Nitrogen deposition and warming – effects on phytoplankton nutrient limitation in subarctic lakes

The aim of this study was to predict the combined effects of enhanced nitrogen (N) deposition and warming on phytoplankton development in high latitude and mountain lakes. Consequently, we assessed, in a series of enclosure experiments, how lake water nutrient stoichiometry and phytoplankton nutrient limitation varied over the growing season in 11 lakes situated along an altitudinal/climate gradient with low N‐deposition (<1 kg N ha^?1 yr^?1) in northern subarctic Sweden. Short‐term bioassay experiments with N‐ and P‐additions revealed that phytoplankton in high‐alpine lakes were more prone to P‐limitation, and with decreasing altitude became increasingly N‐ and NP‐colimited. Nutrient limitation was additionally most obvious in midsummer. There was also a strong positive correlation between phytoplankton growth and water temperature in the bioassays. Although excess nutrients were available in spring and autumn, on these occasions growth was likely constrained by low water temperatures. These results imply that enhanced N‐deposition over the Swedish mountain areas will, with the exception of high‐alpine lakes, enhance biomass and drive phytoplankton from N‐ to P‐limitation. However, if not accompanied by warming, N‐input from deposition will stimulate limited phytoplankton growth due to low water temperatures during large parts of the growing season. Direct effects of warming, allowing increased metabolic rates and an extension of the growing season, seem equally crucial to synergistically enhance phytoplankton development in these lakes.     

Dissolved oxygen and thermal regimes of a Ugandan crater lake

This paper quantifies the temporal pattern of thermal stratification and deoxygenation in Lake Nkuruba, a small (3 ha), deep (maximum depth = 38 m) crater lake in western Uganda. Dissolved oxygen penetrated to an average depth of 9 m and a maximum depth of 15 m below which the lake was permanently anoxic over the 2 years of study. Although surface oxygen levels were correlated with both surface water temperature and rainfall, seasonal cycles of dissolved oxygen were not well-defined and may have been obscured by the high frequency of short-term fluctuations and by inter-annual variations caused by shifts in rainfall. Surface water temperature averaged 23.3±0.7 °C (S.D.) and varied directly with air temperature. Both diurnal changes and top-bottom temperature differentials were small averaging 1.7±0.7 °C and 1.6±0.8 °C, respectively. Thermal stability ranged from 101.3 to 499.9 g-cm cm^-2 and was positively related to surface water temperature suggesting that this small protected lake responds rapidly to short-term meteorological changes. Because contribution to the annual heat exchange cycle was confined to upper waters, the lake's annual heat budget was low, 1,073.8 cal cm^-2 yr^-1. However, net primary productivity was relatively high averaging 1.3 g C m^-2d^-1. The region where Lake Nkuruba is situated experienced a very strong earthquake (6.2 on the Richter scale) on 4 February, 1994. Subsequently, water levels dropped markedly in the lake, falling 3.14 m over a 5-month period. This revised version was published online in September 2006 with corrections to the Cover Date.     

Relationships between mercury in lake water,water colour and mercury in fish

The relationship between mercury content in fish (pike and perch), the different fractions of mercury in lake water and water color was investigated in 76, mainly oligotrophic lakes distributed over a large part of Sweden. The lakes were classified in terms of drainage area characteristics, lake morphometry and water chemistry. The dominant fraction of mercury in lake water was RIHg (fraction reducible to elemental mercury by NaBH₄). RIHg and water color were strongly positively correlated. Water color (determined by the comparative method using colored disks) was used as a surrogate for the amount of humic matter in the water. Thus, humic matter appears to be acting as an important carrier of mercury. A positive relationship between mercury content in fish and water color was found only in deep lakes (average depth > 5 m). It is suggested that the bioavailability of mercury attached to humic matter increases due to anoxic conditions, common in the hypolimnion of deep lakes.     

Diatoms as quantitative indicators of pH and water temperature in subarctic Fennoscandian lakes

Weighted averaging (WA) regression and calibrationbased optima and tolerances of lakewater pH andtemperature are presented for diatoms in ecologicallysensitive, subarctic Fennoscandian lakes. The studysites are mostly small, simple, oligotrophic,low-conductivity lakes with a pH range from 5.0 to7.7 and a temperature range (after data screening)from 9.3 to 15.0 °C. Experiments with inverse andclassical deshrinking, with or without tolerancedownweighting, were used to identify the bestcalibration functions. The model estimates wereadjusted by jackknifing procedures. WA by inversedeshrinking and with tolerance downweighting performedbest for pH prediction, whereas simple WA wasmarginally superior for predicting water temperature.The established pH model is accurate to within±0.39 H units, and the temperature model towithin ±0.88 degrees Celcius. Fifteen diatom taxawere identified as potential indicator species for pHand three for temperature.     

Consequences of dam‐altered thermal regimes for a riverine herbivore's digestive efficiency,growth and vulnerability to predation

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Relationship of tree growth and climatic factors at treeline of Picea likiangensis var. balfouriana forest in Basu County,Xizang

 与传统方法相比, 利用树木年轮学方法研究树线过渡区树木生长温度敏感性高低的问题更注重比较树木个体间的生长情况, 从各个树轮序列间的生长一致性程度和树轮序列对气候因素(气温、降水)的响应一致性程度可探讨树线过渡区树木生长的温度敏感性。为了认识高山树线过渡区内树木生长的温度敏感性问题, 选择西藏昌都地区八宿县的一条川西云杉树线过渡区为研究对象, 比较了过渡区内树木个体间的生长一致性, 分析了树木生长与气候因素的相关性及其在个体间的异同。结果显示: 树线过渡区内树轮生长在个体间的一致性较低, 树轮生长与气温的关系在树木个体间的一致性也较低, 而树轮生长与当年4-9月降水的关系相对较强。西藏八宿树线过渡区属于干旱区, 相对于气温而言, 降水对树木生长的影响更大。此外, 小生境的异质性及干扰事件的发生也有可能降低树木对温度的敏感性。在全球变暖及极端气候事件增加的背景下, 树木生长的温度敏感性被高估可能会导致对树线过渡区位置及树线过渡区内群落生产力等的预测产生偏差, 这一问题应该在区域生态模拟研究和相关林业经营与管理上得到重视。     

Assessment of water quality and health risks for toxic trace elements in urban Phewa and remote Gosainkunda lakes,Nepal

The concentration of 13 metals (Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Pb, and Hg) and their associated health risk assessment was performed for two Himalayan lakes, urban Phewa and remote Gosainkunda, from Nepal. Water Quality Index (WQI), Metal Index (MI), Hazard Quotient (HQ), Hazard Index, and Cancer Risk were calculated in order to evaluate the water quality of these lakes. Correlation analysis revealed that Mn and Fe were derived from natural geological weathering processes and Pb, V, Cr, Co, Ni, Cu, Zn, and Cd might have originated from anthropogenic sources. The results revealed that WQI of the remote lake fell into excellent water quality and urban lake fell into poor water quality, which is also supported by the MI calculation. Moreover, the HQ of Mn in urban lake showed values greater than unity suggesting its health risk to the local inhabitants. The cancer index values indicated “high” risk due to Cr, whereas Cd possesses “very low” cancer risk on local population residing nearby areas. This study provides the useful database and suggests for the regular assessment and policy formulation for safeguarding the natural water bodies in the region.     

西藏八宿川西云杉树线过渡区树木生长与气候关系的一致性

与传统方法相比, 利用树木年轮学方法研究树线过渡区树木生长温度敏感性高低的问题更注重比较树木个体间的生长情况, 从各个树轮序列间的生长一致性程度和树轮序列对气候因素(气温、降水)的响应一致性程度可探讨树线过渡区树木生长的温度敏感性。为了认识高山树线过渡区内树木生长的温度敏感性问题, 选择西藏昌都地区八宿县的一条川西云杉树线过渡区为研究对象, 比较了过渡区内树木个体间的生长一致性, 分析了树木生长与气候因素的相关性及其在个体间的异同。结果显示: 树线过渡区内树轮生长在个体间的一致性较低, 树轮生长与气温的关系在树木个体间的一致性也较低, 而树轮生长与当年4-9月降水的关系相对较强。西藏八宿树线过渡区属于干旱区, 相对于气温而言, 降水对树木生长的影响更大。此外, 小生境的异质性及干扰事件的发生也有可能降低树木对温度的敏感性。在全球变暖及极端气候事件增加的背景下, 树木生长的温度敏感性被高估可能会导致对树线过渡区位置及树线过渡区内群落生产力等的预测产生偏差, 这一问题应该在区域生态模拟研究和相关林业经营与管理上得到重视。     

From dimictic to monomictic: Empirical evidence of thermal regime transitions in three deep alpine lakes in Austria induced by climate change

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Growth of pikeperch in relation to lake characteristics: total phosphorus, water colour, lake area and depth

The growth of pikeperch Sander lucioperca was studied in 41 lakes in central Finland. The backcalculated average total length of 3 year‐old pikeperch was used as an indicator of growth. The growth correlated positively with total phosphorus and water colour and negatively with lake area and depth. The reason for differences in growth may be differences in the amount of suitable food, foraging success or temperature dynamics in different lakes.     

Importance of recent shifts in soil thermal dynamics on growing season length, productivity, and carbon sequestration in terrestrial high-latitude ecosystems

In terrestrial high‐latitude regions, observations indicate recent changes in snow cover, permafrost, and soil freeze–thaw transitions due to climate change. These modifications may result in temporal shifts in the growing season and the associated rates of terrestrial productivity. Changes in productivity will influence the ability of these ecosystems to sequester atmospheric CO₂. We use the terrestrial ecosystem model (TEM), which simulates the soil thermal regime, in addition to terrestrial carbon (C), nitrogen and water dynamics, to explore these issues over the years 1960–2100 in extratropical regions (30–90°N). Our model simulations show decreases in snow cover and permafrost stability from 1960 to 2100. Decreases in snow cover agree well with National Oceanic and Atmospheric Administration satellite observations collected between the years 1972 and 2000, with Pearson rank correlation coefficients between 0.58 and 0.65. Model analyses also indicate a trend towards an earlier thaw date of frozen soils and the onset of the growing season in the spring by approximately 2–4 days from 1988 to 2000. Between 1988 and 2000, satellite records yield a slightly stronger trend in thaw and the onset of the growing season, averaging between 5 and 8 days earlier. In both, the TEM simulations and satellite records, trends in day of freeze in the autumn are weaker, such that overall increases in growing season length are due primarily to earlier thaw. Although regions with the longest snow cover duration displayed the greatest increase in growing season length, these regions maintained smaller increases in productivity and heterotrophic respiration than those regions with shorter duration of snow cover and less of an increase in growing season length. Concurrent with increases in growing season length, we found a reduction in soil C and increases in vegetation C, with greatest losses of soil C occurring in those areas with more vegetation, but simulations also suggest that this trend could reverse in the future. Our results reveal noteworthy changes in snow, permafrost, growing season length, productivity, and net C uptake, indicating that prediction of terrestrial C dynamics from one decade to the next will require that large‐scale models adequately take into account the corresponding changes in soil thermal regimes.     

蒙古高原草地不同深度土壤碳氮磷化学计量特征对气候因子的响应

研究不同深度土壤碳(C)、氮(N)、磷(P)含量及其化学计量比对气候因子(年降水量(MAP)和年平均气温(MAT))的响应差异,对于理解气候变化如何影响生态系统功能具有重要意义。通过对蒙古高原干旱半干旱草地44个样点的野外调查,探讨了不同深度(0–20、20–40、40–60、60–80 cm)土壤C、N、P含量及其化学计量比与MAP和MAT的关系。主要结果:(1)随土壤深度的增加,土壤C和N含量逐渐减少,土壤P含量不变;土壤C:P和N:P逐渐降低,土壤C:N相对稳定。(2)土壤C、N、P含量以及土壤C:P、N:P与MAP显著正相关,与MAT显著负相关,土壤C:N与MAP显著负相关,与MAT无相关性;随着土壤深度的增加,土壤C、N、P含量及其化学计量比与气候因子的相关性均逐渐减弱。(3) MAP和MAT对不同深度土壤C、N、P含量和化学计量比的影响存在显著差异;随着土壤深度的增加, MAP和MAT对土壤C、N、P含量及其化学计量特征变化的总解释度逐渐减少。该研究表明气候因子对土壤元素化学计量特征具有自上而下的调控作用,蒙古高原草地土壤表层C、N、P含量及其化学计量比与MAP和MAT的关...     

Depth and temperature drive patterns of spatial overlap among fish thermal guilds in lakes across Ontario,Canada

Aim

The spatial distribution of ectotherms is strongly dependent on the temperature of their environments. In temperate lakes, fishes with different thermal optima can become spatially segregated during summer stratification. This habitat partitioning, or niche complementarity, may play a role in the coexistence of trophically similar species; however, the extent of partitioning is dependent on the resources available within each habitat. Although habitat partitioning of fish thermal guilds has been studied in individual lakes, broad-scale patterns of spatial overlap and segregation are not yet understood. In this study, we explore the patterns and drivers of spatial overlap among thermal guilds (cold-, cool-, and warm-water) at a broad scale.

Location

Ontario, Canada.

Methods

We built a multivariate regression tree to explore patterns and environmental drivers of spatial overlap in freshwater fishes across three thermal guilds from 438 lakes.

Results

We identified five clusters of lakes exhibiting different patterns of spatial overlap among the three thermal guilds. Temperature (growing degree days) and maximum lake depth were strong drivers of the spatial overlap patterns.

Main Conclusions

These findings provide a better understanding of broad-scale patterns of spatial overlap and allow us to predict how spatial overlap, and ultimately species interactions and competition, may change under a warming climate.     

Water‐level fluctuations affect sediment properties,carbon flux and growth of the isoetid Littorella uniflora in oligotrophic lakes

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Climate‐diameter growth relationships of black spruce and jack pine trees in boreal Ontario,Canada

To predict the long‐term effects of climate change – global warming and changes in precipitation – on the diameter (radial) growth of jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana [Mill.] B.S.P.) trees in boreal Ontario, we modified an existing diameter growth model to include climate variables. Diameter chronologies of 927 jack pine and 1173 black spruce trees, growing in the area from 47°N to 50°N and 80°W to 92°W, were used to develop diameter growth models in a nonlinear mixed‐effects approach. Our results showed that the variables long‐term average of mean growing season temperature, precipitation during wettest quarter, and total precipitation during growing season were significant (alpha = 0.05) in explaining variation in diameter growth of the sample trees. Model results indicated that higher temperatures during the growing season would increase the diameter growth of jack pine trees, but decrease that of black spruce trees. More precipitation during the wettest quarter would favor the diameter growth of both species. On the other hand, a wetter growing season, which may decrease radiation inputs, increase nutrient leaching, and reduce the decomposition rate, would reduce the diameter growth of both species. Moreover, our results indicated that future (2041–2070) diameter growth rate may differ from current (1971–2000) growth rates for both species, with conditions being more favorable for jack pine than black spruce trees. Expected future changes in the growth rate of boreal trees need to be considered in forest management decisions. We recommend that knowledge of climate–growth relationships, as represented by models, be combined with learning from adaptive management to reduce the risks and uncertainties associated with forest management decisions.     

Snow cover and extreme winter warming events control flower abundance of some,but not all species in high arctic Svalbard

The High Arctic winter is expected to be altered through ongoing and future climate change. Winter precipitation and snow depth are projected to increase and melt out dates change accordingly. Also, snow cover and depth will play an important role in protecting plant canopy from increasingly more frequent extreme winter warming events. Flower production of many Arctic plants is dependent on melt out timing, since season length determines resource availability for flower preformation. We erected snow fences to increase snow depth and shorten growing season, and counted flowers of six species over 5 years, during which we experienced two extreme winter warming events. Most species were resistant to snow cover increase, but two species reduced flower abundance due to shortened growing seasons. Cassiope tetragona responded strongly with fewer flowers in deep snow regimes during years without extreme events, while Stellaria crassipes responded partly. Snow pack thickness determined whether winter warming events had an effect on flower abundance of some species. Warming events clearly reduced flower abundance in shallow but not in deep snow regimes of Cassiope tetragona, but only marginally for Dryas octopetala. However, the affected species were resilient and individuals did not experience any long term effects. In the case of short or cold summers, a subset of species suffered reduced reproductive success, which may affect future plant composition through possible cascading competition effects. Extreme winter warming events were shown to expose the canopy to cold winter air. The following summer most of the overwintering flower buds could not produce flowers. Thus reproductive success is reduced if this occurs in subsequent years. We conclude that snow depth influences flower abundance by altering season length and by protecting or exposing flower buds to cold winter air, but most species studied are resistant to changes.     

Snowy plover nest site selection,spatial patterning,and temperatures in the Southern High Plains of Texas

Snowy plover (Charadrius nivosus) populations have declined throughout their range, in part because of habitat degradation and poor nest success, making information regarding regionally specific nest site selection and spatial patterns important when considering habitat conservation and management guidelines. We determined nest site selection characteristics (n = 180) and examined spatial patterns (n = 215) of snowy plover nests in saline lakes in the Southern High Plains (SHP) of Texas. At 104 nests, we examined the influence of substrate type on nest temperatures and heat mitigation. Snowy plover nests were more likely to be found near an object, on pebble substrate, and with fewer plants than random sites. High use areas were generally located in areas with pebble substrate and on human-made or natural islands, berms, and peninsulas. Overall, nests placed on pebble substrate had lower temperatures during the day than nests placed on sand substrates. Nest placement on pebble substrate may be valuable to nesting snowy plovers, providing thermal advantages to incubating adults and depressing potentially high nest predation rates. Management guidelines for this region should emphasize the importance of addressing key elements of snowy plover nesting habitat including the presence of pebble substrate and reducing vegetation encroachment. © 2012 The Wildlife Society.