A model for inferring dissolved organic carbon (DOC) in lakewater from visible-near-infrared spectroscopy (VNIRS) measures in lake sediment

We developed an inference model to infer dissolved organic carbon (DOC) in lakewater from lake sediments using visible-near-infrared spectroscopy (VNIRS). The inference model used surface sediment samples collected from 160 Arctic Canada lakes, covering broad latitudinal (60–83°N), longitudinal (71–138°W) and environmental gradients, with a DOC range of 0.6–39.6 mg L⁻¹. The model was applied to Holocene lake sediment cores from Sweden and Canada and our inferences are compared to results from previous multiproxy paleolimnological investigations at these two sites. The inferred Swedish and Canadian DOC profiles are compared, respectively, to inferences from a Swedish-based VNIRS-total organic carbon (TOC) model and a Canadian-based diatom-inferred (Di-DOC) model from the same sediment records. The 5-component Partial Least Squares (PLS) model yields a cross-validated (CV) R_CV² R_{CV}^{2}  = 0.61 and a root mean squared error of prediction (RMSEP _CV ) = 4.4 mg L⁻¹ (11% of DOC gradient). The trends inferred for the two lakes were remarkably similar to the VNIRS-TOC and the Di-DOC inferred profiles and consistent with the other paleolimnological proxies, although absolute values differed. Differences in the calibration set gradients and lack of analogous VNIRS signatures in the modern datasets may explain this discrepancy. Our results corroborate previous geographically independent studies on the potential of using VNIRS to reconstruct past trends in lakewater DOC concentrations rapidly.     

Ecological distribution of scaled-chrysophyte assemblages from the sediments of 54 lakes in Nova Scotia and southern New Brunswick,Canada

Chrysophyte scales have been used in several paleolimnological studies to track long-term environmental change, however little data exist for the many lakes in the Maritime provinces of eastern Canada. As part of a multi-disciplinary investigation of acidification and other environmental stressors in the Maritimes, chrysophytes scales were identified and enumerated from the sediments of 52 lakes from Nova Scotia and two lakes from New Brunswick. A total of 25 chrysophyte taxa were identified from the surface sediments, reflecting the modern-day chrysophyte assemblages. The dominant species included Mallamonas duerrschmidtiae and Mallomonas acaroides. Taxa of the genus Synura were present in some lakes, but mainly in the more southern sites. In general, the floras were less diverse than those recorded from similar studies in other temperate regions. This may be related to the fact that the calibration lake set contained only a relatively short limnological gradient, and the assemblages reflect the acidic to circumneutral conditions of these lakes. Synura petersenii, a taxon that has been linked to imparting taste and odor problems to lakes, and had been shown to increase in the recent sediments of many other Canadian lakes, was only rarely present. In contrast to other studies, scaled chrysophytes were very rare in the pre-industrial sediments, with substantial nineteenth century populations only present in four relatively deep (>19 m) lakes. Detailed stratigraphic analyses of eight sediment cores revealed that scaled-chrysophyte assemblages increased dramatically during the latter part of the twentieth century. Limnological changes associated with climate (e.g. increased thermal stratification due to a 1.5°C temperature increase since ~1850) may have influenced chrysophyte distributions in these lakes.     

A high resolution multi-proxy record of pronounced recent environmental change at Baker Lake,Nunavut

Arctic aquatic systems are considered to be especially sensitive to anthropogenic disturbance, which can have cascading effects on biological communities as aquatic food-web structure is altered. Bio-indicators that respond to major limnological changes can be used to detect and infer major environmental change, such as climate warming, with the use of paleolimnological techniques. A multi-proxy approach was used to quantify recent environmental changes at Baker Lake, Nunavut, Arctic Canada. Analyses of fossilized remains of chironomids and diatoms were conducted on a sediment core of 20 cm in length sampled at 0.5-cm intervals. A new surface sediment training set of subfossil chironomid assemblages from 65 lakes across the eastern Canadian Arctic generated a robust (r _jack² = 0.79) surface water paleotemperature transfer function. The transfer function was applied to stratigraphic intervals from the Baker Lake sediment core to generate a paleotemperature reconstruction of sub-decadal resolution. The surface water temperature reconstruction inferred a 2°C increase in mid-summer surface water temperature for Baker Lake over the last 60 years, which was corroborated by the local instrumental record spanning the period of 1950–2007 AD. The chironomid record shows a recent decline of several cold-water taxa and appearance of warm-water indicators. This shift in community structure began circa 1906 AD, and intensified after 1940 AD. The corresponding fossil diatom record showed an increase in small planktonic Cyclotella taxa over the past 60 years, intensifying in the last 5 years, which also suggests a warmer climate and longer ice-free periods. The shifts in the diatom assemblages began later than the shifts in the chironomid assemblages, and were of lower magnitude, reflecting differences in the mechanisms in which these two indicators respond to environmental change.     

Use of subfossil <Emphasis Type="Italic">Chaoborus</Emphasis> mandibles in models for inferring past hypolimnetic oxygen

Assemblages of subfossil Chaoboridae mandibles from 80 thermally-stratified shield lakes in southern central Canada were examined to explore the influence of subfossil Chaoborus on subfossil Chironomidae-based paleolimnological inference models of deepwater oxygen, as volume-weighted hypolimnetic oxygen (VWHO). Inclusion of subfossil Chaoborus in subfossil Chironomidae-based VWHO models only improved model performance modestly, however it produced substantively better inferences of hypolimnetic oxygen in anoxic lakes, because Chaoborus had a much stronger positive relationship with low VWHO compared to chironomid taxa indicative of anoxic conditions, such as Chironomus. A Chaoborus mandible:Chironomidae head capsule ratio (chaob:chir) may be a useful index in paleolimnological studies, as chaob:chir in a surface sediment training set was significantly related to VWHO, and displayed little co-variation with other limnological variables such as trophic status (e.g. TP, TN) or lake depth (e.g. Z _max). Chaob:chir values in a stratigraphic analysis tracked chironomid-inferred VWHO, however the use of chaob:chir in regional ‘top–bottom’ paleolimnological studies must be used with caution.     

Hydrodynamic change recorded by diatoms in sediments of Liuxihe Reservoir, southern China

Diatoms are used widely for paleolimnological studies in lakes, but their use for studying the environmental history of reservoirs has not been tested extensively. Reservoirs have hydrodynamic characteristics intermediate between those of rivers and lakes. This study assessed the utility of diatom assemblages as recorders of long-term changes in hydrodynamics and spatial gradients in Liuxihe Reservoir, an impoundment in southern China. Four sediment cores were collected in the reservoir, from the riverine, transition and lacustrine zones. Each core was sectioned at 2-cm intervals to investigate the stratigraphic distribution of accumulated diatoms. Varve counting was used to develop a chronology for one of the cores. The unique characteristics of Liuxihe Reservoir, including its large size, great depth, long narrow morphology and strong thermal stratification for 10 months of the year, limit secondary sedimentation processes and preserve the varves, enabling development of an accurate chronology. Damming profoundly altered the physical environment of the former river, especially in the lacustrine zone, where the change is clearly illustrated by diatoms in the sediment. Diatom abundance increased as a consequence of nutrient enrichment after construction of the dam in 1958, but later decreased as the new impoundment stabilized. After damming, relative abundance of Cyclotella increased along with a simultaneous decrease of Navicula and Achnanthes in the lacustrine zone, most significantly in 1963, when Cyclotella replaced Navicula as the dominant genus. This switch was indicative of a general shift from a lotic to a lentic habitat. A longitudinal gradient was apparent in the patterns of sedimentation and diatom accumulation at different sites in the reservoir, with diatom abundance highest in the transition zone. In the long term, water discharge from the reservoir showed a weak, but significant negative correlation with diatom abundance in the lacustrine zone (r = −0.320, P = 0.03). In summary, diatom assemblages in the Liuxihe Reservoir sediments recorded past changes in hydrodynamics, suggesting that paleolimnological study of some impoundments is feasible.     

Response of Pediastrum in German floodplain lakes to Late Glacial climate changes

Subfossil remains of the Pediastrum-algae group (Fam. Hydrodictyaceae) are frequently found in lake sediments, but very few studies have used them as palaeoclimate and palaeolimnological indicators. This study explored the species composition of Pediastrum assemblages in shallow floodplain lakes in northern Germany during two transitional periods from cold to temperate climate conditions (GS-2/GI-1 and GI-1/Holocene). We identified Pediastrum taxa to subspecies level and used multivariate statistics (constrained clustering, principal component analysis and redundancy analysis) to show that shifts in taxonomic composition reflected the strong Late Glacial climate oscillations. The Pediastrum assemblages indicate that climate amelioration already begun shortly before the main GS-2/GI-1 summer-temperature transition. In contrast to previous studies that identified trophic state as the main driver of change in Pediastrum species composition, we identified climate shifts and related factors as the major drivers of community change. Water depth and trophic state probably acted as secondary factors that were responsible for differences in Pediastrum response between the first and second investigated climate transitions. During cold periods, Pediastrum algae composition was controlled mainly by environmental variables, whereas during warm periods, Pediastrum assemblages may have been influenced to a greater extent by intra- and intergeneric competition. This study contributes to knowledge about ecological niches of Pediastrum species. Size-measurements on Pediastrum coenobia reveal that one response of Pediastrum algae to climate shifts probably was size change. Coenobia of Pseudopediastrum boryanum var. boryanum were significantly smaller during cold climate periods than during temperate periods. The shifts in Pediastrum species composition and coenobium size suggest this algae group has great potential for enabling palaeoecological and palaeoclimate inferences.     

Diatoms indicate that calcium decline,not acidification,explains recent cladoceran assemblage changes in south-central Ontario softwater lakes

In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.     

Reconstruction of the recent history of a large deep prealpine lake (Lake Bourget,France) using subfossil chironomids,diatoms, and organic matter analysis: towards the definition of a lake-specific reference state

This paper presents the recent history of a large prealpine lake (Lake Bourget) using chironomids, diatoms and organic matter analysis, and deals with the ability of paleolimnological approach to define an ecological reference state for the lake in the sense of the European Framework Directive. The study at low resolution of subfossil chironomids in a 4-m-long core shows the remarkable stability over the last 2.5 kyrs of the profundal community dominated by a Micropsectra-association until the beginning of the twentieth century, when oxyphilous taxa disappeared. Focusing on this key recent period, a high resolution and multiproxy study of two short cores reveals a progressive evolution of the lake’s ecological state. Until AD 1880, Lake Bourget showed low organic matter content in the deep sediments (TOC less than 1%) and a well-oxygenated hypolimnion that allowed the development of a profundal oxyphilous chironomid fauna (Micropsectra-association). Diatom communities were characteristic of oligotrophic conditions. Around AD 1880, a slight increase in the TOC was the first sign of changes in lake conditions. This was followed by a first limited decline in oligotrophic diatom taxa and the disappearance of two oxyphilous chironomid taxa at the beginning of the twentieth century. The 1940s were a major turning point in recent lake history. Diatom assemblages and accumulation of well preserved planktonic organic matter in the sediment provide evidence of strong eutrophication. The absence of profundal chironomid communities reveals permanent hypolimnetic anoxia. From AD 1995 to 2006, the diatom assemblages suggest a reduction in nutrients, and a return to mesotrophic conditions, a result of improved wastewater management. However, no change in hypolimnion benthic conditions has been shown by either the organic matter or the subfossil chironomid profundal community. Our results emphasize the relevance of the paleolimnological approach for the assessment of reference conditions for modern lakes. Before AD 1900, the profundal Micropsectra-association and the Cyclotella dominated diatom community can be considered as the Lake Bourget reference community, which reflects the reference ecological state of the lake.     

Cyanobacterial influence on diatom community lifeform dynamics in shallow subtropical lakes of Florida USA

Fragilarioid diatom taxa are often deemed ubiquitous in shallow lake systems. Their presence has been described as contributing to statistical noise in paleolimnological studies of cold-temperate lakes. In shallow, warm-temperate lakes of Florida, long-term transitions from assemblages dominated by Aulacoseira spp. to fragilarioid taxa, particularly Pseudostaurosira brevistriata, Staurosira construens var. venter, and Staurosirella pinnata, often occur. Distinctly higher limnetic nutrient optima are demonstrated by these fragilarioid taxa than by planktonic Aulacoseira spp. Community successions occur during eutrophication, and progressive replacement of Aulacoseira spp. and other planktonic taxa by fragilarioid taxa is concurrent with and apparently related to the onset of cyanobacterial dominance. We examine successions from Aulacoseira-dominated to fragilarioid-dominated assemblages in sediment cores from subtropical Florida lakes that have undergone eutrophication. Diatom profiles are compared with sedimented pigments, nitrogen stable isotopes of organic matter, and with silica accumulation rates. These study lakes have little if any macrophyte presence. Their light-extinction depths are extremely shallow, yet diatom communities are dominated by bottom-dwelling rather than planktonic taxa. Frequent wind-generated mixing, sometimes to lake bottoms, is sufficient to sustain the light needs of benthic and tychoplanktonic taxa. We conclude that assemblage changes generally are not caused by reduced water depths, silica limitation, nor increased incipient stratification, but that cyanobacteria are responsible for reducing planktonic Aulacoseira in favor of fragilarioid taxa. Cyanobacteria blooms persist over a wide seasonal range because of warm climate and high limnetic nutrient concentrations in Florida lakes. Cyanobacteria progressively displace and outcompete Aulacoseira and other planktonic taxa as eutrophication proceeds. Reduced light availability, changes in mineral/nutrient availability, and other aspects of competitive exclusion, such as cyanobacterial allelotoxins, might contribute to observed changes. Climate warming is not likely to account for Aulacoseira reduction as in colder regions because it is less pronounced in this subtropical district. Lakes with low nutrient levels and less cyanobacteria still sustain large Aulacoseira populations, and decreases in limnetic nutrients sometimes lead to the return of planktonic Aulacoseira. Rather than simply representing statistical noise for paleolimnological reconstructions, shifts to certain fragilarioid taxa indicate when subtropical Florida lakes progressed to hypereutrophic conditions that were marked by cyanobacterial proliferation.     

<Emphasis Type="Italic">Pediastrum</Emphasis> species as potential indicators of lake-level change in tropical South America

We explored the potential for using Pediastrum (Meyen), a genus of green alga commonly found in palaeoecological studies, as a proxy for lake-level change in tropical South America. The study site, Laguna La Gaiba (LLG) (17°45′S, 57°40′W), is a broad, shallow lake located along the course of the Paraguay River in the Pantanal, a 135,000-km² tropical wetland located mostly in western Brazil, but extending into eastern Bolivia. Fourteen surface sediment samples were taken from LLG across a range of lake depths (2–5.2 m) and analyzed for Pediastrum. We found seven species, of which P. musteri (Tell et Mataloni), P. argentiniense (Bourr. et Tell), and P. cf. angulosum (Ehrenb.) ex Menegh. were identified as potential indicators of lake level. Results of the modern dataset were applied to 31 fossil Pediastrum assemblages spanning the early Holocene (12.0 kyr BP) to present to infer past lake level changes qualitatively. Early Holocene (12.0–9.8 kyr BP) assemblages do not show a clear signal, though abundance of P. simplex (Meyen) suggests relatively high lake levels. Absence of P. musteri, characteristic of deep, open water, and abundance of macrophyte-associated taxa indicate lake levels were lowest from 9.8 to 3.0 kyr BP. A shift to wetter conditions began at 4.4 kyr BP, indicated by the appearance of P. musteri, though inferred lake levels did not reach modern values until 1.4 kyr BP. The Pediastrum-inferred mid-Holocene lowstand is consistent with lower precipitation, previously inferred using pollen from this site, and is also in agreement with evidence for widespread drought in the South American tropics during the middle Holocene. An inference for steadily increasing lake level from 4.4 kyr BP to present is consistent with diatom-inferred water level rise at Lake Titicaca, and demonstrates coherence with the broad pattern of increasing monsoon strength from the late Holocene until present in tropical South America.     

Variability in diatom and chrysophyte assemblages and inferred pH: paleolimnological studies of Big Moose Lake,New York,USA

We measured variability in the composition of diatom and chrysophyte assemblages, and the pH inferred from these assemblages, in sediment samples from Big Moose Lake, in the Adirondack Mountains of New York. Replicate samples were analyzed from (1) a single sediment core interval, (2) 12 different intervals from each of 3 separate cores, and (3) 10 widely spaced surface sediment samples (0–1 cm). The variability associated with sample preparation (subsampling, processing, and counting) was relatively small compared to between-core and within-lake variability. The relative abundances of the dominant diatom taxa varied to a greater extent than those of the chrysophyte scale assemblages. Standard deviations of pH inferences for multiple counts from the same sediment interval from diatom, chrysophyte, and diatom plus chrysophyte inference equations were 0.04 (n=8), 0.06 (n=32), and 0.06 (n=8) of a pH unit, respectively. Stratigraphic analysis of diatoms and chrysophytes from three widely spaced pelagic sediment cores provided a similar record of lake acidification trends, although with slight differences in temporal rates of change. Average standard deviations of pH inferences from diatom, chrysophyte and diatom plus chrysophyte inference equations for eight sediment intervals representing similar time periods but in different cores were 0.10, 0.20, and 0.09 pH unit, respectively. Our data support the assumption that a single sediment core can provide an accurate representation of historical change in a lake. The major sources of diatom variability in the surface sediments (i.e., top 1.0 cm) were (1) differences in diatom assemblage contributions from benthic and littoral sources, and (2) the rapid change in assemblage composition with sediment depth, which is characteristic of recently acidified lakes. Because scaled chrysophytes are exclusively planktonic, their spatial distribution in lake sediments is less variable than the diatom assemblages. Standard deviations of pH inferences for 10 widely spaced surface sediment samples from diatom, chrysophyte and diatom plus chrysophyte inference equations were 0.21, 0.09, and 0.16 of a pH unit, respectively.     

Recent environmental changes on Banks Island (N.W.T., Canadian Arctic) quantified using fossil diatom assemblages

Banks Island (N.W.T.) has become a focal point for climate change studies in the Canadian Arctic. However, long-term climatic and environmental data are very sparse from this large island, as they are for the entire southwestern region of the Canadian Arctic Archipelago. In this paleolimnological study, diatom species assemblage shifts documented in cores collected from a pond and a lake on Banks Island were interpreted to represent a response to climate warming commencing in the nineteenth century. We found that, although the timing and overall nature of the species changes in the two cores were consistent, the signal was muted in the deeper site likely as a result of differences in ice cover extent and duration between lakes and ponds. A high-resolution study was also conducted from a second pond, at sub-decadal resolution, that only spanned the last ∼60 years. In the deeper lake site, Fragilaria construens and F. pinnata dominated the assemblages, similar to those noted in other high Arctic regions where lakes are characterized by extended ice cover. In contrast, Denticula kuetzingii dominated the shallower ponds and, in the case of the pond core representing the longer time period, this taxon increased in the post-1850 sediments, likely coincident with climate warming. In all cores, diatom assemblages became more diverse and Achnanthes species (particularly A. minutissima) increased from ∼1850 to the present, similar to changes documented in other Arctic regions. Beta diversity values calculated for the diatom species changes indicated that assemblage shifts in the Banks Island cores were of similar magnitude to those recorded in other Arctic regions with high species turnover, such as Ellesmere Island. A diatom-based Total Nitrogen (TN) transfer function previously developed for Banks Island was applied to the three ²¹⁰Pb dated cores as an exploratory tool for inferring past changes in nitrogen concentrations. In both the lake and pond cores, diatom-inferred TN concentrations tended to increase in the more recent sediments, as may be expected with warming; however these trends were not very distinct.     

A diatom-based Holocene record of human impact from a coastal environment: Tuckean Swamp, eastern Australia

Diatom-based paleolimnological studies are being increasingly used to track anthropogenic change in estuaries. Little is known, however, about the direction and nature of long-term environmental changes in Australian estuaries. In this study, shifts in diatom assemblages preserved in a ²¹⁰Pb and C¹⁴ AMS dated sediment core from Tuckean Swamp were analysed to determine environmental changes that had taken place as a result of changing land-use practices. Prior to European impact, the diatom assemblage remained relatively stable and was dominated by Actinocyclus normanii and Diploneis smithii. An increasing dominance of Cyclotella meneghiniana correlates well with changed land use activities in the catchment area and indicates an increase of freshwater influence in the swamp’s environment. A major shift in species composition began ∼1970, Eunotia flexuosa becoming dominant. The assemblage shifts recorded at this site appear to be consistent with environmental changes triggered by human activities such as vegetation clearance, drainage and the construction of a barrage. This study demonstrates the use of paleolimnoology in an estuarine environment to provide pre-impact data necessary for management of the aquatic environment.     

WA-PLS diatom-based pH, TP and DOC inference models from 42 lakes in the Abitibi clay belt area (Québec, Canada)

The relationship between surficial sediment diatom species and measured environmental variables was explored in lakes from the Abitibi region of western Quebec. Diatom assemblages in 42 lakes were identified and their relationship with measured environmental variables was examined using multivariate statistical methods. Canonical correspondence analysis with forward selection and Monte Carlo permutation tests revealed that the three environmental variables pH, TP and DOC each accounted for statistically significant fractions of the variation in diatom taxa. A training set with 164 modern diatom taxa was used to derive transfer functions for lake-water pH, TP and DOC using weighted-averaging-partial-least-squares (WA-PLS) techniques. The models were developed to infer lake water pH, TP and DOC within the ranges 4–8 for pH, 2.75–30.0 g l^–1 for TP, and 2.9–18.5 mg l^–1 for DOC. These quantitative inference models may now be used to help identify and estimate the effects of natural disturbances on the biogeochemistry of Abitibi lakes during their historical development.     

Holocene climate change and carbon cycling in a groundwater-fed, boreal forest lake: Dune Lake, Alaska

We compared cladoceran assemblages in modern and pre-industrial sediments from cores taken in 48 softwater lakes located in four main regions of Nova Scotia (Canada) to evaluate the impacts of acidification and other recent environmental stressors. Lakes in Kejimkujik National Park showed significant increases in Holopedium relative abundances and significant decreases in Alona and other chydorids since pre-industrial times, which appear to be related to declines in pH and calcium (Ca) concentrations caused by acidic deposition. Lakes in Bridgewater also showed a significant decrease in Alona, as well as a significant increase in macrothricid (Acantholeberis and Ophryoxus) taxa that cannot be explained by declines in pH, although declines in [Ca] have been recorded. Lakes in Yarmouth did not show any significant regional changes in major cladoceran species groups. Still, pre-industrial assemblages in these lakes significantly differed from modern assemblages, with assemblage changes being lake-specific and likely related to interactions between local and regional stressors acting on individual lakes. Finally, lakes in Cape Breton Highlands National Park, located on the taiga plateau, have received historically lower levels of sulphate deposition relative to other regions in the province, and recorded a significant decrease in Alona similar to Bridgewater lakes. The province-wide decrease in Alona across different acid deposition rates suggests that limnological changes related to climate warming may be responsible. Overall, this study shows that acidification history is an important predictor of cladoceran assemblage changes since pre-industrial times in Nova Scotia lakes, although multiple stressors result in complex Cladocera responses in some regions.     

A record of hypolimnetic oxygen conditions in a temperate multi-depression lake from chemical evidence and chronomid remains

A multiproxy paleolimnological study of Douglas Lake, Michigan, was undertaken to elucidate the history of productivity and oxygen depletion in three basins of this multi-depression lake. Indicators investigated in three dated cores included chlorophyll a, Fe and Mn stratigraphy, and fossil chironomid assemblages. The coring sites were chosen to correspond to modern studies of oxygen depletion rates, and to determine if conclusions reached in these studies were supported by paleolimnological evidence. Stratigraphies of chlorophyll a, Fe and Mn indicate that two of the basins, South Fishtail Bay and Fairy Island, have been eutrophic and anoxic for a long period of time, predating European settlement. The third basin, Grapevine Point, has been consistently less productive, and had less severe oxygen depletion. Results of the chironomid analysis agree with these conclusions, including a change from mesotrophic to eutrophic indicator taxa in the Grapevine Point basin. All three cores show evidence of increasing trophic state in the most recent sediments, supporting some of the conclusions reached in the modern studies. It is also demonstrated that deforestation of the watershed had profound effects on littoral chironomid assemblages. Paleolimnological investigations also demonstrated the individual nature of the separate basins in Douglas Lake.     

A reference typology of low alkalinity lakes in the UK based on pre-acidification diatom assemblages from lake sediment cores

This paper has two aims: (1) to show for the first time how a natural typology can be established using palaeoecological methods; and (2) to show how it can be used in lake restoration studies with respect to the definition of recovery targets for acidified lakes. By defining the characteristic reference assemblages for low alkalinity site types rather than for a specific site it allows success to be measured more broadly, unconstrained by the specific composition of the pre-acidification flora. We analyse statistically the pre-acidification diatom assemblages of sediment cores from 121 low alkalinity lakes in the UK in order to assess whether a reference typology for such lakes can be defined on the basis of their diatom floras. We use samples dating to ~1850 AD to represent pre-acidification conditions. The results show that three main clusters can be identified, two dominated by benthic taxa (Clusters 1 and 3) and one dominated by planktonic taxa (Cluster 2). Cluster 1 is characterised by taxa such as Brachysira vitrea, Cymbella microcephala and Fragilaria spp., Cluster 2 by Cyclotella comensis, C. radiosa, Asterionella formosa, Aulacoseira subarctica and Achnanthes minutissima and Cluster 3 by Eunotia incisa, Frustulia rhomboides var. saxonica, Fragilaria virescens var. exigua, and Cymbella perpusilla. Although environmental data for 1850 AD are not available it is apparent from the contemporary distribution of the taxa in the different clusters that Cluster 2 represents the most alkaline pre-acidification conditions. Some sites in this cluster have been acidified, but some, especially the larger, deeper lakes have been enriched. Cluster 1 includes sites that contain diatoms with relatively high pH optima (pH 6.4–7.4) whereas Cluster 3 sites contain diatoms with the lowest pre-acidification pH conditions in the data-set. Sites in this cluster also have the lowest base cation concentrations at the present day and include the sites in the UK that have been most affected by acid deposition.     

Diatom-based total phosphorus (TP) and pH transfer functions for the Irish Ecoregion

A 72-lake diatom training set was developed for the Irish Ecoregion to examine the response of surface sediment diatom assemblages to measured environmental variables. A variety of multivariate data analyses was used to investigate environmental and biological data structure and their inter-relationships. Of the variables used in determining a typology for lakes in the Irish Ecoregion, alkalinity was the only one found to have a significant effect on diatom assemblages. A total of 602 diatom taxa were identified, with 233 recorded at three or more sites with abundances ≥1%. Generally diatom data displayed a high degree of heterogeneity at the species level and non-linear ecological responses. Both pH and total phosphorus (TP) (in the ranges of 5.1–8.5 and 4.0–142.3 μg l⁻¹ respectively) were shown to be the most significant variables in determining the surface sediment diatom assemblages. The calibration models for pH and TP were developed using the weighted averaging (WA) method; data manipulation showed strong influences on model performances. The optima WA models based on 70 lakes produced a jack-knifed coefficient of determination (r ² _jack) of 0.89 with a root mean squared error (RMSEP) of 0.32 for pH and r ² _jack of 0.74 and RMSEP of 0.21 (log₁₀ μg l⁻¹) for TP. Both models showed strong performances in comparison with existing models for Ireland and elsewhere. Application of the pH and TP transfer functions developed here will enable the generation of quantitative water quality data from the expanding number of palaeolimnological records available for the Irish Ecoregion, and thus facilitate the use of palaeolimnological approaches in the reconstruction of past lake water quality, ecological assessment and restoration.     

Distribution and Autecology of Chrysophyte Cysts from High Arctic Svalbard Lakes: Preliminary Evidence of Recent Environmental Change

Chrysophycean stomatocyst assemblages were analysed from the sediments of 17 lakes and ponds from Svalbard as one component of a multi-proxy investigation of recent environmental change in the high Arctic. Sediment cores and water chemistry were collected from each of the study lakes, and chrysophyte stomatocysts were investigated from the top 0.25 cm of sediment (present-day) and bottom (i.e., bottom of short sediment core, pre-industrial) sediment samples. This study represents the first undertaking of chrysophyte cyst morphology and distribution on Svalbard. A total of 153 cyst morphotypes were described with light microscopy and/or scanning electron microscopy, of which 21 are new forms. Canonical correspondence analysis indicates that the present-day distribution of cysts is significantly related to pH (p= 0.02), altitude (p= 0.02), and Na⁺ (p= 0.04). Marked shifts in chrysophyte cyst assemblages were recorded between the top and bottom sediment samples of most lakes. A recent study has demonstrated that Svalbard lakes receive atmospheric contaminants from both local and remote sources. The observed assemblage shifts may be the result of the combined effects of these point sources and long-range pollutants, or the effects of recent climate change, or both.