N. John Anderson

Miniview: Diatoms, temperature and climatic change

The development of weighted averaging models for diatoms and lake-water temperature is reviewed in relation to other factors controlling diatom abundance in lakes. It is argued that diatom–temperature models are weaker than those developed for salinity, pH and phosphorus. Moreover, even where a statistically significant model is developed for water temperature, its application over long time periods requires caution. For example, over long time periods (such as the Holocene), soil and vegetation will have changed considerably, altering lake-catchment hydrochemistry and hence the lake biota. Applications of diatom-temperature models in these instances may result in inferred temperatures that are merely artefacts of changing lake-water chemistry. However, palaeolimnological studies using variance partitioning analysis of diatom profiles in sediment cores and independent climate data show that climate signals are recorded in lake sediments. It is suggested, therefore, that the link between changing climate and diatoms be addressed via an alternative method than that of temperature-inference models. The situation demands a more complete understanding of the interactions between lake hydrodynamics and nutrient inputs from catchments over a variety of timescales.

Diatoms, temperature and climatic change

The development of weighted averaging models for diatoms and lake-water temperature is reviewed in relation to other factors controlling diatom abundance in lakes. It is argued that diatom–temperature models are weaker than those developed for salinity, pH and phosphorus. Moreover, even where a statistically significant model is developed for water temperature, its application over long time periods requires caution. For example, over long time periods (such as the Holocene), soil and vegetation will have changed considerably, altering lake-catchment hydrochemistry and hence the lake biota. Applications of diatom-temperature models in these instances may result in inferred temperatures that are merely artefacts of changing lake-water chemistry. However, palaeolimnological studies using variance partitioning analysis of diatom profiles in sediment cores and independent climate data show that climate signals are recorded in lake sediments. It is suggested, therefore, that the link between changing climate and diatoms be addressed via an alternative method than that of temperature-inference models. The situation demands a more complete understanding of the interactions between lake hydrodynamics and nutrient inputs from catchments over a variety of timescales.

CONTROLS OF ALGAL ABUNDANCE AND COMMUNITY COMPOSITION DURING ECOSYSTEM STATE CHANGE

Shallow lake ecosystems can shift between clear-water, macrophyte-rich conditions and turbid states with abundant phytoplankton. However, little is known about the controls of algal community composition and primary production before, during, and after ecosystem state change, because long time series that monitor biological change through the transition are scarce. Using proxy data sets derived from sediment cores from two shallow hypertrophic lakes in Denmark, variance-partitioning analysis (VPA) was used to determine the relative importance of changes in total phosphorus (diatom inferred), planktivorous fish density (zooplankton inferred), and submerged macrophyte communities (as macrofossil abundance) as determinants of algal abundance and community composition (as sedimentary pigments) over ecosystem state transitions since 1750 (CE) for Lake Lading and 1900 for Lake Sobygaard. Past variation in densities of planktivorous fish explained 12.3% and 18.2% of historical algal community change in lakes Lad...

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.

Climate Versus In-Lake Processes as Controls on the Development of Community Structure in a Low-Arctic Lake (South-West Greenland)

The dominant processes determining biological structure in lakes at millennial timescales are complex. In this study, we used a multi-proxy approach to determine the relative importance of in-lake versus indirect processes on the Holocene development of an oligotrophic lake in SW Greenland (66.99°N, 50.97°W). A 14C and 210Pb-dated sediment core covering approximately 8500 years BP was analyzed for organic–inorganic carbon content, pigments, diatoms, chironomids, cladocerans, and stable isotopes (δ13C, δ18O). Relationships among the different proxies and a number of independent controlling variables (Holocene temperature, an isotope-inferred cooling period, and immigration of Betula nana into the catchment) were explored using redundancy analysis (RDA) independent of time. The main ecological trajectories in the lake biota were captured by ordination first axis sample scores (18–32% variance explained). The importance of the arrival of Betula (ca. 6500 years BP) into the catchment was indicated by a series of partial-constrained ordinations, uniquely explaining 12–17% of the variance in chironomids and up to 9% in pigments. Climate influences on lake biota were strongest during a short-lived cooling period (identified by altered stable isotopes) early in the development of the lake when all proxies changed rapidly, although only chironomids had a unique component (8% in a partial-RDA) explained by the cooling event. Holocene climate explained less variance than either catchment changes or biotic relationships. The sediment record at this site indicates the importance of catchment factors for lake development, the complexity of community trends even in relatively simple systems (invertebrates are the top predators in the lake) and the challenges of deriving palaeoclimate inferences from sediment records in low-Arctic freshwater lakes.

Holocene records of effective precipitation in West Greenland

Holocene changes in effective precipitation (precipitation-evaporation; P-E) were reconstructed for the Søndre Strømfjord region of southwest Greenland using the sediment records of two neighbouring closed-basin ‘saline’ lakes. Past lakewater conductivities (a proxy for P-E balance) were estimated using a diatom-inferred conductivity model. Broadly similar changes in both lake records corroborate the use of DI conductivity as a regional climate proxy. An increasing DI conductivity trend through the Holocene indicated net negative or balanced P-E ratios in this region, even during most of the Neoglacial period (post-4000 yr BP). This contrasts with other regions of West Greenland, where conditions became more humid in the Neoglacial period. The reconstructions presented here therefore suggest a high degree of spatial variability in the P-E balance, likely caused by region-specific orographic features. While interpret ation of the mid-Holocene P-E balance was limited by dissolution of the diatom record, other sediment indicators suggest a period of extremely negative effective precipitation (~7000–5600 yr BP), followed by a period of positive effective precipitation lasting until 4700 yr BP. This contrasts markedly with the later Holocene, after c. 4000 yr BP, when high-frequency oscillations in DI conductivity probably reflect short-term climatic variations (amplified by in-lake processes connected with meromixis) but no long-term trends in the P-E balance.

Aulacoseira subarctica: taxonomy, physiology, ecology and palaeoecology

Knowledge of the morphology, taxonomy, physiology, ecology and palaeoecology of the filamentous freshwater diatom Aulacoseira subarctica (O. Müller) Haworth is reviewed from the literature. The species is widely distributed across Northern Europe, Scandinavia and North America, Japan, China, Australia and New Zealand, in the last usually as fo. subborealis. The species is rare in the Tropics and positive identifications are lacking for Africa. The importance of correct identification is stressed because literature reports of its occurrence often need revision. A comparison is made between two sites with detailed records of its seasonal cycle. Blelham Tarn, England, is a clear-water seasonally stratified lake; Lough Neagh, Northern Ireland, is a very large turbid and unstratified lake. In spite of their contrasting characters, Aulacoseira subarctica succeeds in both lakes. In Blelham Tarn, there is a strong seasonal cycle of growth, dormancy on the sediment and further growth on re-suspension. In Lough Neagh, there is a large deposition of cells at the end of the spring bloom but few of these cells survive grazing by benthic animals. Fossil occurrences are summarized from the species’ initial appearance in the Pliocene and its later occurrence during the last inter-glacial (Eemian) deposits in NW Europe, where it is an important ecological indicator. Palaeolimnological studies are reviewed to describe the contemporary distribution and ecological preference of this widespread diatom. Aulacoseira subarctica usually appears in response to moderate increases in nutrients but is disadvantaged by further enrichment. In Blelham Tarn the species only became abundant during the twentieth century, reaching a peak between 1930 and 1960. With nutrient enrichment, abundance has declined. In Lough Neagh, it has been present for much of the post-glacial period and still thrives today in spite of the highly enriched state of the lake. The species has disappeared from many eutrophic lakes and its sustained existence in Lough Neagh is anomalous. Lough Neagh may continue to offer a favourable habitat in spite of the poor light climate because the water depth, lack of stratification and long water retention time reduce losses to a sustainable level.

Natural versus anthropogenic charge in lakes: The role of the sediment record

Abstract Recent methodological developments permit the quantitative reconstruction of water chemistry variables from microfossil assemblages preserved in lake sediments. These reconstructions can be used to identify the extent and timing of disturbance to lake ecosystems. Combined with appropriate sampling strategies, lake sediments permit water chemistry variables and community rates of change to be estimated at a variety of timescales. Sediments predating major cultural impacts offer the possibility of inferring lake history before anthropogenic interference, and can, therefore, contribute to current debates about timescales of natural variance in lakes, as well as the response of lake communities to natural environmental perturbations. Such an approach has relevance to many contemporary environmental problems, e.g. acidification, eutrophication and climate change.Recent methodological developments permit the quantitative reconstruction of water chemistry variables from microfossil assemblages preserved in lake sediments. These reconstructions can be used to identify the extent and timing of disturbance to lake ecosystems. Combined with appropriate sampling strategies, lake sediments permit water chemistry variables and community rates of change to be estimated at a variety of timescales. Sediments predating major cultural impacts offer the possibility of inferring lake history before anthropogenic interference, and can, therefore, contribute to current debates about timescales of natural variance in lakes, as well as the response of lake communities to natural environmental perturbations. Such an approach has relevance to many contemporary environmental problems, e.g. acidification, eutrophication and climate change.

Using the past to predict the future: lake sediments and the modelling of limnological disturbance

Abstract Most lakes have been disturbed to varying degrees but for an individual lake the timescale of these disturbances is rarely known. Lake sediments, however, can be used as natural archives of perturbation histories, e.g. acidification and eutrophication. At present the use of simple weighted averaging models permits the reconstruction of a variety of water chemical variables from diatom and other microfossils preserved in lake sediments (pH, total phosphorus, salinity and lakewater temperature). Sediment records can, therefore, provide lake-specific background data for lake management as well as information about their ecological histories. The common models used in palaeolimnology (dating, transfer-functions) are reviewed and their role in environmental monitoring discussed. Predictions of future lake water quality following lake restoration methods tend to be made from dynamic mathematical models, but they are also used for hindcasting (e.g. the MAGIC model of catchment acidification). A problem with using dynamic models is that they are often site-specific and require calibration for a given lake. Combined with reliable dating, chemical reconstructions from microfossil-based transfer functions offer the possibility of testing hindcast predictions derived from dynamic mathematical models, e.g. for salinity, TP and pH. In this way, sediment microfossil-based models can assist with the parameterization of more complex, dynamic models of contemporary processes. In this review, comparisons between the two approaches (sediment-based and dynamic models) are given and possible future interactions outlined. Validation of mathematical models by palaeolimnological data might enhance their predictive ability when used for forecasting lake recovery. There is clearly, however, a need for a more rigorous approach to palaeolimnology, i.e. critical hypothesis generation. Multidisciplinary studies of lake disturbance, that combine palaeolimnology, dynamic modelling and contemporary process studies, would also be beneficial.

Isotopic variation in modern lake waters from western Greenland

The stable-isotope composition of lake waters along Sïndre Strïmfjord in western Greenland have been measured both spatially (coast to ice sheet) and temporally (up to five times between 1998–2000) and the data compared to local climatic information. There is a climatic gradient along Sïndre Strïmfjord, with the head of the fjord characterised by a low Arctic continental climate, while the coast has a more maritime climate (increased precipitation and reduced annual temperature range). The change in climate from continental to maritime is reflected in the isotopic composition of the lakes. The inland lakes have δ18O and dD values which lie on evaporation trends away from meteoric water composition. The coastal lakes have δ18O and dD compositions close to meteoric water. Short-term seasonal trends are superimposed on longer term variation. For example, the relatively dry years (precipitation lower than the long-term annual mean) of 1998 and 1999 are seen in the smaller lakes as isotopic enrichment, but the larger lakes are buffered in their response to short-term changes. The small coastal lakes tend to have depleted δ13C derived predominantly from organic material, while the inland lakes have contributions from both organic carbon and from exchange with isotopically heavier atmospheric CO2. This study demonstrates the importance of characterizing local hydrological settings in the interpretation of lacustrine isotope records. In the Arctic, this type of information is especially useful for identifying lakes that have the potential to record arid intervals since precipitation/evaporation adjustments, especially toward greater aridity, may not be recorded in the Greenland ice-core records.