Improving lake water quality in Slapton Ley National ... - Hydrologie.org

Freshwater Contamination (Proceedings of Rabat Symposium S4, April-May 1997). IAHS Publ. no. 243, 1997

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Improving lake water quality in Slapton Ley National Nature Reserve, south Devon, UK — amelioration by wetlands or drainage basin source management?

STEPHEN TRUDGILL Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN,

TIM BURT Department of Geography, University of Durham DHl 3LE, UK

PENNY JOHNES Department of Geography, University of Reading, Reading RG6 2AB, UK

LOUISE HEATHWAITE Department of Geography, University of Sheffield, Sheffield S10 2TN, UK

Abstract The efficiency of wetlands in reducing nutrient and sediment concentrations in runoff from agricultural areas can be used as an argument for not tackling nutrient and sediment losses at source within the basin. However, evidence from the Slapton basin shows that this efficiency varies seasonally and is much less marked in the winter than in the summer, pointing to the need for basin management during winter flows. In addition, while wetlands may reduce nutrient and sediment concentrations, thus improving downstream water quality, the accumulation of nutrients and sediments in the wetland can lead to progressive eutrophication and, in addition, terrestrialization of valued wetland habitats, further reinforcing the need for basin source control.

INTRODUCTION: POLICY AND PROCESS Runoff water quality is a function of two sets of factors: (i) inherent basin characteristics involving soil, topographic and climatic variables which interact to determine the degree and duration of solid-solution contact and the entrainment of particulate and soluble material, and (ii) land-use type and practices which (a) influence the operation of the variables in (i) and (b) modify the inputs and outputs by temporal and spatial additions and subtractions. Research has to take into account both the inherent and the management influences on water quality. If water quality is found to be undesirable (which in itself may be a reflection of the efficiency of the temporal and spatial monitoring network), then the management influences are in theory far easier to tackle than the inherent influences. The challenges lie in establishing the links between the management actions and the water quality effects so it can be reasonably predicted that changing management might have the desired results.

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Stephen Trudgill et al.

However, influences upon management are by no means purely scientific. Other agendas exist, especially economic, social and political agendas. The role of science then becomes one of not simply suggesting what appears to be scientifically sensible — even if that can be established in complex situations — but more one of assessing the options which are viable given the economic, social and political constraints. Many water quality scientists would tend to agree that tackling the sources of unacceptable water quality seems the most sensible course of action, but where this may conflict with other agendas, then tackling the downstream effects may sometimes be preferable. This may especially be the case when the effects are cumulative and the situation at any one source is in itself not undesirable. This means that if environmental management is to be successful, an understanding of environmental processes is required, and we should also be able to judge the evidence concerning the need for, and effectiveness of, any management action. Such understanding and evidence can be termed strategic information because it can be used as a basis for devising a strategy or plan of management action. Environmental situations are, however, often not simple or easy to understand and the evidence is not always clear or available. Thus, we often have to judge what might be the most reasonable course of action while being aware of the limitations of any evidence and also of the complexities involved, especially in terms of the interlinkages which exist within and between environmental systems. It is also necessary to take into account any implications for the different individuals and groups which might be involved. An understanding of peoples' livelihoods, viewpoints and attitudes (and what influences their understanding) is just as important in environmental management as is an understanding of environmental processes. In this paper, therefore, the role of wetlands in diminishing nutrient loading as an alternative to drainage basin manipulation, which may impinge upon individual economic activity, is investigated.

THE RESEARCH AREA The Slapton basin is located in south Devon and is described by Burt & Heathwaite (1996a, b). The area is underlain by lower Devonian slates which have a low permeability (Dineley, 1961). The slates have weathered to yield a deep regolith of 1-2 m and soils of an acid brown earth type, which have a pH of 4-5 and a texture comprising 30-40% silt, 40-30% clay, and the remainder as sand (Trudgill, 1983). Plateau tops drain down steep (8-30°) slopes into incised valleys, where the lower reaches are infilled with marshland (Mercer, 1966; Burt et al., 1983; Burt & Butcher, 1985). Rivers from the dominantly agricultural basin drain into a freshwater lake, Slapton Ley (Johnes & Wilson, 1996), which is impounded from the sea by a shingle bar (Morey, 1976). The area has a mild, moist, maritime climate with a mean annual temperature of 10.5°C, and mean temperatures of 5.7°C in February and 15.8°C in July. On average, air temperatures fall below freezing on only 22 occasions per year (Ratsey, 1975). This means that there is a long growing season. Mean annual rainfall for the period 1961-1985 was 1035 mm; over 1 mm of rainfall falls on average on 135 days

Improving lake water quality in Slapton Ley National Nature Reserve, south Devon, UK

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of the year, with over 10 mm of rainfall on 35 days. Although there is an ample supply of moisture for agriculture, average annual evaporation losses exceed 400 mm, and drought conditions may occur in some summers. Four main river systems drain to the lake: the northern River Gara (73% of the inflow to the lake), the Slapton Wood stream (1%), the Start stream (23%) and the Stokeley Barton stream (3%). The former two join and flow into the northern Higher Ley which then flows into the southern Lower Ley. This 77 ha area of open water also receives the Start and Stokeley Barton streams (Burt et al., 1983; van Vlymen, 1979). Stream water quality and sediment yield are described by Burt et al. (1996) and Foster et al. (1996). Under specific consideration in the present paper is the role of the vegetated Higher Ley in ameliorating the quality of input river water from the Gara catchment before it flows into the open water Lower Ley and thence to the outflow at Torcross. Wetland, lake and woodland areas are owned by the Whitley Wildlife Conservation Trust and managed as a National Nature Reserve by the Field Studies Council and English Nature (Riley, 1988, 1994; Trudgill, et al, 1996), with a total freshwater habitat of 155 ha. The Higher Ley is a wetland area with reed beds and islands supporting willow and alder carr (Brooks & Burns, 1969; Burns, 1996), and it is much valued not only for its wetland flora but also for its associated bird life (Elphick, 1996) and as a haunt for otter (Riley, 1996). The Lower Ley is predominantly open water, though reed beds are present round the fringes. The relevant history of the area is summarized in Table 1 and is broadly one of rising sea level which impounded a freshwater lagoon that later became eutrophic. In the Lower Ley there is a valued but dynamic and unstable fish population which appears sensitive to trophic status (Kennedy, 1996).

MONITORING The principal input into the lake ecosystem, the River Gara (Fig. 1), has been monitored for a number of years at Higher North Mill (HNM). This system then gains some further small input from the Slapton Wood stream and flows through a vegetated wetland system, the Higher Ley, to a second monitoring point at Slapton Bridge (SB). From this point the system then flows into the open water of the Lower Ley, with inputs from the Start Stream and Stokeley Barton, and on to the output point at Torcross (TX), which is the final monitoring point (Fig. 1). Although it might be preferable to express budget calculations in terms of loads, the focus in this paper is on concentrations because these are used as criteria when attempting to meet water quality standards. The critical sequence of the three monitoring points discussed here is: (i) river input at Higher North Mill (HMN) from the agricultural catchment to the wetland system, which then traverses through the wetland system of the Higher Ley to (ii) the output from the wetland system and input to the open water at Slapton Bridge (SB), which then passes through the open water lake of the Lower Ley to provide (iii)the output of the open water lake at Torcross(TX). Simply expressed, if solute concentrations show a pattern whereby those in the

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wetland input exceed those in the wetland output, which in turn exceed those in the lake output (i.e. HNM > SB > TX), then there is a downstream decrease in concentrations and the system can be seen as acting to improve water quality. Specifically, if concentrations are greater in the wetland input than the output (HNM > SB), then the vegetated wetland of the Higher Ley is assisting in the protection of the water quality of the Lower Ley and its associated fish populations.

Table 1 The history of Slapton Ley (adapted from a variety of sources including Field Studies, vol. 1996; Burt, 1993; Cannell, 1992; Heathwaite 1993). 10 000 BC Flandrian transgression. 5000 BC Brackish lagoons and estuarine conditions. One freshwater peat sample indicates proto-ephemeral Ley. 980 BC Freshwater peats in Lower Ley (Morey, 1976). 175 AD Brief marine incursion. 1522 Drawbridge present at Slapton Bridge. C17 Use of lime and clover in agriculture. C18 Causeway built at Slapton Bridge. 1856 Construction of road along ridge, weir and culvert built at Torcross. Depth of Ley increased; increased residence time, periodic deoxygenation. Influx of CaC0 3 (peaks of authigenic Ca, Mg and Mn). 1886 First OS map. 13 ha of Higher Ley shown as open water, reedbed managed between Strete and Slapton Wood.

1945 Intensification of agriculture. Increased influx of topsoil and allogenic phosphorus. Aerial photograph shows Higher Ley at 84% managed reedbed free of carr, 6% as "floating islands", 5% scrub and 4% open water. 1950 Shallow clear eutrophic lake. 1951 Aerial photograph shows rapid growth of floating islands and open water (?renavigation of channel). 1953 Slapton Sewage treatment works installed. Increase in biogenic silica and authigenic phosphorus. 1967 Year of peak sediment yields in the Lower Ley, increasing ploughing and grazing in the catchment. 1970s Eutrophy, Lake increasingly productive, cyanobacteria abundant, algal blooms. 1973 Aerial photograph. Continued loss of reedbed to non-island carr; growth of islands with carr coverage to 38%. 1976 Severe drought, peak in authigenic N.

1905 Second OS map. Higher Ley shown as wholly reed covered. 1908 Increase in rate of erosion of soil material into Ley. Ley shallower, deoxygenation ceased, water residence time decreased.

1980s Hypertrophy. Ley turbid, centric diatoms abundant, increasing influx of biogenic silica and authigenic phosphorus; occasional deoxygenation. Fish numbers decline. Algae very evident. "A Dying Lake". Calls for action.

1920 Anecdotal evidence of raising of weir at Torcross, deepening Ley and detachment of lake bed fragments, up valley ponding and sedimentation (Deer Bridge and lower Gara).

1990 Aerial photograph. Decline of reedbed growth with non-island carr to 5.5%; rapid sedimentation of Gara delta; islands to 2.3 ha with carr cover to 73 %.

1930 Existence of island carr in 1945 suggests initial colonization about now.

1992 Improvement at sewage works (May), mainly for improved DO.

1943 Allied bombing of Ley; may have loosened lake bed deposits.

1995-96 Fish apparent again in numbers.


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River input

I HNM

I

—. Higher Ley - - wetland

Tributary SW

Tributary DB

->SWKS

->1B

Tributary StBt

TX outlet

Fig. 1 Schematic diagram of sampling points. The River Gara is sampled at Higher North Mill (HNM), Slapton Bridge (SB) and Torcross (TX). The tributary Start Stream is sampled at Deer Bridge (DB) and below the sewage works (SWKS) at Iron Bridge (IB). The Slapton Wood stream(SW) and the Stokeley Barton Stream (StBt) are also tributary to the River Gara.

Conversely, if the downstream sequence for water quality parameters shows concentrations in the wetland input to be less than or equal to those of the wetland output, which in turn are less than or equal to those in the lake output (HNM < or = SB < or = TX) then this means that the Higher Ley wetlands are not acting to improve lake water quality. In these circumstances, inputs from the River Gara and from tributaries and other sources (including riparian contributions, sources from built-up areas and possibly also inputs from wetlands) are dominant, and this implies that action on the sources in the drainage basin is essential to improve lake water quality in the Lower Ley. The water quality parameters investigated included conductivity (measured using an electrode and expressed in uS), nitrate (measured using an electrode and by an autoanalyser in a technique based on copper-hydrazine reduction, and expressed as NO3-N inmg l"1), ammonium (measured by autoanalyser and expressed as NH4-N in

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mg l"1), phosphate (measured by autoanalyser in a technique based on molybdate blue, and expressed as P04-P in mg l"1) and suspended sediment (measured by filtration and expressed as g 1'1). The data reported below have been gained by a scheme of weekly sampling, with the attendant qualifications which apply to any such spot sampling, especially in terms of the limitations of representativeness during any highly temporally variable conditions. There is also the question of travel times of the water between sampling sites which are inevitably longer than the times between sample collection. However, the sites were sampled in a downstream time sequence, and earlier work on travel times of water in the Higher Ley (Trudgill et al., 1991) indicate that a sample interval of a few hours may indeed be comparable with the travel times of the fastest flowing water. This study presents a simple comparison of downstream and upstream concentrations for the weekly samples taken in the 189 weeks from 31 March 1986 to 21 January 1990, and also some data for the period from 14 February 1979 to 16 December 1981. A further qualification is that the samples have only been analysed for the bioavailable fractions of N and P and not the total N and P (which includes the particulate/organic fractions). Therefore any changes described here may represent a change of phase rather than an actual loss (Heathwaite & Johnes, 1996). THE AMELIORATION OF WATER QUALITY BY WETLANDS The data for ranked differences in concentration are shown in Fig. 2, and are summarized in Table 2. In Fig. 2, data bars plotting to the left of the central vertical axis indicate downstream increases in concentration while data plotting to the right indicate downstream decreases in concentration.

Table 2 Downstream changes (% occurrence) in water quality parameters. Percentages refer to proportion of weekly observations in period 31 March 1986 to 21 January 1990. Numbers of weeks shown in parentheses under determinand name. Values are rounded to nearest whole number; and emboldened values show highest percentage. HMN to SB Higher Ley, vegetated Loss ( > ) None (=) 11 21

Gain( ) None ( = ) 24 10

Gain( SB

^ljj

conductivity

305

Fig. 2 Ranked downstream changes in determinand concentrations. Bars plotting to the right of the vertical axis indicate downstream decreases, bars to the left indicate downstream increases. Left hand column shows effects of the wetland system, right hand column that of the lake system. HNM, SB and TX defined in Fig. 1.

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In contrast to conductivity, which dominantly increases downstream (presumably through additional inputs and evaporation as well as a probable maritime sea salt influence), nitrate and phosphate show a predominance of downstream decreases in concentration in both the vegetated Higher Ley and the open water Lower Ley, and this occurs despite further inputs from tributaries. For nitrate there is a greater frequency of decreases (68% of the 189 observations) in the open water system than in the vegetated system (53%). The few available data for ammonium are more evenly balanced but with a slight dominance of increases. Phosphate shows marginally greater decreases in the vegetated system than in the open water. However, the change from the data for 1979-1981 to the data for 1986-1990 is most marked, with gains occurring for nearly one-third of the observations in this latter period as opposed to under 10% in the former, and c. 90% losses in 1979-81 compared with c. 55-60% in 1986-1990. For suspended sediment, around half the occurrences showed a decrease in concentrations in the vegetated Higher Ley, but also about one-third of the occurrences showed that the area could also act as a source for sediments rather than as a sink. Downstream losses in suspended sediment concentrations were not marked in the Lower Ley, and indeed the area and its tributaries appeared to act as a source for around two-thirds of the observations. In terms of amelioration in the Higher Ley acting to protect the Lower Ley against nutrient inputs from the Gara, the fact that 50-60% of the 1986-1990 observations show losses of nitrate, phosphate and sediment suggests there is not a clear mandate for either: (i) inaction in the drainage basin, leaving wetlands to improve water quality (option A), or (ii) increased catchment management on the grounds of the dominant inefficiency of the wetlands in removing nutrients (option B). Additional management considerations in respect of option A are firstly that the wetland system may have a finite capacity for nutrient retention. This may already be exceeded, as the lower levels of decreases in 1986-1990 compared with 1979-1981 may indicate. Even if this is not the case, the capacity may be exceeded in the future. Secondly, increasing retention could also increase vegetation productivity and aid the terrestrialization of the wetland system, reducing its conservation value as an aquatic ecosystem. It is clear that there may be merit in a more detailed analysis of the data than the simple comparison of input and output concentrations. Of particular interest are firstly the analysis of differences in relation to absolute concentrations and secondly analysis of the differences in relation to season (and thus differences in biological activity). A priori, while there is rarely a simple relationship between concentration and discharge, it can be suggested that either removal is less efficient at high concentrations (because there is more to remove) or at low concentrations which are liable to be associated with higher flows and thus shorter residence times and less opportunity for transformations and uptake. It can also be suggested that removal should be more efficient in the summer for the reasons of greater biological uptake and of lower flows and greater residence times. In Fig. 3, concentrations are plotted on the vertical axis and differences on the horizontal axis. Data points to the left of the origin again indicate downstream

Improving lake water quality in Slapton Ley National Nature Reserve, south Devon, UK (a) Conductivity

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SB > HNM HNM > SB 1

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0.2

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Fig. 3 Relationship of downstream changes in determinand concentrations (horizontal axis) against actual concentrations (vertical axis). Points plotting to the right of the vertical axis indicate downstream decreases, points plotting to the left indicate downstream increases. Left hand column shows effects of the wetland system, right hand column that of the lake system. HNM, SB and TX defined in Fig. 1.

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Fig. 4 Time series of downstream changes in determinand concentrations for period 30 March 1986 to 21 January 1990, except (e) and (f) which are for 14 February 1979 to 16 December 1981. Bars plotting above the horizontal axis represent downstream decreases and bars plotting below the horizontal axis represent downstream increases. Arrows represent the duration of the warmer months from April to September. Left hand column shows effects of the wetland system, right hand column that of the lake system. HNM, SB and TX defined in Fig. 1.


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increases in concentration and those to the right downstream decreases. Data lying along a 1:1 line in these plots indicate complete removal such that the higher the concentration, the higher the matching decrease. Data plotting to the left of this line, but in the top right hand quadrant, indicate a partial decrease, while those points in the top left hand quadrant indicate an addition. For nitrate there is no clear relationship, and the highest concentrations around 10 mg f' show both minimal and also substantial downstream decreases. For phosphate, there is a clear indication that the high concentrations are decreased completely, especially in the Higher Ley (wetland input > wetland output; HNM > SB) while partial decreases occur at the low end of the concentration scale. This also tends to be true for sediment. It is also evident from Fig. 3 that the actual phosphate concentration data (vertical axis) for 1986-1990 are higher than for 1979-1981. While Fig. 2 indicates a lower frequency of decreases for the 1986-1990 data, this is evidently at the lower end of the concentration scale because Fig. 3(g) shows a clear near 1:1 plot of high concentrations matched with high decreases. The decreases in concentration do not therefore appear to vary with actual concentrations except that, if anything, it is the lower concentrations which show the lesser decreases. This hints at the possibility of lower rates of decrease at higher flows with low concentrations, which may well be associated with winter conditions. In Fig. 4, the possibility of seasonal effects is examined by plotting differences between upstream and downstream concentrations against time by a series of bars. Those bars which have positive values above the horizontal axis represent a downstream decrease in concentration while those with negative values below the horizontal axis represent downstream increases. The period of the summer months (April to September) are represented by horizontal arrows. Although there are exceptions, the plots in Fig. 4 reveal that many of the values rising above the horizontal axis (downstream decreases) occur during the summer periods (arrowed) and many of the negative values (downstream increases) are coincident with the winter months (gaps between the extent of the arrows). This tends to suggest that there is indeed a seasonal effect and that vegetation cover plays an important part in water quality improvement. Figures 3(i) and 3(j) also suggest that sediment concentrations generally decrease during the passage of water through the Higher Ley but that in winter the Lower Ley does show some downstream increases in sediment concentrations in the lake (from SB to TX).

CONCLUSIONS The implications for improvement of water quality are thus that option A (inaction and allowing wetlands to decrease concentrations) might well be viable for the summer months. Option B (catchment source management) is clearly, however, important for the winter months when the mechanical and biological effects of a vegetation cover are minimal. There are, however, additional considerations in terms of habitat management. Nutrient concentration decreases in the Higher Ley can represent a number of processes, including denitrification (and thus loss of N to the

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atmosphere) but also plant uptake and adsorption onto sediments, especially of P. Thus, in many ways a decrease in eutrophication in the Lower Ley is at the expense of eutrophication in the vegetated Higher Ley. This, coupled with increased sedimentation, acts to encourage plant productivity, the accumulation of organic debris and terrestrialization of the aquatic system. Such a progression is already evident from Table 1 and is generally perceived as undesirable in terms of habitat management. Wetland habitat is seen as especially valuable in the area while the more common terrestrialized habitats are less valued. It is concluded that while catchment management is evidently desirable during the winter months in terms of water quality improvement, it is also desirable yearround in terms of maintaining the valued wetland habitat. Initially we suggested that a "downstream" management option might be preferable in terms of not affecting people's livelihoods in the (agricultural) drainage basin. However, if such an option leads to habitat degradation (and associated tourist/recreation impacts and loss of wildlife value) it becomes clear that a balance should be found whereby catchment management is effected without adversely affecting people's livelihoods while also maximizing habitat and wildlife potential in the wetlands. Given a certain capacity for water quality amelioration within the wetland area, the aim of the basin management should be not only to promote a general reduction in nitrate, phosphate and sediment losses in order to reduce accumulation in a valued wetland system but specifically to target source control in winter runoff periods when the wetland is least efficient in nutrient and sediment retention. There are methodological qualifications of using weekly sampling data in a simple input-output analysis and also the use of a simple spatial sampling network. Both temporal and spatial changes may be missed, especially changes due to inputs between the sampling points and also in the lake system between specific inputs such as the sewage works and the output. However, the evidence, thus qualified though it may be, is that the wetland system of the Higher Ley is in itself not sufficient to protect the aquatic system of the Lower Ley. Indeed, the reported high sensitivity of the fluctuations in the fish populations to basin changes in land use and changes in land-use practice (Kennedy, 1996 and see Table 1) would tend to support this conclusion. While there is evidence that nutrient and sediment decreases do indeed occur in the Higher Ley wetland, rather than indicating a reassuring efficiency of the wetland in protecting the Lower Ley, the balance of the evidence is more one of increasing sedimentation and eutrophication of the valued Higher Ley wetland system and of the sensitivity of the Lower Ley to inputs from basin sources. Notwithstanding the possibilities of other management options, such as dredging or the use of buffer strips, the data presented in this paper support the view of Johnes & Heathwaite (1997) that the Best Practicable Environmental Option lies in giving attention to the high nutrient export risk source areas within the basin, despite any possible impingement on human economic activity. Wetlands cannot necessarily be seen as a panacea, absolving us from any action in the drainage basin, and indeed the wetlands can be seen as deteriorating as a result of basin processes. Given this situation, in broader terms the implementation of any management option depends upon the balance between the economic effects of basin management and the value placed upon the wildlife habitats. Currently economic


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activity in the basin appears to take precedence over wetland habitat conservation; preliminary investigations (Perkins, 1995) suggest that while there is some willingness in the drainage basin for people to pay for valued wildlife habitats, the matters are rather finely balanced. Acknowledgements The data reported in this paper have been gathered by a large number of workers at Slapton Ley Field Centre over several years under the organization of Keith Chell and Chris Riley and also formerly by Tony Thomas, all under a scheme initiated by Bob and Lorna Troake.

REFERENCES Brooks, B. S. & Burns, A. (1969) The natural history of Slapton Ley Nature Reserve III. The flowering plants and ferns. Field Studies 3, 125-157. Burns, A. (1996) The vascular plants: an update. Field Studies 8, 663-664. Burt, T. P. (ed.) (1993) A field guide to the geomorphology of the Slapton Region. Occasional Papers of the Field Studies Council 27. Burt, T. P. & Butcher, D. P. (1985) The role of topography in controlling soil moisture distributions. J. Soil Sci. 36, 469-486. Burt, T. P., Butcher, D. P., Coles, N & Thomas, A. D. (1983) The Natural History of Slapton Ley Nature Reserve XV. Hydrological processes in the Slapton Wood catchment. Field Studies 5, 731-752. Burt, T. P. & Heathwaite, A. L. (1996a) Long-term study of the natural environment at Slapton Ley. Field Studies 8, 533-542. Burt, T. P & Heathwaite, A. L. (1996b) The hydrology of the Slapton catchments. Field Studies 8, 543-557. Cannell, S. (1992) Slapton Higher Ley — its history, ecology and conservation. Unpublished report, Slapton Ley Field Centre. Dineley, D. L. (1961) The Devonian system in south Devonshire. Field Studies 1, 121-140. Elphick, D. (1996) A review of 35 years of bird-ringing at Slapton Ley (1961-1995) together with a brief historical review of ornithological observations. Field Studies 8, 699-725. Foster, I. D. L., Owens, P. N. & Walling, D. E. (1996) Sediment yields and sediment delivery in the catchments of Slapton Lower Ley, South Devon. Field Studies 8, 629-661. Heathwaite, A. L. (1993) Catchment controls on the recent sedimentation history of Slapton Ley, south-west England. In: Landscape Sensitivity (ed. by D. S. G. Thomas & R. Allison), 241-259. J. Wiley, Chichester, UK. Heathwaite, A. L. & Johnes, P. J. (1996) The contribution of nitrogen species and phosphorus fractions to stream water quality in agricultural catchments. Hydrol. Processes 10, 971-983. Johnes, P. J. & Heathwaite, A. L. (1997) Modelling the impacts of land use changes on water quality in agricultural catchments. Hydrol. Processes, (in press). Johnes, P. J. & Wilson, H. M. (1996) The limnology of Slapton Ley. Field Studies 8, 585-612. Kennedy, C. R. (1996) The fish of Slapton Ley. Field Studies 8, 685-697. Mercer, I. D. (1966) The natural history of Slapton Ley Nature Reserve. I. Introduction and morphological description. Field Studies 2, 385-405. Morey, C. R. (1976) The natural history of Slapton Ley nature Reserve. IX. The morphology and history of the lake •basin. Fidd Studies 4, 191-206. Perkins, R. (1995) Survey of attitudes to wildlife in the Slapton area. Unpublished dissertation, Department of Geography, University of Oxford. Ratsey, S. (1975) The climate at Slapton Ley. Field Studies 4, 191-202. Riley, C. (1988) A management plan for Slapton Ley Nature Reserve. Unpublished report (FSC/EN/WWCT). Slapton Ley Field Centre. Riley, C. (1994) Slapton Ley NNR management plan — revision. Unpublished report (FSC/EN/WWCT). Slapton Ley Field Centre. Riley, C. (1996) Mammals and other animals. Field Studies 8, 665-676. Trudgill, S. T. (1983) The natural history of Slapton Ley Nature Reserve. XVI. The soils of Slapton Wood. Field Studies 5, 833-840. Trudgill, S. T., Chell, K. & Riley, C. (1996) Education and conservation issues in the Slapton Ley NNR. Field Studies 8, 727-746. Trudgill, S. T., Heathwaite, A. L. & Burt, T. P. (1991) The natural history of Slapton Ley Nature Reserve. XIX. A preliminary study of the control of nitrate and phosphate pollution in wetlands. Field Studies 7, 731-742. van Vlymen, C. D. (1979) The natural history of Slapton Ley Nature Reserve. XIII. The water balance of Slapton Lev Field Studies 5, 59-84.