Introduction

This paper will review the field and laboratory data on the chemical factors which affect fish and fishery survival in acid waters, and an attempt will be made to relate the two sets of data where this is possible. In general, the field data being considered are predominantly from Norway where the fish species is mainly brown trout, but, where relevant, data from North America will be described. Data from the UK are dealt with by Turnpenny (this volume).

The main chemical factors, other than pH (hydrogen ion concentration) that will be discussed are calcium (hardness) and aluminium concentration. The aquatic chemistry of aluminium, which is generally present in higher concentrations in more acid waters, is complex and it is necessary at this stage to summarise the details on the subject given by Freeman & Everhard (1977), Burrows (1977), Hunter et al. (1980), Spry et al. (1981) and O'Donnell et al. (1983).

Solubility of aluminium is a direct function of ambient pH, being at a minimum at around pH 5.5, and increasing towards both extremes of the pH scale. Soluble cationic species e.g. A13+, A1OH2+ and Al(OH)2+, are formed at pH levels less than 5.5, and soluble aluminate species, e.g. Al(OH)4-, predominate at pH levels greater than 5.5. In natural waters, aluminium has a strong tendency to form complexes with other anions capable of forming coordinate bonds – for example, six different fluoride complexes are known (Burrows, 1977).

Introduction

There is some consensus now that the death of many fish species, exposed to acid water, is caused by a chain of events starting with the loss of body electrolytes and eventually leading to osmoregulatory and cardiovascular failure (Muniz & Leivestad, 1980; McDonald & Wood, 1981; McDonald, 1983). Sublethal exposure to acidified water often leads to transient or chronic hypo-osmolarity of the blood plasma, mainly caused by reduced Na+ and Cl- levels (McWilliams, 1980; McDonald, 1983; Wendelaar Bonga, Van der Meij & Flik, 1984a). The severity and duration of these effects are determined by both external and internal factors. External factors, such as the calcium and aluminium concentration of the water and the presence of heavy metals, are dealt with by Wood, Potts & McWilliams, and McDonald et al. (this volume). The rate and degree of change of the environmental pH is also important. Internal factors, in particular hormones, are the subject of this chapter.

The endocrine system is of pre-eminent importance for the control of physiological processes that enable animals to adjust to changes in their environment. Since acidification of the water deeply affects many aspects of fish physiology, pronounced and multiple responses of the endocrine system may be envisaged. When it is taken into account that a predominant deleterious effect of acid water on fish is disturbed water and ion balance, it is not surprising that the hormones with osmoregulatory actions in fish, in particular cortisol, ACTH and prolactin, are given greatest consideration in this chapter. Studies on the effects of acid on fish endocrines are still scarce, and limited to a few species.

Introduction

Most measurements of the effects of acid toxicity on aquatic animals concentrate upon changes in body fluid pH and the flux of ions between water and blood. One problem posed by a low blood pH is the potential acidification of the tissues which could cause undesirable deviation from the optimum pH of intracellular enzymes. This chapter examines the techniques and results of investigations into the regulation of intracellular pH (pHi).

Relatively little is known about pHi in most animals, experiments being so far generally confined to those with large neurones. This is because until recently the only way of following pHi over long periods was with pH-sensitive microelectrodes. This method is still the best, but requires both skill and a large cell. For small cells fluorescent dyes are very promising.

In this chapter I will describe some of the evidence on which the present understanding of pHi regulatory mechanisms in snail, crayfish and leech neurones is based. I will confine this chapter to these preparations because they are reasonably typical, and I lack the space for a full review. I will then consider the effects of external acidification before concluding that maintenance of a constant pHi depends very much on a constant external pH. For a detailed review of intracellular pH, see Roos & Boron (1981), but for shorter and more recent accounts of the subject see Thomas (1984, 1986).

Introduction

The physiological effects of environmental acid stress on fish have been thoroughly reviewed in recent years (Muniz & Leivestad, 1980a; Fromm, 1980; Haines, 1981; Spry, Wood & Hodson, 1981; Brown, 1982; Leivestad, 1982; Wood & McDonald, 1982; McDonald, 1983a; Howells, Brown & Sadler, 1983; Howells, 1984); there are certainly not enough new data to justify yet another compendium. Instead, I will first summarize our current knowledge on the acute toxic mechanisms of pure acid stress to adult fish, and in so doing attempt to correct the widely held misconception that external water acidity must cause internal acidosis in the animal. By means of this brief summary, I hope to illustrate that external acidity has proven to be an exceptionally useful probe of normal physiological processes in freshwater teleosts. Secondly, I will describe some of our recent findings on the physiological responses to long term, low level acid stress, and acid-aluminium interactions, both of which may have greater ecological relevance than short term acid stress for ultimate survival in the wild.

Acute responses to pure acid stress

Background

Relatively short term depressions to pH = 4.0–4.5, usually as a result of snowpack melt in the spring, or highly acidic runoff in the summer and autumn, have often been observed in natural soft waters of both northern Europe and eastern North America (e.g. Jeffries, Cox & Dillon, 1979; Harvey et al., 1981; Christophersen, Rustad & Seip, 1984; Marmorek et al., 1985).

Introduction

During the last decade a small library of books and papers has accumulated dealing with the effects of acid waters on fish. In this review we shall attempt to discuss and, where possible, interpret a small part of this work in terms of what is known of the physiology of fresh water fishes.

The blood plasmas of freshwater fishes contain around 150 mequiv Na+ and 130 mequiv Cl-l-1 in addition to lower concentrations of other ions, particularly Ca2+, K+ and HCO3-. In contrast, soft fresh waters, in which acidification may be a problem, usually contain less than 0.1 mequiv NaCl l-1 and much lower concentrations of other ions. Although fish obtain some salts from their food, most species, including all salmonids, are dependent on the active uptake of salt from the medium to balance diffusion and urinary losses. Freshwater fishes produce a dilute urine and urinary losses are usually less than 10% of the total losses, most of which take place across the body wall, particularly across the gills, where the blood plasma and external medium are separated by only a thin layer of respiratory epithelium. The gills are also the main site of salt uptake, although in some marine teleosts salt transport takes place at other sites, particularly on the inside of the operculum, and the possibility of salt uptake and loss at other sites in freshwater fishes should not be overlooked.

Introduction

Acid rain is a short-hand term that covers a set of highly complex and controversial environmental problems. It is a subject in which emotive and political judgements tend to obscure the underlying scientific issues which are fairly easily stated but poorly understood. In this article I shall deal solely with the scientific problems involved in the acidification of surface waters, attempt to establish the facts, describe the present state of knowledge and understanding and discuss what research is needed to provide a firm basis for remedial action.

Although the term acid rain is commonly used to describe all acid deposition from the atmosphere that may cause damage to trees, vegetation, fisheries, buildings, etc., in fact rain (and snow) brings down only about one third of the total acids over the UK, two thirds being deposited in the dry state as gases and small particles. But wet or dry, there is little doubt that acid deposition from the atmosphere poses an ecological threat, especially to aquatic life in streams and lakes on hard rocks and thin soils in southern Scandinavia and in some parts of Scotland and North America. A great deal of research is being undertaken in these three areas but this account is based largely on work in Scandinavia and the United Kingdom where more than 30 research groups from a wide variety of disciplines and institutions are working in a closely integrated and coordinated programme under the author's direction.

Introduction

Although most attention has been focused on the impoverishment of fish populations in acidified lakes, numerous ecological studies have shown that phytoplankton, zooplankton and benthic invertebrates have decreased in diversity in recently acidified waters. There is ample evidence to suggest that the pH is a major variable in determining the distribution of species, although recent work includes other parameters, like the Ca2+ content and the metal concentrations into the list of distribution-limiting factors. Very elegant work in this respect has been performed by Økland and Økland (Økland, J., 1980; Økland, K.A., 1980; Økland & Økland, 1980) who studied in detail the gastropod fauna in acid-polluted and non-polluted lakes in Norway. The literature describing in situ studies of the pH influence on invertebrate distribution, mortality and diversity, being large and quite impressive, will not be quoted in detail in this text, and the reader is advised to study the reports of the SNSF Project in Norway (Leivestad et al., 1976; Overrein et al., 1980), or the review paper of Sutcliffe & Hildrew (this volume) covering the subject.

Laboratory experiments on mortality of freshwater fauna reveal clearly that there exists a critical pH value below which survival is significantly reduced. This critical value is species-dependent but it is also determined by the composition of the testwater.

Whilst the subject of ‘acid rain’ has received a great deal of attention from the mass media, focusing on dramatic toxic effects and the ensuing political arguments, the scientific studies have been slowly forging ahead. This book concentrates on one aspect of the problem, that of the effect of acid toxicity on the group of animals most severely affected - the aquatic animals living in fresh water.

The book results from a Symposium held at the University of Nottingham in March 1986 by the Society of Experimental Biology, when a number of scientists with expertise in this particular subject met to discuss the results of recent research into the problems. They also provided a series of articles on which this book is based. The conference was greatly helped by a financial contribution from SWAP (Surface Waters Acidification Project) – a research organisation jointly administered by the Royal Society of London, The Norwegian Academy of Science and Letters and the Royal Swedish Academy of Sciences.

The aim of the book is to cover a wide range of investigations taking place in both field and laboratory and to provide the necessary scientific background information for present and possible future approaches to the problems facing animals living in acidified fresh water. Starting initially with the environment in order to assess why problems have arisen in particular areas, the volume then deals with field and survival studies on invertebrates and vertebrates in order to examine the extent of the biological problem and the attempts which have been made to relate water quality and the susceptibility of animals.

Introduction

The devastating effects of acid precipitation (resulting from the conversion of atmospheric oxides of sulphur (SO2) and nitrogen (NOX) to sulphuric and nitric acid), on lake fauna and flora have been well documented (Likens & Borman, 1974; Leivestadt et al., 1976; Harvey & Lee, 1982). Losses of freshwater fish populations from such affected areas have been recorded (see, for example, Leivestadt & Muniz, 1976; Harvey, 1979, 1982) and possibly because of their great economic value many physiological studies have concentrated on the effects of acid exposure on fish (reviews by Wood & McDonald, 1982; McDonald, 1983; Wood, this volume). Decreases in invertebrate populations have also been reported (Abrahamsson, 1972; Aimer et al., 1974; France, 1983, 1985), but this has stimulated rather few studies on physiological disturbances resulting from acid exposure in invertebrate animals. Interestingly, the few studies which have been undertaken have shown invertebrates to be more tolerant to acid exposure than fish, or more accurately, than those few, mostly salmonid, fish on which physiological studies have concentrated.

A second common feature of the fish studies is that they have mostly used sulphuric acid (H2SO4) as the acid stressor. While H2SO4 precipitation still remains a most serious problem, in recent years emission of SO2 have declined while those of NOX have continued to rise. Despite this, few studies of the physiological effects of nitric acid (HNO3) pollution on aquatic animals have been carried out.

Introduction

The areas of North America and Northern Europe from which decline of fish populations in acid waters (pH ≤5.6) has been reported are usually characterised by high levels of precipitation and by weathering-resistant bedrock, resulting in poor soils and surface waters very low in dissolved substances ([Ca] ≤100 μmol l-1, 4 mg l-1; conductivity ≤60 μS cm-1) (reviews: Haines, 1981; Spry et al., 1981; Harvey, 1982; Harvey & Lee, 1982; Howells, 1983; see also papers in this volume by Brown & Sadler; Mason; Turnpenny).

The streams and lakes in these predominantly mountainous areas are, consequently, vulnerable to rapid and large changes in their chemistry. The soil may frequently be waterlogged or frozen, and may be thin or absent altogether (at bedrock outcrops and scree slopes). Episodes of heavy rainfall or snowmelt may therefore result in more or less direct runoff into the water bodies. The low concentration of dissolved substances in the water bodies ensures that the ability to buffer changes in acidity is poor, and concentration changes caused by runoff episodes are likely to be relatively large. The pH of the water may fall dramatically during such episodes (Tables 1, 2).

From time to time, fishkills (particularly of Salmonidae) are observed in such waters (Table 2), occurring over short periods (hours or days) and coinciding with increased water discharge associated with heavy rain, snowmelt or release of artificially impounded water.

Introduction

‘Acid Rain’ as a scientific and political issue has advanced since early claims of ecological damage were first formulated in the 1950s and 1960s from the initial, rather simplistic, assumptions and interpretations that could be formulated as a sequence of events initiated by the emission of SO2 from fossil fuel combustion, the formation of acidic radicles in the atmosphere which were then removed by rain and deposited; the rain falling in unbuffered waters rendering them acid by progressively consuming their alkalinity.

Following this early conceptualisation, the contributions of N gases and ammonia were later accepted, as well as the complexity of chemical reactions in the atmosphere and the alternate mechanisms of deposition. Hydrological and geological characteristics of catchments were also recognised as major influences determining the quality of surface waters which were thus distinguished from atmospheric deposition. The ‘susceptibility’ of a lake or stream was seen to be dependent not only on the rate of acidic deposition but also on the supply and/or depletion of alkaline materials leached from soil and weathered from rock. Where loss is not matched by supply from leaching and weathering, these materials become depleted in surface waters. It is recognised that this may eventually become intolerable for some aquatic species although others may be able to exploit the resultant loss of competition. Initial concern was expressed for fish, reported as lost from the affected lakes and rivers: other biological components of the aquatic ecosystem are reported to be similarly restricted.

Introduction

Stony streams normally contain a characteristic fauna of benthic macroinvertebrates, prominent among which are insects, especially Ephemeroptera, Plecoptera, Trichoptera and Diptera. Also present are flatworms, oligochaetes, a few malacostracan crustaceans and molluscs. In streams with moderate alkalinity and a pH of about 6 or more, the fauna is rich in total numbers of species (usually there are some 70–90 taxa) and many of these may be abundant. In acid streams where the mean pH is below 5.7–5.4, however, there is a distinct change in faunal composition. Some taxa disappear or become scarce, particularly mayflies, some caddisflies, crustaceans and molluscs. The fauna is thus impoverished in species and may contain only half the number of taxa found in less acid (soft water) streams.

For instance, in the upper River Tywi in west Wales a diverse fauna of 60–78 taxa occurs in streams containing mean concentrations of at least 85 μequiv l-1 calcium and magnesium, with mean pH 5.6 or above. At mean pH 5.3 or below, and slightly less calcium, only 46 taxa were found and these were generally sparse (Stoner, Gee & Wade, 1984). A similar reduction in numbers of taxa occurred at reduced pH in lowland streams of the Ashdown Forest in southern England (Townsend, Hildrew & Francis, 1983; Hildrew, Townsend & Francis, 1984).

Introduction

Adjustment of pH is one of the central tasks for homeostatic regulation. Deviations from certain set-point values may result in reduced metabolic performance, due to the enzyme activity of metabolic energy-producing processes having pronounced pH optima. Accordingly, any net endogenous production or exogenous induction of acid–base relevant ions has to be counteracted by equivalent removal from the body fluids in order to maintain steady state conditions.

The regulatory mechanisms available for this purpose are in principle the same in all classes of animals, but are (or can be) utilized to variable extents. The situation of fishes is characterized by their intimate contact with the aqueous environment, including utilization of water as a gas exchange medium. Immersion in water favours ion transfer mechanisms supporting acid–base regulation, but also entails severe restrictions for the regulation of Pco2 in the body fluids. This chapter will briefly delineate and discuss the basic principles of acid–base regulation in fishes with respect to their theoretical limitations, and to their relative importance for the regulation of acid–base homoiostasis in fishes. The selection of references was limited by space, and review articles have been cited wherever possible to provide greater access to the subject. This chapter leads directly into Chapter 6 by J.N. Cameron, which explores the exogenous and endogenous variables affecting acid–base regulation in fishes.

Buffering

Buffering is a mechanism for transient acid–base regulation. Surplus H+ are transferred into the non-dissociated state and masked by association with buffer bases.

Introduction

The term ‘acid water’ is variously defined in the literature, but it is usual to adopt an upper limit of pH 5.6, equivalent to the pH of pure water in equilibrium with atmospheric carbon dioxide. The UK Acid Waters Review Group (1986) thus defines three categories of water with respect to acidity:

1. (i) ‘permanently acid’: pH usually < 5.6, alkalinity zero or close to zero;

2. (ii) ‘occasionally acid’: pH occasionally < 5.6, low alkalinity;

3. (iii) ‘never acid’: pH never < 5.6, well buffered.

Although no survey of acid water distribution in the UK has ever been undertaken, the likely distribution can be inferred from the combination of solid and drift geology and soil type. Maps of these characteristics are given by the UK Acid Waters Review Group (1986), and a composite map is shown in Figure 1. This indicates a probable distribution of acid waters in the UK, predominantly to the west and north.

As far as fish are concerned, the definition of an acid water is arbitrary. Fish have been shown to exist in a wide range of pH levels, from 3.5 to 11; also, their response to acid conditions is determined by a complex of other factors such as related increases in toxic metal levels or indirect effects upon food organisms (Alabaster & Lloyd, 1980; Howells, 1983). The purpose of field studies is to establish the net effect of these combining or competing factors on fishery status.