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AIM: Lakes in the Ecuadorean Andes span different altitudinal and climatic regions, from inter Andean plateau to the high‐elevation paramo, which differ in their historical evolution in the several centuries since the pioneering Humboldt expeditions. Here, we evaluate temporal and spatial patterns of change in diatom assemblages between historical (palaeolimnological) and modern times. LOCATION: Ecuadorean Andes METHODS: We compared historical (pre‐1850) and modern (2017) diatom assemblages from 21 lakes and determined the relative role of environmental (water chemistry and climate) and spatial factors (distance‐based Moran's eigenvectors maps) on both assemblages using non‐metric multidimensional scaling (NMDS) with environmental fitting. In addition, we used redundancy analysis (RDA) with variance partitioning to estimate the historical (measured using downcore assemblage composition) effects on modern diatom assemblages and identified diatom species that contributed most to dissimilarity between the two times. RESULTS: Diatom changes between the two time points were limited across the group of lakes, as indicated by the NMDS ordination. Variance partitioning indicated that modern diatom assemblages were affected by environmental and spatial effects, but with non‐significant effects of past diatom species composition. Ordination results showed that variables related to elevation and water chemistry affected both modern and historical diatom assemblages. Diatom species with the best fit on NMDS axes (i.e. >70%) were influenced by elevation and climatic variables. The most distinctive change between the two time periods was the higher relative abundance of planktic diatom species in top‐core assemblages of some lakes, but in a highly variable fashion across gradients of increased elevation and water depth. MAIN CONCLUSIONS: Landscape palaeolimnological analyses of varied Ecuadorean Andean lakes demonstrate both environmental and spatial controls on diatom metacommunities. The multi‐faceted ecological control of the altitudinal gradient on both historic and contemporary diatom assemblages suggests species sorting and dispersal constraints operating at centennial time‐scale. Although a few individual lakes show substantive change between the 1850s and today, the majority of lakes do not, and the analysis suggests the resilience of lakes at a regional scale. We emphasize the potential of diatom palaeolimnological approaches in biogeography to test ecologically relevant hypotheses of the mechanisms driving recent limnological change in high‐elevation tropical lakes.

The Yellowstone National Park ecosystem is a product of dynamic earth system processes, which have been of interest to scientists and the public since the park's discovery. Here, we outline the history of two successive generations of scientific collaboration in Yellowstone National Park. Early collaboration was spurred by the discovery of an unknown diatom species found in Yellowstone Lake. This prompted the first coring project in 1992 that described the morphological evolution of that species and the paleoenvironmental conditions during which it evolved. About twenty years later, the group was brought together again, with the addition of early career scientists, for a coring project focused on hydrothermal activity in the Yellowstone Lake basin. We discuss the ongoing research and analyses of core material, and conclude with the benefits of working in multigenerational interdisciplinary research groups. (Less)

Summary 1. The biological structure of arctic lakes is changing rapidly, apparently in response to global change processes such as increasing air temperatures, although altered nutrient stoichiometry may also be an important driver. Equally important, however, are local factors (e.g. landscape setting, hydrological linkages and trophic interactions) that may mediate responses of individual lakes at the regional scale. Despite general acknowledgement of the importance of local factors, there has been little focus on among-lake variability in the response to environmental change. 2. Sedimentary pigments, organic carbon and nitrogen, and biogenic silica (BSi) in 210Pb and 14C-dated sediment cores from three contrasting lakes in the Kangerlussuaq area (c. 67°N, 51°W) of south-west Greenland were used to reconstruct algal and phototrophic bacterial ecological change during the late-Holocene. Water chemistry for the individual lakes varies in terms of conductivity (range: 30–3000 μS cm−1) and stratification regimes (cold monomictic, dimictic and meromictic), linked with their position along the regional climate gradient from the coast and to the present ice sheet margin. 3. Despite essentially similar regional climate forcing over the last c. 1000 years, marked differences among lake types were observed in the phototrophic communities and their temporal variability. Considerable short-term variability occurred in an oligosaline, meromictic lake (SS1371), dominated by purple sulphur bacterial pigments, most likely due to a tight coupling between the position of the chemocline and the phototrophic community. Communities in a lake (SS86) located on a nunatak, just beyond the edge of the present ice sheet shifted in a nonlinear pattern, approximately 1000 cal. years BP, possibly due to lake-level lowering and loss of outflow during the Medieval Climate Anomaly. This regime shift was marked by a substantial expansion of green sulphur bacteria. 4. A dilute, freshwater coastal lake (SS49) dominated by benthic algae was relatively stable until ca. 1900 AD when rates of community change began to increase. These changes in benthic algal pigments are correlated with substantial declines (1.3–0.44‰) in δ15N that are indicative of increased deposition of atmospheric inputs of industrially derived NOx into the atmosphere. 5. Climate control on lake ecosystem functioning has been assumed to be particularly important in the Arctic. This study, however, illustrates a complex spatial response to climate forcing at the regional scale and emphasises differences in the relative importance of changes in the mass (m, both precipitation and nutrients) and energy flux (E) to lakes for the phototrophic community structure of low-arctic Greenland lakes.

Dissolved organic matter (DOM) of 27 prairie saline lake ecosystems was investigated in the Northern and Central Great Plains of the United States using absorbance, fluorescence, lignin concentration, and stable C isotope values. The majority of variation in DOM fluorescence among lakes was due to humic (peak C) and microbially formed (peak M) fluorescent components, which appear to be derived from autochthonous primary production. Strong correlations between peak M and nutrients allow us to model total phosphorus (TP) concentration using peak M fluorescence and chromophoric dissolved organic matter (CDOM) absorption. The rate of primary production (PP) was positively correlated with peak M fluorescence and negatively with lignin concentration. Lignin phenol yields in the DOM were generally smaller than those of most freshwater systems. δ13C values of dissolved organic carbon (DOC) ranged from −25.0‰ to −20.1‰ and were generally enriched relative to typical freshwaters (ca. −27‰). Terrestrial DOM is degraded in prairie lakes, spanning a gradient from mixotrophic to eutrophic, as determined by a color–nutrient model. The photodegradation of autochthonous DOM was significant: CO2 fluxes from these prairie lakes, modeled from peak M fluorescence, ranged from 5 to 228 mmol C m−2 d−1 (median, 37 mmol C m−2 d−1) and was similar to community respiration estimated from protein fluorescence (median, 50 mmol C m−2 d−1). The combined estimates were about 50% of the global mean total C release previously reported for saline lake ecosystems. The implication of these new results is that the global C release from saline lake ecosystems is likely underestimated.

Contemporary precipitation patterns in the Caribbean region are spatially variable, and the small number of Holocene paleoclimatic records may not adequately capture patterns of variation in the past. The hydrological history of Grenada was inferred from paleolimnological analyses of sediment cores from two crater lakes on the island. The basins were formed by volcanic activity some time during the Last Termination, but were dry between ca. 13 000 and ca. 7200 cal. a BP. After filling, the lakes were initially very shallow, and sedimentation was interrupted by a hiatus ca. 6300-5500 cal. a BP, followed by deposition of a thick tephra in both sites. After 5500 cal. a BP, lake level shows considerable multi-centennial variability, superimposed upon a long-term trend of generally higher lake level after 3200 cal. a BP. The pattern of lake-level variation in Grenada shows some similarity with other Caribbean paleoclimatic records in terms of the timing of transitions, but differs from several classic studies in the sign of inferred precipitation change. The differences among records may reflect spatially variable precipitation patterns in the past in response to the position of the Intertropical Convergence Zone and to sea surface temperature influences on the trade winds and Caribbean low-level jet. Copyright (C) 2011 John Wiley & Sons, Ltd. (Less)

The role of the Arctic in future global change processes is predicted to be important because of the large carbon (C) stocks contained in frozen soils and peatlands. Lakes are an important component of arctic landscapes although their role in storing C is not well prescribed. The area around Kangerlussuaq, SW Greenland (66–68°N, 49–54°W) has extremely high lake density, with ~20 000 lakes that cover about 14% of the land area. C accumulation rates and standing stock (kg C m −2 ), representing late- to mid-Holocene C burial, were calculated from AMS 14 C-dated sediment cores from 11 lakes. Lake ages range from ~10 000 cal yr bp to ~5400 cal yr bp, and reflect the withdrawal of the ice sheet from west to east. Total standing stock of C accumulated in the studied lakes for the last ~8000 years ranged from 28 to 71 kg C m −2 , (mean: ~42 kg C m −2 ). These standing stock determinations yield organic C accumulation rates of 3.5–11.5 g C m −2 yr −1 (mean: ~6 g C m −2 yr −1 ) for the last 4500 years. Mean C accumulation rates are not different for the periods 8–4.5 and 4.5– 0 ka, despite cooling trends associated with the neoglacial period after 4.5 ka. We used the mean C standing stock to estimate the total C pool in small lakes (

The chemical and biological composition of laminated lake-sediments deposited between about 2,500 and 5,500 years B.P. in Diss Mere, Norfolk, England is described. The distribution of Ca, Fe, S, and Si in pale and dark laminae is established by energy-dispersive X-ray microanalysis. The pale laminae consist primarily of calcium carbonate and the dark layers contain mainly Fe, S, Si, and dark organic material. The diatom composition of 16 individual light and dark laminae shows no consistent differences between light and dark layers. There are, however, higher frequencies of chrysophycean cysts in seven of the eight dark layers, suggesting these layers were deposited in late summer or fall. Pollen analyses of the pale and dark layers show that pollen percentages of early-flowering trees and shrubs are highest in the dark laminae and that pollen values of late-flowering plants are highest in the pale layers. The biological data indicate that the pale layers were formed in the late spring and early summer, whereas the dark laminae were deposited in late summer, fall, winter, and the early spring. Seasonal supply of CaCO3 was the major variable responsible for couplet formation.

Diatom dissolution in surface sediment samples from two regional lake datasets in the Northern Great Plains (NGP; n 5 64) and West Greenland (n 5 40) is assessed using a morphological approach categorizing valves during routine diatom analysis. Two dissolution indices are derived to parameterize diatom dissolution, and, when compared between two analysts in a blind test, show good correspondence and are closely correlated to diatom fragmentation. We explore the relationships between hydrochemical and physical lake parameters (including meromixis) on dissolution within both lake regions using multivariate methods and modeled with logistic regression. Salinity is the sole significant predictor of dissolution in West Greenland but salinity, carbonate concentration ([CO3 22 ]) and meromixis are significant predictors in the NGP. Limnological parameters explain 40–59% of variation in dissolution in both regions for both dissolution indices. The dissolution index methodology is applied to a short sediment sequence from Devils Lake (North Dakota), where diatom-inferred salinity inferences can be compared with a historical record of salinity fluctuations over the 20th century. Absolute errors in paleosalinity estimates are strongly correlated with diatom dissolution, with salinity overestimated in 8 out of 11 poorly preserved samples. Preservation does appear to constrain the reliability of the inferred paleosalinity at this site and may also affect the quality of diatom-based paleoenvironmental inferences elsewhere (including estimates of biogenic silica), where preservation state is often not explicitly considered.

A growing number of sites in the Northern Hemisphere show centennial- to millennial-scale climate variation that has been correlated with change in solar variability or with change in North Atlantic circulation. However, it is unclear how (or whether) these oscillations in the climate system are manifest in the Southern Hemisphere because of a lack of sites with suitably high sampling resolution. In this paper, we reconstruct the lake-level history of Lake Titicaca, using the carbon isotopic content of sedimentary organic matter, to evaluate centennial- to millennial-scale precipitation variation and its phasing relative to sites in the Northern Hemisphere. The pattern and timing of lake-level change in Lake Titicaca is similar to the ice-rafted debris record of Holocene Bond events, demonstrating a possible coupling between precipitation variation on the Altiplano and North Atlantic sea-surface temperatures (SSTs). The cold periods of the Holocene Bond events correspond with periods of increased precipitation on the Altiplano. Holocene precipitation variability on the Altiplano is anti-phased with respect to precipitation in the Northern Hemisphere monsoon region. More generally, the tropical Andes underwent large changes in precipitation on centennial-to-millennial timescales during the Holocene. Copyright © 2005 John Wiley & Sons, Ltd.

Three-dimensional modeling of lake morphometry enables the calculation of lake volume, planar surface area, and basin surface area, which are critical components of a conceptual model of how planktic and benthic habitat areas change with changing lake level. We have applied three-dimensional modeling to Foy Lake, Montana, and compared model results of changing ratios of planktic and benthic habitat areas at all possible lake levels to sediment cores recovered from the lake. The model allows us to produce semiquantitative depth reconstructions and greatly improves lake-level reconstruction of this morphometrically complex basin. The conceptual model also was modified to examine the influence of resuspension versus gravity redistribution of diatomaceous material on the sedimentary record. Comparison of the model results with both long and short sedimentary records from Foy Lake, Montana, suggests that planktic : benthic ratios may be a better predictor of mean lake depth than maximum lake depth. The model developed here is simple enough to be easily implemented, has few requirements, can be adapted to suit any lake basin, and can be augmented to examine a variety of different lake processes that affect habitat availability.

Widespread drought is among the most likely and devastating consequences of future global change. Assessment of drought impacts forecast by atmospheric models requires an understanding of natural drought variability, especially under conditions more arid than today. Using high-resolution lake-sediment records from the northern Great Plains, we show pronounced 100- to 130-yr drought cycles during the arid middle Holocene (8000 calendar yr BP). During drought phases, grass productivity declined, erosion and forbs increased, and fuel limitation reduced fire importance. Intervening humid decades saw grass production rise, with stabilization of soils and renewed fire as fuels became abundant. Although both C3 and C4 grasses declined during droughts, a lasting shift to C3 dominance occurred during a single drought -8200 calendar yr BP. During the more humid Late Holocene (2800 calendar yr BP), climate was less variable and without evident drought cyclicity. Consequently, drought severity during past, and possibly future, arid phases cannot be anticipated from the attenuated climate variability evident during contemporary humid phases. Our study demonstrates that agriculturally important grassland ecosystems respond sensitively to drought variability, uncertainty in which has profound implications for the future of these ecosystems.

1. A series of resource competition experiments were performed with four diatom taxa isolated from saline lakes in the Northern Great Plains of North America. Three variables were tested in a factorial design: N/P ratio (6 : 1, 12 : 1, 24 : 1, 48 : 1), salinity (5, 8, 11‰), and anion composition (sulphate versus bicarbonate-dominated medium). 2. Coexistence of Chaetoceros elmorei, Cyclotella quillensis, and Cymbella pusilla occurred at the two lowest N/P ratios. These three taxa have similar N requirements and thus competitive exclusion by the taxon with the lowest N requirement may have required more time. 3. Anomoeoneis costata, which had the highest nitrogen requirements and slowest growth rate of these four taxa, declined in all cases. Cyclotella biovolume concentration consistently declined at the two highest N/P ratios, indicating a higher P requirement, which is consistent with its distribution pattern. 4. In general, we found opposite trends with respect to all variables for Chaetoceros, a taxon that mainly inhabits sulphate systems, versus Cymbella, a bicarbonate-associated taxon. Our results suggest that Cymbella may be found mainly in bicarbonate systems because of a higher C requirement, whereas Chaetoceros may be less abundant in these systems, in part, because of competitive interactions with bicarbonate-associated taxa.

Lacustrine fossil records provide long time series of data on limnological and climatic conditions; these data are useful for establishing natural patterns of climate variability and for generating testable hypotheses about atmospheric circulation and climate-ecosystem linkages. Shoreline features can indicate past lake-level fluctuations that may reflect changes in moisture balance, but often these records are discontinuous and are evidence of only extreme conditions. The organisms, geochemistry, and sedimentology of lake sediments may provide a more continuous sequence of direct and indirect lake-climate interactions in the past. The most clearly interpretable paleolirnnological records of climatic change are those that use several lines of evidence to corroborate a climatic hypothesis and are from sites near an ecotone or in regions of extreme climate. In all cases, hydrologic setting mediates a lake's response to climate and must be considered in interpreting sedimentary sequences. Discussion of the impact of recent and future climatic changes necessitates a baseline against which the magnitude of observed or predicted changes can be measured. Commonly we look to recent history as a reference point for comparison; however, the landscape of recent decades, or in some cases even centuries, has been altered greatly by human activities. Thus, to determine whether recent or future climatic change is unique or unusual, we need long natural (preanthropogenic) time series against which observed or predicted climatic states and variability can be compared. Paleolimnological archives are a tool for looking at environmental patterns prior to anthropogenic impact and show a range of climatic states far greater than those documented by written records. Former strand lines of pluvial lakes Bonneville and Lahontan in the western U.S., for example, testify to moisture extremes unsurpassed in recent centuries or even millennia. In addition, one can us: the paleolimnological record as an independent climatic proxy against which patterns and rates of terrestrial response to climatic change can be compared. Here, I present a broad overview of paleoclimatic reconstruction in North America from lacustrine records, with a focus on the late Quaternary. The review is limited to climatic inference from paleolirnnological records, a term I use in a restrictive sense to refer to records of past lake states (physical, biological, and geochemical), as opposed to records of past stream hydrology (e.g. Ely et al. 1993; Knox 1993) or terrestrial conditions. I summarize paleolimnological tools available for reconstruction of past climate, the capabilities and limitations of these tools

Resource-based physiology of the eight important planktonic diatom species in the large lakes of the Yellowstone region can be used to explain their relative abundances and seasonal changes. The diatoms are ranked along resource ratio gradients according to their relative abilities to grow under limitation by Si, N, P, and light. Hypotheses based on resource physiology can be integrated with observations on seasonal changes in diatom assemblages to explain the present distributions of diatoms and to test the causal factors proposed to explain diatom distributions over the Holocene. Knowledge of the limnology of these lakes and process-oriented physiology provide the basis for a more detailed interpretation of the paleorecord and a firmer basis for landscape-level transfer functions for fine-scale climate reconstruction.

Estimates of past lake-water salinity from fossil diatom assemblages were used to infer past climatic conditions at Moon Lake, a climatically sensitive site in the northern Great Plains. A good correspondence between diatom-inferred salinity and historical records of mean annual precipitation minus evapotranspiration (P - ET) strongly suggests that the sedimentary record from Moon Lake can be used to reconstruct past climatic conditions. Century-scale analysis of the Holocene diatom record indicates four major hydrological periods: an early Holocene transition from an open freshwater system to a closed saline system by 7300 B.P., which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate; a mid-Holocene period of high salinity from 7300 to 4700 B.P., indicating low effective moisture (P - ET); a transitional period of high salinity from 4700 to 2200 B.P., characterized by poor diatom preservation; and a late Holocene period of variable lower salinity during the past 2,200 yr, indicating fluctuations in effective moisture.