowing time and irrigation

With the view to extending the cultivation of common buckwheat to Mediterranean environments, we investigated the responses of two varieties to three sowing times, early spring, late spring and late summer, in rainfed and irrigated conditions. Plants were harvested at two ripening stages for forage production and at maturity for grain yield. The crop cycle lasted 82–88 days independent of sowing time, whereas the thermal time was ~1000 degree-days in early spring and late summer sowings, and 1200 degree-days when sown in late spring. Forage yield increased up to 75% between ripening stages. Early spring was the best sowing time for forage (4 t ha–1 dry weight) and grain yield (2 t ha–1 dry weight) in rainfed conditions. Late spring sowings give the highest forage yield when irrigated (6 t ha–1 dry weight), but were not suitable for producing grain, for the adverse effect of high summer temperatures on seed set and seed filling. Late summer sowings produced acceptable grain yield (1.5 t ha–1 dry weight), whereas short days and low temperatures limited forage production. Thus, in Mediterranean environments, buckwheat could be profitably introduced as a minor summer crop, sown in early spring for grain production and in late spring for forage production.

Thermal stress in Arctic charr Salvelinus alpinus broodstock: a 28 year case study.

Temperature and egg viability data from an Arctic charr Salvelinus alpinus hatchery covering a period of 28 years were analysed. During the study period, there was a significant increase in the mean water temperature in May, July, August and September of c. 2° C. Independent of year, the egg viability showed a negative correlation with the mean monthly temperatures in July, August and September as well as with the temperature difference between October and November. The negative effect of high summer temperatures was further supported by a comparison of egg viability from replicate broodstock reared at two sites differing mainly in summer water temperature. The eggs from the colder site were, on average, significantly larger (4·4 mm compared with 4·0 mm) and had higher hatching rates (57% compared with 37%). These results suggest that unfavourable temperature conditions during the summer and autumn can explain much of the excessive egg mortality experienced at the main facility used for the Swedish S. alpinus breeding programme. The main effect was supra-optimal temperatures during the period July to September, but there also appears to have been an effect from the temperature regime before and during spawning (October to November) that was unrelated to the summer temperatures. These findings emphasize the importance of site selection and sustainable management of aquaculture hatcheries in the light of the ongoing climate change.

Effects of high summer temperatures on mortality in 50 Spanish cities.

BackgroundPeriods of high temperature have been widely found to be associated with excess mortality but with variable relationships in different cities. How these specifics depend on climatic and other characteristics of cities is not well understood. We assess summer temperature-mortality relationships using data from 50 provincial capitals in Spain, during the period 1990–2004.MethodsPoisson time series regression analyses were applied to daily temperature and mortality data, adjusting for potential confounding seasonal factors. Associations of heat with mortality were summarised for each city as the risk increments at the 99th compared to the 90th percentiles of the whole-year temperature distributions, as predicted from spline curves.ResultsRisk increments averaged 14.6% between both centiles, or 3.3% per 1 Celsius degree. Although risk increments varied substantially between cities, the range of temperature from the 90th to 99th centile was the only characteristic independently significantly associated with them. The heat increment did not depend on other city climatic, socio-demographic and geographic determinants.ConclusionsCities in Spain are partially adapted to high mean summer temperatures but not to high variation in summer temperatures.

Modelling the relative importance of the urban heat island and the thermal quality of dwellings for overheating in London

The aim of this study was to assess variations in indoor temperatures in London dwellings during periods of hot weather, and the degree to which those dwelling-to-dwelling variations are explained by the thermal characteristics of the dwelling and location within the urban heat island (UHI).Indoor temperatures during periods of hot weather were modelled using the EnergyPlus simulation programme, taking as input data the building characteristics of 15 notional dwelling archetypes broadly representative of the London housing stock, and assessed under warm future weather conditions at two locations within London. Data on dwelling types and characteristics were determined from Geographic Information System databases, national level domestic building surveys and other sources. External weather data were derived from the London Site-Specific Air Temperature model under the UK Climate Impacts Programme (UKCIP) 2002 2050s Medium–High emissions scenario.There was substantial variation in indoor temperatures across built forms. The thermal quality of a dwelling has an appreciably greater effect on indoor temperatures during the ‘hot’ period studied than the UHI itself.The effects of built form and other dwelling characteristics appear to be more important determinants of variation in high indoor temperatures than the location of a dwelling within London’s UHI. This observation suggests that policies aimed at protection against the adverse effects of high summer temperatures may need to focus more on dwelling design and construction than on the amelioration of the UHI.

Interdependent effects of habitat quality and climate on population growth of an endangered plant

Summary 1. To predict the viability of populations, it is essential to clarify how performance depends both on large-scale environmental changes, such as climate warming, and on the local habitat. However, in spite of their potential importance, effects of interactions between large-scale environmental changes and the local environment on population viability have rarely been examined. 2. We investigated how population dynamics of the endangered alpine plant Dracocephalum austriacum depend on local habitat quality and climatic variation, as well as how effects of climate depend on local habitat. We used lasso regression shrinkage and integral projection models to identify effects on vital rates and population growth rates in seven populations over seven annual transitions. 3. Populations on steeper slopes had lower survival and stochastic population growth rate than populations on more gentle slopes. In years with low spring temperatures and high summer temperatures, survival and population growth rate were lower. In addition, the negative effects of high summer temperatures did depend on local habitat quality, being more negative in populations on steeper slopes. 4. Combining the net positive effects of high spring temperature and the net negative effects of high summer temperature on plant vital rates with predicted climate change over the next 30 years suggested that effects on D. austriacum would be relatively small. 5. Synthesis. Our results show that different aspects of a warmer climate may have opposing effects on populations, and that climatic effects may depend on local habitat quality. Such interactive effects should be accounted for when determining effects of large-scale environmental changes on population and community dynamics.

An Analysis of the Growth of Fire-Damaged Pinus koraiensis Trees in a Northern Temperate Forest

This paper evaluates the effects of a groundfire on the radial growth of fire tolerance species Korean pine (Pinus koraiensis) trees growing in the Changbai Mountain Nature Reserve which occurred more than 90 years.. The study material includes damaged and undamaged sample trees approximately with the same age. The trees are growing in close vicinity to each other and are subject to the same climatic conditions. The study material is rather homogenous and allowing analysis of the long-term effects of ground fire on the growth of important tree species. Two tree-ring chronologies from 25 fire-damaged and 24 undamaged trees were analyzed. The results showed that temperature and precipitation during the previous winter had a significant effect on the mean annual radial growth of both groups. Trees which had been damaged by an old fire were more sensitive to the negative effect of high summer temperatures and to the positive effect of high spring temperatures than the undamaged group. In contrast to previous studies, their growth was also considerably more affected by competition than the growth of the undamaged individuals. The study material is rather homogenous and probably unique, allowing such a temporal and spatial analysis of the long-term effects of a ground fire on the growth of an important tree species in a northern temperate forest.

Quantifying the effects of high summer temperatures due to climate change on buckling and rail related delays in south‐east United Kingdom

AbstractExtreme high temperatures are associated with increased incidences of rail buckles. Climate change is predicted to alter the temperature profile in the United Kingdom with extreme high temperatures becoming an increasingly frequent occurrence. The result is that the number of buckles, and therefore delays, expected per year will increase if the track is maintained to the current standard. This paper uses a combination of analogue techniques and a weather generator to quantify the increase in the number of buckles and rail related delays in the south‐east of the United Kingdom. The paper concludes by assigning a cost to the resultant rise in delays and damage before making recommendations on how these effects can be mitigated. Copyright © 2008 Royal Meteorological Society

Differential grazer‐mediated effects of high summer temperatures on pico‐ and nanoplankton communities

We investigated the role of a macrograzer (the filter feeding mussel Dreissena polymorpha) in mediating effects of high summer temperatures on the dominant components of natural river plankton (i.e., bacteria, algae, and heterotrophic flagellates) in flow channel experiments. Effects of adaptation (by comparing mussels from a southern and a northern population) and thermal acclimation of the mussels were considered. Both heterotrophic flagellates and algae are released from grazing pressure and increase in abundance at temperatures above 20°C. Bacterial abundance, however, decreased with increasing temperature, suggesting a trophic cascade (mussel‐flagellates‐bacteria) that is altered by the temperature response of the mussel ingestion rate. Warm acclimation of the mussels did not change the outcome of the experiments. The dreissenids from the southern population showed a significantly higher ingestion rate than those from the northern population only in July. The general pattern (i.e., decreasing ingestion rates at high temperatures) was found in both populations. Microbial communities controlled by macrofauna can experience substantial changes in warm summers because of differential development of direct and indirect grazing effects with increasing temperature.