We feel our world in crisis. We walk around and sense an emptiness in our way of living and the course which we follow. Immediate, spontaneous experience tells us this: intuition. And not only intuition, but information, speaking of the dangers, comes to us daily in staggering quantities.

How can we respond? Has civilisation simply broken away hopelessly from a perfection of nature? All points to a bleak and negative resignation.

But this is only one kind of intuition – there is also the intuition of joy.

Arne Naess gives a lecture somewhere in Oslo. After an hour he suddenly stops, glances quickly around the stage, and suddenly leaves the podium and approaches a potted plant to his left. He quickly pulls off a leaf, scurries back to the microphone, and gazes sincerely at the audience as he holds the leaf in the light so all can see. ‘You can spend a lifetime contemplating this’, he comments. ‘It is enough. Thank you.’

In 1969, Naess resigned his professorship in philosophy after over thirty years of work in semantics, philosophy of science, and the systematic exposition of the philosophies of Spinoza and Gandhi.

Introduction

In freshwater fish the physiological regulation of the major electrolytes is very sensitive to environmental stressors. Low pH environments in both the laboratory and field cause electrolyte losses in a number of fish species and, indeed, plasma electrolytes have proven to be a fairly reliable indicator of sublethal acid stress (e.g. Leivestad & Muniz, 1976). Similarly, there are now several studies on the toxic trace metals showing that disturbances to ion regulation are either a primary or at least a secondary consequence of exposure to a particular metal. Our objective then is to examine how mixtures of trace metals and H+ might toxically interact to cause ionic disturbances. We have placed emphasis on sublethal effects upon gill function rather than toxicity per se. We first examine the chemical and biological bases for metal and H+ interactions and then present some examples which illustrate the nature of these interactions. It is not our intention to review exhaustively metal and H+ toxicity but rather to point out how one might examine or even predict the interactions of untested metal/H+ mixtures. For a more general and thorough treatment of metal and acid toxicity to aquatic biota the reader is referred to the recent review by Campbell & Stokes (1985).

In terrestrial animals, the toxicity of a particular metal is mainly related to its dose; if a metal is not absorbed then it is not toxic, irrespective of its reactivity in aqueous solution.

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).

Recent advances in modelling plant stands have emphasised the importance of the structural and functional properties of plant canopies, as distinct from those of the constituent parts. In response to proposals made following the 1984 meeting on the ‘control of leaf growth’, which resulted in Seminar Series Publication 27, the Environmental Physiology Group held a series of sessions on plant canopies during the March 1986 meeting of the Society for Experimental Biology at Nottingham. All the invited speakers at these sessions have contributed chapters to this volume either individually or with collaborators.

Chapters have been included on all the major processes occurring in canopies, although there has been space neither for consideration of the manipulation of canopies by chemical or genetical means, nor for discussion of the canopy as habitat for micro-organisms, insects or vertebrates. A policy decision was made at an early stage of planning to encourage authors to look at a diverse range of canopy types and geographical distribution in order to avoid any bias introduced by, for example, considering only temperate zone cereal crops. The reader can decide how successful this policy has been. Some omissions represent genuine areas of ignorance, but it is a matter of regret that space was not available to allow consideration of stands of mixed species either in agricultural intercropping systems or in natural communities.

It is a pleasure to acknowledge the financial and other support of the Environmental Physiology Group, the Association of Applied Biologists and the British Ecological Society.

I would like to record the contributors’ co-operation during the meeting and to thank them for all the time they and their collaborators devoted to preparing and revising their manuscripts.

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

A synthesis of canopy processes can be accomplished at various levels of detail. If historical data are available, then a statistical analysis of that data may provide a kind of synthesis; however, in this case the synthesis is implicit in the statistical tool used, yielding limited insight to us. Alternatively, a mechanistic approach can be used and each relevant process described by appropriate, state-of-the-art, quantitative relations with explicit integration (or synthesis) to achieve an ‘integrated whole’. Clearly, statistical and mechanistic approaches represent extremes of a continuum where all intermediate states are possible. Thus a clear statement of objectives, guiding rules for pursuing these objectives, definition of the system, and evaluation criteria are prerequisites for beginning an orderly synthesis of canopy processes.

This chapter represents an attempt at an orderly synthesis of canopy processes with a reasonably mechanistic approach. The plant-environment model entitled Cupid (Norman & Campbell, 1983; Norman, 1979; Norman, 1982) is used as an example.

Rules for constructing a model

A system of rules for pursuing a synthesis of processes can aid one in resisting the temptation to ‘over sell’ and thus avoid having either to resort to short-term expediency when failure is in sight, or to justify the means deceptively with an end result that was essentially known before the modelling was begun.

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

Green plants utilise the sun's energy to synthesise organic compounds from carbon dioxide and water. The pioneering work, concurrently carried out by Liebig in Germany and Lawes & Gilbert in the UK more than a century ago, conclusively showed that plants must take up inorganic nutrients from the soil to produce these organic components. Since that discovery it has been established that many elements are necessary for optimum functioning of the biochemical machinery of the plant. Most of these are necessary in such small amounts, however, that the supply from the seed, or from natural sources suffices. In agriculture the situation is often different for the macro-elements nitrogen, phosphorus and potassium that are needed in such large quantities, especially where crop management practices aim at very high yields, that the supply from natural sources falls far short of the demand. Fertiliser experiments show that, up to a certain level, addition of these elements from a fertiliser bag leads to higher yields. Unfortunately, interpretation of these fertiliser experiments seldom exceeds the derivation of the optimum nutrient application rate for the conditions of the experiment, either in physical or in economic terms. The lack of explanatory conclusions hinders the use of such results for predictive purposes, for example, in the formulation of fertiliser recommendations for the farmer.

Introduction

Plant canopies modify their own microclimate. The heat and vapour released into the atmosphere at plant surfaces changes the temperature and humidity of the air in contact with those surfaces. These changes in temperature and humidity, in their turn, modulate the fluxes of heat and vapour from the vegetation. The importance of this ‘atmospheric feedback’ depends, amongst other things, on the area of the plant canopy (Jarvis & McNaughton, 1986). Small areas of vegetation modify shallow layers of the atmosphere, and local changes in microclimate are small. The influence of a single field extends upwards for perhaps 10 metres. The gradients of temperature and humidity through this layer have been studied in detail by canopy meteorologists.

If a uniform canopy covers an area of some hundreds of square kilometres then the effect of the vegetation will be felt throughout the whole of the turbulent planetary boundary layer, up to a kilometre or so above the ground. On this regional scale, processes affecting the surface energy balance have received very little scientific attention. This situation is now changing under pressure from hydrologists, who want methods for estimating regional evaporation, and climatologists, who must model the surface energy balance to improve predictions from their models of the global circulation of the atmosphere.

The purpose of this chapter is to review efforts to extend canopy energy balance models to the regional scale. First is a brief descriptive account of atmospheric transport processes in the whole planetary boundary layer, to set the scene.

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.

Introduction

This chapter is about turbulent transfer between a plant community and the atmosphere, especially the transfer of heat, water vapour, CO2 and other scalar entities. We consider the way in which turbulent transfer influences the microclimate within the plant community, in particular the mean scalar concentrations, including temperature and humidity. The second section provides a brief, qualitative overview, to establish the connections between the turbulent transfer process in a plant canopy and exchange processes at both smaller scales, those of individual leaves, and larger scales, those of the entire planetary boundary layer of the atmosphere. Then, with frequent reference to the observed properties of turbulence in plant canopies, the third and fourth sections review two kinds of theory for describing turbulent transfer. The more common Eulerian theories consider the behaviour of the turbulent transfer process at a grid of points fixed in space. Less common, but of increasing importance, are the Lagrangian theories: these describe turbulent transfer by considering the statistical behaviour of the wandering blobs of fluid which actually carry the transferred entity.

Overview

Common experience shows that the air motion within a plant canopy is highly erratic and intermittent. The origin of this behaviour lies in the planetary boundary layer (PBL), the turbulent layer of the atmosphere which extends from the ground to a height of the order of a kilometre (within a factor of three or so).