Abundance, body composition and reproductive output of Gammarus minus (Crustacea: Amphipoda) in ten cold springs differing in pH and ionic content

1. A survey of thirty-two rheocrene springs in central Pennsylvania revealed that, Like Gammarus in lakes and streams, Gammarus minus is absent from springs with pH <6.0 and conductivity <25μS cm^?1 (total range in pH = 4.6–7.7, and in conductivity = 14–411 μS cm^?1). 2. In ten springs G. minus density was positively correlated with Ca²⁺ and Mg²⁺ hardness, but not with pH, unless three springs with either an exceptionally high velocity or extremely high densities of the potentially competing snail Fontigens nickliniana were omitted. 3. Adults were larger in the springs with few or no large predators than in those with more predators. In all ten springs, adult dry mass was unrelated to spring pH and ionic content, but brooding female dry mass covaried positively with Ca²⁺ and Mg²⁺ hardness in the five predator-poor springs. 4. Body water, Na and Ca contents and body mass/length ratios varied independently of spring pH and ionic content. Water content was inversely correlated with fat content, but even when expressed as a percentage of fat-free wet mass, it was unrelated to water chemistry. 5. In juveniles, males and non-brooding females, fat content varied independently of spring pH and ionic content, but in brooding females it was correlated with alkalinity and Ca²⁺ and Mg²⁺ hardness. The cost of reproduction in brooding females may have been a factor here; they had significantly lower per cent fat than did non-brooding females. Juvenile fat content did not differ significantly among spring populations, whereas adult fat content did. The per cent fat of brooding females covaried positively with body size among springs, and this was marginally true for non-brooding females, as well. The residuals of brooding female per cent fat against dry mass were not related to water chemistry. 6. Brood size (number of embryos in a brood) and brood mass varied significantly among populations, but independently of spring pH and ionic content. Both covaried positively with maternal body size among springs. The residuals of these relationships were unrelated to water chemistry, as was the percentage of females brooding. 7. G. minus from a pH 6 spring survived better and lost less body mass in acidic soft water than did those from a pH 7.6 spring. However, although G. minus has apparently been able to adapt (or acclimate) to pH 6 water it has failed to adapt to more acidic waters. A physiological or structural constraint may be involved because this species has probably had ample opportunity to evolve resistance to dilute acidic water. This hypothesis is consistent with the threshold effect observed: above pH 6 G. minus shows very little evidence of osmotic or metabolic stress, but beiow pH 6 viable populations apparently cannot survive at all. However, the gradual linear decrease in population density of G. minus with decreasing alkalinity and Ca²⁺ and Mg²⁺ hardness suggests that other factors may also be involved (e.g. a decrease in food quality).     

Effects of natural disturbance on stream communities: a habitat template analysis of arctic headwater streams

1. We used the habitat template approach to test the hypothesis that substratum stability, freezing and nutrient supply were determinants of community structure in 19 headwater streams of arctic Alaska. Streams were selected from five categories: glacier (n = 3), mountain (4), mountain spring (4), tundra spring (4) and tundra (4). 2. Bed movement among streams ranged from 0 to 97% during a ～2‐month summer season. Glacier and mountain streams had significantly higher bed movement than tundra and spring streams (P < 0.001). 3. All glacier and tundra streams froze solid during winter; all mountain spring streams remained unfrozen. Freezing among mountain and tundra spring streams was variable, with a subset of streams flowing throughout winter. 4. With the exception of glacier streams, which showed high concentrations of NH₄⁺ and NO₃^? (P < 0.001), differences in nutrient concentrations among stream types were not significant. 5. Algal taxon richness was greatest in tundra springs (13 taxa) and lowest in glacier streams (five taxa, P < 0.001), as was algal biovolume (7350 versus 687 mm³ m^?2, P < 0.001). Macroinvertebrate taxon richness was lower in glacier streams (4.7 ± 1.7, P < 0.005) than the other stream types (20.5–25.0 taxa), and biomass was greater in mountain springs (4837 mg m^?2) and tundra springs (3367 mg m^?2, P < 0.001). 6. Multidimensional scaling and multiple regression analyses of macroinvertebrate (biomass) and periphyton (biovolume) indicated that a 2‐dimensional habitat template with bed movement and freezing as axes provides an accurate model of major factors controlling the community structure of headwater streams in arctic Alaska.     

Nocturnally retained zeaxanthin does not remain engaged in a state primed for energy dissipation during the summer in two Yucca species growing in the Mojave Desert

Differently oriented leaves of Yucca schidigera and Yucca brevifolia were characterized in the Mojave Desert with respect to photosystem II and xanthophyll cycle activity during three different seasons, including the hot and dry summer, the relatively cold winter, and the mild spring season. Photosynthetic utilization of a high percentage of the light absorbed in PSII was observed in all leaves only during the spring, whereas very high levels of photoprotective, thermal energy dissipation were employed both in the summer and the winter season in all exposed leaves of both species. Both during the summer and the winter season, when energy dissipation levels were high diurnally, xanthophyll cycle pools (relative to either Chl or other carotenoids) were higher relative to the spring, and a nocturnal retention of high levels of zeaxanthin and antheraxanthin (Z + A) occurred in all exposed leaves of both species. Although this nocturnal retention of Z + A was associated with nocturnal maintenance of a low PSII efficiency (F_v/F_m) on a cold winter night, pre‐dawn F_v/F_m was high in (Z + A)‐retaining leaves following a warm summer night. This indicates nocturnal engagement of Z + A in a state primed for energy dissipation throughout the cold winter night – while high levels of retained Z + A were not engaged for energy dissipation prior to sunrise on a warm summer morning. Possible mechanisms for a lack of sustained engagement of retained Z + A for energy dissipation at elevated temperatures are discussed.     

Carbon dynamics in a Boreal land-stream-lake continuum during the spring freshet of two hydrologically contrasting years

We studied in 2013 and 2014 the spring carbon dynamics in a Boreal landscape consisting of a lake and 15 inflowing streams and an outlet. The first year had weather and a hydrological regime typical of past years with a distinct spring freshet connected with the thaw of the average snowpack. The latter year had higher air temperatures which did not permit snow accumulation, despite similar winter precipitation. As such, there was hardly any spring freshet in 2014, and stream discharge peaked in January, i.e., the conditions resembled those predicted in the future climate. Despite the hydrological differences between the years, there were only small interannual differences in the stream CO₂ and DOC concentrations. The relationship between the concentrations and discharge was stronger in the typical year. CO₂ concentrations in medium-sized streams correlated negatively with the discharge, indicating dilution effect of melting snowpacks, while in large-sized streams the correlation was positive, suggesting stronger groundwater influence. The DOC pathway to these streams was through the subsurface soil layers, not the groundwater. The total amount of carbon transported into the lake was ca. 1.5-fold higher in the typical year than in the year with warm winter. In 2013, most of the lateral inputs took place during spring freshet. In 2014, the majority of inputs occurred earlier, during the winter months. The lateral CO₂ signal was visible in the lake at 1.5 m depth. DOC dominated the carbon transport, and in both years, 12% of the input C was in inorganic form.

     

How will increased temperature and nutrient enrichment affect primary producers in sub‐Arctic streams?

1. Spring‐fed streams, with temperatures ranging from 7.1 to 21.6 °C, in an alpine geothermal area in SW Iceland were chosen to test hypotheses on the effects of nutrients and temperature on stream primary producers. Ammonium nitrate was dripped into the lower reaches of eight streams, with higher reaches being used as controls, during the summers of 2006 and 2007. Dry mass of larger primary producers, epilithic chlorophyll a and biovolumes of epilithic algae were measured. 2. Bryophyte communities were dominated by Fontinalis antipyretica, and biomass was greatest in the warmest streams. Jungermannia exsertifolia, a liverwort, was found in low densities in few samples from cold streams but this species was absent from the warmest streams. 3. Nutrient enrichment increased the biomass of bryophytes significantly in warm streams. No effects of the nutrient addition were detected on vascular plants. The biomass of larger filamentous algae (mainly Cladophora spp.) was significantly increased by nutrient enrichment in cold streams but reduced by nutrients in warm streams. Thalloid cyanobacteria (Nostoc spp.) were not affected by nutrients in cold streams but decreased with nutrient addition in warm streams. Epilithic algal chlorophyll a was increased by nutrients in all streams and to a greater extent in 2007 than in 2006. Nutrient addition did not affect the epilithic chlorophyll a differently in streams of different temperatures. 4. There were small differential effects of nutrients, influenced by pH and conductivity, on different epilithic algal groups. 5. As global temperatures increase, animal husbandry and perhaps crop agriculture are likely to increase in Iceland. Temperature will directly influence the stream communities, but its secondary effects, manifested through agricultural eutrophication, are likely to be much greater.     

Seasonal succession of the travertine‐forming desmid Oocardium stratum

The calcifying Conjugatophyte Oocardium stratum occurs exclusively in spring‐associated limestones (SAL) with active meteogene limestone deposition. The macroscopic colonies of Oocardium stratum form hemispherical, pinhead‐like structures with a diameter of 0.5–2.0 mm. As its autecology is still poorly understood, we focused on the seasonal development of Oocardium stratum and linked environmental factors to its abundance. The study was conducted in a rivulet in Lunz/See (Austria) for 16 months on a weekly (growing season) to monthly (winter season) basis. Oocardium colonies were found throughout the whole year, with maximum abundance during the mid‐summer months July and August. Repeated macro‐mapping of three SAL sites measuring 750 cm² each showed a maximum Oocardium cover of around 30% in August; two smaller peaks developed in early summer and late autumn with ~10% cover. Diatom mats dominated by Cymbella excisiformis occurred in spring, autumn and winter, with more than 75% cover. The seasonal change between Oocardium and diatoms in limestone‐precipitating springs causes a typical sequence pattern of limestone layers. Redundancy analysis revealed water temperature and bicarbonate content as the main structuring factors; these control the occurrence and growth of Oocardium, reflecting season as a background variable. Optimum growth conditions for Oocardium were an alkalinity around 4.7 meq · L^?1 and a water temperature around 13°C. Site openness, nitrate and dissolved carbon dioxide were inversely related to Oocardium biomass, the opposite for diatoms. Other environmental factors such as total ions or soluble reactive phosphorus had no significant influence on Oocardium stratum abundance.     

Bacterial metabolism in small temperate streams under contemporary and future climates

1. We examined the detailed temperature dependence (0–40 °C) of bacterial metabolism associated with fine sediment particles from three Danish lowland streams to test if temperature dependence varied between sites, seasons and quality of organic matter and to evaluate possible consequences of global warming. 2. A modified Arrhenius model with reversible denaturation at high temperatures could account for the temperature dependence of bacterial metabolism and the beginning of saturation above 35 °C and it was superior to the unmodified Arrhenius model. Both models overestimated respiration rates at very low temperatures (<5 °C), whereas Ratkowsky's model – the square root of respiration – provided an excellent linear fit between 0 and 30 °C. 3. There were no indications of differences in temperature dependence among samples dominated by slowly or easily degradable organic substrates. Optimum temperature, apparent minimum temperature, Q₁₀‐values for 0–40 °C and activation energies of bacterial respiration were independent of season, stream site and degradability of organic matter. 4. Q₁₀‐values of bacterial respiration declined significantly with temperature (e.g. 3.31 for 5–15 °C and 1.43 for 25–35 °C) and were independent of site and season. Q₁₀‐values of bacterial production behaved similarly, but were significantly lower than Q₁₀‐values of respiration implying that bacterial growth efficiency declined with temperature. 5. A regional warming scenario for 2071–2100 (IPCC A2) predicted that mean annual temperatures will increase by 3.5 °C in the air and 2.2–4.3 °C in the streams compared with the control scenario for 1961–1990. Temperature is expected to rise more in cool groundwater‐fed forest springs than in open, summer‐warm streams. Mean annual bacterial respiration is estimated to increase by 26–63% and production by 18–41% among streams assuming that established metabolism–temperature relationships and organic substrate availability remain the same. To improve predictions of future ecosystem behaviour, we further require coupled models of temperature, hydrology, organic production and decomposition.     

Biorecovery of gold using cyanobacteria and an eukaryotic alga with special reference to nanogold formation – a novel phenomenon

Pro- and eukaryotic algal genera, i.e. Lyngbya majuscula, Spirulina subsalsa (Cyanophyceae) and Rhizoclonium hieroglyphicum (Chlorophyceae), were used for bio-recovery of gold (Au) out of aqueous solution. Au (III) spiked with ¹⁹⁸Au was used for the experiment. Batch laboratory experiments indicated quick metabolic independent binding of Au to the algae followed by active accumulation and subsequent reduction. Gold accumulation by different algal genera was found in order of R. hieroglyphicum > L. majuscula > S. subsalsa (3.28, 1.93 and 1.73 mg g^-1, respectively). It was observed that the algal biomass and the media used for the experiment turned purple in colour indicating reduction of Au (III) to Au (0) at intra- and extracellular level. This was confirmed by TEM studies of L. majuscula biomass exposed in HAuCl₄ solution where <20-nm-sized gold particles were found both inside as well as on the surface of the cell. Up to 90–100% of accumulated gold was recovered from the algal biomass by using nitric acid and acidic thiourea solution.     

Effects of spring acid episodes on macroinvertebrates revealed by population data and in situ toxicity tests

1. While streams in Europe and North America are now recovering chemically from chronic acidification, severe episodic acidification continues to threaten sensitive biota. To appraise further the biological importance of episodic acidification, we surveyed the distribution of the mayfly Baetis alpinus in streams in the Southern Alps (Canton Ticino, Switzerland) in relation to runoff acidity during spring floods. Moreover, to improve mechanistic understanding, in situ toxicity assays were carried out on nymphal B. alpinus during low flows and spring floods, both in streams prone to acid episodes and in well‐buffered controls. 2. Streams surveyed for invertebrates represented three groups which contrasted in susceptibility to episodic acidity. Group one included streams that were acid (alkalinity <0) in spring; group two streams were susceptible to acid episodes because of low base‐flow alkalinity (<200 μeq L^?1); and group three streams were well‐buffered and unlikely ever to be acid. The abundance of B. alpinus was similar among groups during stable flows in winter and summer, but was significantly lower in groups one and two following spring snowmelt. 3. During the bioassays, control streams remained circumneutral to alkaline (pH >6.4). By contrast, episodic streams were circumneutral at low flows, but became acid (pH 4.5–5.6 and total dissolved aluminium to 276 μg L^?1) during intense spring snowmelt. After 15‐day exposures, nymphal B. alpinus survival in the circumneutral control streams exceeded 92% irrespective of flow. In the episodic streams, survival matched the controls during low flows, but declined significantly to 10–20% during acid episodes in spring. Shorter exposure (2–4 days) to pH 5.5–5.7 did not cause significant mortality either during the exposure or over the following 7 days, indicating that B. alpinus might recover from short acid episodes. 4. Our data suggest that the spring distribution of B. alpinus in acid sensitive parts of the Alps directly reflects the toxicity of acid runoff during snowmelt. Our study illustrates that even mild episodic acidification can have significant consequences in Alpine streams for one of the most important invertebrate indicators of acidity.     

Vegetative survival of some wall and soil blue-green algae under stress conditions

Lyngbya major (a wall alga), survived throughout year, maximally to >80 % at atmospheric temperature (AT) of 17-36 degrees C and relative humidity (RH) 60-100 % in rainy and spring seasons, but the survival was 43-64 % in winter when AT decreased to 5 degrees C and RH was 65-98 %, and 15-23 % in summer when AT reached 48 degrees C and RH was 23-60 %. All soil algae (Lyngbya birgei, Aphanothece pallida, Gloeocapsa atrata, Oscillatoria subbrevis, O. animalis) survived >90 % in rainy season when soil moisture content (SMC) was 89-100 %. Lowering of SMC to a minimum of 55 % in spring and 39 % in winter led L. birgei, O. subbrevis and O. animalis to survive from 75, 66, and 65 %, respectively, in spring and 12, 14, and 20 % in winter, and A. pallida and G. atrata not at all in both seasons. All soil algae did not survive in summer when SMC was 12-30 %. Myxosarcina burmensis survived only in rainy and spring seasons when pond water temperature (PWT) was 19-25 degrees C and 18-26 degrees C, respectively, and not in winter and summer when PWT was 2-14 degrees C and 25-36 degrees C, respectively. L. major and A. pallida survived almost equally well under both submerged and air-exposed conditions for 15 d but less if submerged for more time than air-exposed on moist soil surface, while L. birgei, G. atrata, O. subbrevis, and O. animalis survived submergence in liquid medium better and longer than air-exposure on moist soil surface. Pond alga M. burmensis survived submergence better than air-exposure, true to its aquatic habitat. All algae survived less and died without forming any resistant cells when exposed to physical and physiological water stress (imposed by growing them on highly agarized media or in salinized liquid media), light stress (at 0, 2 and 10 mumol m(-2) s(-1) light intensity) or following UV shock (0.96-3.84 kJ/m(2)). A. pallida and G. atrata cells did not divide on 8 % agarized solid media, in >/=0.3 mol/L salinized liquid media, and in darkness. The presence of sheath over L. major and L. birgei filament cells and mucilage cover over A. pallida and G. atrata cells protect them against physical desiccation to some extent but not against UV shock.     

Diurnal variations in the carbon chemistry of two acidic peatland streams in north-east Scotland

1. Two acidic peatland upland streams in north‐east Scotland draining catchments of 1.3 and 41.4 km² were sampled each season for 2 years to investigate diurnal variations in dissolved and gaseous forms of carbon. Stream metabolism, alkalinity, discharge, pH, air and water temperatures were measured to aid data interpretation. 2. Free CO₂ showed marked diurnal variation with lowest concentrations during the period from late morning to early afternoon and highest during the hours of darkness. Although alkalinity and pH also showed some diurnal fluctuations, in comparison with other more productive alkaline systems, variation was small. Dissolved organic carbon (DOC) showed no significant diurnal pattern. However, significant changes in stream discharge influenced DOC concentrations, as well as over‐riding diurnal patterns of free CO₂, alkalinity and pH. 3. The highest diurnal ratios (maximum concentration/minimum concentration) in CO₂, gross primary productivity (GPP) and community respiration (CR) occurred in spring and summer and the lowest in autumn and winter. Variation in biotic in‐stream processes caused changes in CO₂ concentrations and temperature affected both the solubility of CO₂ and changes in up‐stream CO₂ inputs. There was no significant difference in diurnal fluctuations between the two orders of stream studied. 4. The mean GPP (as CO₂) was 0.81 g CO₂ m^?2 day^?1 and mean CR 2.67 g CO₂ m^?2 day^?1. The mean primary production/respiration (P/R) ratio was 0.26 ± 0.09 and 0.33 ± 0.15 in the first and second order streams, respectively. These values are low compared with published data because these heterotrophic headwater streams are dominated by benthic respiration and upstream allochthonous inputs with little autotrophic metabolism, particularly during the colder autumn and winter months. 5. The results have implications for the calculation of dissolved inorganic carbon (DIC) fluxes in streamwater. Samples taken during daylight hours tend to have lower concentrations of free CO₂ and HCO₃^? than samples taken during darkness. During spring, concentrations of free CO₂ were measured up to 2.4 (annual mean 1.8) times higher at night than during the day at a similar discharge. It is suggested that fluxes based on daytime measurements alone will under‐estimate the annual flux of these determinands in streamwater by as much as 40%.     

Microbiology and geochemistry of Little Hot Creek,a hot spring environment in the Long Valley Caldera

A culture‐independent community census was combined with chemical and thermodynamic analyses of three springs located within the Long Valley Caldera, Little Hot Creek (LHC) 1, 3, and 4. All three springs were approximately 80 °C, circumneutral, apparently anaerobic and had similar water chemistries. 16S rRNA gene libraries constructed from DNA isolated from spring sediment revealed moderately diverse but highly novel microbial communities. Over half of the phylotypes could not be grouped into known taxonomic classes. Bacterial libraries from LHC1 and LHC3 were predominantly species within the phyla Aquificae and Thermodesulfobacteria, while those from LHC4 were dominated by candidate phyla, including OP1 and OP9. Archaeal libraries from LHC3 contained large numbers of Archaeoglobales and Desulfurococcales, while LHC1 and LHC4 were dominated by Crenarchaeota unaffiliated with known orders. The heterogeneity in microbial populations could not easily be attributed to measurable differences in water chemistry, but may be determined by availability of trace amounts of oxygen to the spring sediments. Thermodynamic modeling predicted the most favorable reactions to be sulfur and nitrate respirations, yielding 40–70 kJ mol^?1 e^? transferred; however, levels of oxygen at or below our detection limit could result in aerobic respirations yielding up to 100 kJ mol^?1 e^? transferred. Important electron donors are predicted to be H₂, H₂S, S⁰, Fe²⁺ and CH₄, all of which yield similar energies when coupled to a given electron acceptor. The results indicate that springs associated with the Long Valley Caldera contain microbial populations that show some similarities both to springs in Yellowstone and springs in the Great Basin.     

Effects of winter conditions on Daphnia dynamics and genetic diversity in a dimictic temperate reservoir

1. Winter conditions shape plankton dynamics and community composition in temperate regions, but their effect on dynamics and genetic composition of cyclical parthenogens like Daphnia is largely unclear. 2. For 5 years, we studied the dynamics, hatching from resting eggs and genetic structure of a D. galeata × longispina hybrid complex in a dimictic, temperate reservoir. Our main hypothesis was that higher spring densities and an earlier population peak will be observed after warmer winters, with a lower genetic diversity because of a lower contribution of resting eggs to population growth. 3. The study period could clearly be categorised into cold‐winter years (n = 3) and warm‐winter years (n = 2). Daphnia densities at the end of spring overturn were ～10‐fold lower after cold winters than after warm ones, but no pattern emerged concerning the timing and the height of the population peak in early summer. 4. Hatching intensity from resting eggs was higher and contributed up to 8.5% to Daphnia abundance in a cold‐winter year compared to a negligible contribution in a warm‐winter year. Consistent with this finding, new multilocus genotypes (MLGs) adding to the overwintering stock after the end of spring overturn and presumably originating from resting eggs increased genetic diversity and attained high frequencies within the population only after a cold winter. New MLGs were recorded also after warm winters, but they never gained dominance and no shift in genetic diversity was observed. However, genetic diversity was not generally reduced after warm winters. 5. Our results confirm earlier findings that winter conditions have only a limited effect on the main growth phase and the peak of Daphnia during late spring and early summer. However, winter conditions determine the contribution of resting eggs to the population development, which may profoundly alter the genetic composition of the population compared to the previous season.     

Physical and Chemical Conditions in Long Pond,St. John's,Newfoundland: A Pond Receiving Both Rural and Urban Runoff

The physico-chemical environment was studied from June 1971 to May 1973 at five stations in Long Pond, St. John's, Newfoundland. This pond receives both rural and urban runoff and is characterized by a high flushing rate. Of the parameters studied, concentrations of total suspended matter, biochemical oxygen demand (BOD), total CO₂, and free CO₂ were significantly greater in a small pool prior to the main body of the pond near the entry of Learys Brook. Current speed is noticeably reduced at this point. No significant differences were found between stations with respect to pH, dissolved O₂, total dissolved solids (TDS), total hardness, Ca hardness, Mg hardness, alkalinity, ammonium-N, nitrite-N, nitrate-N, orthophosphate and polyphosphate. Water quality in Long Pond is compared with unpolluted lakes and streams in the area; nutrient levels are indicative of eutrophic conditions.     

Seasonal succession of ciliates in lake constance

We found a recurrent seasonal pattern in abundance and composition of planktonic ciliates in Lake Constance, FRG, over a three-year period. Abundance peaks occurred in early spring and summer/autumn, while ciliate numbers were low in late spring (clear-water phase) and winter. Prostomatida and Oligotrichida dominated in early spring. They responded immediately to the phytoplankton spring bloom, while Haptorida, Peritrichida, and large Scuticociliatida (Histiobalantium) were delayed by 1 to 2 weeks. The spring community broke down at the onset of the clear-water phase.Pelagohalteria viridis containing symbiontic algae appeared shortly after this event. A highly diverse community was recorded in summer/autumn. Peritrichida, small Oligotrichida, and large Scuticociliatida reached their maxima during this season. Small Scuticociliatida were rare throughout the year and contributed moderately to total ciliate numbers only during the cold season. The observed seasonal sequence of pelagic ciliates in Lake Constance is discussed in relation to simultaneously collected data on potential food organisms and grazers.     

Regional and temporal variation in pH, alkalinity and carbon dioxide in Danish streams, related to soil type and land use

SUMMARY 1. The Weichsel glaciation has divided Denmark into two regions with different susceptibility to acidification. East of the Weichselian terminal moraine, soils are usually clayey and calcareous, and the streams are alkaline (mean alkalinity 2.24 mmol 1-^-1) and resistant to inputs of acidifying substances. 2. Trend analysis of pH and alkalinity of water samples taken over 15 years in two streams with alkalinities above 1.5 mmol 1^?1 in eastern Jutland, showed no trends of acidification. 3. West of the terminal moraine the soils are sandy and leached and alkalinity is lower (mean 0.59 mmol 1^?1). Although such streams with medium alkalinity are believed not to be vulnerable to acidification, we have documented significant decreases in their pH and alkalinity over 12 years. 4. Trends of pH and alkalinity in four western streams with mean alkalinities between 0.05 and 0.79 mmol 1^?1 showed annual decreases of 0.027 pH units and 4.7 nmol 1^?1 in alkalinity. 5. Overall, Danish streams contain about 7.9 times more calculated free CO₂ (pCO₂=10^?2.⁶ atm) than water in equilibrium with air (pCO₂= 10^?3.⁵ atm). The calculated free CO₂ content has increased significantly in western Danish streams over the study period (6.9 μmol 1^?1 yr^-1). This increase cannot be explained by the prevailing global increase in atmospheric pCO₂ which only can account for 0.54 pmol 1^?1 yr^?1 at maximum. 6. Reasons for the ongoing stream acidification in the western part of Denmark are discussed. We suggest that atmospheric deposition causes stream acidification in a heath-covered catchment without agriculture. In heavily cultivated regions the main acidification factor is argued to be proton production in the soil through nitrification of ammonium-containing fertilizers.     

Spatial and seasonal variations in stem CO2 efflux of Pinus canariensis at their upper distribution limit

We calculated stem CO₂ efflux (E _S) of Pinus canariensis at a timberline site in Tenerife, Canary Islands, from March 7, 2008 and February 9, 2009. E _S varied markedly throughout the year. Although E _S generally followed the seasonal trends in temperature, we observed seasonal adjustment of E _S in both E _S normalized to temperature (E _S10) and the temperature sensitivity (Q ₁₀) resulting in lower E _S10 values during the warm and dry season as compared during the cold and wet season; the latter corresponding with the period of stem growth. The Q ₁₀ by contrast, was higher during the warm and dry summer than during the cold and wet season, an observation suggesting that climate extremes such as summer drought in the Mediterranean may override the observation that Q ₁₀ declines with increasing temperature. As concurrent measurements estimating the potential impact of sap flow on E _S revealed no interaction in P. canariensis, the Q ₁₀ values were used along with E _S10 and stem temperature records to predict annual total E _S and for partitioning total E _S into its maintenance (E _m) and a growth (E _g) component.     

Age-dependent response of boreal forest to temperature and rainfall variability

We used measurements of CO₂ exchange by six Canadian boreal forest stands to determine how sites of contrasting age respond to interannual temperature and precipitation variation. The stands ranged in age (time since last burn) from 4 to 155 years. The study included years that were anomalously cool and moist, warm and dry, cool and dry, and warm and moist. Warmer than average springs accelerated the onset of older stand (> 20 years) photosynthesis by as much as 37 days and younger stand (< 20 years) photosynthesis by as much as 25 days. The warm–dry year resulted in a marked reduction of mid‐summer CO₂ uptake by the younger, but not older, stands. The mid‐summer decline in young stand photosynthesis reflected the combination of warmth and drought; similar declines were not observed during the cool–dry or warm–moist years. The annual carbon gain by the oldest stands was greatest during the warm–dry year as a result of the expanded growing season. The annual carbon gain by the youngest stands was consistent from year to year, largely as a result of offsets between increased spring photosynthesis and reduced mid‐summer photosynthesis during the warm–dry year. Night‐time CO₂ efflux increased by 2–29% during the warm–moist year relative to the warm–dry year. This increase was not systematically related to age. We conclude that the major source of interannual CO₂ exchange variation at the landscape scale is the ability of older, evergreen canopies to respond to warm springs by advancing the onset of photosynthesis. Drought‐related reductions in photosynthesis, moisture‐driven respiratory losses, and the effects of spring warmth on young‐stand photosynthesis are of secondary importance. The advantage conferred on older, evergreen stands during warmer years carries implications for the possible effects of climate change on boreal forest succession. Warmer temperatures may increase the competitive advantage of late successional species relative to early successional species, and also the incidence of fire, thereby accelerating plant succession and shortening the fire‐return interval.     

Supercooling ability and cold hardiness of the pollen beetle Meligethes aeneus

Supercooling point (SCP) and cold‐hardiness of the pollen beetle Meligethes aeneus (Fabricius) (Coleoptera: Nitidulidae) were investigated. Mature eggs from the oviduct were supercooled on average to ?28.0 °C and from oilseed rape buds to ?24.4 °C; first instars were supercooled to ?21.0 °C and second instars to ?16.8 °C. Despite their high supercooling ability, none of the eggs survived 24 h exposure to ?2.5 °C. The supercooling ability of adults varied significantly among feeding and non‐feeding beetles: high SCPs prevailed during the whole warm period, being about ?12 °C; low values of SCP of ?20 °C dominated in non‐feeding beetles. In spring and autumn, beetles displayed the same acclimation efficiency: after 1 week of exposure at 2.0 °C with no access to food their SCPs were depressed equally by about 3 °C. Meligethes aeneus beetles have a different response to low temperatures depending on the season. The lowest tolerance was found in reproductively active beetles after emergence from overwintering sites; the time needed to kill 50% of individuals (Ltime₅₀) was 56.2 h at ?7 °C and the lower lethal temperature needed to kill 50% (Ltemp₅₀) after 24 h exposure was ?8.6 °C. Cold hardiness increased from midsummer to midwinter; Ltime₅₀ was 80 h in August, 182.8 h in September, and 418.1 h in January. Lethal temperature after 24 h exposure was ?9.1 °C in August and ?9.8 °C in September. In February, after diapause, the beetles started to loose their cold tolerance, and Ltemp₅₀ was slightly increased to ?9.5 °C. Hibernating beetles tolerated long exposure at ?7 °C well, but mortality was high after short exposure if the temperature dropped below ?9 °C for 24 h. Despite the season, the beetles died at temperatures well above their mean SCP; consequently, SCP is not a suitable index for cold hardiness of M. aeneus.     

Molecular phylogeny and detailed morphological analysis of two freshwater Rhizoclonium strains from contrasting spring types in Egypt and Italy

Two Rhizoclonium strains thriving in contrasting spring types (slightly hot spring/ambient, shaded pool spring) and biogeographic areas (El-Farafra Oasis in the Western Desert of Egypt/River-Po Plain in Northern Italy) were studied in depth based on field and cultured material, bright-field and fluorescence microscopy, and phylogenetic molecular analyses (SSU and LSU rRNA gene sequences). This polyphasic approach revealed that the two Rh. strains clearly belonged to the Rhizoclonium clade but differed in some of their key diagnostic features. The Egyptian Rhizoclonium strain, isolated from the spring-fed (Ain El-Balad) agricultural ditch in El-Farafra Oasis, was described with the working name ‘Rhizoclonium sp. 10.6 μm from a desert, slightly-hot spring’ based on its smaller cell diameter, ecological and phylogenetic molecular traits compared to allied morphospecies. Moreover, it was highlighted that the L/D ratio can be more than 3.0 within this group. The Italian Rhizoclonium strain, collected from the Fontanile Valle Re-shaded, pool ambient spring (Emilia-Romagna Region), was virtually identical to other strains identified as Rh. hieroglyphicum from China and Japan, due to its highly supported congruence in morphological characteristics and phylogenetic position. This study provided the first partial LSU and SSU rRNA gene sequences for European Rh. hieroglyphicum based on available literature. Phenotypic plasticity of rhizoid formation was also observed in both Rhizoclonium strains studied using culturing approaching techniques. Our investigation also confirms that a lot of work with a variety of approaches is still needed to assess the ecological preferences, morphological plasticity, and phylogenetics of freshwater Rhizoclonium taxa worldwide.