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32 A. A. SAVAGE

THE DISTRIBUTION OF CORIXIDAE IN RELATION TO THE WATER QUALITY OF BRITISH LAKES: A MONITORING MODEL

ALAN A. SAVAGE

(Dr A. A. Savage, Department of Biological Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK.)

(This article is based on a talk given at the FBA's Scientific Meeting on "Freshwater biodiversity and water quality", September 1992.)

Introduction

This article is an attempt to devise a method of using certain species of Corixidae as a basis for the assessment of general water quality in lakes. An empirical graphical representation of the distribution of populations or communities of Corixidae in relation to conductivity, based mainly on English and Welsh lakes, is used as a predictive monitoring model to establish the "expected" normal community at a given conductivity, representing the total ionic concentration of the water body. A test sample from another lake of known conductivity is then compared with the "expected" community. The "goodness of f i t" is examined visually or by calculation of indices of similarity based on the relative proportions of the constituent species of each community. A computer programme has been devised for this purpose. The interpretation of deviations from expectation is assisted by notes on species whose presence have particular ecological implications on phenomena such as changes in trophic status and fish stocks.

Most of the article consists of the presentation and discussion of evidence related to the development of the empirical model. The argument is based principally on patterns of distribution as there is little information on causal processes. Three aspects are considered: water quality and its relationships with some aspects of lake ecology, a general description of the distribution of Corixidae in relation to environmental parameters, and an account of the distribution of "open water" species in relation to water quality. My article is completed by a brief description of the empirical monitoring model, together with its use and interpretation.

The ideas presented here have been developed from an earlier article on the use of Corixidae in the classification of freshwater lakes or, more correctly, in defining the point in the continuum covering the ecology of larger lentic habitats (Savage 1982b). Later personal observations have confirmed the distribution patterns already described and collections in

THE DISTRIBUTION OF CORIXIDAE 33

some Irish lakes are in accordance with my proposals (O'Connor et al. 1985). Furthermore, recent collections of Corixidae in both inland and coastal saline waters have provided sufficient information for the inclusion of such habitats in the present scheme. A further stimulus for the use of macroinvertebrates for the assessment of water quality in lakes has been their widespread and successful use in rivers (e.g. Wright et al. 1984; Hellawell 1986). Indeed, at present, there appears to be a more general development of interest in Biomonitoring (e.g. Spellerberg 1991; Rosenberg & Resh 1993).

Data sources used in developing the model

The data considered here comprise collections of Corixidae, altitudes, areas and linear dimensions of water-bodies, and conductivity and analyses of major and nutrient ions. The published data sources are as follows: Corixidae in Cumbrian lakes (Macan 1955); Corixidae in North West Midlands, North Wales, Anglesey and Scotland (Savage 1971, 1979, 1981, 1982a, b, 1985, 1990; Savage & Pratt 1976); areas of waterbodies, conductivity, major and nutrient ions (Macan 1970; Savage 1971, 1979, 1981, 1982b, 1990; Savage & Pratt 1976; Reynolds 1979; Carrick & Sutcliffe 1982; Savage & Gazey 1987; Savage et al. 1992). Unpublished data from the North West Midlands, Cambridgeshire, Leicestershire, Northamptonshire, Cumbria, Yorkshire and Lincolnshire have also been included. Details of methods of collection of Corixidae, water samples and chemical analysis may be found in the original papers. A complete list of sites where Corixidae were collected is given in an appendix. The relationships discerned from the data have been expressed by product-moment regresssion analysis, with logarithmic transformation where appropriate, and Spearman's rank correlation coefficients (rs). The four permutations in two-way analysis are decribed as linear, logarithmic (x, log; y, linear), exponential (x, linear; y, log) and power (x, log; y, log) relationships.

Water quality in terms of its major chemical components

The chemical water quality of a lake may be defined in terms of three factors: conditions, resources and pollutants. Conditions are determined by pH and the major ions which provide a chemical backcloth for the living organisms inhabiting a water-body and whose concentrations are so high that they are largely unaffected when used by those organisms. In contrast, resources are those plant nutrients, namely silicate, phosphate and nitrate ions, together with any naturally occurring inorganic and organic substances present in relatively small concentrations, which may

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be severely depleted by algal and macrophyte growth. Pollutants comprise a nebulous group of chemical substances which may include those ions named above but also include a wide variety of others such as pesticides, herbicides, heavy metals and hydrocarbons.

This article is concerned primarily with the chemical conditions in lakes, together with one ionic resource, nitrate. Here, the concentrations of calcium, magnesium, sodium, potassium, sulphate, chloride and bicarbonate (alkalinity) ions constitute the conditions. Some data were available on phosphate ion concentrations but no significant relationships could be discerned with conductivity or the concentrations of major ions. There was a lack of data on silicate. However, as will be seen later, deductions concerning the relationships between conditions and resources may be attempted by an examination of phytoplankton production. The lakes studied showed few signs of pollution, apart from minor eutrophication, and pollutants are not considered in detail here.

Electrical conductivity of water

Electrical conductivity represents a summation of the dissolved ions in water. There are significant positive correlations between conductivity and the concentrations of most major ions, reflecting chemical conditions in fresh water in both the North West Midland meres and Cumbrian lakes (Figs la,b, 2a, 3a and Table 1; Reynolds 1979; Savage 1982b; Savage & Gazey 1987). Among resources, the concentrations of nitrate showed a significant correlation with conductivity in Cumbrian lakes (Fig. 2b; Table 1) and in samples collected during winter (November-February) in the North West Midlands, but not with samples collected throughout the year (Fig. 1c; Table 1). It is notable that there was a logarithmic relationship in winter concentrations in the North West Midlands, thus demonstrating unexpectedly high concentrations of nitrate at intermediate conductivities and perhaps suggesting a contribution from agricultural practices. In the saline waters of the North West Midlands, chloride and sodium showed highly significant positive correlations with conductivity, but concentrations of the remaining major ions remained virtually constant (Savage 1982 a).

Major sources of freshwater supply to lakes and meres

The sources of water in a lake are ombrogenous, topogenous and soligenous, each being defined, respectively, as from direct rainfall (precipitation), overground flow, and percolation through adjacent drift soils. Gorham (1958) and Sutcliffe & Carrick (1983b) have demonstrated that ombrogenous water (precipitation) contains strong acids but is


essentially very dilute sea water, at least in areas with an oceanic climate such as the British Isles, and thus contains relatively high concentrations of sodium and chloride ions. In contrast, soligenous water which, as in the North West Midlands, has percolated through a base-rich glacial drift, has absolutely high concentrations of calcium, magnesium and bicarbonate (alkalinity) but relatively low concentrations of sodium and chloride, often related to the solid geology of the catchments. Topogenous water shows an intermediate state (Sutcliffe & Carrick 1983a, 1988). For convenience, these three types of water may be reduced to two, ombrogenous-topogenous and soligenous, and these may be represented by the absolute concentrations of chloride and bicarbonate (alkalinity) (Fig. 3a; Table 1). Similarly, their relative concentrations, expressed as ratios of bicarbonate/chloride, reflect the type(s) and hence

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FIG. 2. Cumbrian freshwater lakes. Relationships between conductivity (μS cm-1 k25) and concentrations of: a, total cations (Ca2+ + Mg2 + + Na+ +K+, m. equiv. 1 -1) (data from Carrick & Sutcliffe 1982, means for years 1972-1974 only in nine lakes); b, nitrate ( N O , mg 1 -1) (data source as for a); c, standing crop of phytoplankton (arbitrary units) (data for conductivity as for a and phytoplankton from Lund 1957). d, Relationships between the standing crop of phytoplankton (μg 1-1 dry weight) and total cation concentrations (Ca2+ + Mg2 + + Na+ + K+, m. equiv. 1-1) (data from Gorham et al. 1974). See Table 1 for regresssion data.


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the major sources of water supplies to any particular water-body. The Cumbrian lakes, in a mountainous area with thin soils, have low HCO3

-/Cl - ratios, reflecting their ombrogenous-topogenous sources. In contrast, the North West Midlands meres, lying in base-rich glacial drifts, have high HCO3

-/Cl- ratios, reflecting their (mostly) soligenous supply of water (Fig. 3c; Table 1). However, there are exceptions to this general principle which suggest that the differences are not only dependent on spatial geographical factors but also on local differences in the geology of catchments, as revealed by the overlap in ratios (between ca. 0.2 and 1.8) for Cumbrian and North West Midlands fresh waters (Fig. 3c). In addition, lakes with an ombrogenous-topogenous water supply generally have lower total concentrations of major ions, including chloride and bicarbonate, and hence lower conductivities than those with a soligenous supply. However, since the bicarbonate concentration increases more rapidly than that of chloride, the HCO3

-/Cl- ratio shows a highly significant correlation coefficient against conductivity when expressed as a logarithmic relationship (Fig. 3c; Table 1).

The general preponderance of a soligenous water supply in the North West Midlands freshwater meres is confirmed by the significant negative linear relationship between conductivity and altitude (Fig. 2d; Table 1). These meres lie at the level of the water table and it seems probable that water will have percolated through larger volumes of glacial drift at lower altitudes and hence contain larger quantities of dissolved chemical substances (Savage et al. 1992).

Inland saline lakes

The saline lakes of the North West Midlands are maintained by natural brine springs or industrial effluent. Both types have a similar water chemistry. This consists of a base-rich fresh water similar to that of the meres, on which is superimposed varying concentrations of sodium and chloride. Again, the relationship between these two components may be represented by differences in the absolute concentrations of bicarbonate and chloride (Fig. 3b; Table 1). A study of borehole data shows that the two components are almost entirely derived from distinct aspects of a soligenous source. The upper layer of the water table provides the base-rich freshwater component represented by bicarbonate. The lower layer provides the source for the brine springs, composed principally of sodium and chloride, and here represented by chloride, which appear at the surface as a consequence of hydrodynamic forces. The saline waters show a markedly low HCO3

-/Cl- ratio whose relationship with conductivity may be expressed by a power regression equation (Fig. 3d; Table 1). These ratios may be used as a basis for a definition to distinguish

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between fresh water and saline water. It is proposed that water should be described as saline when the positive regression relationship between conductivity and the HCO3

-/Cl-ratio, found in fresh waters (Fig. 3c), becomes negative (Fig. 3d). This change occurs when the concentrations of sodium and chloride ions begin to exceed the sum of the remaining major ions when these are expressed in milliequivalents per litre and at conductivities of 1000-2000 microsiemens per centimetre at 25°C. The phenomenon is in reasonable agreement with observed changes in communities of Corixidae. Savage & Pratt (1976) suggested that a conductivity of 2000 μS cm-1 k25 was a demarcation point for fresh and saline water. In fact, 1000 μS cm-1 k25 has been chosen as the demarcation point for the empirical model considered below.

Conductivity as a measure of lake status and productivity

In lakes, primary production of both phytoplankton and macrophytes shows close relationships with the chemical composition of water (e.g. Pearsall 1921; Lund 1957; Gorham et al. 1974). Regression analysis of the data shows an exponential relationship between aspects of algal production and water chemistry (Fig. 2c,d; Table 1).

Conductivity as a limnological estimate

The above evidence indicates that conductivity, itself a summation of the ionic composition of a water-body, may be used as a single convenient measure of a number of aspects of lake chemistry and biology which allow reasonable extrapolations concerning more general aspects of limnology such as the geology and topography of the catchment and the position of the lake in the oligotrophic-eutrophic series.

The general ecological distribution of Corixidae

The patterns of distribution of British Corixidae in relation to certain environmental parameters are well established. The first tentative steps in this descriptive process are apparent in the accounts of Saunders (1892) and Butler (1923). Macan (1939) also provided general ecological notes on each species in his key to Corixidae. In the third and fourth decades of the twentieth century a comprehensive nationwide survey of water bugs was undertaken by members of the Society for British Entomology, which provided basic information on the geographical and ecological distribution of Corixidae (e.g. Pearce & Walton 1939; Brown 1943, 1948; Popham 1943, 1949, 1950; Walton 1943). Most of this information was summarised by Macan (1956, 1965) and Southwood & Leston (1959).


Since that time many other workers have contributed studies of areas not included in the initial survey and the whole has been included in a tabular summary (Savage 1989).

The earliest formal demonstration of a relationship between the distribution of species of Corixidae and a specific environmental parameter was by Macan (1938) when he provided clear evidence of a pattern related to the hydrosere succession of macrophytes, expressed as the percentage organic matter content of the substratum in Windermere. Later, he showed a distinct, though comparable, pattern involving different species, in two far more eutrophic lakes in Shropshire (Macan 1967) which has been confirmed and extended (Savage & Pratt 1976; Savage 1990). We now know that separate successions of species of macrophytes and Corixidae occur in the different water qualities of oligotrophic and eutrophic lakes (summary in Savage 1989).

Macan (1954a) made a comprehensive survey of miscellaneous data on the distribution of Corixidae. By mathematical examination of associations between species he arranged them in three groups, viz. pools and ponds on base-poor soils (oligotrophic and dystrophic waters), productive (eutrophic) ponds, and lakes together with the lower lake-like reaches of rivers. Again, water quality, as indicated by oligotrophic or eutrophic conditions, which may be represented by low and high conductivities respectively, was shown to be important and an additional factor, water-body size, was incorporated as an equally important parameter. A study of a series of inland lakes of varying salinity showed that further species of Corixidae were present in relation to the distinct water quality of such habitats (Savage 1971, 1979, 1981, 1985). Thus, three environmental parameters, viz. water quality, percentage organic matter in the substratum and water-body size are of fundamental importance in relation to the distribution patterns of Corixidae.

This article is concerned with the distribution of Corixidae in lakes in relation to water quality only. Thus, the patterns of distribution associated with water-body size and percentage organic matter in the substratum related to the hydrosere succession, must be eliminated so far as is possible. Here, lakes are defined as permanent lentic water-bodies where area (hectares) x width/length is greater than unity (1) (Savage 1982b, 1989, 1990 and see p. 48).

In turn, the species associated with the later stages of the hydrosere succession and, hence, high percentages of organic matter in the substratum, are excluded from the empirical model presented later (Fig. 10). This is easily achieved by restricting collections to the open water margins of emergent vegetation where only "open water" species occur. This practice does not totally eliminate variation in percentage organic matter content in the substratum but the remaining variation appears to

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be directly related to conductivity, being low in oligotrophic lakes and rather higher in eutrophic lakes (Savage 1989). Observations have shown that the hydrosere succession is poorly developed and subject to seasonal drought in many lakes and the only species of Corixidae present are those associated with the open water margin (Savage & Pratt 1976; Savage 1982b, 1990). Thus, limiting the model to "open water" species allows for its more universal application.

The ecology of the "open water" species

Six species, Sigara scoff; (Douglas & Scott), 5. distincta (Fieber), S. dorsalis (Leach), 5. falleni (Fieber), Callicorixa praeusta (Fieber) and S. concinna {Fieber) may be described as "open water" species in freshwater lakes with conductivities up to 1000 μS cm-1 k25 (Savage 1982b). All six species show significant distribution relationships with conductivity, both as Spearman's rank correlation coefficients (rs) and as logarithmic relationships with product moment regression analysis. S. scoffi (Fig. 4a; Table 2) is restricted to water of low conductivity and its distribution shows a significant negative correlation with conductivity. It was the only species found in Llyn Cwm Bychan in Mid Wales (Fig. 6) and Loch a' Bhaillidh in Scotland (Savage & Pratt 1976) and it formed significant parts of the corixid community in Cumbrian lakes with low conductivities (Macan 1955). A few specimens were taken in Oak Mere in Cheshire, which has one of the lowest conductivities and the lowest base status of all the North West Midland meres (Savage 1990). In contrast, 5. concinna (Fig. 4b; Table 2) occurs at the highest conductivities of the freshwater range and shows a significant positive correlation with conductivity. It was always relatively uncommon in fresh water compared with 5. scoffi but the distributions of these two species form a symmetrical pattern occupying opposite ends of the conductivity range (Figs 4a,b). Macan (1954a) suggested that S. concinna might be associated with slightly saline conditions. It has now been shown to be abundant in a slightly saline lake, Watch Lane Flash in Cheshire (Fig. 9), at conductivities of 3000-7000μS cm - 1 k25. Thus an increase in numbers of S. concinna is also associated with a change to saline waters (Savage 1971, 1979).

The distributions of S. distincta (Fig. 4c; Table 2) and C. praeusta (Fig. 4d; Table 2) also show significant negative and positive correlations with conductivity respectively. Their distributions again form a symmetrical pattern in relation to conductivity but are less extreme, with some overlap, than those of 5. scoff; and S. concinna. There is evidence that both 5. distincta and C. praeusta are associated with higher percentages of organic matter in the substratum than the other four species (Savage 1990).

S. dorsalis and 5. falleni are the most abundant species in the more productive freshwater lakes of both Cumbria and the North West Midlands and frequently occur together (Macan 1954a; Savage & Pratt 1976; Savage 1982b, 1990). S. dorsalis shows a negative correlation with conductivity whilst S. falleni shows a positive one. In both cases the Spearman rank correlation coefficients (rs) are highly significant, thus suggesting that their distributions are very closely linked to the conductivity gradient (Fig. 5a; Table 2). The extremes of this gradient may be exemplified, for instance, by Tal y Llyn in Mid Wales (conductivity ca. 60 μS cm-1 k25) where only 5. dorsalis was found and Pick Mere in Cheshire (ca. 870 μS cm-1 k25) where 94% 5. falleni was found (Fig. 7). However, it should be remembered that S. dorsalis is usually replaced by 5. scoffi at low conductivities and S. falleni is replaced by S. dorsalis and S. concinna in slightly saline waters with conductivities higher than 2000 μS cm-1 k25. Savage (1971, 1989, 1990) showed that S. falleni occupies an intermediate position within the total range of 5. dorsalis. Sigara fossarum (Leach) occurs in moderately productive lakes in the second quartile of the conductivity range, occasionally in considerable numbers. In an earlier study (Savage 1982b) it was omitted because Macan's (1954a) evidence suggested that it was not an "open water" species, being associated with moderate percentages of organic matter in the substratum, and also because I was unable to demonstrate a significant correlation with conductivity. It is now included in the empirical model simply because it is there!

On page 41 a lake was defined as a lentic water body where area (ha) x width/length was greater than 1. This definition is convenient when using Corixidae as indicator organisms owing to the distribution patterns of S. dorsalis and S. falleni (Fig. 5b). In the North West Midlands S. dorsalis


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was frequently the most numerous species in large ponds (0.1-0.7 ha) with conductivities in the range 300-1000 μS cm-1 k25 (e.g. Savage 1979, 1981). It was also common in Sweat Mere (0.2 ha) (Macan 1967). It has already been shown that S. falleni was the most numerous species in meres (2-50 ha) in that conductivity range. It was also noticed that 5. dorsalis formed an important element of the population in long, narrow lakes which showed no effects of wave action; examples are Moston and Crabmill Flashes in Cheshire (Savage 1971). Thus, I have attempted to relate the distribution of the two species to a "shelter factor". There are significant positive and negative correlations, respectively, in the distributions of S. falleni and S. dorsalis with the "shelter factor" (waterbody area (ha) x width/length) (Fig. 5b; Table 2). In contrast, Macan (1954b) has shown that the large eutrophic Danish lakes had large populations of Sigara striata (L), a close relative of 5. dorsalis, even though chemically they are virtually identical with the North West Midlands meres. Similarly, for instance, Pitsford Reservoir in Northamptonshire has large numbers of 5. dorsalis. It could therefore be argued that a large lake, such as Windermere, would still contain S. dorsalis even if it had a much higher conductivity. Thus 5. falleni is restricted to water-bodies of intermediate size and the relationship with conductivity demonstrated here relates only to such places (Fig. 5c).

It has already been argued that a convenient division between fresh water and saline water occurs at conductivities of 1000-2000 μS cm-1

k25, and 1000 μS has been chosen as the point of division. Unfortunately, some species of Corixidae do not respect my division. S. dorsalis is a common member of corixid communities at conductivities up to 2000 |xS where it is associated with 5. falleni (Fig. 8). These communities therefore show similarities with those at conductivities of 100-200 μS. If Corixidae are used as indicators of water quality this distribution pattern is potentially a source of confusion. This problem has been avoided by adopting the two sections, fresh and saline water, already indicated, and treating the two sections separately in the empirical model. At conductivities greater than 3000 μS cm-1 k25 a clearly defined sequence of communities occurs which is distinct and associated with more saline conditions. The species concerned are 5. concinna, Sigara lateralis (Leach) and either Sigara stagnalis (Leach) or Sigara selecta (Fieber), the latter two tending to occur in northern and southern parts of the British Isles respectively. The latter three species are not confined to open water but may be regarded as open water species in the context of this article. It is possible to use regression analysis of community composition with conductivity as a means of demonstrating relationships, as was adopted for fresh water, providing a somewhat arbitrary and "convenient" choice of conductivity ranges is made.

However, Watch Lane Flash, a lake whose salinity has varied through time, provides more realistic evidence of the distribution of corixid communities in relation to conductivity (Figs 8, 9). In 1968-70 S. concinna was the commonest corixid at annual mean conductivities of approximately 5000μS cm-1 k25. From 1971 to 1976 it was replaced by S. lateralis and S. stagnalis at annual mean conductivities of 8000-12000 μS cm-1 k25. S. stagnalis was the only corixid species in the nearby Red Lane Flash at conductivities of 20000-29000 μS cm-1 k25 and was a source of immigrants since the species did not breed in Watch Lane Flash except for 1976 (Savage, 1971, 1979, 1985). In 1975 Gammarus tigrinus Sexton was introduced. During the hot summers of 1975-76 G. tigrinus reached high population densities and, since it was a voracious predator of corixid nymphs, reduced the corixid communities to a very low level from 1977 to 1979. Thus, these years are omitted from Fig. 8. By 1980 the conductivity had fallen to approximately 5000 μS cm-1 k25 and S. concinna was again the commonest species, although numbers remained low compared with 1968-76. S. concinna remained the commonest

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species until 1984. From 1985 to 1988 the numbers of individuals increased and by 1989 were similar to 1968-76. The species composition of the corixid communities fluctuated markedly during the period when total numbers of individuals increased but still showed relationships with conductivity. From 1989 to 1992 the conductivity was very low, 1200-1700 μS cm-1 k25, and the corixid community consisted principally of 5. falleni and 5. dorsalis. Indeed, except for the presence of considerable numbers of S. dorsalis it was scarcely distinguishable from the freshwater communities at higher conductivities.

S. stagnalis and 5. selecta were confined to conductivities greater than 10000 μS cm-1 k25 except as noted above.

It will be apparent from the above account that each species occurs at a wide range of conductivities in both fresh and saline waters and hence it is needlessly imprecise to use the presence or absence of a single species as an indicator of water quality. Use of the significant quantitative changes in each species demonstrated above would provide a more precise indicator but it is probable that a combination of their distributions, expressed as the percentage species composition of a corixid community, will provide the most robust relationships with conductivity in particular and water quality in general.

An empirical monitoring model

The model is designed as a means of testing whether a given community of Corixidae from a lake of known conductivity conforms with the expected pattern derived from collections in a series of natural unpolluted waters.

The model (Fig. 10) comprises the percentage frequencies of ten species of Corixidae, 5. selecta and S. stagnalis being treated as a single catergory, in each of twelve divisions representing consecutive ranges of conductivity. The twelve divisions were produced by using a logarithmic function of conductivity (logio) to encompass the observed total range so that the representation of the relative ionic concentrations experienced by the organisms would be more physiologically realistic than one using a linear scale. For example, there is the same 10-fold increase in total ionic concentration between conductivities of 100 (logio = 2.0) and 1000 (logio = 3.0,) and between 1000 and 10000 (logio = 4.0) but a linear scale based on 100 unit divisions would produce nine and 90 divisions respectively. In contrast, a logarithmic scale (logio) with 0.25 unit divisions produces four divisions in each of the above ranges. Therefore a logarithmic scale ranging from 1.50 to 4.50, with 0.25 divisions, was used to produce the twelve divisions which have an overall conductivity range of approximately 32 to 32000 μS cm-1 k25. The model is then

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subdivided, at a conductivity of 1000 μS cm-1 k25, into a lower freshwater section and a higher salinewater section, each containing six divisions of conductivity.

The species comprising collections of Corixidae from each water-body have been expressed as percentages of the total numbers found, excluding those with a frequency of less than 2%. Two methods of mathematical treatment have been used: indices of similarity and dendrograms (Savage & Pratt 1976).

The indices of similarity for each conductivity division of the empirical model (Fig. 10), compared with other divisions, were calculated using the equation:

If the same species are present in identical percentages in two divisions then the index of similarity is 1; if there are no species in common then the index is 0. The proposed function of the above equation in this paper is to find the highest index of similarity or the "best fit" of a new sample with a particular division in the empirical model (Fig. 10).

The indices of similarity are also used to construct dendrograms for each of the two sections of the model in order to illustrate the relationships of the corixid communities within each section (Fig. 11). The method of dendrogram construction is from Mountford (1962) and may also be found in Spellerberg (1991).

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Procedure for use of the model

(1). Collect a total of at least 50 individuals from five separate stations at the open water edge of any vegetation present in the lake. Avoid collection in places devoid of vegetation, if possible, as few corixids may be found. Collect a surface water sample.

(2). Calculate the percentage frequency of the constituent species of the total samples, excluding those which form less than 2% of the total. Measure pH and conductivity of the water sample (μS cm-1 k25) and express the latter as common logarithms (logio)-

(3). Compare the sample percentage distribution with each conductivity division in the appropriate section of the model (fresh water or saline water) and select the best fit. This comparison may be done visually using Fig. 10 or, more precisely, by calculation of indices of similarity between the sample and each division in the appropriate section of the model (percentage species composition is shown in Fig. 10). Again, the best fit should be chosen, i.e. the highest index of similarity. If the conductivity of the water sample is near the point of separation of the two sections (ca. 1000 ΜS cm-1 k25), or more than 10% of S. lateralis are present, then both sections should be used.


Interpretation of results

The relationships between the corixid communities within each of the two sections do not follow a regularly changing pattern, as is shown by the two dendrograms (Fig. 11). In the freshwater section the dendrogram shows two distinct groups of communities; one at lower conductivities where S. dorsalis is common (F1, F2, F3) and a second at higher conductivities where S. fallen! is common (F4, F5, F6). However, the first group is partially subdivided into a group at the lowest conductivities where S. scotti is common (F1). Indeed, some lakes contain 5. scoffi alone. The salinewater section is divisible into five groups. The first group (S1), where 5. falleni is common, is closely related to the freshwater groups F4, F5 and F6. The second group (S2), where S. dorsalis is common, is similarly related to groups F2 and F3. The next two groups (S3, S4) contain large numbers of S. concinna and S. lateralis respectively. It should be noted that these four species all occur in fresh water, although the latter two, S. concinna and 5. lateralis, usually occur under special circumstances, as noted later. The final group (S5, S6) is peculiar to saline water, containing S. stagnalis and 5. selecta. These relationships should be noted when using the model.

If there is agreement in both the corixid communities and conductivity between sample and model then it is believed that the lake is in a normal state. If there is a lack of agreement, or agreement by best fit where the index of similarity is less than 0.5, then the notes on individual species set out below should be consulted prior to an assessment of a possible cause of the discrepancy and further investigations on aspects of water quality or limnology.

Notes on individual species

Micronecta power! (Douglas & Scott). Associated with high quality water in a natural state and low percentages of organic matter in the substratum. It is likely to be common where there is little vegetation (Macan 1938; Jansson 1977a,b, 1987).

Micronecta scholtzi (Fieber). Has a distribution similar to M. power! but in water of higher conductivity.

Sigara scotti. Associated with base-poor conditions. Occurs at higher conductivities than is usual if pH is low (ca. pH 5) (fig. 10), (Savage 1990).

Sigara dorsalis. Occurs in large lakes (Fig. 5b) where 5. fallen! is the commonest species in the model (Fig. 10) (Macan 1954a; Savage 1989, 1990).

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Sigara falleni. In my experience this species is associated with natural eutrophic conditions in British lakes. However, studies of lakes in Poland indicate that S. falleni may be associated with organic pollution (Biesiadka & Tabaka 1990).

Sigara fallenoidea (Hungerford). Is confined to Ireland where it appears to have similar ecological requirements to 5. falleni (O'Connor & Norton 1978; Tully et al. 1991).

Callicorixa praeusta. Common in newly created habitats (Macan 1954a) and where there is an unusually high organic content in the substratum (Savage 1990).

Sigara concinna. May occur in numbers in freshwater, at conductivities lower than 1000 ΜS cm-1 k25, if there has been temporary saline pollution such as runoff from salted roads.

Sigara lateralis. When it occurs in water with a conductivity of less than 5000 ΜS cm-1 k25, it is associated with former saline pollution or organic pollution such as fouling of water by cattle, waterfowl or silage effluent (Fig. 10; Macan 1954a; Savage & Pratt 1976).

Corixa punctata (llliger). Associated with sheltered areas of lakes such as deep bays; normally a pond species (Macan 1954a; Savage & Pratt 1976).

Claenocorisa propinqua (Fieber). A pelagic species which is prone to fish predation and usually only present in lakes where fish are absent. It became the commonest pelagic species in many Scandinavian lakes where fish have become extinct owing to acidification (Henrikson & Oscarson 1978, 1981, 1985; Oscarson 1987). Increasing numbers over a period of time are likely to be an indication of depletion or stress in fish populations.

Arctocorisa carinata (Sahlberg, C). An upland species (altitudes >300m) except in Scotland where it may appear with G. propinqua or S. dorsalis in some lowland lakes (Savage 1989).

Arctocorisa germari (Fieber). Associated with upland lakes, at altitudes >300m, and stony lakes with a deep littoral zone (Crisp 1962). Occurs occasionally in the North West Midlands meres.

Acknowledgements

I am obliged to the many landowners who kindly gave permission to sample the water bodies and to British Soda Company Limited for allowing me to examine borehole data. Dr M. Diamond, Mr A. Rock and Miss Jane Oldroyd allowed me to use their unpublished data while Mr J. Howell devised appropriate computer programmes. Figs 1 and 2 were


redrawn mainly from a paper submitted to the Proceedings of the Silver Jubilee Conference of the Cheshire Conservation Trust (now Cheshire Wildlife Trust) through the courtesy of Dr J. W. Eaton. Miss Stephanie Heath and Mrs Kay Bedford typed the manuscript. My son, Mr D. A. Savage, took some of the photographs.

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