Introduction

As White and Frady (1995) say in the Preface of their recent international directory on experts in toxic and harmful algae, ‘Toxic and harmful algal blooms present a growing global problem for fisheries, aquaculture, and public health.’ With entries from 58 countries, they list 22 countries with names and addresses of people working with harmful diatoms and/or their toxins: Australia (3), Canada (29), Chile (1), Croatia (1), Denmark (4), Germany (3), India (3), Israel (1), Japan (6), Netherlands (3), New Zealand (3), Norway (4), People's Republic of China (17), Republic of Korea (2), Romania (1), Russian Federation (1), Spain (6), Thailand (1), Turkey (1), United Kingdom (2), United States of America (29), and Vietnam (1), for a total of 122 workers around the world. One such list of international specialists was compiled by Woods Hole Oceanographic Institution Sea Grant Program in 1990 (subsequently updated), so that the consequences of outbreaks of toxic and harmful algal bloom events on fisheries and public health could be reduced. The purpose of this summary chapter is to serve as a resource for those faced with the challenges brought about by the changing dominant coastal diatom flora. In the study of toxic and harmful diatoms, there are applications for fisheries, public health institutions, mariculture, and tourism.

Harmful blooms

‘A first step in applied ecology is to accurately define the present state of the environment’, according to Rowe (1996, p. 7), and progress in this endeavor has been made.

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 the hydrologic budget, caused by either climatic change or human activity, have the potential to alter lake level and lake chemistry. These, in turn, may affect the physiological responses and species composition of the lake's biota, including those of 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 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, that is lakes without surface outflow, often show changes in level associated with changes in the balance between precipitation and evaporation (P – E). The magnitude of response to fluctuations in P – E depends on the relative contribution of groundwater inflow and outflow to the hydrologic budget; lake-level change is greatest in terminal basins, which have neither surface nor groundwater outflow.

At this point it is appropriate, if not necessarily wise, to attempt a brief view into the future. By its very nature, scientific research is not usually kind to those who would attempt to plan it, or even forecast its future direction.

Be this as it may, it seems to us that some of the immediate future directions in applied diatom studies seem almost foreordained. It is very clear that a good deal of effort needs to be devoted to the formalities of taxonomy and nomenclature, which have been sadly neglected for the past century. Great strides have been made very recently in the alpha level taxonomy of diatoms (e.g., Lange-Bertalot & Metzelin, 1996). It has also become much more common for diatomists to document their work in published iconographs (e.g., Douglas & Smol, 1993; Cumming et al., 1995) followed by deposition of properly vouchered material from major studies. Even more promising, the application of modern systematic techniques to diatoms (e.g., Kociolek & Stoermer, 1989; Kociolek et al., 1989; Theriot & Stoermer, 1984; Williams, 1985) is becoming more and more established. However, it is also true that relatively few diatomists are formally trained systematists. The very increase in interest in diatom taxonomy, particularly that part fueled by practical applications, has left behind it a virtual morass of nomenclatorial problems. Unfortunately, many well-intentioned attempts to alleviate the situation have resulted in inappropriate synonymies, conservations, and circumscriptions, which only serve to further complicate it.

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 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.

Introduction

One of the goals of modern archeology is to understand how past communities interacted spatially, economically, and socially with their biophysical environment (Butzer, 1982). To this end, archeologists have developed strong links with zoologists, botanists and geologists to provide information on the environment of past societies and to help understand the complex relationships between culture and environment. This chapter reviews the role of diatom analysis in such studies, and discusses how the technique can be applied at a range of spatial and temporal scales to place archeological material in its broader site, landscape and cultural context. In particular, we examine applications to the provenancing of individual archeological artefacts, the analysis of archeological sediments and processes of site formation, the reconstruction of local site environments, and the identification of regional environmental processes affecting site location and the function of site networks. We have chosen a small number of examples that best illustrate these applications; other case studies directly motivated by archeological problems may be found in recent reviews by Battarbee (1988), Mannion (1987) and Miller and Florin (1989), while diatom-based studies of past changes in sea level, climate, land-use, and water quality that are also relevant to archeological investigation are reviewed elsewhere in this volume (e.g., Bradbury; Cooper; Denys & de Wolf; Fritz et al.; Hall & Smol).

Analysis of archeological artefacts

The direct application of diatom analysis to archeological artefacts is best represented in the field of pottery sourcing and typology.

Introduction

In 1846 Charles Darwin wrote:

Darwin sent some of this dust to Ehrenberg (1844) who found diatoms. Windblown material is widespread (Pye, 1987) and the study of aeolian diatoms can indicate past changes in atmospheric circulation. The most abundant aeolian diatoms are freshwater ones blown from North Africa into the intertropical Atlantic, where they often outnumber the marine ones (Pokras & Mix, 1985).

Most diatoms grow in water bodies where the surface tension film protects them from light winds. Stronger winds pick up water and diatoms which can act as nuclei for raindrops and so often drop out. Schlichting (1964) trapped more soil algae from updrafts on cloudy days; but he caught few live diatoms. Marshall and Chalmers (1997) only trapped broken cells of Pinnularia borealis Ehrenberg; when alive these motile diatoms avoid dry surfaces. Diatom remains from dry fossil deposits and algal flakes from dried-up water bodies are more readily picked up by the wind.

Introduction

The legal process has used scientific procedures for many years in its various deliberations. Some of these, for instance the DNA profiling of body fluids, are now essential and routine practice. The use of diatoms in forensic science is naturally much smaller, but in certain types of investigation, diatom taxonomy and ecology play a significant role. The diatomologist may be able to provide the investigation with evidence, which will enable the court to reach its verdict, and may be used by either the prosecution or the defence.

Drowning

The most frequent application of diatoms in forensic science is in the diagnosis of death by drowning. Drowning is a very common accidental cause of death, and thousands die each year in this fashion. The majority of these individuals die in circumstances which are not contentious, where there are witnesses, or strong indications of suicide such as a note. Where the circumstances surrounding the individual's death are less clear, then it is often important to be as certain as one can be of how death occurred.

Where the body is fresh, the pathologist may have little difficulty in reaching a verdict of drowning. However, the histopathological signs of drowning are often transient and overlaid by the grosser effects of decomposition. Additionally, in cases where an individual has been severely injured before being immersed in water, it is obviously important to determine whether death is due to these injuries or because of drowning.

Introduction

Timber line represents the most prominent ecotone in mountainous and arctic regions. It is characterized by the transition from closed forest to the most advanced solitary trees (i.e., timber line), to single tree islands (i.e., treeline), and eventually to unforested vegetation. This biological boundary can vary in width between tens of meters and many kilometers. In northern Europe it is formed by deciduous trees (Betula, Alnus, Populus), whereas coniferous trees (Pinus, Picea, Larix) form treeline in the Alps, northern North America and Eurasia.

Treeline is primarily related to cold temperatures but a complex set of different climatic factors, as well as the specific adaptation of trees, actually defines the forest limit (e.g., Tranquillini, 1979). This is evident from the decrease in altitude of treeline from subtropical to arctic regions and, on a smaller scale, by the higher forest limit on southern slopes compared to northern slopes (Ellenberg, 1986). In the Alps, timber line represents the transition between the subalpine and the alpine belts (Ozenda, 1985; Ellenberg, 1986). The lower boundary of the alpine belt, however, is difficult to locate as human impact, grazing and climatic oscillations have lowered natural tree limit by several hundred meters in the last millennia (e.g., Lang, 1994; Tinner et al., 1996).

In the north, physical and biotic features are sufficiently distinct to unequivocally separate ‘arctic’ from ‘boreal’ regions. However, great disparity exists among definitions as to where the Boreal region ends and the Arctic region begins (Larsen, 1989).

Introduction

Water level changes result from a variety of geological, biological or climatic processes. Many of these changes occur over long periods of time; however, some may be rapid or result from catastrophic events. In glaciated regions, water level changes are influenced by the geological process of isostatic rebound. As the weight of glacial ice is removed from the Earth's surface, depressed surfaces adjust upward, while adjacent areas may subside (i.e., Larsen, 1987). Lakes are often formed as embayments become isolated from larger water bodies, and existing lakes can reinvade subsiding regions. Differential rates of rebound within large lake basins may affect drainage patterns and water levels. Transport of sediments across river mouths or embayments can also create lakes and increase water levels.

In most cases, diatom microfossils from lake sediments can be used to identify the above changes. The clearest diatom signals are found in areas where lakes are isolated from marine or brackish waters (Denys & de Wolf, this volume). In freshwater systems, however, salinity gradients are absent, and signals are generally recorded as increases of deep-water or planktonic forms. Water level changes in lakes affected by isostatic rebound and sediment-transport isolation are common in the Laurentian Great Lakes region of North America (e.g., Yang & Duthie, 1995a; Wolin, 1996) and in the North Sea and Baltic region of Europe (e.g., Digerfeldt, 1988).

One of the most important factors controlling water level is a change in hydrological conditions.

Introduction

Historically, there has been a lack of appreciation of the severity of human impacts on estuaries, and of how important these systems are to human society. The demand for resources and the products and residues generated as human populations grow will continue to cause cultural, economic, aesthetic and environmental problems, especially in coastal areas. Understanding the processes surrounding these problems is important for managing the continuing impacts of growing populations (National Research Council, 1993). Environmental issues relevant to estuaries include eutrophication, anoxia, harmful algal blooms, industrial pollution, loss of habitats such as wetlands and submerged aquatic vegetation, land-use effects on turbidity and sedimentation, and invasion of exotic species (National Safety Council, 1993).

Paleoecology offers powerful techniques with which to study historical changes due to human influences in depositional environments, including estuaries. A paleoecological approach makes it possible to define the naturally occurring state of an ecosystem, against which human influences can be measured (Smol, 1992). Diatoms are particularly useful not only because they are preserved in the sediment record but because they have a rapid reproductive rate and respond quickly to changes in nutrient availability and other water quality conditions. In addition, diatoms are abundant in aquatic environments, generally cosmopolitan in distribution, and have a fairly well-studied taxonomy and ecology.

Paleoecological studies in estuarine environments have lagged behind paleolimnology, in large part because of the more dynamic nature of coastal ecosystems.

Introduction

Assessments of environmental conditions in rivers and streams with diatoms have a long history, which has resulted in the development of the two basic conceptual and analytical approaches used today. First, based on the work of Kolkwitz and Marsson (1908), autecological indices were developed to infer levels of pollution based on the species composition of assemblages and the ecological preferences and tolerances of taxa (e.g., Butcher, 1947; Fjerdingstad, 1950; Zelinka & Marvan, 1961; Lowe, 1974; Lange-Bertalot, 1979). Second, Patrick's early monitoring studies (Patrick, 1949; Patrick et al., 1954; Patrick & Strawbridge, 1963) relied primarily on diatom diversity as a general indicator of river health (i.e., ecological integrity), because species composition of assemblages varied seasonally and species richness varied less. Thus, the concepts and tools for assessing ecosystem health and diagnosing causes of impairment in aquatic habitats, particularly rivers and streams, were established and developed between 50 and 100 years ago.

The many advances in the use of diatoms and other algae for monitoring stream and river quality have been reviewed by Patrick (1973) and, more recently, by Stevenson and Lowe (1986), Round (1991), Whitton et al. (1991), Coste et al. (1991), Whitton and Kelly (1995), Rosen (1995), and Lowe and Pan (1996). There are three major objectives for this chapter. First, we emphasize the importance of designing environmental assessments so that many approaches are used and results are based on rigorous statistical testing of hypotheses. Second, we review the many characteristics of diatom assemblages that could be used in assessments and the methods and indices of assessment.

This book is about the uses of diatoms (Class Bacillariophyceae), a group of microscopic algae abundant in almost all aquatic habitats. There is no accurate estimate of the number of diatom species. Estimates on the order of 104 are often given (Guillard & Kilham, 1977), and Mann and Droop (1996) point out that this estimate would be raised to at least 105 by application of modern species concepts. Diatoms are characterized by a number of features, but are most easily recognized by their siliceous (opaline) cell walls, composed of two valves, that together form a frustule (Fig. 1.1). The size, shape, and sculpturing of diatom cell walls are taxonomically diagnostic. Moreover, because of their siliceous composition, they are often very well preserved in fossil deposits, and have a number of other industrial uses.

This book is not about the biology and taxonomy of diatoms. Other volumes, for example, Round et al. (1990) and the review articles and books cited in the following chapters, provide introductions to the biology, ecology, and taxonomy of diatoms. Instead, we focus on the applications and uses of diatoms, with a further focus on environmental and earth sciences. Although this book contains chapters on direct applications, such as uses of fossilized diatom remains in industry, oil exploration, and forensic applications, most of the book deals with using these indicators to decipher the effects of long-term ecological perturbations, such as climatic change, lake acidification, and eutrophication.

Introduction

It is curious that diatoms, whose short lifespans and capacity for rapid regeneration make them especially suitable for short-term paleoenvironmental studies, would also have a significant role as indicators of long-term environmental change. This chapter explores the nature of long diatom records, their relation to global environmental changes, guidelines for their interpretion, and problems common to such records.

Definitions and concepts

To examine the use of continental diatoms as proxies of long-term environmental change, it is first necessary to define what is meant by ‘long-term’. ‘Long’ for this paper refers to lake records that encompass several glacial / interglacial cycles (e.g., IGBP, 1992). At a minimum, long records reach the preceding interglacial, the Eemian or Sangamon, and correlate to the marine oxygen isotope stage 5e, about 125000 years ago.

It is also relevant to discriminate between ‘long-term environmental change’ as opposed to short-term environmental changes that have occurred over long time periods. Long-term environmental change results from processes which operate along uninterrupted trends of thousands or millions of years. Only three basic processes actually operate at this scale: (i) Deep-seated lithosphere convection caused by radioactive decay that drives continental drift, tectonism and volcanism; (ii) Variations in orbital relations between the earth and sun that govern insolation and long-term climate change; (iii) Slowing of the earth's rotation and the increasing distance between earth and moon; very long-term (109 years) changes irrelevant to the scope of this chapter.

Introduction

Eutrophication refers to enrichment of aquatic systems by inorganic plant nutrients (Wetzel, 1983; Mason, 1991). 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 lake productivity. Causes of eutrophication include human (anthropogenic eutrophication) and non-human (natural eutrophication) disturbances. Marked natural eutrophication events are rare and may result from dramatic episodes, such as forest fire (e.g., Hickman et al., 1990) and tree die-off (Boucherle et al., 1986; Hall & Smol, 1993). Climatic shifts, such as droughts, may also concentrate lakewater nutrients or give rise to an increased contribution of nutrient-rich groundwater (e.g., Webster et al., 1996). In most cases, however, water-quality problems are caused by anthropogenic nutrient inputs from domestic and industrial sewage disposal, farming activities and soil erosion.

Eutrophication is the most widespread form of lake pollution on a global scale, and has many deleterious impacts on aquatic systems (Harper, 1992). In addition to increasing overall primary production, eutrophication causes considerable changes to biochemical cycles and biological communities. Marked changes occur at all levels in the food web and entire communities can change or die out. For example, changes in the ratio of N:P often results in primary production shifting from primarily diatoms and other smaller edible algae towards larger cyanobacteria that are better competitors for N (Tilman et al., 1986), and more resistant to grazing (Reynolds, 1984).