Summer Accumulation Research Articles

Climate change and glacier retreat in northern Tien Shan (Kazakhstan/Kyrgyzstan) using remote sensing data

This paper presents an analysis of precipitation and temperature trends and a GIS-supported investigation of the related glacier change in the mountain ridges Zailiyskiy and Kungey Alatau, which represent an important part of the northern Tien Shan. The recent glacier coverage was delineated in a semi-automated way using a TM4/TM5 ratio image of a Landsat ETM Scene from the year 1999 and a merged ASTER/SRTM3-DEM. The extent of these glaciers is compared to that of the glaciers in the Soviet Glacier Inventory [UdSSR, Academica Nauk (1966 to 1983). Katalog Lednikov SSSR (in Russian), Gidrometeoizdat. Leningrad], which represents the situation in study area in approx. 1955. Regionalization of temperature and precipitation as well as solar radiation calculation was conducted in order to determine the climate situation at the glaciers. Trend and correlation analysis for the period from 1879 to 2000 at 16 climate stations showed a temperature increase, which have become pronounced since the 1950s. Another strong increase occurred at the beginning of the 1970s and since around 1980, the temperatures have generally stayed at this high level. The trend coefficient was about 0.8 K/100a for the period 1900 to 2000 and about 2.0 K/100a on average for the second half of the last century. The increase was about two times higher than the global average in northern Tien Shan from 1950 until 2000, where the increase was mainly due to temperature rise in autumn and winter. The increase is less pronounced in the mountainous areas, but still obvious. However, the higher temperature increase at the lower stations, located for the most part in cities or larger settlements, may be due in part to increased urbanization. For precipitation, there was a small increase on average, but no clear trend. On the average, the decrease in glacier extent was more than 32% between 1955 and 1999 in the investigated valleys of Zailiyskiy and Kungey Alatau. The glacier retreat was not homogeneous, but depended strongly on the size, location and climate regime at the glaciers. The area loss of the continental-type glaciers with very predominant summer accumulation, as for those situated in the deeply incised Chon-Kemin valley between Zailiyskiy and Kungey Alatau, was conspicuously less, in parts, than the loss at the more maritime glaciers on the northern slope of Zailiyskiy Alatau. This is consistent with the small increase in summer temperatures. However, under dryer conditions with high solar radiation input, such as with glaciers in the Chon-Aksu valley in Kungey Alatau, the area retreat of the continental-type glaciers can be even more pronounced than that of the more maritime glaciers.

Retention and early diagenetic transformation of phosphorus in Lake Arendsee (Germany) - consequences for management strategies

Repeated sediment core investigations over one decade, mass balance calculations, and vertical flux measurements by traps and dialysis samplers, were used to determine P retention rates, release potential, and early diagenetic transformation processes in the sediment of Lake Arendsee (Germany). Sediment cores were dated by varve counting, by 137 cesium, and by a distinct layer originating from a restoration attempt in 1995, which involved the capping of the sediment with calcium rich material from the lake shore. P retention rates and the internal P cycle have not been altered by the sediment capping. The sharp decline of total P content within the first two centimeters of the sediment shows that diagenetic P mobilization is a rapid process. The temporary P pool in the sediment, calculated from core analysis (mean ± SE: 709 ± 82mg m -2 , n = 7), was small compared to the rates of hypolimnetic SRP increase (10.7 ± 0.45 mg m -2 d -1 , 1992-1997) and total P losses in the epilimnion caused by sedimentation (11.7 ± 0.53 mg m -2 d -1 , 1992-1997), both of the latter calculated by mass balances during summer stratification. Without additional supply of freshly settled material, the temporary P pool in the sediment would be exhausted in less than three months. The fast P release of freshly settled material was also demonstrated during summer by the three times higher vertical P sedimentation rates calculated on the basis of mass balance data, compared to rates determined on the basis of cylindrical traps, in which some P was released during four weeks exposure time. The driving process for the rapid P release in Lake Arendsee is the remineralisation of organic P. This study demonstrates that high hypolimnetic P accumulation rates are not always correlated with a large total or potentially mobile P pool in the sediment. In lakes with small temporary P pools in the sediment, a decrease of P in the water body would immediately decrease the hypolimnetic P accumulation in summer; capping or dredging as in-lake measures are ineffective in such lakes.

To live or to survive in Doñana dunes: Adaptive responses of woody species under a Mediterranean climate

The Dune System of Donana National Park (SW Spain) exhibit a mosaic of environmental characteristics, with different plant communities, all under the same Mediterranean climate, creating an interesting field laboratory for the study of plant responses to stressing conditions. Fourteen woody plant populations were selected, belonging to either xerophytic or hygrophytic plant communities on stabilised dunes, where topography causes differences in soil water availability. Plants were tagged and morphological and ecophysiological measurements were recorded in winter and summer of 1999. Seasonal differences in ecophysiological measurements together with morphological variables were used as plant traits to identify the main adaptive responses of the species. Cluster analysis of traits separated three groups of plant strategies: spiny legume species; sclerophyll, and semideciduous species. In addition, another two kind of strategies have been found in the semideciduous group of species, those withstanding water shortages, attaining very negative water potentials, low photochemical efficiency, and leaf proline accumulation in summer, as opposed to those tolerating water deficit, with moderate seasonal differences in water potential, proline content and photochemical efficiency. The results of this study indicate that legume species behave as a different functional group and drought-semideciduous species present different adaptive responses under the same environmental stress. Ecophysiological measurements must be used as plant traits to detect functional groups under Mediterranean climate.

Seasonal cardenolide production and Dop5βr gene expression in natural populations of Digitalis obscura

Productivity variations and seasonal fluctuations of cardenolides have been studied in 10 natural populations of Digitalis obscura distributed in three bioclimatic belts. Main cardenolides in D. obscura plants are those of the series A and such predominance (ca. 80–85%) over the series B metabolites is independent of the population studied or the degree of maturity of the leaves. Primary glycosides represent ca. 50–60% of total cardenolides; this percentage did not vary among populations or with the leaf age but increased in summer and decreased in winter. A correlation analysis between plant biomass and cardenolide content showed a positive relationship of these parameters, which, according to the bioclimatic distribution of the populations, suggests that certain environmental conditions may cause marked decreases in plant biomass together with a reduction in productivity. Cardenolide contents changed in the timecourse of the four seasons as a multiple response to distinct plant and/or environmental factors. The lowest production was recorded in May, followed by a fast cardenolide accumulation in summer, a decreasing phase in autumn, and a stationary phase in winter. We also analysed the seasonal expression of the gene encoding the progesterone 5β-reductase, enzyme producing the required 5β-configured intermediaries of cardenolides. A fragment of the isolated partial genomic sequence was used as a probe for Northern analysis to study the seasonal gene expression in selected populations. The expression pattern showed increasing levels from February to July and a further reduction in autumn, although harmful climatic conditions seems to induce overexpression of this gene.

Ice dynamics and basal properties of Sofiyskiy glacier, Altai mountains, Russia, based on DGPS and radio-echo sounding surveys

AbstractCentral Asian glaciers, compared to other glaciers in the world, should exhibit a different response to changing climate owing to (i) the extreme continentality that gives rise to aridity and large seasonal temperature variations, and (ii) the coincidence of accumulation and ablation seasons in summer. A detailed 4year field survey has been carried out on Sofiyskiy glacier in the south Chuya range(Altai mountains), Russia. Field observations reveal that this glacier retreated steadily during the 20th century at a rate of 18.3 m a–1, considerably faster than Maliy Aktru glacier, 30km to the north. Based on radio-echo sounding (RES), the subglacial topography of Sofiyskiy glacier has been reconstructed, and stake and snow-pit measurements enable the determination of surface velocity and mass balance. Basal sliding plays an important role in Sofiyskiyglacier’s behaviour, as is shown by a force-balance analysis. An analysis of RES measurements yields the bed reflection power (BRP). Using the three-layer reflectivity model of Born and Wolf (1986), the BRP is compared with the theoretical bed reflectivity for two contrasting layers of varying porosity and thickness. Results indicate that Sofiyskiy glacier possibly exhibits a polythermal regime.

Features of meteorological events preserved in a high-resolution Law Dome (East Antarctica) snow pit

AbstractSnow-pit and shallow firn-core records of oxygen isotope ratios (δ18O) and trace ion species were generated at a high-accumulation site on Law Dome, East Antarctica. Concordance between accumulation events identified in records up to 7.7 km a part confirms that the observed glaciochemical variations are the result of regional rather than local surface effects. This allows calibration of the snow-pit records with measured meteorological parameters. Net accumulation periods that comprise the snow-pit record are identified using hourly snow-accumulation measurements from a co-located automatic weather station (AWS). Particular focus is given to three net accumulation periods preserved during austral summer 1999/2000 that correspond to the top 0.5 m of the snow pit. Local meteorological conditions recorded during the summer accumulation periods by the AWS are combined with regional and synoptic-scale meteorology derived from Casey station (110 km away) and Advanced Very High Resolution Radiometer satellite imagery to identify potential source regions for chemical signals preserved in summer snow at Law Dome.

Summer Accumulation of Tall Fescue at Low Elevations in the Piedmont: I. Fall Yield and Nutritive Value

Summer accumulation of tall fescue (Festuca arundinacea Schreb.) for fall and winter use in the Piedmont, a region of the USA of moderate fall and winter temperatures and open winters of frequent rainfall, has not been well documented. We determined in this 5‐yr study the yield and nutritive value in mid‐November of tall fescue accumulated monthly from 1 June to 1 September. A fifth treatment included an additional 67 kg N ha−1 applied at the 1 July accumulation date. The site was a Cecil clay loam (fine, kaolinitic, thermic Typic Kanhapludults) soil and the treatments were arranged in a randomized complete block design with four replicates. Summer accumulations yielded from 3280 to 4130 kg ha−1 with a mean linear (P ≤ 0.05) reduction in dry matter of 195 kg ha−1 for each week's delay in accumulation from 1 June until 1 September. In two of the three years, highest (P ≤ 0.05) dry matter yields (DMY) resulted from the 1 June date (4070 to 5440 kg ha−1) and lowest (P ≤ 0.05) from the 1 September date (1010 and 860 g kg−1). Repeated summer accumulations did not alter DMY (P < 0.01) in subsequent falls or in the following spring. At mid‐November in vitro dry matter disappearance (IVDMD) had increased 30 g kg−1 for each 30‐d delay in accumulation from 1 June to 1 September, but no change occurred in crude protein (CP) (mean = 120 g kg−1). Summer accumulation of tall fescue for fall grazing can be practiced in the lower Piedmont with accumulation beginning as early as 1 June.

Summer Accumulation of Tall Fescue at Low Elevations in the Humid Piedmont: II. Fall and Winter Changes in Nutritive Value

The mild but variable temperatures and predominant rainfall during the winter could reduce the nutritive value in the winter of summer‐accumulated tall fescue (Festuca arundinacea Schreb.). The objective of our 3‐yr study was to determine the nutritive value of tall fescue accumulated from 1 June, 1 July, 1 July + N (67 kg N ha−1), 1 August, and 1 September and sampled from October to March. The treatments were arranged in a randomized complete block design with four replicates. Delaying accumulation from 1 June to 1 September increased in vitro dry matter disappearance (IVDMD) linearly (P ≤ 0.05) at each monthly sampling from October to March. Highest IVDMD (717 g kg−1) was obtained from the 1 September accumulation sampled in October and declined to 623 g kg−1 in March. Forage accumulated from 1 June and 1 July was lowest in IVDMD and averaged 590 g kg−1 in October and declined to 539 g kg−1 in March. Crude protein (CP) concentrations showed little change from November to March (mean = 121 g kg−1). Green tissue in accumulated forage retained high IVDMD (mean = 714 g kg−1) throughout the winter, but the proportion shifted from about 73% green in November to 36% in January. Dead tissue, consistently low in IVDMD (mean = 393 g kg−1), reduced canopy IVDMD from 26 to 55 g kg−1 for each 10 percentage unit increase. Tall fescue can be accumulated during the summer in the Piedmont and can provide forage of high nutritive value until January or until dead tissue dominates in the forage.

Effect of summer accumulation on glacier mass balance on the Tibetan Plateau revealed by mass-balance model

AbstractThe characteristics and sensitivities of a cold-based glacier on the Tibetan Plateau, where the summer monsoon provides most of the mass input to glaciers, are discussed using an energy-balance model incorporating the process of water refreezing. The model accurately represents the observational results related to the mass balance of Xiao Dongkemadi glacier on the central plateau during 1992/93. Our data revealed that the mass balance of cold glaciers cannot simply be described by the surface mass/heat balances, because about 20% of infiltrated water is refrozen and thus does not run off from the glacier. Model calculations demonstrate that glaciers in an arid environment can maintain their mass since the monsoon provides precipitation during the melting season. Snowfall in summer keeps surface albedo high and largely restrains ablation. Nevertheless, the calculations also make clear that glaciers on the plateau are more vulnerable than those of other regions because of summer accumulation. In the monsoon climate, warming would cause not only a decrease in accumulation, but also a drastic increase in ablation in combination with surface-albedo lowering. Therefore, although glaciers on and around the plateau can be sustained by summer accumulation, they are more vulnerable to warming than winter-accumulation-type glaciers.

Summer Accumulation of Tall Fescue at Low Elevations in the Piedmont

Summer Accumulation of Tall Fescue at Low Elevations in the Piedmont

Mass balance of Xiao Dongkemadi glacier on the central Tibetan Plateau from 1989 to 1995

AbstractData on the mass balance of Xiao Dongkemadi glacier in the Tanggula mountains, central Tibetan Plateau, were obtained over 5 5 years from 1989 to 1995. These are the first continuous mass-balance data for a continental-type glacier on the Tibetan Plateau, where the glacier accumulates during the summer monsoon (summer-accumulation-type glacier). Mass-balance vs altitude profiles were steeper in the negative than in the positive mass-balance years. This is considered to have resulted from the effect of summer accumulation. The annual mass balance is compared with air temperature, precipitation, and black-body temperature in the area including the glacier, which is calculated from infrared radiation observations by theJapanese Geostationary Meteorological Satellite. It was found that the interannual variation in the glacier mass balance was not closely related to maximum monthly mean air temperature, while it did have a relatively good correlation with maximum monthly mean black-body temperature.

Asynchronous glaciation at Nanga Parbat, northwestern Himalaya Mountains, Pakistan

Research Article| May 01, 2000 Asynchronous glaciation at Nanga Parbat, northwestern Himalaya Mountains, Pakistan William M. Phillips; William M. Phillips 1Department of Geography, University of Edinburgh, Edinburgh EH8 9XP, Scotland, UK Search for other works by this author on: GSW Google Scholar Valerie F. Sloan; Valerie F. Sloan 2Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309, USA Search for other works by this author on: GSW Google Scholar John F. Shroder, Jr.; John F. Shroder, Jr. 3Department of Geography and Geology, University of Nebraska, Omaha, Nebraska 68182, USA Search for other works by this author on: GSW Google Scholar Pankaj Sharma; Pankaj Sharma 4PRIME Lab, Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA Search for other works by this author on: GSW Google Scholar Michèle L. Clarke; Michèle L. Clarke 5School of Geography, University of Nottingham, University Park, Nottingham NG7 2RD, UK Search for other works by this author on: GSW Google Scholar Helen M. Rendell Helen M. Rendell 6Department of Geography, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (5): 431–434. https://doi.org/10.1130/0091-7613(2000)28<431:AGANPN>2.0.CO;2 Article history received: 28 Oct 1999 rev-recd: 14 Feb 2000 accepted: 23 Feb 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation William M. Phillips, Valerie F. Sloan, John F. Shroder, Pankaj Sharma, Michèle L. Clarke, Helen M. Rendell; Asynchronous glaciation at Nanga Parbat, northwestern Himalaya Mountains, Pakistan. Geology 2000;; 28 (5): 431–434. doi: https://doi.org/10.1130/0091-7613(2000)28<431:AGANPN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present a new glacial chronology demonstrating asynchroneity between advances of Himalayan glaciers and Northern Hemisphere ice-sheet volumes. Glaciers at Nanga Parbat expanded during the early to middle Holocene ca. 9.0–5.5 ka. No major advances at Nanga Parbat during the last global glacial stage of marine oxygen isotope stage 2 (MIS-2) between 24 and 11 ka were identified. Preliminary evidence also indicates advances between ca. 60 and 30 ka. These periods of high ice volume coincide with warm, wet regional climates dominated by a strong southwest Asian summer monsoon. The general lack of deposits dating from MIS-2 suggests that Nanga Parbat was too arid to support expanded ice during this period of low monsoon intensity. Advances during warm, wet periods are possible for the high-altitude summer accumulation glaciers typical of the Himalayas, and explain asynchronous behavior. However, the Holocene advances at Nanga Parbat appear to have been forced by an abrupt drop in temperature ca. 8.4–8.0 ka and an increase in winter precipitation ca. 7–5.5 ka. These results highlight the overall sensitivity of Himalayan glaciation to orbital forcing of monsoon intensity, and on millennial or shorter time scales, to changes in North Atlantic circulation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Summer Accumulation of Tall Fescue at Low Elevations in the Humid Piedmont

Summer Accumulation of Tall Fescue at Low Elevations in the Humid Piedmont

Mass Balance Features Derived from a Firn Core at Hielo Patagónico Norte, South America

Glaciological research was carried out in November and December 1996 in the accumulation area of Glaciar Nef, an eastward-facing glacier from Hielo Patagónico Norte (Northern Patagonia Icefield), South America. A 14.5-m-deep firn core (9.7 m water equivalent: w.e.) was obtained at 1500 m a.s.l., and air temperature and melting rate were also measured. Values of δ18O of 1- or 2-cm-thick samples in the upper 6 m of the firn fluctuated from −16 to −10±, which are explained by variations in δ18O of precipitation. Below 6 m in the firn, δ18O was almost constant, due to percolation of large amounts of meltwater. It was considered that the upper 6 m was deposited during the prior winter; thus winter balance in 1996 was derived as +3.5 m w.e. Based on the measurement results and climatic data, winter and summer ablations were estimated at 0.0 and 3.4 m w.e., respectively, and summer accumulation at 2.1 m w.e. Then, net balance in the calendar year 1996 was estimated at about +2.2 m w.e. Comparison with previous studies and candidate drilling sites are also discussed.

Mass Balance Features Derived from a Firn Core at Hielo Patagonico Norte, South America

Glaciological research was carried out in November and December 1996 in the accumulation area of Glaciar Nef, an eastward-facing glacier from Hielo Patagónico Norte (Northern Patagonia Icefield), South America. A 14.5-m-deep firn core (9.7 m water equivalent: w.e.) was obtained at 1500 m a.s.l., and air temperature and melting rate were also measured. Values of δ18O of 1- or 2-cm-thick samples in the upper 6 m of the firn fluctuated from −16 to −10±, which are explained by variations in δ18O of precipitation. Below 6 m in the firn, δ18O was almost constant, due to percolation of large amounts of meltwater. It was considered that the upper 6 m was deposited during the prior winter; thus winter balance in 1996 was derived as +3.5 m w.e. Based on the measurement results and climatic data, winter and summer ablations were estimated at 0.0 and 3.4 m w.e., respectively, and summer accumulation at 2.1 m w.e. Then, net balance in the calendar year 1996 was estimated at about +2.2 m w.e. Comparison with previous studies and candidate drilling sites are also discussed.

Seasonal variations of glaciochemical, isotopic and stratigraphic properties in Siple Dome (Antarctica) surface snow

AbstractSix snow-pit records recovered from Siple Dome, West Antarctica, during 1994 are used to study seasonal variations in chemical (major ion and H202), isotopic (deuterium) and physical stratigraphic properties during the 1988-94 period. Comparison of δD measurements and satellite-derived brightness temperature for the Siple Dome area suggests that most seasonal SD maxima occur within ±4 weeks of each 1 January. Several other chemical species (H2O2, non-sea-salt (nss) SO42-, methanesulfonic acid and NO3-) show coeval peaks with SD, together providing an accurate method for identifying summer accumulation. Sea-salt-derived species generally peak during winter/spring, but episodic input is noted throughout some years. No reliable seasonal signal is identified in species with continental sources (nssCa2+ nss Mg2+), NH4+ or nssCl-. Visible strata such as large depth-hoar layers (&gt;5 cm) are associated with summer accumulation and its metamorphosis, but smaller hoar layers and crusts are more difficult to interpret. A multi-parameter approach is found to provide the most accurate dating of these snow-pit records, and is used to determine annual layer thicknesses at each site Significant spatial accumulation variability exists on an annual basis, but mean accumulation in the sampled 10 km2 grid for the 1988-94 period is fairly uniform.

Seasonal variations in the lipid and fatty acid composition of the euphausiid Meganyctiphanes norvegica from the Ligurian Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 186:199-210 (1999) - doi:10.3354/meps186199 Seasonal variations in the lipid and fatty acid composition of the euphausiid Meganyctiphanes norvegica from the Ligurian Sea Patrick Mayzaud*, Patti Virtue, Eric Albessard Observatoire Océanologique, Océanographie Biochimique et Ecologie, ESA-CNRS 7076, BP 28, 06230 Villefranche-sur-Mer, France *E-mail: mayzaud@ccrv.obs-vlfr.fr ABSTRACT: Lipid and fatty acid composition of the northern krill Meganyctiphanes norvegica from the Mediterranean Sea were studied with respect to sex and season. Detailed composition of the lipid classes is reported for both males and females during the spring reproductive season and the late summer non-reproductive season. Both sexes showed similar lipid structure, with neutral lipids dominated by triacylglycerols. Polar lipids were dominated by phosphatidylcholine. Fatty acid compositions of both polar lipids and triacylglycerols showed little change with sex or season. Polar lipids were dominated by HUFA (20:5n-3 or EPA and 22:6n-3 or DHA) as well as palmitic and oleic acid. Neutral lipid fatty acid profiles were dominated by 22:6(n-3) and palmitic acid, with a significant contribution of oleic, myristic, stearic and vaccenic acids as well as 20:5(n-3). Seasonal changes in total lipids showed maximum accumulation in early summer and minimum levels in early winter. Changes in fatty acid composition appeared related to the succession of populations with varying levels of triacylglycerols, as illustrated by the contrast between the early winter and spring periods in a correspondence analysis. Increased proportions of myristic, palmitoleic and 18:4(n-3) acids during the spring period are characteristic of the triglyceride fraction while the main descriptor of the winter period is DHA, which is the key constituent of the polar lipids. Comparison with literature data illustrated the clear decrease in lipid content of northern krill with latitude and the specific nature of the seasonal pattern recorded in the Ligurian Sea. The regional variability in fatty acid composition and the significance of fatty acids as potential markers of feeding behaviour are discussed in relation to the omnivorous character of M. norvegica. KEY WORDS: Northern krill · Lipids · Ligurian Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 186. Publication date: September 17, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Root production, turnover and respiration under two grassland types along an altitudinal gradient: influence of temperature and solar radiation.

We have measured the rates of root production and death and of root respiration in situ under two grasslands along an altitudinal gradient in the northern Pennines, UK, represented by a lowland site at 171 m in an agricultural setting, and three upland sites between 480 and 845 m. One grassland was dominated by Festuca ovina and was on a brown earth soil; the other was dominated by Juncus squarrosus and Nardus stricta and occurred on a peaty gley. The natural altitudinal gradient was extended by transplantation. Although root biomass and root production (estimated using minirhizotrons) both showed pronounced seasonal peaks, there was no simple altitudinal gradient in either variable, and neither root production nor root death rate was a simple function of altitude. Increased root accumulation in summer was a function of change in the length of the growing season, not of soil temperature. Root populations in winter were similar at all sites, showing that increased production at some sites was accompanied by increased turnover, a conclusion confirmed by cohort analyses. Respiration rate, measured in the field by extracting roots and measuring respiration at field temperature in an incubator, was unrelated to temperature. The temperature sensitivity of respiration (expressed as the slope of a plot of log respiration rate against temperature) showed no simple seasonal or altitudinal pattern. Both root growth (under Festuca) and respiration rate were, however, closely related to radiation fluxes, averaged over the previous 10 days for growth and 2 days for respiration. The temperature sensitivity of respiration was a function of soil temperature at the time of measurement. These results show that root growth and the consequent input of carbon to soil in these communities is controlled by radiation flux not temperature, and that plants growing in these upland environments may acclimate strongly to low temperatures. Most carbon cycle models assume that carbon fluxes to soil are powerfully influenced by temperature, but that assumption is based largely on short-term studies and must be reassessed.

The influence of discharge on entrainment of bank litter in a headwater stream

The fate of summer accumulations of leaf litter at Langrivier, a second-order, acid, headwater stream was monitored during the first ten weeks of winter rains (April-June 1991), in an attempt to describe the quantity of litter available and its movement through the stream during, and between, winter spates. Litter mass was measured at three distances from the stream edge, on banks with different gradients. A closed-system release and recapture method was employed to quantify leaf movement after each spate in a continuous 76-day study. Langrivier had a mean bank-litter mass of 1.04 kg m -2 at the start of winter. Bank litter provided 0.005-0.54 kg m -2 to the stream during the study period. Leaf movement was spate-driven, and was not significantly affected by wind or non-spate flows, although some litter was incorporated due to bank collapse during larger spates. Large spates (average discharge, 1.32 m 3 s -1 ) scoured the stream of debris, while moderately large spates (average, 0.8 m 3 s -' ) led to the reincorporation of new loads of bank litter into the stream. It is concluded that the dynamics of leaf detritus in this headwater stream during winter are characterised by the sequential removal (scour) and replacement (incorporation from banks) of litter in spate-driven pulses.

Methylmercury production in the anoxic hypolimnion of a Dimictic Seepage Lake

Experimental results and field data indicated that methyl-Hg was produced within a layer of bacterioplankton near the top of the anoxic hypolimnion of Pallette Lake. In situ incubations at ambient Hg concentrations indicated that the net flux of methyl-Hg from the layer was between 50 and 100 pmol/m2*d. This input was sufficient to account for the summer accumulation of methyl-Hg in the entire hypolimnion and it exceeded atmospheric inputs by 2 orders of magnitude. Maximum rates of net methylation occurred in the same region of the water column where we observed maximum rates of sulfate reduction. The measured rates were: 100 fmol rriethyl-Hg/L*d and 90 nmol SO4/L*d. Sulfate reducing enrichment cultures isolated from the hypolimnion were also able to methylate Hg in the laboratory. Sulfate reduction did not occur in anoxic profundal sediments during summer and we infer from ancillary data that methylation in profundal sediments was also low. Whole-lake rates of sulfate reduction in the hypolimnetic layer and shallow sediments were roughly equivalent, but we cannot yet compare methylation rates at these sites due to large uncertainties in the littoral flux of methyl-Hg. We propose that zones of Hg methylation and SO4 reduction follow the oxic/anoxic boundary in both the watercolumn and sediments. The relative importance of watercolumn and sediment processes will depend on the physical and chemical structure of a given lake.