Storm impacts on phytoplankton community dynamics in lakes

In many regions across the globe, extreme weather events such as storms have increased in frequency, intensity, and duration due to climate change. Ecological theory predicts that such extreme events should have large impacts on ecosystem structure and function. High winds and precipitation associated with storms can affect lakes via short‐term runoff events from watersheds and physical mixing of the water column. In addition, lakes connected to rivers and streams will also experience flushing due to high flow rates. Although we have a well‐developed understanding of how wind and precipitation events can alter lake physical processes and some aspects of biogeochemical cycling, our mechanistic understanding of the emergent responses of phytoplankton communities is poor. Here we provide a comprehensive synthesis that identifies how storms interact with lake and watershed attributes and their antecedent conditions to generate changes in lake physical and chemical environments. Such changes can restructure phytoplankton communities and their dynamics, as well as result in altered ecological function (e.g., carbon, nutrient and energy cycling) in the short‐ and long‐term. We summarize the current understanding of storm‐induced phytoplankton dynamics, identify knowledge gaps with a systematic review of the literature, and suggest future research directions across a gradient of lake types and environmental conditions.     

Climate and human water use diminish wetland networks supporting continental waterbird migration

Migrating waterbirds moving between upper and lower latitudinal breeding and wintering grounds rely on a limited network of endorheic lakes and wetlands when crossing arid continental interiors. Recent drying of global endorheic water stores raises concerns over deteriorating migratory pathways, yet few studies have considered these effects at the scale of continental flyways. Here, we investigate the resiliency of waterbird migration networks across western North America by reconstructing long‐term patterns (1984–2018) of terminal lake and wetland surface water area in 26 endorheic watersheds. Findings were partitioned regionally by snowmelt‐ and monsoon‐driven hydrologies and combined with climate and human water‐use data to determine their importance in predicting surface water trends. Nonlinear patterns of lake and wetland drying were apparent along latitudinal flyway gradients. Pervasive surface water declines were prevalent in northern snowmelt watersheds (lakes ?27%, wetlands ?47%) while largely stable in monsoonal watersheds to the south (lakes ?13%, wetlands +8%). Monsoonal watersheds represented a smaller proportion of total lake and wetland area, but their distribution and frequency of change within highly arid regions of the continental flyway increased their value to migratory waterbirds. Irrigated agriculture and increasing evaporative demands were the most important drivers of surface water declines. Underlying agricultural and wetland relationships however were more complex. Approximately 7% of irrigated lands linked to flood irrigation and water storage practices supported 61% of all wetland inundation in snowmelt watersheds. In monsoonal watersheds, small earthen dams, meant to capture surface runoff for livestock watering, were a major component of wetland resources (67%) that supported networks of isolated wetlands surrounding endorheic lakes. Ecological trends and human impacts identified herein underscore the importance of assessing flyway‐scale change as our model depictions likely reflect new and emerging bottlenecks to continental migration.     

Patterns of fish species richness in China's lakes

Aim To document the patterns of fish species richness and their possible causes in China's lakes at regional and national scales. Location Lakes across China. Methods We compiled data of fish species richness, limnological characteristics and climatic variables for 109 lakes across five regions of China: East region, Northeast region, Southwest region, North‐Northwest region, and the Tibetan Plateau. Correlation analyses, regression models and a general linear model were used to explore the patterns of fish species richness. Results At the national scale, lake altitude, energy availability (potential evapotranspiration, PET) and lake area explained 79.6% of the total variation of the lake fish species richness. The determinants of the fish richness pattern varied among physiographic regions. Lake area was the strongest predictor of fish species richness in the East and Southwest lakes, accounting for 22.2% and 82.9% of the variation, respectively. Annual PET explained 68.7% of the variation of fish richness in the Northeast lakes. Maximum depth, mineralization degree, and lake area explained 45.5% of the fish variation in the lakes of the North‐Northwest region. On the Tibetan Plateau, lake altitude was the first predictor variable, interpreting 32.2% of the variation. Main conclusions Lake altitude was the most important factor explaining the variation of fish species richness across China's lakes, and accounted for 74.5% of the variation. This may stem in part from the fact that the lakes investigated in our study span the largest altitudinal range anywhere in the world. The effects of the lake altitude on fish species richness can be separated into direct and indirect aspects due to its collinearity with PET. We also found that the fish diversity and its determinants were scale‐dependent. Fish species richness was probably energy‐determined in the cold region, while it was best predicted by the lake area in the relatively geologically old region. The independent variables we used only explained a small fraction of the variations in the lake fish species richness in East China and the Tibetan Plateau, which may be due to the effects of human activity and historical events, respectively.     

Water‐level fluctuations regulate the structure and functioning of natural lakes

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The Influence of Hydrologic Residence Time on Lake Carbon Cycling Dynamics Following Extreme Precipitation Events

The frequency and magnitude of extreme events are expected to increase in the future, yet little is known about effects of such events on ecosystem structure and function. We examined how extreme precipitation events affect exports of terrestrial dissolved organic carbon (t-DOC) from watersheds to lakes as well as in-lake heterotrophy in three north-temperate lakes. Extreme precipitation events induced large influxes of t-DOC to our lakes, accounting for 45–58% of the seasonal t-DOC load. These large influxes of t-DOC influenced lake metabolism, resulting in lake net heterotrophy following 67% of the extreme precipitation events across all lakes. Hydrologic residence time (HRT) was negatively related to t-DOC load and heterotrophy; lakes with short HRT had higher t-DOC loads and greater net heterotrophy. The fraction of t-DOC mineralized within each lake following extreme precipitation events generally exhibited a positive relationship with lake HRT, similar to the previous studies of fractions mineralized at annual and supra-annual time scales. Event-associated turnover rate of t-DOC was higher than what is typically reported from laboratory studies and modeling exercises and was also negatively related to lake HRT. This study demonstrates that extreme precipitation events are ‘hot moments’ of carbon load, export, and turnover in lakes and that lake-specific characteristics (for example, HRT) interact with climatic patterns to set rates of important lake carbon fluxes.     

Acid rain recovery may help to mitigate the impacts of climate change on thermally sensitive fish in lakes across eastern North America

From the 1970s to 1990s, more stringent air quality regulations were implemented across North America and Europe to reduce chemical emissions that contribute to acid rain. Surface water pH slowly increased during the following decades, but biological recovery lagged behind chemical recovery. Fortunately, this situation is changing. In the past few years, northeastern US fish populations have begun to recover in lakes that were historically incapable of sustaining wild fish due to acidic conditions. As lake ecosystems across the eastern United States recover from acid deposition, the stress to the most susceptible populations of native coldwater fish appears to be shifting from acidification effects to thermal impacts associated with changing climate. Extreme summer temperature events – which are expected to occur with increasing frequency in the coming century – can stress and ultimately kill native coldwater fish in lakes where thermal stratification is absent or highly limited. Based on data from northeastern North America, we argue that recovery from acid deposition has the potential to improve the resilience of coldwater fish populations in some lakes to impacts of climate change. This will occur as the amount of dissolved organic carbon (DOC) in the water increases with increasing lake pH. Increased DOC will reduce water clarity and lead to shallower and more persistent lake thermoclines that can provide larger areas of coldwater thermal refuge habitat. Recovery from acidification will not eliminate the threat of climate change to coldwater fish, but secondary effects of acid recovery may improve the resistance of coldwater fish populations in lakes to the effects of elevated summer temperatures in historically acidified ecosystems. This analysis highlights the importance of considering the legacy of past ecosystem impacts and how recovery or persistence of those effects may interact with climate change impacts on biota in the coming decades.     

中国湖泊水量调节能力及其动态变化

湖泊水量调节是指湖泊生态系统通过洪水蓄积和径流补给实现水资源的再分配,进而减轻洪旱灾害。在围湖造田、退田还湖和气候变化影响下,科学评估我国湖泊水量调节能力现状及变化情况,是实现湖泊洪水调蓄功能和水资源调节功能评价的重要基础。基于全国湖泊调查数据,将全国划分为5个评价区,探讨了面向全国尺度的湖泊水量调节能力评价方法,在此基础上对全国湖泊的水量调节能力及其动态变化进行了分析评价。结果表明:(1)我国湖泊(面积1 km2)水量调节总量为1500.02亿m3,其中东部平原区和青藏高原区的调节量最高,分别占全国总量的44.46%和43.63%;(2)湖泊调节水量的效能以东部平原区最高(310.19万m3/km2),其次是东北平原与山区(191.19万m3/km2),围湖造田/退田还湖将导致该区湖泊水量调节能力明显削弱/增强;(3)近几十年来,我国湖泊水量调节能力呈小幅增长(增长量9.76亿m3,增幅0.65%),5个评价区仅蒙新高原区湖泊水量调节能力明显削弱,其余区域均呈不同程度增强,且以东部平原区增加最多,东北平原与山区增幅最大。研究可以为评估我国湖泊生态系统水量调节能力、分析土地利用变化对流域洪水调蓄和水资源调节功能的影响提供参考。     

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.     

Biological invasion by a benthivorous fish reduced the cover and species richness of aquatic plants in most lakes of a large North American ecoregion

Biological invasions are projected to be the main driver of biodiversity and ecosystem function loss in lakes in the 21st century. However, the extent of these future losses is difficult to quantify because most invasions are recent and confounded by other stressors. In this study, we quantified the outcome of a century‐old invasion, the introduction of common carp to North America, to illustrate potential consequences of introducing non‐native ecosystem engineers to lakes worldwide. We used the decline in aquatic plant richness and cover as an index of ecological impact across three ecoregions: Great Plains, Eastern Temperate Forests and Northern Forests. Using whole‐lake manipulations, we demonstrated that both submersed plant cover and richness declined exponentially as carp biomass increased such that plant cover was reduced to <10% and species richness was halved in lakes in which carp biomass exceeded 190 kg ha^?1. Using catch rates amassed from 2000+ lakes, we showed that carp exceeded this biomass level in 70.6% of Great Plains lakes and 23.3% of Eastern Temperate Forests lakes, but 0% of Northern Forests lakes. Using model selection analysis, we showed that carp was a key driver of plant species richness along with Secchi depth, lake area and human development of lake watersheds. Model parameters showed that carp reduced species richness to a similar degree across lakes of various Secchi depths and surface areas. In regions dominated by carp (e.g., Great Plains), carp had a stronger impact on plant richness than human watershed development. Overall, our analysis shows that the introduction of common carp played a key role in driving a severe reduction in plant cover and richness in a majority of Great Plains lakes and a large portion of Eastern Temperate Forests lakes in North America.     

Carbon accumulation and sequestration of lakes in China during the Holocene

Understanding the responses of lake systems to past climate change and human activity is critical for assessing and predicting the fate of lake carbon (C) in the future. In this study, we synthesized records of the sediment accumulation from 82 lakes and of C sequestration from 58 lakes with direct organic C measurements throughout China. We also identified the controlling factors of the long‐term sediment and C accumulation dynamics in these lakes during the past 12 ka (1 ka = 1000 cal yr BP). Our results indicated an overall increasing trend of sediment and C accumulation since 12 ka, with an accumulation peak in the last couple of millennia for lakes in China, corresponding to terrestrial organic matter input due to land‐use change. The Holocene lake sediment accumulation rate (SAR) and C accumulation rate (CAR) averaged (mean ± SE) 0.47 ± 0.05 mm yr^?1 and 7.7 ± 1.4 g C m^?2 yr^?1 in China, respectively, comparable to the previous estimates for boreal and temperate regions. The SAR for lakes in the East Plain of subtropical China (1.05 ± 0.28 mm yr^?1) was higher than those in other regions (P < 0.05). However, CAR did not vary significantly among regions. Overall, the variability and history of climate and anthropogenic interference regulated the temporal and spatial dynamics of sediment and C sequestration for lakes in China. We estimated the total amount of C burial in lakes of China as 8.0 ± 1.0 Pg C. This first estimation of total C storage and dynamics in lakes of China confirms the importance of lakes in land C budget in monsoon‐influenced regions.     

Loss of trophic complexity in saline prairie lakes as indicated by stable-isotope based community-metrics

Variations in climate, watershed characteristics and lake-internal processes often result in a large variability of food-web complexity in lake ecosystems. Some of the largest ranges in these environmental parameters can be found in lakes across the northern Great Plains as they are characterized by extreme gradients in respect to lake morphometry and water chemistry, with individual parameters often varying over several orders of magnitude. To evaluate the effects of environmental conditions on trophic complexity in prairie lake food-webs, we analyzed carbon and nitrogen stable isotopes of fishes, zooplankton and littoral macroinvertebrates in 20 lakes across southern Saskatchewan. Our two-year study identified very diverse patterns of trophic complexity, with was predominantly associated with among-lake differences. Small but significant temporal effects were also detected, which were predominantly associated with changes in productivity. The most influential parameters related to changes in trophic complexity among lakes were salinity, complexity of fish assemblage, and indicators of productivity (e.g. nutrients, Chl a). Generally, trophic diversity, number of trophic levels, and trophic redundancy were highest in productive freshwater lakes with diverse fish communities. Surprisingly, mesosaline lakes that were characterized by very low or no predation pressure from fishes were not colonized by invertebrate predators as it is often the case in boreal systems; instead, trophic complexity was further reduced. Together, prairie lake food-webs appear to be highly sensitive to changes in salinity and the loss of piscivorous fishes, making freshwater and mesosaline lakes most vulnerable to the impacts of climate variability. This is particularly important as global circulation models predict future climate warming to have disproportionate negative impacts on hydrologic conditions in this area.     

Species richness and distribution of eastern Australian lake chironomids and chaoborids

1. Chironomids and chaoborids were collected across eastern Australia and Tasmania in dune, glacial, sinkhole and maar lakes. Based on sampling exuviae from these relatively undisturbed freshwater lakes, we observed that species richness on the Australian continent was substantially greater than previously reported, and challenge the long‐standing view that chironomid species richness is depauperate in Australian lakes, compared with the northern hemisphere. 2. While chironomid species richness was equivalent across the four geographical regions sampled (tropical northern Queensland, Fraser Island, south‐eastern mainland Australia and Tasmania), there were only five ‘cosmopolitan’ species found across all regions. In general, species distributions were more closely associated with geographical region than with lake characteristics, and there were species assemblage differences among biogeographical regions. More than half of the 134 identified species were restricted to a single geographical region. Overall, Tasmanian lakes had the highest proportion of locally endemic species. 3. Latitude and altitude more strongly influenced species assemblages than did lake chemistry, although species richness sometimes varied among lake geomorphic types within a region.     

Nutrient limitation of periphyton growth in arctic lakes in south-west Greenland

Many arctic lakes are oligotrophic systems where phototrophic growth is controlled by nutrient supply. Recent anthropogenic nutrient loading is associated with biological and/or physico-chemical change in several lakes across the arctic. Shifts in nutrient limitation (nitrogen (N), phosphorus (P), or N + P) and associated effects on the growth and composition of algal communities are commonly reported. The Kangerlussuaq region of south-west Greenland forms a major lake district which is considered to receive little direct anthropogenic disturbance. However, long-range transport of pollutant N is now reaching Greenland, and it was hypothesised that a precipitation gradient from the inland ice sheet margin to the coast might also deliver increased N deposition. In situ nutrient bioassays were deployed in three lakes across the region: ice sheet margin, inland (close to Kangerlussuaq) and the coast (near Sisimiut), to determine nutrient limitation of lakes and investigate any effects of nutrients on periphyton growth and community composition. Nutrient limitation differed amongst lakes: N limitation (ice sheet margin), N and P limitation (inland) and N + P co-limitation (coast). Factors including variation in N supply, ice phenology, seasonal algal succession, community structure and physical limnology are explored as mechanisms to explain differences amongst lakes. Nutrient limitation of arctic lakes and associated ecological impacts are highly variable, even across small geographic areas. In this highly sensitive region, future environmental change scenarios carry a strong risk of significantly altering nutrient limitation; in turn, potentially severely impacting lake structure and function.     

Microbiology of Lonar Lake and other soda lakes

Soda lakes are saline and alkaline ecosystems that are believed to have existed throughout the geological record of Earth. They are widely distributed across the globe, but are highly abundant in terrestrial biomes such as deserts and steppes and in geologically interesting regions such as the East African Rift valley. The unusual geochemistry of these lakes supports the growth of an impressive array of microorganisms that are of ecological and economic importance. Haloalkaliphilic Bacteria and Archaea belonging to all major trophic groups have been described from many soda lakes, including lakes with exceptionally high levels of heavy metals. Lonar Lake is a soda lake that is centered at an unusual meteorite impact structure in the Deccan basalts in India and its key physicochemical and microbiological characteristics are highlighted in this article. The occurrence of diverse functional groups of microbes, such as methanogens, methanotrophs, phototrophs, denitrifiers, sulfur oxidizers, sulfate reducers and syntrophs in soda lakes, suggests that these habitats harbor complex microbial food webs that (a) interconnect various biological cycles via redox coupling and (b) impact on the production and consumption of greenhouse gases. Soda lake microorganisms harbor several biotechnologically relevant enzymes and biomolecules (for example, cellulases, amylases, ectoine) and there is the need to augment bioprospecting efforts in soda lake environments with new integrated approaches. Importantly, some saline and alkaline lake ecosystems around the world need to be protected from anthropogenic pressures that threaten their long-term existence.     

Lake regionalization and diatom metacommunity structuring in tropical South America

Lakes and their topological distribution across Earth's surface impose ecological and evolutionary constraints on aquatic metacommunities. In this study, we group similar lake ecosystems as metacommunity units influencing diatom community structure. We assembled a database of 195 lakes from the tropical Andes and adjacent lowlands (8°N–30°S and 58–79°W) with associated environmental predictors to examine diatom metacommunity patterns at two different levels: taxon and functional (deconstructed species matrix by ecological guilds). We also derived spatial variables that inherently assessed the relative role of dispersal. Using complementary multivariate statistical techniques (principal component analysis, cluster analysis, nonmetric multidimensional scaling, Procrustes, variance partitioning), we examined diatom–environment relationships among different lake habitats (sediment surface, periphyton, and plankton) and partitioned community variation to evaluate the influence of niche‐ and dispersal‐based assembly processes in diatom metacommunity structure across lake clusters. The results showed a significant association between geographic clusters of lakes based on gradients of climate and landscape configuration and diatom assemblages. Six lake clusters distributed along a latitudinal gradient were identified as functional metacommunity units for diatom communities. Variance partitioning revealed that dispersal mechanisms were a major contributor to diatom metacommunity structure, but in a highly context‐dependent fashion across lake clusters. In the Andean Altiplano and adjacent lowlands of Bolivia, diatom metacommunities are niche assembled but constrained by either dispersal limitation or mass effects, resulting from area, environmental heterogeneity, and ecological guild relationships. Topographic heterogeneity played an important role in structuring planktic diatom metacommunities. We emphasize the value of a guild‐based metacommunity model linked to dispersal for elucidating mechanisms underlying latitudinal gradients in distribution. Our findings reveal the importance of shifts in ecological drivers across climatic and physiographically distinct lake clusters, providing a basis for comparison of broad‐scale community gradients in lake‐rich regions elsewhere. This may help guide future research to explore evolutionary constraints on the rich Neotropical benthic diatom species pool.     

2000-2014年我国西北地区湖泊面积的时空变化

以2000-2014年Landsat TM、ETM+以及OLI影像为基础,通过人工目视解译方法提取我国西北地区111个面积10.0 km²以上湖泊矢量数据,并对湖泊面积变化总体趋势、空间分异特征以及成因进行分析。结果表明：（1）2000-2014年期间,西北地区湖泊总面积呈整体增加的态势,由1.58×10⁴ km²增加为1.74×10⁴ km²。（2）在研究时段内,我国西北地区湖泊面积在整体增加的同时也表现出明显的空间差异。根据湖泊面积变化不同,研究区可以被划分为稳定、扩张、萎缩3种类型和5个区域：新疆维吾尔自治区北部与青海省南部为湖泊面积稳定区;塔里木盆地、昆仑山北麓、阿尔金山、柴达木盆地、可可西里地区、祁连山南麓至三江源北部大片区域为湖泊面积扩张区;由准噶尔盆地和吐鲁番盆地组成的北疆中部地区和喀喇昆仑山北坡山区为湖泊萎缩区。（3）受研究区复杂的自然人文环境影响,在不同区域引起湖泊面积变化的原因不尽相同：总体而言,降水增加、气温上升导致的冰川融水增加和冻土水分释放是湖泊面积呈现扩张趋势地区的最主要原因;在湖泊面积稳定区,湖泊所处的地形条件以及湖泊与河流间强烈的水量交换起着关键作用;日益增强的人类活动是湖泊萎缩区的主要影响要素,但在喀喇昆仑山北坡地区,冰川物质平衡状态的变化也可能发挥着重要作用。     

Identifying resilience mechanisms to recurrent ecosystem perturbations

The complex nature of ecological systems limits the unambiguous determination of mechanisms that drive resilience to natural disturbance or anthropogenic stress. Using eight-year time series data from boreal lakes with and without bloom formation of an invasive alga (Gonyostomum semen, Raphidophyceae), we studied resilience of phytoplankton communities in relation to recurring bloom impacts. We first characterized phytoplankton community dynamics in both lake types using univariate metrics of community structure (evenness, species richness, biovolume and Simpson diversity). All metrics, except species richness, were substantially altered and showed an inherent stronger variability in bloom lakes relative to reference lakes. We assessed resilience mechanisms using a multivariate time series modelling technique. The models captured clear successional dynamics of the phytoplankton communities in all lakes, whereby different groups of species were substituted sequentially over the ice-free period. The models also identified that G. semen impacts in bloom lakes were only manifested within a single species group, not across species groups, highlighting the rapid renewal of the phytoplankton communities upon bloom collapse. These results provide empirical support of the cross-scale resilience model. Cross-scale resilience could provide an explanation for the paradox that similar species richnesses are seen in bloom-forming lakes and reference lakes despite the clear difference between the community features of the two different sets of lakes investigated.     

Quantifying the potential effects of climate change and the invasion of smallmouth bass on native lake trout populations across Canadian lakes

Climate change and invasive species are two stressors that should have large impacts on native species in aquatic and terrestrial ecosystems. We quantify and integrate the effects of climate change and the establishment of an invasive species (smallmouth bass Micropterus dolomieu ) on native lake trout Salvelinus namaycush populations. We assembled a dataset of almost 22 000 Canadian lakes that contained information on fish communities, lake morphologies, and geography. We examined the pelagic-benthic and littoral forage fish community available to lake trout populations across three lake size classes in these aquatic ecosystems. Due to the decreased presence of alternate prey resources, lake trout populations residing in smaller lakes are more vulnerable to the effects of smallmouth bass establishment. A detailed spatially and temporally explicit approach to assess smallmouth bass invasion risk in Ontario lakes suggests that the number of Ontario lakes with vulnerable lake trout populations could increase from 118 (~1%) to 1612 (~20%) by 2050 following projected climate warming. In addition, we identified nearly 9700 lake trout populations in Canada threatened by 2100, by the potential range expansion of smallmouth bass. Our study provides an integration of two major stressors of ecosystems, namely climate change and invasive species, by considering climate-change scenarios, dispersal rates of invasive species, and inter-specific biotic interactions.     

Spatial Variation among Lakes within Landscapes: Ecological Organization along Lake Chains

Although limnologists have long been interested in regional patterns in lake attributes, only recently have they considered lakes connected and organized across the landscape, rather than as spatially independent entities. Here we explore the spatial organization of lake districts through the concept of landscape position, a concept that considers lakes longitudinally along gradients of geomorphology and hydrology. We analyzed long-term chemical and biological data from nine lake chains (lakes in a series connected through surface or groundwater flow) from seven lake districts of diverse hydrologic and geomorphic settings across North America. Spatial patterns in lake variables driven by landscape position were surprisingly common across lake districts and across a wide range of variables. On the other hand, temporal patterns of lake variables, quantified using synchrony, the degree to which pairs of lakes exhibit similar dynamics through time, related to landscape position only for lake chains with lake water residence times that spanned a wide range and were generally long (close to or greater than 1 year). Highest synchrony of lakes within a lake chain occurred when lakes had short water residence times. Our results from both the spatial and temporal analyses suggest that certain features of the landscape position concept are robust enough to span a wide range of seemingly disparate lake types. The strong spatial patterns observed in this analysis, and some unexplained patterns, suggest the need to further study these scales and to continue to view lake ecosystems spatially, longitudinally, and broadly across the landscape.     

Functional ecology and palaeolimnology: using cladoceran remains to reconstruct anthropogenic impact

The field of lake palaeoecology has undergone significant changes. Powerful quantitative techniques have been developed to investigate anthropogenic impacts on lakes. Inclusion of zooplankton and benthic chydorid cladocerans has provided previously unavailable information on the historical development of planktivorous fish populations, submerged macrophytes and lake production, and has been used to document exotic species introductions, rapid genetic evolution and human disturbance of lakes. In particular, new techniques now allow a more complete evaluation of changes in past and present trophic structure to be made, and provide insights on the rapid evolutionary responses of aquatic invertebrate communities to anthropogenic perturbation of lakes.