Kathleen M. Rühland

Lake diatom responses to warming: reviewing the evidence

Algae, the dominant primary producers in many aquatic ecosystems, are critical to global biogeochemical cycling, and changes in their abundance and composition can cascade throughout aquatic food webs. Diatoms often dominate the algal communities in many freshwater systems. Their population dynamics are affected by a variety of environmental variables, many of which are linked to changes in water column properties and habitat availability, which themselves can be linked to shifts in ice cover, length of the growing season, thermal stability and stratification, vertical mixing patterns, habitat alterations, and the availability of resources such as light and nutrients. Climate has strong moderating controls on all of these fundamental aquatic processes, which can directly and indirectly alter species composition, abundance and seasonal dynamics of both periphytic and planktonic diatoms. In this review, we examine the role that climate-mediated alterations in inter-related lake processes have played on diatom community composition, dynamics and size structure, with particular attention to the recent success of planktonic diatom species relative to heavier tychoplanktonic and small benthic diatoms. We focus primarily on paleolimnological records, but also reference a wide spectrum of limnological and physiological studies to review and discuss how climate-driven shifts in lake properties may affect diatom assemblage reorganization. Understanding the limnological and historical context of these often complex diatom changes is key to making scientifically defensible interpretations of paleolimnological records. We further evaluate the plausibility of alternative explanations (e.g. atmospheric nitrogen deposition) for the recent success of small cyclotelloid species by examining trends in these planktonic diatoms from a large number of sites. Using a weight-of-evidence approach, we conclude that recent climate change is the main driver that has led to ecological tipping points resulting in the recent success of small planktonic diatoms that have been reported in many aquatic systems.

Global change revealed by palaeolimnological records from remote lakes: a review

Over recent decades, palaeolimnological records from remote sites have provided convincing evidence for the onset and development of several facets of global environmental change. Remote lakes, defined here as those occurring in high latitude or high altitude regions, have the advantage of not being overprinted by local anthropogenic processes. As such, many of these sites record broad-scale environmental changes, frequently driven by regime shifts in the Earth system. Here, we review a selection of studies from North America and Europe and discuss their broader implications. The history of investigation has evolved synchronously with the scope and awareness of environmental problems. An initial focus on acid deposition switched to metal and other types of pollutants, then climate change and eventually to atmospheric deposition-fertilising effects. However, none of these topics is independent of the other, and all of them affect ecosystem function and biodiversity in profound ways. Currently, remote lake palaeolimnology is developing unique datasets for each region investigated that benchmark current trends with respect to past, purely natural variability in lake systems. Fostering conceptual and methodological bridges with other environmental disciplines will upturn contribution of remote lake palaeolimnology in solving existing and emerging questions in global change science and planetary stewardship.

Past environmental and climatic changes related to tree-line shifts inferred from fossil diatoms from a lake near the Lena River Delta, Siberia

Changes in diatom assemblages in a sediment core taken from a tundra lake near the Lena River Delta, northeastern Siberia, reflect climatic and environmental changes throughout the Holocene. Significant limnological changes occurred at the same time as trees advanced into the region c. 9100 cal. yr BP (8500 radiocarbon yr BP) and subsequently retreated c. 3800 cal. yr BP (3500 radiocarbon yr BP). Prior to this tree-line shift, diatom assemblages were dominated by small benthic Fragilaria species, and diatom-inferred alka linity values were high, suggesting that climatic conditions were cool and relatively dry, and that lake pro ductivity was probably low. Coincident with tree-line advance into the region c. 9100 cal. yr BP, inferred alkalinity decreased and remained low throughout the forested period, reflecting soil development and increased organic runoff associated with the shift in catchment vegetation. The establishment of a larch forest in the catchment corresponded with an initial period of limnological change between 9100 and 7700 cal. yr BP, suggesting that soil and forest development took place over approximately 1400 years. Between c. 7700 cal. yr BP (c. 7000 radiocarbon yr BP) and 3800 cal. yr BP, a stable diatom assemblage dominated by Achnanthes species and low diatom-inferred alkalinity indicate the influence of organic runoff associated with a fully developed forest. A return to cooler conditions and reversion to shrub tundra c. 3800 cal. yr BP coincided with a shift back to a Fragilaria-dominated diatom assemblage and an increase in diatom-inferred alkalinity. Recent conditions have been marked by declining alkalinity estimates and a minor change in diatom assemblages, most likely reflecting an influx of humic substances from catchment peats. The close correlation between trends in reconstructed alkalinity and catchment vegetation suggests that diatom-inferred alkalinity may provide a sensitive record of soil and forest development in unpolluted circumneutral or alkaline tree-line lakes.

FRESHWATER DIATOMS FROM THE CANADIAN ARCTIC TREELINE AND DEVELOPMENT OF PALEOLIMNOLOGICAL INFERENCE MODELS1

Relationships between surface sediment diatom assemblages and measured environmental variables from 77 lakes in the central Canadian arctic treeline region were examined using multivariate statistical methods. Lakes were distributed across the arctic treeline from boreal forest to arctic tundra ecozones, along steep climatic and environmental gradients. Forward selection in canonical correspondence analysis determined that dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), total nitrogen (TN), lake surface area, silica, lake‐water depth, and iron explained significant portions of diatom species variation. Weighted‐averaging (WA) regression and calibration techniques were used to develop inference models for DIC, DOC, and TN from the estimated optima of the diatom taxa to these environmental variables. Simple WA models with classical deshrinking produced models with the strongest predictive abilities for all three variables based on the bootstrapped root mean squared errors of prediction (RMSEP). WA partial least squares showed little improvement over the simpler WA models as judged by the jackknifed RMSEP. These models suggest that it is possible to infer trends in DIC, DOC, and TN from fossil diatom assemblages from suitably chosen lakes in the central Canadian arctic treeline region.

Global warming triggers the loss of a key Arctic refugium

We document the rapid transformation of one of the Earths last remaining Arctic refugia, a change that is being driven by global warming. In stark contrast to the amplified warming observed throughout much of the Arctic, the Hudson Bay Lowlands (HBL) of subarctic Canada has maintained cool temperatures, largely due to the counteracting effects of persistent sea ice. However, since the mid-1990s, climate of the HBL has passed a tipping point, the pace and magnitude of which is exceptional even by Arctic standards, exceeding the range of regional long-term variability. Using high-resolution, palaeolimnological records of algal remains in dated lake sediment cores, we report that, within this short period of intense warming, striking biological changes have occurred in the regions freshwater ecosystems. The delayed and intense warming in this remote region provides a natural observatory for testing ecosystem resilience under a rapidly changing climate, in the absence of direct anthropogenic influences. The environmental repercussions of this climate change are of global significance, influencing the huge store of carbon in the regions extensive peatlands, the worlds southern-most polar bear population that depends upon Hudson Bay sea ice and permafrost for survival, and native communities who rely on this landscape for sustenance.

Evaluating the Response of Cladocera to Recent Environmental Changes in Lakes from the Central Canadian Arctic Treeline Region

ABSTRACT Arctic and subarctic ecosystems have undergone considerable environmental changes in recent years as the result of climate warming. Fossil records of freshwater diatoms in lakes throughout the circumpolar Arctic, including the central Canadian Arctic treeline region, have revealed marked directional shifts in diatom assemblages in recent lake sediments. These algal changes have been linked to longer growing seasons, decreased duration of ice cover, and/or increased thermal stability. The effects of these recent environmental changes on higher trophic levels, such as the Cladocera, are still unclear. Using cladoceran remains preserved in the sediments of 50 lakes, which were previously examined for changes in diatoms, we show that significant changes in cladoceran species composition have occurred from pre-industrial times to the present. However, these changes are considerably muted compared to the more substantial changes observed in the diatom record. We found no consistent patterns of change in planktonic cladocerans (i.e. Daphnia, Bosmina) within our study lakes, and the response of the Cladocera to environmental changes does not appear to be strongly coupled to recent changes in diatom communities, thus further confirming that the previously observed diatom changes were related to bottom-up limnological controls. These results highlight the complex response of Arctic freshwater food webs to climate change, and the need for incorporating multi-trophic studies into climate change investigations.

Response of Diatoms and Other Siliceous Indicators to the Developmental History of a Peatland in the Tiksi Forest, Siberia, Russia

Diatoms and other siliceous microfossils were examined from a 386-cm-long peat core, covering the last ca. 7200 yr, from north-central Siberia to gain insights into peatland developmental history, and to explore the potential of diatoms as proxy indicators in arctic peats. Diatom analyses of arctic peatlands are rare, and so one aspect of this study was to examine the sensitivity of diatom taxa in relation to independent paleoindicators already described from this core. Changes in the relative abundances of diatom taxa divided the core into four zones that closely tracked the ontogeny of the peatland from an open water environment (Zone I: benthic, alkaliphilic taxa), followed by fen environments (Zones II and III; epiphytic, acidophilic taxa), and finally to a better-drained, high-centered bog (Zone IV; aerophilic taxa). In addition to the diatom taxa, observations were made on the relative abundances of siliceous protozoan plates, chrysophyte cysts, and phytoliths. Both the diatoms and these other siliceous microfossils appeared to respond to changes in hydrology and moisture, as well as to fire episodes likely triggered by climatic change. This study demonstrates that diatoms and other siliceous microfossils from arctic peat deposits provide an important source of paleoenvironmental information that can strengthen interpretations derived from other commonly used indicators.

A Multi-Trophic Exploratory Survey of Recent Environmental Changes using Lake Sediments in the Hudson Bay Lowlands, Ontario, Canada

Abstract A multi-proxy paleolimnological survey was performed on 13 lakes in the Hudson Bay Lowlands (HBL) of northern Ontario in order to provide a regional analysis of recent environmental changes in this poorly studied sub-Arctic region. In contrast to the amplified warming experienced by most of the circumpolar Arctic since the mid-19th century, the climate of the Hudson Bay (HB) region has remained relatively cool and stable for hundreds of years. However, since approximately the 1990s, the HBL has experienced rapid and large increases in air temperature and declines in sea ice. Diatom, cladoceran, and chironomid remains preserved in the recent (surface) and pre-1850 sediments of 13 lakes were used to examine whether this new climate regime has resulted in species assemblage changes across multiple trophic levels. Our results indicate clear limnological responses to warming among the freshwater biota of HBL lakes; however, the magnitude of this change varied among both biological indicators and sites. As expected, diatoms exhibited the greatest degree of change, closely followed by chironomids, with relatively little change observed among cladoceran assemblages. Planktonic diatoms were more common in modern assemblages, often including plankters that were previously not recorded in the bottom sediments, and in fact all indicator groups recorded a change in benthic/littoral taxa in the recent sediments indicative of warming-induced increases in habitat availability due to decreased lake ice cover.

An Exploratory Survey of Summer Water Chemistry and Plankton Communities in Lakes near the Sutton River, Hudson Bay Lowlands, Ontario, Canada

Abstract We provide the first assessment of regional water chemistry and plankton (phytoplankton and crustacean zooplankton) for a suite of lakes near the Sutton River region of the north-central Hudson Bay Lowlands (HBL). We use ordination analyses to examine the spatial variation in water chemistry and plankton across lakes, and to explore the factors that may explain this variation. Based on data collected during summer from 2009 to 2011, we found that in addition to geology, water chemistry was strongly influenced by a lakes proximity to salt water and the degree of permafrost development within its catchment. Phytoplankton composition varied across lakes based on differences in water depth and nutrient concentrations, with non-filamentous cyanobacteria and chlorophytes more common in shallow lakes, and deeper lakes dominated by planktonic diatoms or filamentous cyanophytes. Crustacean zooplankton community composition and richness in the HBL lakes was similar to communities found in Ontario lakes in more temperate regions within the Precambrian Shield. These baseline data provide a foundation upon which future surveys in this climatically sensitive region may be compared.

ABSENCE OF EVIDENCE IS NOT EVIDENCE OF ABSENCE: IS STEPHANODISCUS BINDERANUS (BACILLARIOPHYCEAE) AN EXOTIC SPECIES IN THE GREAT LAKES REGION?(1).

The eutrophic, freshwater diatom species Stephanodiscus binderanus (Kütz.) Willi Krieg. has long been considered a nuisance exotic alga introduced from Eurasia to the Great Lakes in North America in the early to mid‐20th century. However, our paleolimnological data from Lake Simcoe, Ontario, provide unequivocal evidence that this taxon has been present in the Great Lakes region since at least the late 17th century. Subfossil diatom valves were identified and enumerated at high resolution in 210Pb‐dated sediment cores from four sites across the lake. The taxonomic identification of S. binderanus was confirmed using SEM. The historical presence of this species in Lake Simcoe indicates somewhat naturally productive conditions and also refutes the idea that S. binderanus is a nonindigenous species to North America. This study underscores the caution that should be applied to questions of diatom (and protistan) distributions in time and space. Clearly, the absence of evidence is not evidence of absence.