Brian F. Cumming

Rainfall and drought in equatorial east Africa during the past 1,100 years

Knowledge of natural long-term rainfall variability is essential for water-resource and land-use management in sub-humid regions of the world. In tropical Africa, data relevant to determining this variability are scarce because of the lack of long instrumental climate records and the limited potential of standard high-resolution proxy records such as tree rings and ice cores. Here we present a decade-scale reconstruction of rainfall and drought in equatorial east Africa over the past 1,100 years, based on lake-level and salinity fluctuations of Lake Naivasha (Kenya) inferred from three different palaeolimnological proxies: sediment stratigraphy and the species compositions of fossil diatom and midge assemblages. Our data indicate that, over the past millennium, equatorial east Africa has alternated between contrasting climate conditions, with significantly drier climate than today during the ‘Medieval Warm Period’ (∼ ad 1000–1270) and a relatively wet climate during the ‘Little Ice Age’ (∼ ad 1270–1850) which was interrupted by three prolonged dry episodes. We also find strong chronological links between the reconstructed history of natural long-term rainfall variation and the pre-colonial cultural history of east Africa, highlighting the importance of a detailed knowledge of natural long-term rainfall fluctuations for sustainable socio-economic development.

The Widespread Threat of Calcium Decline in Fresh Waters

Calcium concentrations are now commonly declining in softwater boreal lakes. Although the mechanisms leading to these declines are generally well known, the consequences for the aquatic biota have not yet been reported. By examining crustacean zooplankton remains preserved in lake sediment cores, we document near extirpations of calcium-rich Daphnia species, which are keystone herbivores in pelagic food webs, concurrent with declining lake-water calcium. A large proportion (62%, 47 to 81% by region) of the Canadian Shield lakes we examined has a calcium concentration approaching or below the threshold at which laboratory Daphnia populations suffer reduced survival and fecundity. The ecological impacts of environmental calcium loss are likely to be both widespread and pronounced.

Diatoms as indicators of surface-water acidity

Lake acidification became an environmental issue of international significance in the late 1960s and early 1970s when Scandinavian scientists claimed that ‘acid rain’ was the principal reason why fish populations had declined dramatically in Swedish and Norwegian lakes (Oden, 1968; Jensen & Snekvik, 1972; Almer et al., 1974). Similar claims were being made at about the same time in Canada (Beamish & Harvey, 1972). However, these claims were not immediately accepted by all scientists. It was argued instead that acidification was due to natural factors or to changes in catchment land-use and management (Rosenqvist 1977, 1978; Pennington 1984; Krug & Frink, 1983). In the scientific debate that followed, diatom analysis played a pivotal role. It enabled the timing and extent of lake acidification to be reconstructed (Charles et al., 1989; Battarbee et al., 1990; Dixit et al., 1992a) and allowed the various competing hypotheses concerning the causes of lake acidification to be evaluated (Battarbee et al., 1985; Battarbee & Charles 1994; Emmett et al., 1994). However, diatoms had been recognized and used as indicators of water pH well before the beginning of this controversy. The ‘acid rain’ issue served to highlight the importance of diatoms and stimulated the advance of more robust and sophisticated techniques, especially the development of transfer functions for reconstructing lakewater pH and related hydrochemical variables. This chapter outlines the history of diatoms as pH indicators, and describes how diatoms are currently used in studies of acid and acidified waters.

Lake sediments record large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia

Six high-resolution climatic reconstructions, based on diatom analyses from lake sediment cores from the northern prairies of North America, show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia. The predominant broad-scale pattern seen at all sites is a major shift in moisture regimes from wet to dry, or vice versa (depending on location), that occurred after a period of relative stability. These large-scale shifts at the different sites exhibit spatial coherence at regional scales. The three Canadian sites record this abrupt shift between anno Domini 500 and 800, and subsequently conditions become increasingly variable. All three U.S. sites underwent a pronounced change, but the timing of this change is between anno Domini 1000 and 1300, thus later than in all of the Canadian sites. The mechanisms behind these patterns are poorly understood, but they are likely related to changes in the shape and location of the jet stream and associated storm tracks. If the patterns seen at these sites are representative of the region, this observed pattern can have huge implications for future water availability in this region.

A 10,000-year high-resolution diatom record from Pilkington Bay, Lake Victoria, East Africa

A new diatom record from Lake Victoria’s Pilkington Bay, subsampled at 21- to 25-year intervals and supported by 20 AMS dates, reveals a ∼10,000 calendar year environmental history that is supported by published diatom and pollen data from two nearby sites. With their chronologies adjusted here to account for newly documented ancient carbon effects in the lake, these three records provide a coherent, finely resolved reconstruction of Holocene climate change in equatorial East Africa. After an insolation-induced rainfall maximum ca. 8800–8300 cal yr B.P., precipitation became more seasonal and decreased abruptly ca. 8200 and 5700 yr B.P. in apparent association with northern deglaciation events. Century-scale rainfall increases occurred ca. 8500, 7000, 5800, and 4000 yr B.P. Conditions after 2700 yr B.P. were generally similar to those of today, but major droughts occurred ca. 1200–600 yr B.P. during Europe’s Medieval Warm Period.

Diatom-salinity relationships in 111 lakes from the Interior Plateau of British Columbia, Canada: the development of diatom-based models for paleosalinity reconstructions

Diatoms were identified and enumerated from the surface sediments of 111 lakes, 45 from the Kamloops region and 66 from the Cariboo/Chilcotin region, located on the southern Interior Plateau of British Columbia, Canada. This paper is an extension of another study which investigated the relationship of diatoms to salinity and ionic composition in 65 lakes from the Cariboo/Chilcotin region. The 111 lakes spanned a large gradient in salinity, ranging from fresh through hypersaline (late-summer salinity values ranged from 0.04 to 369 g l−1), and included both carbonate- and sulphate-dominated lakes with sodium and magnesium as the dominant cations. The Kamloops region had more sulphate-dominated, hypersaline lakes and fewer carbonate-rich lakes than the Cariboo/Chilcotin region. Most lakes had higher salinities in the late-summer compared to the spring.Both salinity and brine-type were important variables that could explain the different diatom assemblages present in the lakes. The majority of diatom taxa had salinity optima in the freshwater to subsaline range (<3 g l−1), and the taxa displayed a range of both narrow and broad tolerances along the salinity gradient. Weighted-averaging regression and calibration, and maximum likelihood techniques were used to develop salinity inference models from the diatom assemblages based on their relationship to the spring, late-summer and average lakewater salinity measurements. Simple weighted-averaging (WA) models generally produced the same or lower bootstrapped RMSEs of prediction than weighted-averaging with tolerance downweighting (WA(tol)) in the two regional and the combined datasets. Weighted averaging partial least squares (WA-PLS) showed little or no improvement in the predictive abilities of the datasets, as judged by the jackknifed RMSE of prediction. In all cases, the combined dataset of 102 lakes performed better than either of the smaller regional datasets, with relatively little difference between spring, average and late-summer salinity models. The maximum likelihood models gave lower apparent RMSEs of prediction in comparison to other methods; however, independent validation of this technique using methods such as bootstrapping were not undertaken because of the computer intensive nature of such analyses. These diatom-based salinity models are now available for reconstructing salinity and climatic trends from appropriately chosen closed-basin lakes in the Interior region of British Columbia.

Diatom assemblages from Adirondack lakes (New York, USA) and the development of inference models for retrospective environmental assessment

Detrended canonical coreespondence analysis (DCCA) was used to examine the relationships between diatom species distributions and environmental variables from 62 drainage lakes in the Adirondack region, New York (USA). The contribution of lakewater pH, Alm (monomeric Al), NH4, maximum depth, Mg, and DOC (dissolved organic carbon) were statistically significant in explaining the patterns of variation in the diatom species composition. Twenty-three and sixteen diatom taxa were identified as potential indicator species for pH and Alm, respectively (i.e. a taxon with a strong statistical relationship to the environmental variable of interest, a well defined optimum, and a narrow tolerance to the variable of interest). Using weighted-averaging regression and calibration, predictive models were developed to infer lakewater pH (r2=0.91), Alm (r2=0.83), DOC (dissolved organic carbon) (r2=0.64), and ANC (acid neutralizing capacity; r2=0.90). These variables are of key importance in understanding watershed acidification processes. These predictive models have been used in the PIRLA-II (Paleoecological Investigation of Recent Lake Acidification-II) project to answer policy-related questions concerning acidification, recovery, and fisheries loss.

SCALED CHRYSOPHYTES (CHRYSOPHYCEAE AND SYNUROPHYCEAE) FROM ADIRONDACK DRAINAGE LAKES AND THEIR RELATIONSHIP TO ENVIRONMENTAL VARIABLES1

The relationships between 23 scaled chrysophyte taxa (Chrysophyceae and Synurophyceae) and measured limnological variables in 62 Adirondack, New York, drainage lakes were examined by canonical correspondence analysis (CCA). The major proportion of variation in chrysophyte species distributions was strongly related to total monomeric Al (Alm) and Mg concentrations, and their close correlates pH, Na, Ca, and acid‐neutralizing capacity (ANC). Total monomeric Al concentrations explain a greater proportion of species variation than pH, suggesting that Alm concentrations may be more important in governing the distribution of chrysophyte taxa in these lakes. Gaussian logit (GL) and linear logit (LL) regressions of the relative percentages of individual chrysophyte taxa to lakewater pH and Alm concentrations and the examination of pH–Alm response surfaces show that many chrysophyte taxa exhibit unique responses to these environmental gradients; taxa can be characterized as alkaline, circumneutral, acidic, and pH indifferent. Within each of these groups, taxa can be characterized further based upon their optima and tolerances to Alm concentrations. Chrysophyte indicator species (i.e. a taxon with a strong statistical relationship to the environmental variable of interest, a well‐defined optimum, and a narrow tolerance to the variable of interest) for pH include Mallomonas hindonii, M. crassisquama, M. pseudocoronata, and Synura uvella; M. hindonii, M. crassisquama, M. pseudocoronata, S. petersenii, and S. spinosa are good indicators of Alm concentrations. Highly significant predictive models were developed to infer lakewater pH and Alm concentrations from the relative percentages of chrysophyte scales in the study lakes. Model evaluation was based on their correlation coefficients and the root‐mean‐squared error of prediction (RMSE) derived from bootstrapping. Weighted averaging regression and calibration with tolerance down‐weighting (i.e. weighting taxa inversely to their variance) produced superior results when compared to the computationally and data‐demanding maximum likelihood methods and to simple weighted averaging regression and calibration.

Development of diatom-based salinity models for paleoclimatic research from lakes in British Columbia (Canada)

Diatoms were identified and enumerated from the surface sediments of 65 lakes located on the Cariboo and Chilcotin Plateaux (British Columbia, Canada). These lakes span a large gradient in lakewater ionic concentration (fresh through hypersaline) and composition, as well as other physical/chemical variables. Almost all of the study lakes had higher salinities in the late-summer than in the spring. The lakes with spring salinities > 8 g l−1 showed the largest seasonal increases in salinity Ionic composition was similar in the spring and late-summer for most lakes. Both ionic concentration (i e salinity) and composition were important environmental variables that could account for the different diatom floras in the lakes. Diatom assemblages characteristic of carbonate-dominated and sulfate-dominated waters were identified. Other variables such as water depth and phosphorus concentration were also important.

Multi-proxy Holocene palaeoclimatic record from a saline lake in the Canadian Subarctic

Multi-proxy palaeolimnological analyses of a postglacial sedimentary sequence at a centennial-scale resolution from an athalassic saline lake in the Yukon were conducted to infer patterns of Holocene climatic change in the Canadian Subarctic, using sediment mineralogy and biostratigraphy (diatoms, pigments). Diatominferred quantitative estimates of palaeosalinity were obtained by use of transfer functions developed from a calibration set of 219 lakes from western North America. The sediment mineralogy and fossil pigments at the base of the core indicated a moderately deep non-stratified lake dominated by clastic influx, probably in a basin fed by glacial meltwater. The early-Holocene history (c. 11 000–8100 14C yr BP) was characterized by a relatively deep mesosaline lake with diatom-inferred salinities approximating 20 g L-1. The occurrence of both aragonite and dolomite, as well as elevated concentrations of chlorophyll and carotenoid pigments, support the interpretation of deepwater anoxia and possibly strong chemical stratification. High concentrations of the chemically stable b-carotene suggest that total algal abundance was particularly high during the early Holocene, when planktonic Cyclotella cf. choctawhatcheeana and Chaetoceros muelleri were the most common diatom taxa. Relatively fresh (2–15 g L-1) eutrophic conditions prevailed during the mid-Holocene period (c. 8000– 2000 yr BP), with four periods of alternating fresh and saline conditions. The diatom-inferred salinity profile reveals significant fluctuations within these cycles, but overall they indicate humid climatic conditions compared to today. Algal abundance is inferred to have declined three-fold relative to the early Holocene, particularly in the case of eukaryotic algae (e.g., diatoms, cryptophytes, chlorophytes). The recent history of the lake (about 2000 years BP until the present day) was marked by important changes in ionic composition (e.g., occurrence of gypsum and Mg-carbonates) and hydrologic conditions. The lakewater composition during the last two millennia was characterized by hypersaline Mg-SO4 brines. The palaeolimnological evidence from most proxies indicates a trend towards drier conditions during the past 2000 years. The various indicators reveal a complex history of frequent and rapid shifts in palaeosalinity and lake palaeoproductivity during the Holocene, and the effects of the Younger Dryas and ‘Little Ice Age’ episodes may be recorded in the palaeoclimate proxy data. The palaeoclimatic interpretation emerging from this high-latitude lake corroborates existing broad trends based on palynological studies in this region but provides evidence for more dynamic climatic change during the mid- and late Holocene.