Introduction

The term eutrophication broadly refers to the enrichment of aquatic systems by inorganic plant nutrients (Mason, 1991; Wetzel, 2001). Lake eutrophication occurs when nutrient supplies, usually phosphorus (P) and nitrogen (N), are elevated over rates that occur in the absence of any system perturbation, and results in increased production. Causes of eutrophication include human (anthropogenic eutrophication) and non-human (natural eutrophication) disturbances. Marked natural eutrophication events are relatively rare and may result from dramatic episodes, such as forest fire (e.g. Hickman et al., 1990), tree die-off (Boucherle et al., 1986; Hall & Smol, 1993; St. Jacques et al., 2000) and prolific returns of spawning salmon to nursery lakes (Gregory-Eaves & Keatley, this volume), to name a few mechanisms. Climatic episodes, such as droughts, may also concentrate lake-water nutrients by increasing contributions of nutrient-rich groundwater (e.g. Webster et al., 1996), or reducing flushing rates and increasing deepwater anoxia leading to elevated internal P loading from sediments to the illuminated surface waters (Brüchmann & Negendank, 2004). Some lakes lie in naturally fertile catchments or receive high natural loads of nutrients from groundwater and are naturally eutrophic (e.g. Hall et al., 1999). In most cases, however, eutrophication is caused by anthropogenic nutrient inputs from domestic and industrial sewage disposal, farming activities, soil erosion, and numerous other activities.

Eutrophication is the most widespread form of lake pollution on a global scale, and has many deleterious effects on aquatic systems (Harper, 1992; Smith et al., 2006). In addition to increasing overall primary production, eutrophication causes considerable changes to biochemical cycles and biological communities (Schelske, 1999). Marked changes occur at all levels in the food web and entire communities can change or die out (Carpenter et al., 1995).

Introduction

Lakes are intricately tied to the climate system in that their water level and chemistry are a manifestation of the balance between inputs (precipitation, stream inflow, surface runoff, groundwater inflow) and outputs (evaporation, stream outflow, groundwater recharge) (Mason et al., 1994). Hence, changes in a lake's hydrologic budget caused by climatic change have the potential to alter lake level and lake chemistry. These changes, in turn, may affect the physiological responses and species composition of the lake's biota, including diatoms. Here we review the use of diatoms as indicators of hydrologic and climatic change, with an emphasis on environmental reconstruction in arid and semi-arid regions. First we discuss linkages among climate, hydrology, lake hydrochemistry, and diatoms that form the foundation for environmental reconstruction, and then we review selected examples of diatom-based studies.

Lake hydrology and hydrochemistry

Lakes vary in their hydrologic sensitivity to climatic change (Winter, 1990). In basins with a surface outlet, lake-level increase is constrained by topography, and any change in input is usually balanced by outflow. Thus, in open basins, lake level fluctuates relatively little, unless hydrologic change is sufficiently large to drop water level below the outlet level. In contrast, closed-basin lakes without surface outflow often show changes in level associated with changes in the balance between precipitation and evaporation (P – E).

Introduction

The significance of relative sea level during the late Quaternary is recognized by disciplines across the Earth sciences. Sea-level histories are important for calibrating and constraining geophysical models of Earth's rheology and glacio-isostatic adjustment (e.g. Peltier, 2004). Sea level is crucial to any study of coastal evolution as it serves as the ultimate baseline for continental denudation (Summerfield, 1991). For human populations, sea levels during the late Quaternary have been an important factor in sustaining coastal communities and may have profoundly influenced the very initiation of human civilization (e.g. Turney and Brown, 2007). Publication of reports from the Intergovernmental Panel on Climate Change (IPCC, 2007) re-emphasized the importance of sea level as a barometer of climate and drew attention to the potentially devastating consequences of future climate-related sea-level change (e.g. Rahmstorf, 2007). However, the IPCC also highlighted the uncertainty with which the driving mechanisms of sea-level change are understood and the disconnection between long-term geological and recent observational trends. Predictions of sea level for the twenty-first century rely on models, and the veracity of model output is based on verification against observations. Interpretation of these observations requires great care in light of the large spatial and temporal variability in relative sea-level change (Milne et al., 2009).

Sea level is far from a constant, planar surface and exhibits spatial and temporal changes at a multitude of scales. To the observer, these changes are manifestations of relative sea level, a term which reflects the uncertainty in separating the often simultaneous contributions from movements of the ocean surface and land (Shennan, 2007).

Introduction

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 (Odén, 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 by some that acidification was due to natural factors or to changes in catchment land use and management (Rosenqvist 1977, 1978; Krug & Frink, 1983; Pennington, 1984).

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 lake-water 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. It then describes how diatom-based paleolimnological methods have been used to trace the pH and acidification history of lakes and how diatoms are being used to monitor acidity trends in streams and lakes.

Introduction

High Arctic environments continue to receive increased attention from the scientific community, policy makers, and the public at large because polar regions are considered to be especially sensitive to the effects of global climatic and other environmental changes (Rouse et al., 1997; ACIA, 2004; IPCC, 2007). Polar lakes and ponds, and the biota they contain, are important sentinels of environmental changes (Pienitz et al., 2004; Schindler & Smol, 2006) and have thus been the focus of many research programs (Vincent & Laybourn-Parry, 2008).

There is considerable potential for using living and fossil diatom assemblages to track environmental trends in High Arctic regions (Smol & Douglas, 1996; Douglas et al., 2004a). A growing number of studies have examined the taxonomy, ecology, and paleoecology of High Arctic diatoms, as lakes and ponds are dominant features of most Arctic landscapes. Given the diversity and vastness of these regions, many exciting research opportunities exist. For example, about 18% (by area) of Canada's surface waters are situated north of 60 °N (Statistics Canada, 1987), and Sheath (1986) estimated that tundra ponds cover approximately 2% of the Earth's surface. The heightened interest in High Arctic environments, coupled with an increased accessibility of these remote regions (e.g. by helicopter), has resulted in a recent surge of interest in Arctic diatom research. Whilst some proxy techniques, such as palynology and dendroecology, have limited applicability in some High Arctic regions due to the paucity of higher plants (Gajewski et al., 1995), paleolimnological approaches using diatoms have become especially important for studies of long-term global environmental change.

Introduction

Diatoms are major contributors to total primary production as well as many important biogeochemical processes in aquatic environments (Falkowski et al., 1998; Smetacek, 1999). Nonetheless, a small number of species (<30) have been recognized as harmful to fisheries, wildlife, or people, through production of either a toxin or various exudates, or via mechanical damage due to cell morphology and/or high biomass accumulation. Fryxell & Villac (1999), and more recently Fryxell & Hasle (2003), have identified several of these harmful taxa and outlined their often devastating impacts on other organisms, ecosystems, and economies. Examples include: oily surface films associated with bird mortalities (Coscinodiscus centralis Ehrenb., Coscinodiscus concinnus W. Smith); surface accumulations on beach surf-zones affecting tourism/recreation (Asterionellopsis glacialis [Castracane] Round, Anaulus australis Drebes & D. Schulz; in Villac & Noronha, 2008); mucilage production causing a condition known as “mare sporco” (or dirty sea) (Ceratoneis closterium Ehrenb., Pseudo-nitzschia pseudodelicatissima [Hasle] Hasle) as well as clogging of bivalve gills (Thalassiosira mala Takano) and fishing nets (Guinardia striata [Stolterfoth] Hasle, Coscinodiscus wailesii Gran & Angst); high biomass accumulations resulting in shading and depletion of oxygen/nutrients (C. wailesii) as well as clogging gills of benthic shellfish and bony fish (Cerataulina pelagica [Cleve] Hendey); and spines/setae inflicting physical damage to fish gills leading to major financial losses for aquaculture operations (Chaetoceros convolutus Castracane and Chaetoceros concavicornis Mangin).

Introduction

The polar regions, both Arctic and Antarctic, show strong evidence of climate change affecting freshwater species, communities, and ecosystems, and are expected to undergo rapid and continued change in the future (IPCC, 2007). Diatoms in the freshwater and brackish habitats of inland waters of the Antarctic provide valuable records of their historic and modern environmental status. Antarctic habitats also contain a unique biodiversity of species many of which are found nowhere else on Earth. In this chapter, we review investigations using diatoms as indicators of environmental change in Antarctic and subantarctic island habitats, including lakes and ponds, streams and seepage areas, mosses and soils, cryoconite holes, brine lakes, and remarkable subsurface glacial lakes.

The Antarctic continent holds the vast majority of the Earth's freshwater, but the water is largely inaccessible because it is in the form of ice. Life is dependent upon liquid water, a substance scarce in Antarctica. Less than 0.4% of the continent is ice free, and it is within these ice-free regions that freshwater lakes and ephemeral streams form, fed by the melting of snow and glacial ice and occasional precipitation. These ice-free regions are located primarily near the Antarctic coastline (Figure 14.1). Of these regions, the “desert oases” of East Antarctica are considered to be the coldest, driest regions on Earth. In the limited parts of these oases where liquid water is available, even if present for only a few short weeks of the year, there is life (McKnight et al., 1999).