Diatom assemblages and their relationship to environmental variables in lakes from the boreal forest-tundra ecotone near Yellowknife,Northwest Territories,Canada

The relationship between diatom (Bacillariophyceae) taxa preserved in surface lake sediments and measured limnological and environmental variables in 22 lakes near Yellowknife (N.W.T.) was explored using multivariate statistical methods. The study sites are distributed along a latitudinal gradient that includes a strong vegetational gradient of boreal forests in the south to arctic tundra conditions in the north. Canonical correspondence analysis (CCA) revealed that lakewater concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) each accounted for independent and statistically significant proportions of variation in the distribution of diatom taxa. Weighted-averaging (WA) models were developed to infer DIC and DOC from the relative abundances of the 76 most common diatom taxa. These models can now be used to infer past DIC and DOC concentrations from diatom assemblages preserved in sediment cores of lakes in the Yellowknife area, which may provide quantitative estimates of changes in lakewater chemistry related to past vegetational shifts at treeline.     

RELATIONSHIPS BETWEEN LITTORAL DIATOMS AND THEIR CHEMICAL ENVIRONMENT IN NORTHEASTERN GERMAN LAKES AND RIVERS1

We explored statistical relationships between the composition of littoral diatom assemblages and 21 chemical and physical environmental variables in 69 lakes and 15 river sites in the lowland of northeastern Germany. Canonical correspondence analysis with single treatment and with forward selection of environmental variables was used to detect 11 important ecological variables (dissolved inorganic carbon [DIC], Na ⁺ , total phosphorus [TP], dissolved organic carbon [DOC], total nitrogen [TN], pH, oxygen saturation, dissolved iron, SO₄^{2 ?} , NH₄ ⁺ , soluble reactive silicium) and maximum water depth or Ca^{2 +} or soluble reactive phosphorus that most independently explain major proportions of the total diatom variance among the habitats. Monte Carlo permutation tests showed that each contributed a significant additional proportion (P < 0.05) of the variance in species composition. Together, these 11 most important environmental variables explained 34% of the total variance in species composition among the sites and captured 73% of the explained variance from the full 21 parameters model. Weighted‐averaging regression and calibration of 304 indicator taxa with tolerance down‐weighting and classic deshrinking was used to develop transfer functions between littoral diatoms and DIC, pH, TP, TN, and Cl ^? . The DOC:TP ratio was introduced and a weighted‐averaging model was developed to infer allochthonous DOC effects in freshwater ecosystems. This diatom‐DOC/TP model was significant (P < 0.001) and explained 7.6% of the total diatom variance among the sites, surpassing the inferential power of the diatom‐TP‐transfer function (7.3% explained variance). The root‐mean‐square errors of prediction of the models were estimated by jack‐knifing and were comparable with published data sets from surface sediment diatom samples. The data set of littoral diatoms and environmental variables allows use of the diatom‐environmental transfer functions in biomonitoring and paleolimnological approaches across a broad array of natural water resources (such as floodplains, flushed lakes, estuaries, shallow lakes) in the central European lowland ecoregion.     

Climate drivers of diatom distribution in shallow subarctic lakes

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Diatoms and their relationship to environmental variables from lakes and ponds on Bathurst Island, Nunavut, Canadian High Arctic

Baseline and historical environmental data are sparse in the High Arctic, however diatom assemblages preserved in high arctic lake and pond sediment profiles can provide proxy data of past environmental changes. Ecological data, however, are still sparse. Diatom taxa preserved in the surficial sediment of lakes and ponds on Bathurst Island (75° 42 N, 97° 21 W), Nunavut, Canadian High Arctic, and their relationship to 34 environmental variables were examined using multivariate statistics. A total of 148 diatom taxa were identified from the surface sediments of 29 study sites. Five environmental variables, Fe³⁺, Total Phosphorus (Unfiltered) (TPU), Total Nitrogen (TN), Temperature (TEMP) and pH significantly (P0.05) accounted for most of the variation in the diatom assemblages. TN was also significantly correlated to other variables (e.g. TPF, DOC, POC). A CCA constrained to TN indicated that this variable best explained the species distributions, and a weighted-averaging (WA) model was developed to infer nutrient levels from the relative abundances of the 58 dominant taxa. Interestingly, previous limnological work indicated that nitrogen most likely limited algal production in some of these high arctic sites. This model may be used to quantitatively estimate past TN levels from diatom assemblages preserved in sediment cores from Bathurst Island, and may provide a means to track past environmental changes in the High Arctic.     

FACTORS INFLUENCING DIATOM DISTRIBUTIONS IN CIRCUMPOLAR TREELINE LAKES OF NORTHERN RUSSIA

Diatom assemblages and limnological data were analyzed from 74 lakes spanning arctic treeline in three geographical regions of northern Russia: near the mouth of the Pechora River, on the Taimyr Peninsula, and near the mouth of the Lena River. Analysis of similarities indicated that diatom assemblages in tundra and forest lakes were significantly different from each other in all regions, with tundra lakes generally associated with higher abundances of small benthic Fragilaria Lyngbye taxa. Canonical correspondence analysis identified variables related to ion concentrations (e.g. Na ⁺ , dissolved inorganic carbon), lake depth, silica concentrations, and surface water temperatures as factors that explained significant amounts of variation in the diatom assemblages. Across treeline, the generally higher surface water temperatures of the forested lakes consistently accounted for a significant proportion of the diatom distribution patterns. Major ion concentrations also explained significant amounts of variation in the diatom assemblages across treeline for all three regions; however, regional trends were most likely influenced by local factors (i.e. ocean proximity or anthropogenic activities). The importance of climatic gradients across treeline (e.g. temperature) diatom distributions provides additional evidence that diatoms may be useful as paleoclimatic indicators. However, combination of the three calibration sets revealed that local water chemistry determinants (e.g. lithology, marine influence) overrode the influence of climatic gradients in explaining diatom distributions, suggesting that regional differences must be minimized for successful combination of geographically separate calibration sets.     

Contrasts between dystrophic and clearwater lakes in the long‐term effects of acidification on cladoceran assemblages

1. Most studies on zooplankton responses to acidification have focused on clearwater lakes with a dramatic acidification history. The role of dissolved organic carbon (DOC) in moderating zooplankton responses to acidification in naturally acidic, dystrophic lakes is less well understood and is partially impeded by a lack of baseline data. 2. Cladocera leave identifiable remains preserved in lake sediments that can be used to provide information on pre‐industrial species assemblages and their responses to environmental stressors such as acidification. Therefore, we used palaeolimnological approaches to track cladoceran assemblage responses to acidification since c.1850 (inferred from sedimentary diatom assemblages) in three acidified lakes in Kejimkujik National Park (Nova Scotia, Canada) that differ markedly in DOC content. These include two highly dystrophic lakes (Kejimkujik and Pebbleogittch lakes), and one clearwater lake (Beaverskin Lake). 3. In dystrophic Pebbleogittch Lake, an increase in the acid‐tolerant, jelly‐clad, pelagic taxon Holopedium glacialis occurred coincident with diatom‐inferred pH (DI‐pH) declines, but no other notable cladoceran assemblage shifts occurred. Similarly, Cladocera assemblages did not appear to respond to lakewater acidification in dystrophic Kejimkujik Lake. 4. In contrast, in the clearwater Beaverskin Lake, several observed shifts in cladoceran assemblage corresponded to DI‐pH declines, including an increase in the proportion of littoral taxa and an increase in Hill’s N2 species diversity. This may indicate increased water clarity as a result of acidification‐related decreases in DOC, which may have enhanced growth of emergent aquatic macrophytes and improved visibility for planktivorous fish, leading to increased predation on pelagic taxa. Species shifts within the littoral assemblage of Beaverskin Lake may reflect the differing tolerances of littoral taxa to low pH and aluminium toxicity. 5. Overall, our results suggest that cladoceran assemblages in naturally acidic, dystrophic lakes may be resilient against additional pH declines related to industrial emissions of acidifying agents, as dystrophic lakes are less vulnerable to increased aluminium toxicity and acidification‐induced increases in water clarity and often have a pre‐industrial cladoceran assemblage already adapted to acidic conditions.     

Comparison of Freshwater Diatom Assemblages from a High Arctic Oasis to Nearby Polar Desert Sites and Their Application to Environmental Inference Models

Arctic oases are regions of atypical warmth and relatively high biological production and diversity. They are small in area (<5 km²) and uncommon in occurrence, yet they are relatively well studied due to the abundance of plant and animal life contained within them. A notable exception is the lack of research on freshwater ecosystems within polar oases. Here, we aim to increase our understanding of freshwater diatom ecology in polar oases. Diatoms were identified and enumerated from modern sediments collected in 23 lakes and ponds contained within the Lake Hazen oasis on Ellesmere Island, and compared with diatom assemblages from 29 sites located outside of the oasis across the northern portion of the island. There were significant differences in water chemistry variables between oasis and northern sites, with oasis sites having higher conductivity and greater concentrations of nutrients and related variables such as dissolved organic carbon (DOC). Taxa across all sites were typical of those recorded in Arctic freshwaters, with species from the genera Achnanthes sensu lato, Fragilaria sensu lato, and Nitzschia dominating the assemblages. A correspondence analysis (CA) ordination showed that oasis sites generally plotted separately from the northern sites, although the sites also appear to plot separately based on whether they were lakes or ponds. Canonical correspondence analysis (CCA) identified specific conductivity, DOC, and SiO₂ as explaining significant (P < 0.05) and additional amounts of variation in the diatom data set. The most robust diatom‐based inference model was generated for DOC, which will provide useful reconstructions on long‐term changes in paleo‐optics of high Arctic lakes.     

Biogeography of copepods in lakes and ponds of subarctic Québec,Canada

We aimed to determine whether the copepod assemblages in lakes and ponds of northern Québec, Canada, were closer in composition to those found in southern Québec, or to those reported from the subarctic and arctic. Six calanoid and five cyclopoid species were identified from 37 ponds and lakes located in the region between 55° N and 59° N. Species diversity was generally low, ranging from 0 to 4 species per lake. Dominant species were Leptodiaptomus minutus and Acanthocyclops vernalis. The species assemblages showed high affinity with those found in forested regions of southern Québec. Exceptions were Leptodiaptomus tyrrelli, previously only recorded west of Hudson Bay, and Hesperodiaptomus arcticus, hitherto recorded north of 58° N. Relationships between the lakes, species, and environmental variables were explored using multivariate analysis. Lakes situated along the coast and on two offshore islands clustered together and were characterised by higher conductivity and pH than those lakes located further inland. Leptodiaptomus tyrrelli was common in these coastal lakes. Canonical correspondence analysis revealed statistically significant relationships between copepod distributions and conductivity, dissolved organic carbon and pH. These three variables accounted for 70% of the variation in the species' distribution.     

Tracing lake trophic history with a chironomid–total phosphorus inference model

1. In the absence of historical water chemistry data, predictive biological indicator groups preserved in lake sediments can be employed to reconstruct the history of lake eutrophication. Diatoms are well established in this role, but to augment diatom‐based inferences of nutrient status we investigate the potential use of chironomid midges (Insecta: Chironomidae). 2. Canonical correspondence analysis (CCA) of modern chironomid assemblages in surface sediments from 44 lakes in the English Midlands and Wales, U.K., shows that five environmental variables (total phosphorus (TP), bottom dissolved oxygen, maximum lake depth, Secchi depth and surface water temperature) make a statistically significant (P < 0.05) contribution to explaining the variance in the chironomid data, of which TP makes the largest contribution (29%). 3. The relationship is used to develop a series of weighted averaging (WA) and partial least squares (PLS), (WA‐PLS) models to infer log₁₀TP. The models are evaluated by leave‐one‐out (jack‐knifing) cross‐validation. The simplest minimal adequate model is provided by WA with unweighted inverse deshrinking of root mean square error of prediction (RMSEP_jack=0.34 and r²_jack=0.60). 4. Using this model, the trophic history of Betton Pool, Shropshire, U.K., is reconstructed from the mid‐19th century to the present day and the results from the chironomid‐TP model are compared with inferences from a diatom‐TP model ( 13 ). Both reconstructions suggest that there was a gradual rise in TP since 1850 AD until about 1974, followed by a more pronounced and rapid increase that has continued until the present. Inferred TP values from the WA chironomid inference model agree with diatom‐inferred values. 5. The study demonstrates that fossil chironomid assemblages can be used to investigate quantitatively the trophic history of lakes.     

Limnological characteristics of lakes located across arctic treeline in northern Russia

Limnological data (e.g., water chemistry, lakewater temperature, vegetation zone and degree of human impact) were collected from lakes spanning the Russian arctic treeline in three regions: on the Taimyr Peninsula and near the mouth of the Lena River, both in central arctic Siberia, and near the mouth of the Pechora River, western arctic Russia. Pearson correlation and canonical variates analyses revealed similar environmental gradients in all three regions. Variables expressing ionic composition of the water (i.e., cations, anions, dissolved inorganic carbon and conductivity) were highly intercorrelated, as were nutrients, chlorophyll a, particulate organic matter and metal (i.e., Fe and Mn) concentrations. Lakewater transparency was related to water colour (i.e., Fe, Mn and dissolved organic carbon) and productivity. Regional differences among the lakes were strong and appeared to reflect differences in geology, hydrology and human impact. For example, Na and Cl concentrations were related to proximity to the ocean in the Lena and Pechora River regions but not in the more inland Taimyr region. Extensive mining and smelting at Norilsk, on the Taimyr Peninsula, has apparently resulted in elevated major ion and metal concentrations in lakes closer to the city. Surface water temperatures, nutrients, and related variables were particularly useful for distinguishing lakes in different vegetation biomes. Forest lakes were typically warmer, with slightly elevated concentrations of dissolved organic carbon (DOC). Lakes in the forest–tundra zone often had higher concentrations of particulate organic matter, Fe and Mn. Tundra lakes were characterized by low nutrient and DOC concentrations. These data will facilitate the development of models that predict the outcome of future climatic change on arctic and subarctic aquatic ecosystems, as well as provide baseline data for future limnological studies in these remote regions.     

Distribution of cladoceran assemblages across environmental gradients in Nova Scotia (Canada) lakes

Cladocera sub-fossils have widely been recognized as useful environmental indicators. Nevertheless, investigations into the distribution and environmental controls on Cladocera are scarce in North America, hindering their use in paleolimnological studies. Here, we examine cladoceran assemblage and size structure from the surface sediments of 49 soft-water Nova Scotia (Canada) lakes to provide ecological data on this key taxonomic group, the first study of its kind for the northern Atlantic coast of North America. We found that Nova Scotia lakes contain a diverse chydorid assemblage, but were generally dominated by the smaller pelagic herbivore Eubosmina longispina. Daphnia, a larger pelagic herbivore, was relatively uncommon in these lakes. Redundancy analyses (RDA) identified maximum lake depth and dissolved organic carbon (DOC) as the environmental variables that best explained the structuring of these cladoceran assemblages. Generalized linear models were then used to better characterize the ecological associations for individual taxa. Body size of the bosminids in these lakes was significantly correlated only to total nitrogen (TN), with larger bosminids in lakes with lower TN values. Bosminid mucro length, an indication of invertebrate and fish predation pressure, was significantly related to TN, maximum lake depth, and lake surface area. The ecological information provided by this study should assist the interpretation of paleolimnological assessments of environmental change in these and other similar lakes.     

Browning affects pelagic productivity in northern lakes by surface water warming and carbon fertilization

Global change impacts important environmental drivers for pelagic gross primary production (GPP) in northern lakes, such as temperature, light, nutrient, and inorganic carbon availability. Separate and/or synergistic impacts of these environmental drivers on pelagic GPP remain largely unresolved. Here, we assess key drivers of pelagic GPP by combining detailed depth profiles of summer pelagic GPP with environmental and climatic data across 45 small and shallow lakes across northern Sweden (20 boreal, 6 subarctic, and 19 arctic lakes). We found that across lakes summer pelagic GPP was strongest associated with lake water temperatures, lake carbon dioxide (CO₂) concentrations impacted by lake water pH, and further moderated by dissolved organic carbon (DOC) concentrations influencing light and nutrient conditions. We further used this dataset to assess the extent of additional DOC-induced warming of epilimnia (here named internal warming), which was especially pronounced in shallow lakes (decreasing 0.96°C for every decreasing m in average lake depth) and increased with higher concentrations of DOC. Additionally, the total pools and relative proportion of dissolved inorganic carbon and DOC, further influenced pelagic GPP with drivers differing slightly among the boreal, subarctic and Arctic biomes. Our study provides novel insights in that global change affects pelagic GPP in northern lakes not only by modifying the organic carbon cycle and light and nutrient conditions, but also through modifications of inorganic carbon supply and temperature. Considering the large-scale impacts and similarities of global warming, browning and recovery from acidification of lakes at higher latitudes throughout the northern hemisphere, these changes are likely to operate on a global scale.     

DIATOM ASSEMBLAGES AS INDICATORS OF LAKE TROPHIC STATUS IN SOUTHEASTERN ONTARIO LAKES1

Canonical correspondence analysis (CCA) was used to explore the relationship between measured environmental variables and surficial diatom (Bacillariophyceae) assemblages in alkaline lakes from southeastern Ontario. Total nitrogen (TN), watershed area, alkalinity, and maximum depth each explain significant (P ≤ 0.05) directions of variance in the distribution of diatom taxa. TN was highly correlated to total phosphorus (TP) (r = 0.92), chlorophyll a (r = 0.86), and Secchi depth (r =0.77). When a series of CCAs were run with the first axis constrained to each of these variables in turn, the ratio of the eigenvalue of the first axis to that of the second axis (λ1/λ2) was highest for TN, indicating that TN best explained the distribution of the diatom assemblages in this set of lakes. Furthermore, results of Monte Carlo permutation tests indicated that these four variables did not act independently on the diatom assemblages. Therefore, TN was selected to represent these four closely related variables to infer lake trophic status. Weighted-averaging regression and calibration (with classical deshrinking) were used to develop transfer functions to infer TN from the relative abundances of 83 diatom taxa recovered from the surficial sediments of 51 lakes. There was a good correlation between diatom-inferred TN concentrations and measured TN concentrations (r²= 0.75, n = 51). The weighted-averaging regression and calibration model was used to infer lake trophic status (represented by TN) from diatom assemblages presented in the sediments from Little Round Lake, Ontario. These data were used in conjunction with historical land-use data in order to quantify the sequence and extent of nutrient enrichment related to human activity in the watershed area.     

BENTHIC DIATOM AUTECOLOGY AND INFERENCE MODEL DEVELOPMENT FROM THE CANADIAN HIGH ARCTIC ARCHIPELAGO1

Diatom assemblages were analyzed from 64 lakes and ponds from Alert, Ellesmere Island and Mould Bay, Prince Patrick Island in the Canadian High Arctic Archipelago. Diverse water chemistry conditions and diatom communities were present in these sites. Small benthic taxa typically dominated diatom communities; however, assemblages were markedly different between Alert and Mould Bay sites in response to disparate water chemistry characteristics in the two regions. The most abundant taxa belonged to the genera Navicula, Cymbella, Achnanthes, Nitzschia, and Pinnularia. Canonical correspondence analysis indicated that pH, specific conductivity, dissolved organic carbon, and total phosphorus were the most important limnological variables in determining species composition. Diatom inference models were developed for pH, specific conductivity, and dissolved organic carbon using weighted averaging and weighted averaging partial least squares techniques; these had root mean square error of prediction/r²_boot values of 0.40/0.77, 0.28/0.70, and 0.24/0.55, respectively. These models are applicable to sites with large ranges of taxonomic and limnological variation and will allow the reconstruction of past changes of climate‐related limnological parameters from biostratigraphic records in future paleolimnological studies.     

Light and nutrient control phytoplankton biomass responses to global change in northern lakes

Global change affects terrestrial loadings of colored dissolved organic carbon (DOC) and nutrients to northern lakes. Still, little is known about how phytoplankton respond to changes in light and nutrient availability across gradients in lake DOC. In this study, we used results from whole‐lake studies in northern Sweden to show that annual mean phytoplankton biomass expressed unimodal curved relationships across lake DOC gradients, peaking at threshold DOC levels of around 11 mg/L. Whole‐lake single nutrient enrichment in selected lakes caused elevated biomass, with most pronounced effect at the threshold DOC level. These patterns give support to the suggested dual control by DOC on phytoplankton via nutrient (positively) and light (negatively) availability and imply that the lakes' location along the DOC axis is critical in determining to what extent phytoplankton respond to changes in DOC and/or nutrient loadings. By using data from the large Swedish Lake Monitoring Survey, we further estimated that 80% of northern Swedish lakes are below the DOC threshold, potentially experiencing increased phytoplankton biomass with browning alone, and/or combined with nutrient enrichment. The results support the previous model results on effects of browning and eutrophication on lake phytoplankton, and provide important understanding of how northern lakes may respond to future global changes.     

Spatial and temporal patterns of light attenuation among lakes of the Mackenzie Delta

SUMMARY 1. The seasonal dynamics of light attenuation, and the relative roles of total suspended solids (TSS), dissolved organic carbon (DOC) and chlorophyll as light attenuators among two sets of lakes in the Mackenzie Delta, were assessed during the open‐water periods of 1998 and 1999. 2. The first set consisted of 40 spatially discrete lakes where the frequency of flooding with river water was controlled by sill height (‘sill‐set lakes’). The second set consisted of a chain of six lakes connected to a main river channel (frequently flooded, all with same frequency), but where riverine influence was controlled by the distance from the channel connection point (‘chain‐set lakes’). 3. As the flooding frequency of lakes decreased (sill‐set), and as the distance from the channel connection point increased (chain‐set), lake water became increasingly transparent and the stability (decreasing temporal variability) of underwater light increased. 4. The effect of flooding on transparency was greater in years with a high minimum summer water level. However, the effect of river flooding on lake water transparency was damped more by an increase in the frequency and duration of flooding than by an increase in distance from the channel connection point. 5. The index of scattering was linearly related to TSS over the common range of concentrations in both sets of lakes. The specific attenuation coefficient for TSS (and scattering) increased substantially from the most turbid to the most transparent waters. 6. During the summer, DOC provided an approximate index of water colour in the sill‐set lakes but not in the chain‐set lakes, where the gradient of DOC ran counter to the gradient of water colour. The specific attenuation coefficient for water colour was roughly constant among both sets of lakes. 7. Calculations of partial attenuation show that, during the spring flood peak, TSS is the dominant attenuator among most lakes, other than those with high sills or positioned far from channel connection points. During the lengthy summer period of open water, however, water colour appeared to be the most important light attenuator among almost all of the lakes in the central delta, with chlorophyll a of only minor importance. 8. Lakes of the Mackenzie Delta may be quite sensitive to changes in climate and ultraviolet‐b (UV‐b) radiation in the circumpolar arctic because of the role of DOC as an attenuator of photosynthetically active radiation and UV‐b irradiance and as an energy source for microbial foodwebs in this system.     

TRACKING LONG-TERM CHANGES IN CLIMATE USING ALGAL INDICATORS IN LAKE SEDIMENTS

Interest in climate change research has taken on new relevance with the realization that human activities, such as the accelerated release of the so‐called greenhouse gases, may be altering the thermal properties of our atmosphere. Important social, economic, and scientific questions include the following. Is climate changing? If so, can these changes be related to human activities? Are episodes of extreme weather, such as droughts or hurricanes, increasing in frequency? Long‐term meteorological data, on broad spatial and temporal scales, are needed to answer these questions. Unfortunately, such data were never gathered; therefore, indirect proxy methods must be used to infer past climatic trends. A relatively untapped source of paleoclimate data is based on hindcasting past climatic trends using the environmental optima and tolerances of algae (especially diatoms) preserved in lake sediment profiles. Paleophycologists have used two approaches. Although still controversial, attempts have been made to directly infer climatic variables, such as temperature, from past algal assemblages. The main assumption with these types of analyses is that species composition is either directly related to temperature or that algal assemblages are related to some variable linearly related to temperature. The second more commonly used approach is to infer a limnological variable (e.g. water chemistry, lake ice cover, etc.) that is related to climate. Although paleolimnological approaches are broadly similar across climatic regions, the environmental gradients that paleophycologists track can be very different. For example, climatic inferences in polar regions have focused on past lake ice conditions, whereas in lakes near arctic treeline ecotones, paleophycologists have developed methods to infer past lakewater‐dissolved organic carbon, because this variable has been linked to the density of coniferous trees in a drainage basin. In closed‐basin lakes in arid and semiarid regions, past lakewater salinity, which can be robustly reconstructed from fossil algal assemblages, is closely tied to the balance of evaporation and precipitation (i.e. drought frequency). Some recent examples of paleophycolgical work include the documentation of striking environmental changes in high arctic environments in the 19th century believed to be related to climate warming. Meanwhile, diatom‐based reconstructions of salinity (e.g. the Great Plains of North America and Africa) have revealed prolonged periods of droughts over the last few millennia that have greatly exceeded those recorded during recent times. Marked climatic variability that is outside the range captured by the instrumental record has a strong bearing on sustainability of human societies. Only with a long‐term perspective can we understand natural climatic variability and the potential influences of human activities on climate and thereby increase our ability to understand future climate.     

Climate‐related changes of soil characteristics affect bacterial community composition and function of high altitude and latitude lakes

Lakes at high altitude and latitude are typically unproductive ecosystems where external factors outweigh the relative importance of in‐lake processes, making them ideal sentinels of climate change. Climate change is inducing upward vegetation shifts at high altitude and latitude regions that translate into changes in the pools of soil organic matter. Upon mobilization, this allochthonous organic matter may rapidly alter the composition and function of lake bacterial communities. Here, we experimentally simulate this potential climate‐change effect by exposing bacterioplankton of two lakes located above the treeline, one in the Alps and one in the subarctic region, to soil organic matter from below and above the treeline. Changes in bacterial community composition, diversity and function were followed for 72 h. In the subarctic lake, soil organic matter from below the treeline reduced bulk and taxon‐specific phosphorus uptake, indicating that bacterial phosphorus limitation was alleviated compared to organic matter from above the treeline. These effects were less pronounced in the alpine lake, suggesting that soil properties (phosphorus and dissolved organic carbon availability) and water temperature further shaped the magnitude of response. The rapid bacterial succession observed in both lakes indicates that certain taxa directly benefited from soil sources. Accordingly, the substrate uptake profiles of initially rare bacteria (copiotrophs) indicated that they are one of the main actors cycling soil‐derived carbon and phosphorus. Our work suggests that climate‐induced changes in soil characteristics affect bacterioplankton community structure and function, and in turn, the cycling of carbon and phosphorus in high altitude and latitude aquatic ecosystems.     

Small lakes dominate a random sample of regional lake characteristics

1. Lakes are a prominent feature of the Northern Highland Lake District (NHLD) of Wisconsin, covering 13% of the landscape. Summarising the physical, chemical, or biological nature of NHLD lakes at a regional scale requires a representative sample of the full size distributions of lakes. In this study, we selected at random 168 lakes from the full size distribution of lakes in the NHLD and sampled each lake for a broad suite of limnological variables.
2. Most lakes were small. The median lake area was 1.1 ha, however, half of the surface area of water was in a relatively small number of lakes larger than 162 ha. Smaller lakes tended to be low in dissolved inorganic carbon (DIC) and high in dissolved organic carbon (DOC). Inclusion of small lakes (<4 ha) in the survey resulted in an acid neutralising capacity (ANC) median (76.5  μ Eq L⁻¹) much lower than previous estimates, and a DOC median (10.1 mg L⁻¹) about 50% higher than it would have been without the smaller lakes. Unlike DOC, total P tended to be evenly distributed across lake sizes.
3. The implications of these findings are that regional summaries of lake characteristics for the NHLD are influenced by the inclusion of small lakes in the sample, even though most of the water surface area is in lakes larger than 162 ha. Excluding small lakes introduces bias in the estimates of organic carbon and inorganic carbon values, for example. Similar biases may be introduced for lake characteristics at the global scale if small lakes are not sampled, because the size distribution of lakes globally is dominated in number by small lakes.