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C O N S E R V A T I O N P R O B L E M S IN T H E N O R F O L K B R O A D S A N D R I V E R S O F E A S T A N G L I A , E N G L A N D - -
P H Y T O P L A N K T O N , B O A T S A N D T H E C A U S E S O F T U R B I D I T Y
BRIAN MOSS
School O] Environmental Sciences, University oJ East Anglia, Norwich, NR4 7T J, Great Britain
ABSTRACT
The NorJolk Broads and rivers of eastern England (Fig. 1 ) comprise an area hitherto [amedjor the diversity of its wildliJe and submerged aquatic plant communities. The latter have progressively disappeared since the earl)' 1950s, until only jbur sites currently retain more than remnants of the original sub-aquatic macrophyte flora and its associated invertebrate Jauna. Increases in turbMity of the water have been associated with the loss oj macrophytes, and these increases have been variously attributed to phytoplankton and to disturbance of sediment by the many boats of visiting tourists and residents. Synoptic surveys of turbidity were carried out in the navigable waterways ojBroadland in summer and winter 1973, and oJphytoplankton in summer 1973. The differential distribution o lphytoplankton is discussed in terms oJ the nutrient loadings on, and flushing coejficients of, the waterway. Highly significant correlations were obtained between phytoplankton numbers and turbidity in the system as a whole and in Broads and rivers considered separately. A very weak correlation between boat activity and turbidity was shown to be non-causative. It is concluded that increase in turbidity is a junction of increased nutrient loading jrom human activities in the catchment area and that boat disturbance does not contribute significantly to the sustained t~rbidity.
INTRODUCTION
The Norfolk Broads are small, shallow lakes excavated in river valleys for peat up to the 13th century (Lambert et al., 1960). Sited either close to the rivers, and connected with them by channels, or isolated by up to 3km from the main waterways, they form a complex of wetland and open water communities long recognised for the richness of its wildlife (Duffey, 1964; Ellis, 1965; Nature
95 Biol. Conserv. (12) (1977)--~'~ Applied Science Publishers Ltd, England, 1977 Printed in Great Britain
96 BRIAN MOSS
Conservancy, 1965). Some of the Broads are National Nature Reserves, others are County Reserves or Sites of Special Scientific Interest (Nature Conservancy, 1965). Two reserves, Hickling Broad NNR and Bure Marshes NNR, are recognised internationally in the RAMSAR convention. The bird diversity, the richness of the reedswamp and fen floras and faunas, and the variety of submerged aquatic macrophytes have been three of the reasons for the great scientific interest of Broadland. It is with the conditions underwater and the latter aspect that this paper is concerned.
In only four (Calthorpe, Martham, Backfleet, Upton) of more than 40 Broads do any substantial stands of submerged macrophytes now remain. This decline has been noted elsewhere (George, 1970; Morgan, 1972; Mason & Bryant, 1975) and has been relatively recent. Macrophytes disappeared from Hickling Broad in 1973/74 and from other Broads from the 1950s onwards. Anecdotal evidence (see Ellis, 1965) contrasts the water clarity of some years ago with the present very turbid water.
The increased turbidity might have been the cause of the macrophyte decline through severe shading of macrophyte propagules growing at the sediment surface. Phillips et al. (1977) have shown, however, that increased development of, and thence shading by, epiphytes and filamentous algae may have been the immediate cause of the decline. Increase in phytoplankton has probably subsequently resulted from the disturbance of a competitive mechanism by which macrophytes previously restricted the growth ofphytoplankton cells. Nonetheless the present high turbidity certainly must be reduced to allow recolonisation of macrophytes and restoration of this aspect of the conservation value of the area.
Increased nutrient loading from human activity in the catchment has been associated with the macrophyte decline (Osborne & Moss, 1977). The proportions of critical nutrients, nitrogen and phosphorus compounds, coming from the three potential sources of sewage effluent, farmyard effluent and field drainage, vary from Broad to Broad. In Barton Broad sewage effluent contributes the major proportion (Osborne & Moss, 1977) whilst in the Hickling Broad-Horsey Mere complex, from whose catchment there is no significant sewage effluent discharge, the increased output, particularly of nitrogen compounds, consequent on the progressive conversion of natural and semi-natural fen and grassland communities to arable land seems to have been critical. Establishment of high algal crops requires also a sufficient supply of phosphorus and this may be supplied by the flocks of black- headed gulls (Larus ridibundus) which now roost nightly on the Broad.
Turbidity in the water is also caused by the mechanical disturbance of sediment by boats. Broadland is a well-known boating holiday area and supports a population of at least 10,000 boats (9247 in 1964, a threefold increase since 1947, Nature Conservancy, 1965), of which about half are hired to visitors in the main spring to autumn holiday season. Bank erosion is attributed to the effects of wash particularly from motor cruisers and agitation by propellers is thought likely to stir up sediment
CONSERVATION PROBLEMS IN THE NORFOLK BROADS 97
from the bottom since many of the waterways are shallow (Nature Conservancy, 1965; Tubbs, 1975). By the very nature of sediment, however, it might be expected to sink back rapidly unless maintained in suspension by a continual passage of boats.
The possibility, nonetheless, that sediment turbidity may be present as well as high populations of phytoplankton, has meant that responsibility for the macrophyte conservation problem could be discreetly attributed by one agent (among the boat hire industry, the water authorities, the farming community and the navigational authority) to another in the absence of firm evidence. In turn the conservation bodies have been hampered in obtaining agreement, even in principle, to corrective measures.
Data are given here on the phytoplankton community of the Broadland, which has not previously been systematically studied, and on the relative effects of phytoplankton and boats in creating turbidity.
METHODS
Eighty-two stations (Fig. 1) were sampled during 2-5 July 1973, and all but those on the River Waveney were examined in a cruise from 11-13 December. At each station light penetration was measured with a secchi disc. The mean depth at which the disc disappeared from view and reappeared on lowering and then raising it, Z s (m), gave a measure of water transparency, or as a reciprocal, of turbidity. In the shallow lowland Loch Leven the secchi disc depth was found to be between one half and one third of the depth of the euphotic zone (Bindloss, 1976). Although not providing as precise a measure of light penetration as would a series of measurements with a sub- aquatic photometer, the secchi disc method is so rapid that many more measurements can be made in a short period over a large area.
Conductivity was measured at each station with a conductivity meter and water samples were collected for phytoplankton determinations, which were made with a Wild inverted microscope on aliquots of water pipetted into settling chambers. Counts were made with a x 40 objective and expressed as cells, filaments, or colonies per ml, dependent on the form in which each species was normally present. Total number of phytoplankters gives an acceptable measure of the size of the community for the purposes of extensive surveys.
A measure of boat activity was given by the number of boats passing given points in either direction (Fig. 1) during the period 0800-2000 h GMT. These data were obtained by the Norfolk County Council planning department on 8 and 9 August 1967 (Broadland Study and Plan, 1971). The data conform to the expected pattern of high activity along the northern rivers (Bure, Ant, Thurne) and lower activity on the Waveney and Yare. The pattern in summer 1973 is not expected to have differed significantly.
98 BRIAN MOSS
NORTH
BARTON
,O TON
'To3dE
STALHAM
HICKLING BROAD~.
NORTH
MARTHAM
SEA
BROAD MALTHOUSE BROAD %
NORWICH BROAD
R . C P E 7 ~ LCODON
R. WAVENEY
BECCLE:
R. BURE
BREYDON
YARMOUTH
Fig. 1. Map of the Broadland waterways. Solid circles indicate sampling stations in July (1973). Stations in December 1973 are indicated on Fig. 3. Encircled figures give numbers of boats passing each way between 0800 h and 2000 h G M T on 8 and 9 August 1967, along reaches indicated by lines crossing the rivers. Small arrows indicate main sewage effluent outfalls and large, solid arrows the arbitrarily defined
limits of the estuary of the rivers.
CONSERVATION PROBLEMS IN THE NORFOLK BROADS 99
The mean discharges of the rivers are shown in Table 1 ; and are based on gauging data obtained in the upper reaches of the rivers extrapolated to cover the entire catchments, determined from l:10000 Ordnance Survey maps.
TABLE 1 ESTIMATED MEAN DISCHARGE OF BROADLAND RIVERS
Catchment area" Mean discharge h (km 2) m 3 sec-i
River Waveney (at entrance to Breydon Water) River Yare (at entrance to Breydon Water) River Ant (at confluence with River Bure) River Thurne (at confluence with River Bure) River Bure (at confluence with River Ant) River Bure (at entrance to Breydon Water)
950 2.71 1475 10-25
163 1.09 143 0.96 475 3.17 931 6"22
"Based on graphical determination on large scale maps. Estimated from data given in the Surface Water Year Book and Supplement (1968).
R ESU LTS
The area surveyed, together with the location of major sewage effluent outfalls and data on boat traffic, are shown in Fig. 1. Only Broads that were sampled are shown. There are a number of other Broads either isolated from the rivers or in connection with the rivers but with restricted access to boats. This survey included only areas of unrestricted public access.
The pattern of conductivity in July is shown in Fig. 2. In December the pattern was similar, varying only in the magnitude of conductivity in the lower estuarine reaches of the Bure, Yare and Waveney, where it varies diurnally with the state of the tide. The upper reaches of the rivers and their associated Broads all had conductivities in the normal range for freshwaters, except for the River Thurne and its Broads. These were slightly saline, with conductivities about ten times higher than those found elsewhere in the upper rivers. Conductivity in the Thurne system was highest in Horsey Mere, which is closest to the sea coast. Although sea water penetrates up the rivers on the highest spring tides under certain weather conditions (Nicholson, 1896; Gurney, 1911 ; lnnes, 1911), the salinity of the Thurne Broads seems most likely to be caused by incursion through the ground water (Pallis, 1911; Goldsworthy, 1972), particularly as porous sands cover much of the area between Horsey Mere and the sea.
The depths at which a secchi disc was just visible are shown in Fig. 3. In general the water was more transparent in December when both boat activity and phytoplankton numbers were minimal. Mean values for secchi disc transparency are shown in Table 2. There was little difference between July and December in the lower (estuary, see Fig. 1) reaches of the rivers or in the River Yare. The River Thurne and
lO0 BRIAN MOSS
. ql Z.LZ ?. .! . ; ,
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Fig. 2.
ol P •
• ~0.I
• ~0.5
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,o •
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CONDUCTIVITY ,umho cm--Zx 10 4
C o n d u c t i v i t y o f the B r o a d l a n d wa te r s , Ju ly 1973. M e a s u r e m e n t s a re expressed a t 20C in/~ Siemens c m - 2 (/~mho c m - 2).
T A B L E 2 MEAN SECCHI DIS(." TRANSPARENCIES, Z s m IN BROADLAND WATERWAYS CALCULATED FROM SYNOF[IC
SURVEYS IN JULY AND DECEMBER 1973
July December
E s t u a r y River T h u r n e a n d its B r o a d s Rivers Bure a n d A n t a n d the i r B r o a d s River Ya re a n d its B r o a d s River W a v e n e y W h o l e sys tem, except River Waveney
0.45 0.39 0 .50 0.60 0 .40 1.08 0.61 0.59 1.12 0.46 0.77
CONSERVATION PROBLEMS IN THE NORFOLK BROADS ] 0 1
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102 BRIAN MOSS
its Broads showed only slightly greater transparency in winter than in summer, and the major seasonal contrast was in the Bure-Ant system. The most transparent area in summer was the River Waveney. Comparable transparencies in the other rivers were reached only in the uppermost navigable stretches, above the locations of the Broads.
The distribution of phytoplankton numbers in July is shown in Fig. 4, where the composition of crops exceeding 1000 ml - ' is also shown. The estuarine reaches have sparse plankton, as also does the River Waveney, except in Oulton Broad, a saline area close to Lowestoft. Phytoplankton crops were also relatively small in the River Yare. As in the Waveney, where an increase occurred below the sewage effluent outfall near Beccles, there was an increase just below the outfall of the Norwich (Whitlingham) sewageworks and in the region of Rockland and Surlingham Broads. On the River Chet, a tributary of the Yare, a similar increase occurred below the Loddon effluent outfall. Diatoms were prominent at all of these stations.
A diatom-dominated plankton became abundant in the lower Bure, above its estuarine section, and was associated with the outfall of a sewage works near Acle, whilst a complex pattern was found in the upper Bure. Plankton was sparse in the river at Wroxham and above it, but increased in Wroxham, Salhouse and Hoveton Little Broads, in the river between them and just above Wroxham Broad. Diatoms again predominated. Downriver from this area, two Broads, Malthouse and South Walsham, supported very large populations of blue green algae (Cyanophyta) which were also relatively abundant in the river below the dykes which connect these Broads and the river.
Moderately abundant diatom and diatom-blue green algal populations developed in quiet tributaries of the River Ant above Barton Broad, but in the Broad itself large blue-green algal populations were found. These persisted down-river until diluted at the confluence with the larger River Bure. A number of sewage effluent outfalls (Fig. 1) discharge into the Bure-Ant system.
The River Thurne and its Broads had a plankton dominated by colonial blue- green algae and green algae (Chlorophyta). The only exception, which also provided the most abundant crop recorded, was the relatively less saline Womack Water, a small tributary into which effluent from Ludham is discharged. Here diatoms (St~7~hanodiscus sp.) formed a major part of the phytoplankton. High algal populations were recorded in Hickling Broad, Horsey Mere, and their interlinking waterways, but Martham Broad, which lies astride the river and almost at its head, had a much smaller population.
The species which comprised the plankton of the River Thurne and its Broads were much smaller than those of the other parts of the system (Fig. 5). This phenomenon, which is immediately apparent on examination of the plankton, is consistent throughout the growing season (B. Moss, R. A. Watson, P. L. Osborne, unpublished data). The colonial forms of the Thurne system are of comparable size (20 30 Itm) to unicells elsewhere, and even the same species, such as Scenedesmus
CONSERVATION PROBLEMS IN THE NORFOLK BROADS ]03
PHYTOPLANKTON~ ~IP~'~.~ (l)
\
E)
® @
@
2000
< ~oooo
0
0
0
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<looooo O OTHERS
. / - -
< 30000 ~ DIATOMS
< 40000 0 CHLOF~OPHYTA / ~ \I o -\ < 5oo0o @
CYANOPH
o ®
Fig. 4. Distribution of phytoplankton in Broadland waterways in July 1973. Symbols give numbers per ml.
104 BRIAN MOSS
BURE / ANT THURNE Fig. 5. Typical phytoplankters of the Bure/Ant system and the Thurne system, July 1973, to same scale.
quadricauda (Turp) Breb, if it occurs both in the Thurne and elsewhere, is much smaller in the former.
The distributions of some major genera and groups are shown in Figs. 6 and 7. In Fig. 6 the particular association of colonial blue-green algae with the Thurne system is clear. Major genera included Aphanocapsa, Aphanothece, Coelosphaerium and Merismopaedia, and were accompanied by abundant Chlorococcolean algae, of which Ankistrodesmus (Monoraphidium) is representative. The nitrogen-fixing genus Anabaena (Cyanophyta) was not particularly abundant, though widespread. Moderate populations occurred in Wroxham and Malthouse Broads on the River Bure.
In Fig. 7 distributions are shown of the three most abundant diatom genera, Melosira, Cyclotella and Stephanodiscus, plus one characteristic of brackish waters, Chaetoceros, and of Oscillatoria (Cyanophyta). Melosira populations were often very large, particularly below Acle on the Bure, in the upper Bure and its Broads and in parts of the upper River Ant. Cyclotella, although widespread, was the most characteristic diatom of the Yare system, but another Centric genus, Stephanodiscus, was characteristic of Oulton Broad and Womack Water. Chaetoceros was typical of the estuarine reaches of the Yare and Waveney, and also occurred in the upper Thurne area. Its incidence was not great at the time of this survey, but more intensive work on Hickling Broad has shown it to be a frequent genus at other times of the year.
C O N S E R V A T I O N P R O B L E M S IN T H E N O R F O L K BROADS 105
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106 BRIAN MOSS
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CONSERVATION PROBLEMS IN THE NORFOLK BROADS 107
Oscillatoria (Fig. 7) was the major component of the dense algal growths in Barton Broad and the lower River Ant, Malthouse Broad and South Walsham Broad. It was also widespread, though much less abundant, in the southern rivers, than in the Bure and its tributaries.
Relationships between turbidity (as the reciprocal ofsecchi disc depth, Z~- 1 m - 1), phytoplankton numbers, and boat activity are shown as linear regressions in Table 3. The mean turbidities and phytoplankton numbers were determined for the river stretches and their Broads (shown on Fig. 1) along which boat flows had been determined, for the purposes of correlation. There were highly significant correlations between phytoplankton number and turbidity for the entire data set, for the Broads stations alone, and for the rivers above their estuarine reaches. In the estuary (Fig. 1) there was a correlation of lesser significance. Boat activity was only weakly correlated with turbidity when the original data were used. When a correction was applied to the value of Z s-~ to subtract turbidity caused by phytoplankton (obtained from the gradient of the phytoplankton-turbidity regression, and the mean phytoplankton number in the appropriate area), the residual turbidity, Z~-~(r), due to factors other than phytoplankton was not strongly correlated with boat flow. It was significant (Student's t test) only at a level between 10 ~o and 25 ~o.
T A B L E 3 REGRESSIONS BETWEEN THE RECIPROCAL OF SECCHI DISC DEPTH Zx- I m 1, PHYTOPLANKTON NUMBERS, N m I - 1 × 10 - 3 AND BOAT FLOWS, B BOATS PASSING IN EITHER DIRECTION PER DAY BETWEEN 0800 h AND 2 0 0 0 h G M T . Z~(r) ~ INDICATESRECIPROCALSECCHIDISCDEPTHSCORRECTEDFORTHECONTRIBUTIONOF
THE PHYTOPLANKTON
Relationship Degrees o/ Sign
108 BRIAN MOSS
may lag behind that of others so that it becomes limiting to production, an increase in productivity demands increases in all nutrients which could potentially be limiting. The summer biomass of phytoplankton, measured as numbers, chlorophyll a or some chemical constituent, is correlated, among a set of sites, with productivity (Schindler, 1971) and hence with nutrient loading. For nutrients such as nitrogen and phosphorus, which are potentially in short supply compared with carbon, potassium, magnesium, sulphur, chlorine and other necessary elements for algal growth, the bulk of the available supply is contained in summer in particulate form (i.e. in phytoplankton and detritus and bacteria associated with it), and only small amounts are detected in available dissolved form in the water. A measure of the concentration of each of these elements present [Mw] is then given by a measure of the phytoplankton biomass.
The overall concentration of an element (in both particulate and dissolved form) depends also on three other factors--the mean depth of water, the replacement or flushing rate, and the sedimentation coefficient. These have been related by Vollenweider (1975) for well-mixed water bodies such as are the Norfolk Broads and rivers in the expression:
[Mw] = L z - l ( p + or)-l
where L is the areal loading (mass per unit time per unit area) of substance m, - is the mean depth, p the flushing coefficient (in units of time- 1, derived from the total volume of water passing through the body per unit time divided by the mean volume present in the body at any one time), and a, the sedimentation coefficient, or the proportion of substance which is lost per unit time to the sediment by both biotic and abiotic processes.
The major components determining the summer phytoplankton biomass (proportional to [Mw]) in the Broadland system are L and p. Although the river channels may be several metres deep in parts, and the Broads are only about 1 m deep, the range of z is very narrow compared with that under which Vollenweider's model has been verified (Dillon & Rigler, 1974, 1975; Vollenweider, 1975) and z can be taken as constant. Little is known yet of the significance of a but it seems to depend directly on L and inversely on p. Its maximum value, since it is a coefficient, is !, and for Barton Broad, Osborne & Moss (1977) found a = 0.17 y- 1 In a system of rivers and associated lakes where flushing coefficients are generally high (e.g. Barton Broad 40 y- ~), a can be neglected. For interpretation of the differential distribution of phytoplankton numbers in Broadland, the approximate expression: N oc L p - 1 can then be used.
There is no doubt that nutrient loadings are now very high over much of Broadland; the predominance of blue-green algae and of Melosira, the obvious colour of the water, the loss of macrophytes, and the incidence of avian botulism (Lloyd et al., 1976) reflect a syndrome typical of extreme eutrophication elsewhere. Even in a relatively undisturbed state, the water, entering the rivers from a lowland catchment
CONSERVATION PROBLEMS IN THE NORFOLK BROADS 109
of chalk and glacial drift providing rich loam soils (Woodward, 1902), would have been fertile, but the loading has been much increased by the entry of sewage effluent from a human population which has been steadily expanding in the last few decades. Sewage effluent in non-industrial areas is an excellent plant growth medium and outfalls are placed on all the rivers except the Thurne above its confluence with Womack Water. The effluent loading has been studied in detail (Osborne & Moss, 1977) at Barton Broad, where a phosphorus loading higher than any other yet recorded in the literature has been measured. The effects of effluent loading on Broads with low flushing coefficients (because they lie at the ends of long 'blind' dykes) are to be seen particularly at South Walsham Broad and Womack Water.
Where flushing is high a given loading is less effective in creating high phytoplankton concentrations. The River Waveney is a good example of this; it has three major outfalls but a sparse phytoplankton. Onlywhen Waveney water rests for a longer time in Oulton Broad than in the main river can crops build up.
High flushing coefficients, which are found in rivers particularly, do not guarantee low phytoplankton crops if they are associated with side channels or Broads where flushing is reduced and where phytoplankton crops can then accumulate. The simplest case is in the River Ant, where phytoplankton crops build up in the relatively stagnant side-channels above Barton Broad, but which are partly diluted in the main river. Water is retained in summer in Barton Broad for several weeks, however, allowing substantial populations to grow which then provide a plankton for the lower River Ant. This inoculum role, which has been described for the River Nile and its man-made lakes (Hammerton, 1972; Tailing & Rzo~ka, 1975), can also be seen in the Rivers Bure and Yare. The latter has a greater discharge (Table 1) than the Waveney and probably a much higher flushing coefficient, but it had a much more abundant phytoplankton. Three main sources of inoculum provide plankton for the river--Surlingham and Rockland Broads and the small River Chet, which develops a rich phytoplankton just below the sewage effluent outfall at Loddon. The Waveney, despite its smaller discharge, has no Broads along it to provide nuclei for plankton development. Compared with that of the smaller River Bure in its stretches above its confluences with the Ant and Thurne, the Yare plankton has a small biomass despite the very large outfall from Whitlingham sewage works, which is situated between Surlingham Broad and Norwich. This can be attributed to the very high discharge of the river (Table 1). Reduction of flushing rate by enclosure of River Yare water in an experimental Broad almost isolated from the river near Surlingham has led to establishment of very large phytoplankton populations (B. Moss, unpublished data).
The inoculum effect of the River Bure Broads is well shown. Despite loading from a sewage works serving the thriving towns of Wroxham and Hoveton, plankton is sparse in the river until just above Wroxham Broad. This does not invalidate the role of the Broads as producers of plankton inoculum for the rivers, because although not necessarily saline these rivers are tidal and water can be pushed back upstream
1 lO aRIAN MOSS
for a short distance at high tide. Wroxham and Salhouse Broads are not so isolated as, for instance, Malthouse and South Walsham Broads and are little more than extensive widenings of the river, separated from the main flow by longitudinal peat islands. Their flushing coefficients, although they must be lower than that of the main river, must be high compared with the more isolated Broads. This is reflected in their only moderate plankton populations dominated by Melosira spp., a diatom genus associated with relatively turbulent water (Lund, 1966). In contrast, South Walsham Broad, situated at the end of a long dyke, had a plankton dominated by Oscillatoria spp., buoyant with gas vesicles, and well adapted to less turbulent conditions.
The inoculum provided to the rivers by the Broads is soon diluted out. The Melosira plankton provided to the River Bure by Wroxham, Salhouse and Hoveton Little Broads is soon lost and replaced by that from the River Ant, Malthouse and South Walsham Broads in the region above the confluence with the River Thurne. Malthouse Broad does not receive sewage effluent directly but the fertile water, which passes into it along the dyke connecting it with the River Bure, at every change of tide, supports the development of large algal crops.
The characteristic plankton of the Thurne Broads persists in the river until diluted out by the larger Bure, whilst a last pulse ofphytoplankton, again rich in Melosira, developed in a sheltered side channel fertilised by Acle sewage effluent. This plankton is washed upstream a little way by the tide but reaches its peak below Acle before being diluted out as the estuary is approached.
The River Thurne system above Womack Water is not supplied with nutrients from sewage works effluent, but there is evidence that its nutrient loading has been increasing for some time (Phillips et al., 1977). Drainage from fertilised agricultural land provides nutrients, though its phosphate content is usually low, and it is phosphorus that is usually the prime limiting nutrient for phytoplankton growth in water bodies that have not been artificially enriched by sewage or farm yard effluent. Agricultural drainage water is received by the entire Broads system but has been shown at Barton Broad to have been insufficient to have caused the major recent changes recorded there. Barton Broad, however, has a high flushing coefficient (40 y- 1). The Hickling Broad-Horsey Mere complex is not flushed through nearly so rapidly (R. A. Watson, pers. comm.) and a relatively low loading would be as effective there as a much higher loading coupled with a high flushing coefficient elsewhere. The low flushing rate is reflected in the persistence of summer phytopiankton into the winter in Hickling Broad (R. A. Watson unpublished data). Martham, the smallest of the Thurne Broads, had clear water and a low plankton crop. This may reflect a low nutrient loading from a relatively small catchment area compared with that of Horsey Mere and Hickling Broad. Dense phytoplankton crops were found in Hickling Broad and Horsey Mere at the time of this survey and are now characteristic for much of the year (R. Watson, unpublished data). Work is proceeding on the construction of a nutrient budget to partition the sources of nutrients necessary to sustain these crops.
CONSERVATION PROBLEMS IN THE NORFOLK BROADS 111
The markedly small size of the phytoplankton cells and colonies of the Thurne system is of interest. Small size conveys a large ratio of surface area to volume which may be advantageous in areas where dissolved nutrients are in very low concentration. Although the Thurne Broads are now very fertile, the low flushing rates mean that dissolved critical nutrients have very low summer concentrat ions-- all of the input load is rapidly incorporated into the particulate fraction. In the more riverine Broads and rivers, with higher flushing rates, dissolved nutrients are not entirely exhausted except for short periods and the concentration available for uptake is higher. In phytoplankton, where rapid cell division and thus nutrient uptake, is essential if populations are to be maintained in an unstable physical environment, small size might be expected to be selected for in all waters. However, small size also confers greater vulnerability to zooplankton grazers (Hutchinson, 1967) so that where dissolved nutrient concentrations are relatively high, large size may then be advantageous.
Very few data exist on the phytoplankton of the Broadland waterways prior to the 1970s. Griffiths (1927) visited the River Bure (Wroxham and Salhouse Broads) and the River Yare (Rockland and Surlingham Broads) in August 1924 but there appears to be no further account of the phytoplankton of the main waterways until the present work. Some species lists have been given for private Broads by Clarke (1960), and Gurney (1970). In August 1924, Griffiths found that the plankton of Wroxham Broad was similar to that of the present day, being dominated by Melosira and other taxa associated with bicarbonate-rich waters. He noted also that the adjacent River Bure had a similar plankton to that of Wroxham Broad but that in the river upstream at Wroxham itself the plankton was much sparser. Wroxham Broad is among the deeper of the Broads and aquatic macrophytes did not dominate the surface water as they did in Rockland and Surlingham Broads on the River Yare. Griffiths notes that the algae present in the water of Surlingham Broad were largely detached epiphytes, but his species list for Rockland Broad includes truly planktonic species, many of which were recorded in the area in 1973. No absolute estimates of the phytoplankton population, which in any case was selectively sampled with a net, were made by Griffiths, so that it is difficult to assess whether any increases have taken place between then and now. In Wroxham and Salhouse Broads, where the high flushing rates may be more critical than an increase in loading in an existing fertile writer, there may have been little change, but in the Yare Broads, the macrophyte-dominated ecosystems described by Griffiths in 1927 have now been replaced by plankton-dominated ones. There is evidence from sediment cores of recent phytoplankton increases in Barton, Hickling and Malthouse Broads on the main waterway (Osborne & Moss, 1977; Phillips et al., 1977; B. Moss, unpublished data) and in the isolated Alderfen Broad (B. Moss, unpublished data). In Broads which still possess macrophytes, examination of sediment cores has shown evidence of increasing enrichment at Upton Broad (Phillips et al., 1977) and Martham Broad (D. F. Eminson, unpublished data). Anecdotal evidence (Ellis, 1965) suggests low phytoplankton populations in much of the system until the 1950s or later.
112 BRIAN MOSS
Turbidity in the Broadland waters is created potentially by three main agents: phytoplankton; sediment resuspension and bank erosion by boats; and natural mixing processes connected with tidal changes. The latter are year-round phenomena, whereas except in the Thurne Broads, both plankton growth and boat activity are largely features of the spring-to-autumn period. The turbidity of vigorously flushed waterways might be expected to be dominated by sediment kept in suspension by natural mixing processes, and should not be greatly different in summer and winter. This was found (Table 2) for the estuary, and for the River Yare which, below Norwich, is the largest and most strongly tidal of the Broadland rivers.
In the Thurne system the turbidity was also similar in July and December. Boat activity there is much reduced in winter but phytoplankton biomass may then still be high (see above). The major seasonal contrast in turbidity was in the Bure-Ant system, whose phytoplankton crops are negligible in winter (P. L. Osborne, unpublished data) but where boat activity is also highest in summer.
Correlation analysis (Table 3) showed a highly significant relationship between turbidity, Z~- 1 and phytoplankton number. This held also for Broads and rivers, above the estuary, considered separately. The intercept constant (1-9) in the regression equation for the entire data set indicates the mean turbidity not accounted for by phytoplankton and is equivalent to a secchi disc depth of 0.53 m in the absence ofphytoplankton. This is not greatly different from the mean December secchi disc depth of 0.46 m, and suggests a predominant role for phytoplankton in creating turbidity in the system.
Though no statistical analysis was attempted because of the few data, Mason & Bryant (1975) found an apparent inverse correlation between phytoplankton (measured as chlorophyll a uncorrected for degradation products) and secchi disc depth in Broads which are inaccessible to cruising boats or from which boats are excluded, but were unable to detect any pattern in data from navigable Broads. From seasonal data (68 sets of observations) on extinction coefficients at 550 nm and 615nm, chlorophyll a concentration, and concentration of suspended matter, obtained in Hickling and Barton Broads (navigable) and Alderfen, Upton Broads and Woodbastwick Fen (boats excluded), Phillips (1976) obtained correlations significant at the 0.1 ~o level between all combinations of these four parameters. This also suggests a prime role for phytoplankton in creating turbidity.
A Weak correlation (Table 3) was obtained between boat activity and turbidity. This correlation may not be causative, however, because when allowance was made for the phytoplankton turbidity the boat flow correlation with residual turbidity was weakened even further. A correlation at the < 25 ~o > 10 ~ probability level is much more likely to be obtained by chance or by very indirect cause than by a real relationship. If boats were a prime cause of turbidity the correlation with residual turbidity should have been strengthened. The correlation between boat activity and total turbidity may result from the fact that the Thurne and Bure-Ant systems, because of their many tourist features and high density of Broads, villages,
CONSERVATION PROBLEMS IN THE NORFOLK BROADS 113
b o a t y a r d s a n d hoste l r ies , a re the m o s t p o p u l a r b o a t i n g areas , as well as the par t s o f
B r o a d l a n d which , fo r o t h e r r e a sons s u p p o r t the la rges t p h y t o p l a n k t o n c r o p s a n d
turb id i t ies . N o s igni f icant c o r r e l a t i o n c o u l d be d e m o n s t r a t e d be tween b o a t f lows
and p h y t o p l a n k t o n n u m b e r s ; the w e a k c o r r e l a t i o n be tween b o a t f low a n d tu rb id i t y
seems t h e r e f o r e to s t em f r o m a m u t u a l c o r r e l a t i o n a m o n g the a t t r ac t iveness o f smal l
r ivers and B r o a d s to i t i ne ran t tou r i s t s a n d sai lors , the re la t ively low f lushing ra tes o f
these areas , wh ich a l low large c rops o f p h y t o p l a n k t o n to deve lop , a n d the s t r o n g
r e l a t i onsh ip be tween p h y t o p l a n k t o n a n d tu rb id i ty . Boa t s can be seen to stir up
s e d i m e n t a n d d o e r o d e m a t e r i a l f r o m r iver banks . T h e d i s t u r b e d ma te r i a l d o e s n o t
c o n t r i b u t e s igni f icant ly to the sus t a ined tu rb id i t y o f the wate r , however .
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