A PRELIMINARY LIMNOLOGICAL INVESTIGATION OF TWELVE SOUTHERN AFRICAN GEOTHERMAL WATERS

SUMMARY

The ionic composition and algal flora of twelve geothermal waters in southern South West Africa and north-western South Africa are described, many for the first time. Water temperatures ranged from 24,9 to 66 °C and salinity values indicated moderate mineralization. A trend of increasing sodium and sulphate dominance with increasing water temperature was evident. Green algae were confined to springs with low water temperatures (below 31 °C) while diatoms and blue-green algae were recorded in all the springs, at temperatures up to 66 °C. Flexibacteria (Chlcroflexus) were recorded only in the hotter springs, above 40 °C.     

Effects of Entrainment on Phytoplankton Primary Production at Four Thermal Electric Generating Stations on the Laurentian Great Lakes

Primary production was used to measure the response of phytoplankton to entrainment in once-through cooling water at thermal electric generating stations. Ambient lakewater temperatures ranged from 1.0 to 20.5 °C. The maximum discharge temperature was 32.0 °C. There was no chlorination of cooling water at the stations studied. On a few occasions, primary production was stimulated following station passage by discharge temperatures which were approximately 10 °C above ambient lakewater temperatures of 4.5 to 8.5 °C. Differences in production levels were not apparent, however, following the return of discharge water to ambient lakewater temperature. There was no consistent response of phytoplankton to the temperature regimes tested, with production levels generally differing by less than 20 % as a result of station passage or temperature elevation alone. Entrainment was considered to have minimal impact on phytoplankton productivity in large open water bodies such as the Great Lakes.     

Methane oxidation activity and diversity of aerobic methanotrophs in pH-neutral and semi-neutral thermal springs of the Kunashir Island,Russian Far East

Aerobic methane oxidation has been mostly studied in environments with moderate to low temperatures. However, the process also occurs in terrestrial thermal springs, where little research on the subject has been done to date. The potential activity of methane oxidation and diversity of aerobic methanotrophic bacteria were studied in sediments of thermal springs with various chemical and physical properties, sampled across the Kunashir Island, the Kuriles archipelago. Activity was measured by means of the radioisotope tracer technique utilizing ¹⁴C-labeled methane. Biodiversity assessments were based on the particulate methane monooxygenase (pmoA) gene, which is found in all known thermophilic and thermotolerant methanotrophs. We demonstrated the possibility of methane oxidation in springs with temperature exceeding 74 °C, and the most intensive methane uptake was shown in springs with temperatures about 46 °C. PmoA was detected in 19 out of 30 springs investigated and the number of pmoA gene copies varied between 10⁴ and 10⁶ copies per ml of sediment. Phylogenetic analysis of PmoA sequences revealed the presence of methanotrophs from both the Alpha- and Gammaproteobacteria. Our results suggest that methanotrophs inhabiting thermal springs with temperature exceeding 50 °C may represent novel thermophilic and thermotolerant species of the genera Methylocystis and Methylothermus, as well as previously undescribed Gammaproteobacteria.     

Effect of acclimation temperature and substrate type on selected temperature,movement and activity of juvenile spiny softshell turtles (Apalone spinifera) in an aquatic thermal gradient

In some turtle species, temperature selection may be influenced by environmental conditions, including acclimation temperature and substrate quality. These factors may be particularly important for softshell turtles that are highly aquatic and often thermoregulate by burying in the substrate in shallow water microhabitats. We tested for effects of acclimation temperature (22 °C or 27 °C) and substrate type (sand or gravel) on the selected temperature and movement patterns of 20 juvenile spiny softhshell turtles (Apalone spinifera; Reptilia: Trionychidae) in an aquatic thermal gradient of 14–34 °C. Among 7–11 month old juvenile softshell turtles, acclimation temperature and substrate type did not influence temperature selection, nor alter activity and movement patterns. During thermal gradient tests, both 22- and 27 °C-acclimated turtles selected the warmest temperature (34 °C) available most frequently, regardless of substrate type (sand or gravel). Similarly, acclimation temperature and substrate type did not influence movement patterns of turtles, nor the number of chambers used in the gradient tests. These results suggest that juvenile Apalone spinifera are capable of detecting small temperature increments and prefer warm temperatures that may positively influence growth and metabolism, and that thermal factors more significantly influence aquatic thermoregulation in this species than does substrate type.     

Responses of the killifish (Aphanius dispar) to long-term exposure to elevated temperatures: growth,survival and microstructure of gill and heart tissues

Some fish species, such as killifish, that normally inhabit temperate water environments are also found in extreme thermal environments such as thermal springs. The extent of the adaptations involved is not known. In the present laboratory study, we exposed killifish (Aphanius dispar) acclimated to a normal thermal environment to elevated temperatures (37–40 °C) in which related killifish species live permanently. Our objective was to determine whether there is evidence that killifish have heat-shock characteristics that make permanent adaptation likely. The fish was exposed to this temperature for a period of 44 days and then compared with control fish kept at their normal temperature (24 °C) with respect to growth, survival and histopathology of gill and heart tissues. At the end of the experimental time, the percentage of body weight gain and specific growth rate were significantly lower in fish kept in thermal stress compared with the control group. Feed conversion ratio (FCR) was also significantly affected by water temperature, so that during thermal stress the values of FCR were negative. Fish condition (Condition factor: CF) did not differ significantly between both groups at the end of the experiment. On days 11 and 33, however, CF was significantly lower in the thermal stress group. The gill showed blood congestion in primary lamellae and shortened secondary lamellae in fish kept at 37–40 °C. No specific alterations were found in the cardiac tissue of fish kept in thermal stress conditions. Under thermal stress, 40% of fish survived until the end of the experiment. A preliminary conclusion drawn from this work is that A. dispar, which lives at normal temperatures, shows evidence of adaptability to elevated temperatures that could be a factor in the ability of killifish to adapt permanently over time to thermal environment.     

Mapping microhabitat thermal patterns in artificial breakwaters: Alteration of intertidal biodiversity by higher rock temperature

Urbanization is altering community structure and functioning in marine ecosystems, but knowledge about the mechanisms driving loss of species diversity is still limited. Here, we examine rock thermal patterns in artificial breakwaters and test whether they have higher and spatially less variable rock temperature than natural adjacent habitats, which corresponds with lower biodiversity patterns. We estimated rock temperatures at mid‐high intertidal using infrared thermography during mid‐day in summer, in both artificial (Rip‐raps) and natural (boulder fields) habitats. We also conducted diurnal thermal surveys (every 4 hr) in four seasons at one study site. Concurrent sampling of air and seawater temperature, wind velocity, and topographic structure of habitats were considered to explore their influence on rock temperature. Rock temperature was in average 3.7°C higher in the artificial breakwater in two of the three study sites, while air temperature was about 1.5–4°C higher at this habitat at summer. Thermal patterns were more homogeneous across the artificial habitat. Lower species abundance and richness in the artificial breakwaters were associated with higher rock temperature. Mechanism underlying enhanced substrate temperature in the artificial structures seems related to their lower small‐scale spatial heterogeneity. Our study thus highlighted that higher rock temperature in artificial breakwaters can contribute to loss of biodiversity and that integrated artificial structures may alter coastal urban microclimates, a matter that should be considered in the spatial planning of urban coastal ecosystems.     

The Altitudinal Limit of Beech Woods in the Northern Apennines (Italy). Its Spatial Pattern and Some Thermal Inferences

The spatial pattern of the altitudinal limit of beech woods in the Northern Apennines and its correlation with the Fagus sylvatica thermal requirements were studied. The study area was the whole northern side of the Northern Apennines (latitude 44° N), including a timberline. The pattern was described on a scale of 1:25,000, using digitized phytosociological vegetation maps. The timberline elevation ranges from 1,200 to 1,825 m a.s.l., with the highest range at 1,525 to 1,725 m and a peak (13%) at 1,600–1,625 m. As suggested by the known beech thermal requirements, the following thermal parameters were considered: mean annual temperature, mean of the coldest month (January), mean of the warmest month (July); days with maximum temperatures >10°C and summer mean (June to September). The values were calculated using data for 1951–2002 from six weather stations. The present upper timberline (1,825 m) corresponds to the following: i) mean annual temperature of 4.5°C; ii) coldest month mean temperature of ?2.3°C; iii) summer mean of 11.5°C; iv) warmest month mean temperature of 13°C; v) 139 days with maximum temperatures of 10°C or higher. The mean temperature of the warmest month corresponds to the July mean temperature in maritime mountains, such as the Appalachians and the New Zealand Alps (13°C). The geographical location of the Northern Apennines close to the Mediterranean Sea seems to indicate that such a conclusion is reliable. With reference to the elevation variability of the upper timberline, the multiple regression shows that the annual mean is the best explaining thermal parameter (P?<?0.05). Our thermal data do not take into account the atmospheric phenomena that can affect the temperature lapse rate with altitude, such as the cloudiness of the chain, and site factors, such as wind-exposed topography and snow cover duration, which play a crucial role in the Northern Apennine local climate.     

Aquatic thermal conditions predict the presence of native and invasive rock pool Aedes (Diptera: Culicidae) in the southern Appalachians,U.S.A.

The native rock pool mosquito, Aedes atropalpus (Coquillett), and the invasive Aedes japonicus (Theobald) have been found in many types of artificial and natural containers throughout North America. Little is known about the ecology of these two species in habitats where they co‐occur, although multiple investigators have reported the decline of the native species concurrent with the introduction and spread of the invasive species. Here we report the results of riverine rock pool collections (n=503) in the southern Appalachian Mountains between 2009‐2015. Surface water temperatures strongly predicted the presence of each species across a broad range of observed temperatures (11‐39.8° C). For every unit of increase in temperature (°C) the odds of collecting Ae. atropalpus larvae increased by 0.34 while the odds of collecting Ae. japonicus larvae decreased by 0.28. No Ae. japonicus larvae or pupae were collected at temperatures greater than 36° C; however, immature Ae. atropalpus were found in rock pools with temperatures up to 39.8° C. In contrast, Ae. japonicus were highly abundant in cooler rock pools (<17° C) where Ae. atropalpus were infrequent or absent. Our findings suggest that in spite of the successful invasion by Ae. japonicus, Ae. atropalpus remains well established in the southern Appalachian Mountains. Given the strong correlation of temperature with the presence of the two species and the contrasting absence of each species at observed temperature extremes, the role of thermal conditions should be carefully explored in the context of other ecological factors likely influencing the range and abundance of these mosquitoes.     

Behavioural adaptations of Rana temporaria to cold climates

Environmental conditions at the edge of a species’ ecological optimum can exert great ecological or evolutionary pressure at local populations. For ectotherms like amphibians temperature is one of the most important abiotic factors of their environment as it influences directly their metabolism and sets limits to their distribution. Amphibians have evolved three ways to cope with sub-zero temperatures: freeze tolerance, freeze protection, freeze avoidance. The aim of this study was to assess which strategy common frogs at mid and high elevation use to survive and thrive in cold climates. In particular we (1) tested for the presence of physiological freeze protection, (2) evaluated autumnal activity and overwintering behaviour with respect to freeze avoidance and (3) assessed the importance of different high-elevation microhabitats for behavioural thermoregulation. Common frogs did not exhibit any signs of freeze protection when experiencing temperatures around 0 °C. Instead they retreated to open water for protection and overwintering. High elevation common frogs remained active for around the same period of time than their conspecifics at lower elevation. Our results suggest that at mid and high elevation common frogs use freeze avoidance alone to survive temperatures below 0 °C. The availability of warm microhabitats, such as rock or pasture, provides high elevation frogs with the opportunity of behavioural thermoregulation and thus allows them to remain active at temperatures at which common frogs at lower elevation cease activity.     

Rock glaciers in crystalline catchments: Hidden permafrost‐related threats to alpine headwater lakes

A global warming‐induced transition from glacial to periglacial processes has been identified in mountainous regions around the world. Degrading permafrost in pristine periglacial environments can produce acid rock drainage (ARD) and cause severe ecological damage in areas underlain by sulfide‐bearing bedrock. Limnological and paleolimnological approaches were used to assess and compare ARDs generated by rock glaciers, a typical landform of the mountain permafrost domain, and their effects on alpine headwater lakes with similar morphometric features and underlying bedrock geology, but characterized by different intensities of frost action in their catchments during the year. We argue that ARD and its effects on lakes are more severe in the alpine periglacial belt with mean annual air temperatures (MAAT) between ?2°C and +3°C, where groundwater persists in the liquid phase for most of the year, in contrast to ARD in the periglacial belt where frost action dominates (MAAT < ?2°C). The findings clearly suggest that the ambient air temperature is an important factor affecting the ARD production in alpine periglacial environments. Applying the paleoecological analysis of morphological abnormalities in chironomids through the past millennium, we tested and rejected the hypothesis that unfavorable conditions for aquatic life in the ARD‐stressed lakes are largely related to the temperature increase over recent decades, responsible for the enhanced release of ARD contaminants. Our results indicate that the ARDs generated in the catchments are of a long‐lasting nature and the frequency of chironomid morphological deformities was significantly higher during the Little Ice Age (LIA) than during pre‐ or post‐LIA periods, suggesting that lower water temperatures may increase the adverse impacts of ARD on aquatic invertebrates. This highlights that temperature‐mediated modulations of the metabolism and life cycle of aquatic organisms should be considered when reconstructing long‐term trends in the ecotoxicological state of lakes.     

Too cold is better than too hot: Preferred temperatures and basking behaviour in a tropical freshwater turtle

Thermoregulation is critical to the survival of animals. Tropical environments can be particularly thermally challenging as they reach very high, even lethal, temperatures. The thermoregulatory responses of tropical freshwater turtles to these challenges are poorly known. One common thermoregulatory behaviour is diurnal basking, which, for many species, facilitates heat gain. Recently, however, a north-eastern Australian population of Krefft's river turtles (Emydura macquarii krefftii) has been observed basking nocturnally, possibly to allow cooling. To test this, we determined the thermal preference (central 50% of temperatures selected) of E. m. krefftii in an aquatic thermal gradient in the laboratory. We then conducted a manipulative experiment to test the effects of water temperatures, both lower and higher than preferred temperature, on diurnal and nocturnal basking. The preferred temperature range fell between 25.3°C (±SD: 1.5) and 27.6°C (±1.4) during the day, and 25.3°C (±2.4) and 26.8°C (±2.5) at night. Based on this, we exposed turtles to three 24 h water temperature treatments (‘cool’ [23°C], ‘preferred’ [26°C] and ‘warm’ [29°C]) while air temperature remained constant at 26°C. Turtles basked more frequently and for longer periods during both the day and night when water temperatures were above their preferred range (the ‘warm’ treatment). This population frequently encounters aquatic temperatures above the preferred thermal range, and our results support the hypothesis that nocturnal basking is a mechanism for escaping unfavourably warm water. Targeted field studies would be a valuable next step in understanding the seasonal scope of this behaviour in a natural environment.     

Thermal patterns in the surface waters of a glacial river corridor (Val Roseg, Switzerland)

SUMMARY 1. We examined the thermal patterns of the surface waters in the catchment of the Roseg River, which is fed by the meltwaters of two valley glaciers. One of the glaciers has a lake at its terminus. The river corridor comprised a proglacial stream reach below one glacier, the glacier lake outlet stream, a 2.5‐km long complex floodplain and a constrained reach extending to the end of the catchment. 2. Temperatures were continuously measured with temperature loggers at 27 sites between 1997 and 1998. Moreover, from 1997 to 1999, spot measurements were taken at 33–165 floodplain sites (depending on water level) at monthly intervals. 3. The temperature regime of glacial streams, including the glacier lake outlet, was characterised by rapidly increasing temperatures in April and May, a moderate decline from June to September (period of glacial melt) and a subsequent fast decline in autumn. During summer, the lake increased temperatures in the outlet stream by 2–4 °C, compared with the adjacent proglacial stream reach. 4. In the main channel (thalweg) of the Roseg River, annual degree‐days (DD) ranged from 176 DD in the upper proglacial reach to 1227 DD at the end of the catchment. 5. Thermal variation among different channels within the floodplain was higher than the variation along the entire main channel. Floodplain channels lacking surface connection to the main channel accumulated up to 1661 annual DDs. 6. Thermal heterogeneity within the floodplain was linked to the glacial flow pulse. With the onset of ice melt, temperatures in the main channel and in channels surface‐connected to the main channel began to decline, whereas in surface‐disconnected channels temperatures continued to increase; as a consequence, thermal heterogeneity at the floodplain scale rose slightly until September. 7. High thermal heterogeneity was not anticipated in the harsh environment of a largely glacierised alpine catchment. The relatively wide range of thermal environments may contribute to the highly diverse zoobenthic community.     

Broad thermal capacity facilitates the primarily pelagic existence of northern fur seals (Callorhinus ursinus)

Thermoregulatory capacity may constrain the distribution of marine mammals despite having anatomical and physiological adaptations to compensate for the thermal challenges of an aquatic lifestyle. We tested whether subadult female northern fur seals (Callorhinus ursinus) experience increased thermoregulatory costs in water temperatures potentially encountered during their annual migration in the Bering Sea and North Pacific Ocean. Metabolic rates were measured seasonally in 6 captive female northern fur seals (2.75–3.5 yr old) in ambient air and controlled water temperatures of 2°C, 10°C, and 18°C. Rates of oxygen consumption in ambient air (1°C–18°C) were not related to environmental temperature except below 2.5°C (winter only). However, metabolism was significantly higher during the fall seasonal trials (September–October) compared to other times of year, perhaps due to the costs of molting. The fur seals appeared thermally neutral in all seasons for all water temperatures tested (2°C–18°C) except during the summer when metabolic rates were higher in the 2°C water. Comparing this broad thermal neutral zone to the average sea surface temperatures potentially encountered during annual migrations indicates wild fur seals can likely exploit a large geographic area without added thermal metabolic costs.     

Differences in leaf thermoregulation and water use strategies between three co‐occurring Atlantic forest tree species

Given anticipated climate changes, it is crucial to understand controls on leaf temperatures including variation between species in diverse ecosystems. In the first study of leaf energy balance in tropical montane forests, we observed current leaf temperature patterns on 3 tree species in the Atlantic forest, Brazil, over a 10‐day period and assessed whether and why patterns may vary among species. We found large leaf‐to‐air temperature differences (maximum 18.3 °C) and high leaf temperatures (over 35 °C) despite much lower air temperatures (maximum 22 °C). Leaf‐to‐air temperature differences were influenced strongly by radiation, whereas leaf temperatures were also influenced by air temperature. Leaf energy balance modelling informed by our measurements showed that observed differences in leaf temperature between 2 species were due to variation in leaf width and stomatal conductance. The results suggest a trade‐off between water use and leaf thermoregulation; Miconia cabussu has more conservative water use compared with Alchornea triplinervia due to lower transpiration under high vapour pressure deficit, with the consequence of higher leaf temperatures under thermal stress conditions. We highlight the importance of leaf functional traits for leaf thermoregulation and also note that the high radiation levels that occur in montane forests may exacerbate the threat from increasing air temperatures.     

Physiological performance of the cold-water coral Dendrophyllia cornigera reveals its preference for temperate environments

Cold-water corals (CWCs) are key ecosystem engineers in deep-sea benthic communities around the world. Their distribution patterns are related to several abiotic and biotic factors, of which seawater temperature is arguably one of the most important due to its role in coral physiological processes. The CWC Dendrophyllia cornigera has the particular ability to thrive in several locations in which temperatures range from 11 to 17 °C, but to be apparently absent from most CWC reefs at temperatures constantly below 11 °C. This study thus aimed to assess the thermal tolerance of this CWC species, collected in the Mediterranean Sea at 12 °C, and grown at the three relevant temperatures of 8, 12, and 16 °C. This species displayed thermal tolerance to the large range of seawater temperatures investigated, but growth, calcification, respiration, and total organic carbon (TOC) fluxes severely decreased at 8 °C compared to the in situ temperature of 12 °C. Conversely, no significant differences in calcification, respiration, and TOC fluxes were observed between corals maintained at 12 and 16 °C, suggesting that the fitness of this CWC is higher in temperate rather than cold environments. The capacity to maintain physiological functions between 12 and 16 °C allows D. cornigera to be the most abundant CWC species in deep-sea ecosystems where temperatures are too warm for other CWC species (e.g., Canary Islands). This study also shows that not all CWC species occurring in the Mediterranean Sea (at deep-water temperatures of 12–14 °C) are currently living at their upper thermal tolerance limit.     

Temperature-related heart rate in water and air and a comparison to other temperature-related measures of performance in the fiddler crab Leptuca pugilator (Bosc 1802)

Performance in poikilotherms is known to be sensitive to temperature, often with a low-sloping increase with temperature to a peak, and a steep decline with increasing temperature past the peak. We complemented past measures of performance by measuring heartbeat rates of the fiddler crab Leptuca pugilator in water and in air as a function of a range of temperatures previously shown to affect other measures of performance. In water over a range of 20–50 °C, heartbeat increased steadily to a peak at 40 °C and then steeply declined to near zero at 50 °C. In air, heartbeat also increased, but to a peak at 35 °C and then with a gentler decline than was found in water. Part of this different response may be due to evaporative water loss, which reduced body temperature in air, and therefore thermal stress, relative to body temperature when crabs were immersed in water. Increased availability of oxygen from air, according to the oxygen and capacity-limited thermal tolerance hypothesis, likely increased aerobic scope past the thermal peak, relative to within water, where oxygen delivery at higher temperatures may have been curtailed.We compared the heart rate performance relations to two previous measures of performance – endurance on a treadmill and sprint speed, both done in air. The peak performance temperature increased in the order: treadmill endurance time, sprint speed, heart rate in air, and heart rate in water, which demonstrates that different performance measures give different perspectives on the relation of thermal tolerance and fitness to temperature. Endurance may therefore be the limiting upper thermal stress factor in male fiddler crabs, when on hot sand flats. Temperature preference, found to be for temperatures <30 °C in air, could be a bet-hedging evolutionary strategy to avoid aerobic scope affecting endurance.     

Temperature acclimation alters cardiac performance in the lobster Homarus americanus

The American lobster is a poikilotherm that inhabits a marine environment where temperature varies over a 25°C range and depends on the winds, the tides and the seasons. To determine how cardiac performance depends on the water temperature to which the lobsters are acclimated we measured lobster heart rates in vivo. The upper limit for cardiac function in lobsters acclimated to 20°C is approximately 29°C, 5°C warmer than that measured in lobsters acclimated to 4°C. Warm acclimation also slows the lobster heart rate within the temperature range from 4 to 12°C. Both effects are apparent after relatively short periods of warm acclimation (3–14 days). However, warm acclimation impairs cardiac function at cold temperatures: following several hours exposure to frigid (<5°C) temperatures heart rates become slow and arrhythmic in warm acclimated, but not cold acclimated, lobsters. Thus, acclimation temperature determines the thermal limits for cardiac function at both extremes of the 25°C temperature range lobsters inhabit in the wild. These observations suggest that regulation of cardiac thermal tolerance by the prevailing environmental temperature protects against the possibility of cardiac failure due to thermal stress.     

Structural-Nanomechanical Property Correlation of Shallow Water Shrimp （Pandalus platyceros） Exoskeleton at Elevated Temperature

This investigation reports the nanomechanical properties of shallow water shrimp exoskeleton at temperatures ranging from 30 ℃ to 80 ℃ measured using nanoindentation experiments. Scanning Electron Microscopy （SEM） measurements suggest that the shrimp exoskeleton has the Bouligand structure in its layers, a key characteristic of the crustaceans. The thickness of the layers and packing density are found to be different from that of lobsters and crabs reported earlier in the literature. Mechanical properties at high temperatures are determined using micro materials nanoindentation test set up combined with the hot stage. The properties measured during nanoindentation test are corrected for the creep and thermal drift during the experiments. The reduced modulus values are found to be around 28 GPa at 30 ℃ that reduces to approximately 24 GPa at 80 ℃. The hardness values also decrease from 1.6 GPa at 30 ℃ to around 1.2 GPa at 80 ℃. The indentation size effect is found to be absent at all temperatures. Creep mechanisms of polymers like materials and its temperature dependence are discussed to give more insight into the deformation mechanism.     

Thermal sensitivity of a Neotropical amphibian (Engystomops pustulosus) and its vulnerability to climate change

A species’ thermal sensitivity and its exposure to climate variation are key components in the prediction of its vulnerability to climate change. We tested the thermal sensitivity of a tropical amphibian that lives in a mild constant climate in which the thermal tolerance range is expected to closely match the experienced environmental temperature. The air temperature that this species is exposed to varies between 21.9 and 31.6°C with an annual mean of 27.2°C. We estimated the microhabitat water temperature variation under vegetation shade, which buffers the temperature by 1.8°C in relation to that of the air, and with open canopy, where the water was 1.9°C warmer than the air temperature. With broods of tadpoles split into five treatments (15°C, 21°C, 28°C, 31°C, and 33°C), we estimated the critical thermal maximum (CTMax) and critical thermal minimum (CTMin) after at least 7 days of acclimation. Both CTMax (42.3°C) and CTMin (11.8°C) were more extreme than the temperature range estimated for the field. We estimated the optimum temperature (T_o = 28.8°C) and the thermal performance breadth (range: 23.3–34.1°C) based on growth rate (g/day). The animals were able to acclimate more extensively to cold than to warm temperatures. These performance curve traits closely matched the air temperature. The estimated vulnerability varied according to the microhabitat prediction model used. The combination of tadpole data on thermal sensitivity and macro‐ and microhabitat variation provides a necessary framework to understand the effects of climate change on tropical amphibians.     

Thermal maxima for juvenile shortnose sturgeon acclimated to different temperatures

Many populations of shortnose sturgeon, Acipenser brevirostrum, in the southeastern United States continue to suffer from poor juvenile recruitment. High summer water temperatures, which may be exacerbated by anthropogenic activities, are thought to affect recruitment by limiting available summer habitat. However, information regarding temperature thresholds of shortnose sturgeon is limited. In this study, the thermal maximum method and a heating rate of 0.1°C min⁻¹ was used to determine critical and lethal thermal maxima for young-of-the-year (YOY) shortnose sturgeon acclimated to temperatures of 19.5 and 24.1°C. Fish used in the experiment were 0.6 to 35.0 g in weight and 64 to 140 days post hatch (dph) in age. Critical thermal maxima were 33.7°C (±0.3) and 35.1°C (±0.2) for fish acclimated to 19.5 and 24.1°C, respectively. Critical thermal maxima significantly increased with an increase in acclimation temperature (p < 0.0001). Lethal thermal maxima were 34.8°C (±0.1) and 36.1°C (±0.1) for fish acclimated to 19.5 and 24.1°C, respectively. Lethal thermal maxima were significantly affected by acclimation temperature, the log₁₀ (fish weight), and the interaction between log₁₀(fish weight) and acclimation temperature (p < 0.0001). Thermal maxima were used to estimate upper limits of safe temperature, thermal preferences, and optimal growth temperatures of YOY shortnose sturgeon. Upper limits of safe temperature were similar to previous temperature tolerance information and indicate that summer temperatures in southeastern rivers may be lethal to YOY shortnose sturgeon if suitable thermal refuge cannot be found.