control of ecosystem state in a shallow, brackish lake: implications for the conservation of...

AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS

Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 221–240 (2008)

Published online 18 September 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/aqc.819

Control of ecosystem state in a shallow, brackish lake:implications for the conservation of stonewort communities

TOM BARKER, KEITH HATTON, MICHAEL O’CONNOR, LES CONNOR,LEE BAGNELL and BRIAN MOSS*

School of Biological Sciences, University of Liverpool, Liverpool, UK

ABSTRACT

1. The alternative stable states hypothesis for the behaviour of shallow lake communities requiresswitches to transform clear-water macrophyte-dominated communities to turbid algal-dominatedones. Such switches have rarely been demonstrated experimentally. This study shows the role ofrising salinity as such a switch while contributing a solution to the conservation problems of animportant nature reserve.2. Hickling Broad changed from a clear-water, charophyte-dominated lake to a turbid,

phytoplankton-dominated lake in the early 1970s, probably owing to guanotrophication by gullsand to increased salinity from more intensive pumping of the agricultural land that separates itsmain inflow from the nearby North Sea. Following a decline in nutrient loading as the gull flockmoved away, the plants began to return during the 1980s and 1990s. In 1998/99, the water clearedand charophytes, including some very rare species, were abundant.3. This was welcome to conservation bodies, but the vigorous growth precluded competitive sailing

and there were conflicts with the local sailing club. The plants, however, began an irregular decline in2000, though nutrient loadings and other conventional chemical drivers have remained steady.4. Our hypothesis was that the unstable nature of the plant community was linked to high salinity,

and that if salinity were lowered there would be vigorous and reliable growth, enabling annualcutting of plants to allow sailing races. In an experiment using mesocosms, salinities straddling thecurrent values in the Broad led to declines in plant biomass, macrophyte species richness andmacrophyte Shannon–Weaver diversity through increased release of phosphorus from the sediments,increased algal turbidity and reduction of zooplankton grazer activity.5. Stabilization of the plant community of Hickling Broad would be achieved by a reduction of present

salinities by about 20%. This would be possible by use of existing Environmentally Sensitive Area (HighLevel Environmental Stewardship) arrangements or diversion of some pumped drainage water to the sea.There remain some uncertainties about the future of the area because of rising sea levels.Copyright # 2007 John Wiley & Sons, Ltd.

Received 5 July 2006; Accepted 29 October 2006

KEY WORDS: alternative states; Chara; Daphnia; macrophytes; mesocosms; salinity; phosphorus; recreation;

restoration

*Correspondence to: Brian Moss, School of Biological Sciences, University of Liverpool, Liverpool L69 3GS, UK.E-mail: [email protected]

Copyright # 2007 John Wiley & Sons, Ltd.

INTRODUCTION

Shallow lake ecosystems can exist in alternative states (Irvine et al., 1989; Scheffer et al., 1993), dominatedeither by submerged plants in clear water or by phytoplankton or suspended sediment in very turbid water.The plant-dominated state is the more biodiverse and is stabilized against a changing environment } forexample, of nutrient loading } by biological mechanisms (Moss et al., 1996). Submerged plants are atsubstantial disadvantage compared with overlying microscopic algae and these mechanisms tip the balancein favour of the plants. The mechanisms can be overcome, however, and many formerly plant-dominatedlakes have switched to phytoplankton dominance. Switches, described from circumstantial evidence,include herbicide or mechanical damage to the plants by cutting or boat propellers, and grazing by exoticcrayfish, fish and birds. They may also include toxic damage to the invertebrate grazers that controlcompeting periphyton or phytoplankton through pesticide leakage, biocides such as boat anti-foulingpaints (Sayer et al., 2006), heavy metals or trace organics present in sewage effluents (Scholten et al., 2005).Increase in water level (Blindow, 1992) or rising salinity (Jeppesen et al., 1994) may also be responsible.

Such switches have generally been inferred but rarely demonstrated by field-scale experiments.Uncertainty over optimal management of Hickling Broad has provided an opportunity for experimentsto examine the role of a switch, while also suggesting appropriate management for the conservation of therare charophyte communities in the lake. Hickling Broad is one of the most heavily designated conservationareas in the UK. It is part of the Upper Thurne Broads and Marshes Site of Special Scientific Interest and aNational Nature Reserve under British legislation, a Special Protection Area and Special Area ofConservation under European legislation, and a Ramsar Site by international agreement.

STUDY SITE

Hickling Broad (18 350 E, 528 440 N), the largest of a group of flooded mediaeval peat and clay pits in easternEngland, was dominated by charophytes at the turn of the nineteenth century and had notably clear waterwith little phytoplankton (Moss, 2001). It retained clear water until the 1960s but the plant and filamentousalgal communities increased in abundance during the first half of the twentieth century and changed in thedirection of vascular plant dominance (Myriophyllum spicatum L., Potamogeton pectinatus L., Najas marinaL.,Hippuris vulgaris L.), with ‘blanket weed’ (Cladophora sauteriKuetz.), though a diversity of charophytespersisted until at least 1961 (Phillips, 1963). Nutrient loadings increased throughout the century owing tothe build-up of a roost of black-headed gulls (Larus ridibundus L.) (Moss and Leah, 1982).

Following the Enclosure Acts around AD 1800, the wetlands surrounding the Broad had been partlydrained to create pastures (‘levels’) for cattle grazing. Hickling Broad and the channels connecting it withother nearby Broads (Horsey Mere, Martham Broad, Figure 1) were (and are) held behind embankments asthe drained land shrank and the peaty soils of the former floodplain wasted on oxidation. In the 1960s,installation of more powerful electrical pumps around the Broad prominently raised the salinity. Thepumps drain particularly the level between the Broad and the North Sea, which has a saline water table(Pallis, 1911). The water pumped into the Brograve Dyke and Horsey Mere (Figure 1) is backed intoHickling Broad (Holdway et al., 1978) on very modest (5–10 cm) physical flood tides created ultimately atthe estuary of the Broadland rivers some 30 km downstream. The term ‘physical tide’ is used to describe amovement upstream of inland water impelled by the tide at the coast but not a local movement of seawater.Salinities in Hickling Broad in 1911 (Gurney, 1911) were around 600mgL�1 chloride (3% of sea water), buthave reached 1800 to 2000mgL�1 chloride (9–10% sea water) since then. Figure 1 gives a synoptic picture.

In the late 1960s therefore, the Broad was faced with increased salinity and increased guanotrophy fromgulls. The plant communities collapsed in the early 1970s and phytoplankton populations became high(Moss and Leah, 1982; Bales et al., 1993). These events were consistent with operation of the alternative

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Copyright # 2007 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 18: 221–240 (2008)

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states hypothesis with salinity proposed as a switching mechanism. A major stabilizing mechanism forfreshwater plant communities involves grazing by large-bodied Cladocera such as Daphnia, which aretolerant only of modest salinities. Daphnia hyalina Leydig and other large cladocerans were formerlyabundant in the Broad (Gurney, 1904; Ellis, 1965; Bales et al., 1993). Cladocera became very scarce from atleast the early 1970s and remain scarce (Bales et al., 1993; Irvine et al., 1993; Hoare, 2002).

An estuarine mysid shrimp, Neomysis integer (Leach) is present in the Broad and may have beenfavoured by the increased salinity. It is an omnivore that grazes periphyton but also feeds on zooplanktersand may have been partly responsible for the Daphnia decline. A brackish-water alga, Prymnesium parvum(Carter) became abundant in the early 1970s. It killed many fish with toxins that are produced when its cellslyse (Holdway et al., 1978). Fish kills would be expected to favour development of large-bodied Cladocera,owing to reduced predation (Hrbacek et al., 1961; Brooks and Dodson, 1965). However, in the 1970s thezooplankton community became and remains dominated by the salinity-tolerant copepod Eurytemoraaffinis (Poppe). The Broad was turbid with only remnants of submerged plants throughout the 1970s.

Figure 1. Distribution of chloride concentrations (mgL�1) in surface waters of the Thurne catchment on 21 April 1984.

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In the 1980s, although the water remained turbid and phytoplankton-dominated, plant colonization wasextending in the Broad (Bales et al., 1993). Gull populations had decreased greatly from peaks of up to 250000in the 1970s, perhaps because a nearby waste-tip, on which they had fed, was completed and covered with soil.Neomysis was abundant and, in controlling periphyton, may have mitigated the shading of the aquatic plantsurfaces by phytoplankton in the overlying water column. Phosphorus concentrations in the water columndeclined. These processes continued in the 1990s with growth of an increasing number of charophyte species.

In 1998 and 1999 there was a dramatic return of charophytes and very clear water. Phosphateconcentrations were much reduced, probably because of uptake by the Chara (Kufel and Kufel, 2002),which included prolific growths of the very rare Chara intermedia A. Braun. Salinities had not changed.More grazing birds, particularly coot (Fulica atra L.) were attracted back into the Broad, which wasapproaching the status intended for it as a candidate Special Area of Conservation under the EC HabitatsDirective (Eaton, 1999).

National and local conservation organizations were delighted but the Hickling Broad Sailing Club wasnot. Its races were impeded, and a difficult conflict developed over the desirability or otherwise of cuttingthe plants. There were angry meetings and direct action was threatened. Matters were alleviated whenturbid water returned in 2000, the charophytes grew less well and there was little demand for cutting. Thereasons for this reversion are not understood and since 1999, with the exception of some recovery in 2003,the plant coverage has irregularly declined. By 2005 only 9.5% of the bottom was covered by dense Charaintermedia lawns, compared with 42% in 1999 (Harris, 2006). Submerged plants essentially disappearedfrom the Broad in 2006 (J. Harris, pers.comm). There is nothing in the physico-chemical data, including thesalinity, that offers any explanation for this decline (Hoare, 2002) though potential influences such as thestate of the fish stock and the zooplankton community have not been monitored.

One possibility is that the system now exists at some breakpoint in the interaction between salinity as a switchmechanism, and nutrient loading. Nutrient loading, although not directly a mechanism for switching betweenstates, is important in influencing the thresholds at which independent switches may act. The conservationbodies are uncertain how to restore and then stabilize the charophytes and thence to judge how much cuttingwill satisfy the sailing club without jeopardizing the plant communities. Reduction in salinity by less pumping ofagricultural water and raising the water table in the levels would be the obvious management experiment.Farmers may join a voluntary government-backed scheme, the Broads Environmentally Sensitive Area (ESA)scheme (now called High Level Environmental Stewardship), and be compensated for maintaining traditionalfarming methods with grazing and high water tables, but those in the crucial part of the Hickling catchment arearable farmers and have so far not chosen to do so. Local and national government authorities feel they have nomandate for pressuring local farmers without direct evidence of a salinity effect. This paper reports a directexperimental determination in replicated mesocosms of the system (Figure 2).

The experiment had four salinity treatments. The first was planned at around 600mgCl L�1, as in theearly years of the 20th century; the second was planned to be around 1000mgL�1, which is about halfthe current value and which was judged to be practicably attainable under current ESA arrangements. Thethird and fourth, at around 1600mgL�1 and around 2500mgL�1, spanned current values. Our hypotheseswere that we would be able to stabilize charophyte-dominated communities at similar plant diversity to thatexperienced in the Broad in 1998/99 by reducing salinity, and that stability, and diversity would be impairedat current and higher salinities.

METHODS

The experiment was set up in 48 fibreglass tanks, each 3m diameter and containing about 3m3 of waterwhen full, located in the botanic gardens of Liverpool University at Ness (538 150 N, 38 030 W), about320 km from Hickling Broad. Sediment was removed, with a mechanical digger mounted on a barge

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Figure 2. (a) Hickling Broad from White Slea Lodge. (b) Removing sediment with a digger fixed onto a barge. (c) Transferringsediment into a lorry for movement to the experimental site at Ness. (d) Receipt of the sediment at Ness and preparation for transferto the mesocosms. (e) Final mixing of sediment. (f) The mesocosm system. (g) Low-salinity mesocosm with clear water and dense bedsof Chara. (h) Mesocosm at high salinity with turbid water. This figure is available in colour online at www.interscience.wiley.com/

journal/aqc.



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(Figure 2), to a depth of about 30 cm in early December 2003 from an area of 240m2 at NGR TG 41462155in Hickling Broad. This area had existing, but not dense, Chara communities the previous summer. Thelocation was prescribed by the conservation bodies responsible for the Broad so as not to damage confluentlawns of Chara. The sediment was loaded into a separate barge that had been carefully precleaned. Digginginevitably included supernatant water so about 80m3 of sediment/water mixture were removed to realizesome 45m3 of consolidated sediment after settling and removal of the supernatant water.

The sediment/water mixture was lifted out of the barge with a second digger and loaded into precleanedlorries for immediate transport to Ness. There it was stored in a temporary lagoon lined with a rubbermembrane. When all loads had been received the sediment was mixed and transferred in a small tractor-driven tanker to a similar lagoon close to the tanks and again mechanically mixed. A pump was then usedto transfer 29 cm depth of the sediment/water mixture to the tanks, where it was allowed to settle overabout a month before the supernatant water was pumped off, and manual transfers were used to establishequal amounts (27 cm depth) in each tank. The sediment then had re-acquired the Hellmann’s (Original)mayonnaise-like consistency that it has in the Broad.

In late January 2004, Hickling Broad water was pumped from offshore (NGR TG 41002250) intoprecleaned tankers and transferred to Ness the same day. The water was stored in a rubber-lined lagoonthen pumped into the tanks, in amounts dependent on the intended salinity for each tank. Polyethylenemembranes temporarily covered the sediment to minimize disturbance. Overwintering plant material wascollected from Hickling Broad and other water bodies in the R. Thurne system by SCUBA divers, underlicence from English Nature (now Natural England), between April 13 and April 16. The plants weresubmerged in netting bags in Hickling Broad until they could be transferred to Ness Gardens, divided intopackages of about 25–50 g wet weight and planted, in groups of three packages, into the tanks inrandomized patterns on April 17 2004. Species introduced were: Chara intermedia, Chara hispida L, Charaglobularis Thuill. Myriophyllum spicatum, Potamogeton pectinatus, Hippuris vulgaris, Callitriche sp., Elodeacanadensis Michx., Ranunculus circinatus Sibth., Lemna trisulca L. and Ceratophyllum demersum L. Theplants were left at the ambient salinity of the Broad until early June, when they had established and startedto put on vigorous growth.

Benthic animals, zooplankters and phytoplankters inevitably came in with the material transferredfrom the Broad. Additional mixed inocula of zooplankton from Hickling Broad and a local lake (toensure presence of Daphnia species) were also added. Analyses were carried out in spring to check foruniformity of water chemistry and then salinities were adjusted between June 17 and June 22, usingcrystallized sea salt (Instant Ocean) and deionized water. Two male sticklebacks (Gasterosteus aculeatus L.)were introduced to each tank and the first additions of nutrients were made. The fish were obtained from alarge pond in Ness Gardens and had been progressively acclimated to the salinity of the tanks over theprevious 2 weeks.

Phosphate was added uniformly to all tanks, but different doses of nitrate were given because a separateexperiment on the effects of different nitrate loadings on plant diversity was incorporated. The nitrate andsalinity treatments were combined in a randomized block design and the results of the nitrate experimentwill be reported elsewhere. Nutrient additions were given as constant loadings of potassium phosphate(KH2PO4) intended to increase the concentration by 50 mg PL�1, and loadings of sodium nitrate calculatedto give additions of 1, 2, 5 and 10mgNL�1 within each of three blocks. Loading was thus controlled ratherthan concentration in the tanks. Twenty-four additions (to mimic the continuous ingress of nutrientsexperienced by the lake through its inflow) were made over 2 years, giving annual loadings well in excess ofthose currently experienced by the Broad. This was to ensure that nutrient availability did not confound theresults and that if clear water was attained it could not be attributed simply to nutrient shortage. TheBroad’s maximum inorganic nitrogen concentration is about 2mgNL�1 and total phosphorusconcentrations are now around 50 mgL�1 (Hoare, 2002), with a flushing rate of about twice per year(Moss and Leah, 1982). Additions of P were thus around six times the current loading and of N between 3

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and 30 times. Ten strips of doubled plastic netting (2 cm wide), of mesh size 1 cm, extending the full depth ofthe water column, were suspended each year in each tank from a rod placed diametrically across the rim forcolonization of periphyton and macroinvertebrates.

Water was collected, using a tube that spanned the water column, every 2 weeks just prior to nutrientadditions, and the effects of the treatments on substrate-associated benthos, algal periphyton,phytoplankton, zooplankton, mysids and plants were monitored at frequent intervals. Standard methodsfor total phosphorus (TP), total nitrogen (TN), alkalinity, pH and chlorophyll a are documented in Stephenet al. (1998). Nitrate was determined on a Dionex DX120 ion chromatograph to a precision of � 3% witha lowest detectable concentration of 0.01mgNL�1. Conductivity was measured with a Jenway 4010 meter.Mohr titrations, using silver nitrate, of field samples were used to relate conductivity to chloride.

Plants were assessed non-destructively as percentage volume infested (PVI), from estimates of cover anddepth of the water occupied, by two independent observers who then agreed the final value. Plants weredestructively harvested only at the end of the experiment in early September 2005, when the tanks werepumped out and all plant material was recovered, sorted and dried. Zooplankton was regularly estimatedfrom 10L of multiple bulked samples of the entire water column taken with a tube, passed through a 64-mmmesh net and preserved in ethanol. Crustacea were counted using a counting tray with a stereomicroscopeand numbers converted to biomass using body lengths and literature-derived length–weight relationships.Attempts were made every 2 weeks to sample mysids using nets placed on the bottom for about 1 hour thensteadily pulled up through the water column, using three nets per tank. However, mysids were not detectedin any treatment in either year. One netting strip per tank was removed and vigorously shaken with acetoneon eight evenly spaced sampling occasions in each growing season. An aliquot was filtered for chlorophyllanalysis, as above, for periphyton algal estimates. The dislodged macroinvertebrates were preserved inalcohol for subsequent identification and counting.

Sampling continued over the winter of 2004/2005 and then the fish community was allowed to risethrough reproduction to carrying capacity by introducing two male and two female sticklebacks to eachtank on 11 March 2005. The eventual fish populations were recorded during the final harvest in earlySeptember 2005. Numbers were converted to biomass based on previous empirically measured data.Evaporation and rainfall inevitably caused small fluctuations in the salinity of the tanks. Salinity wasmonitored as conductivity and converted to chloride concentrations using a regression relationship(chloride ðmg L�1Þ ¼ 0:359 (conductivity (mS cm�1))–275) (r2 ¼ 0:964; p50:0001; n ¼ 48). Salinities wereadjusted back to intended levels on three occasions in 2004 and 2005 using dried sea salt and deionizedwater.

Ancillary experiments included culture of Daphnia magna and D. hyalina from the tanks for estimation oftheir survival and reproduction at different salinities. Animals were cloned from single individuals and fedwith a suspension of green algae. Ten individuals, placed in glass containers at laboratory temperature(178C) with 100mL of a series of sea water dilutions, replicated five times, were examined daily. Survivors(those that moved on disturbance) were counted and corpses removed daily. Intrinsic rates of increase werecalculated from a logarithmic growth equation.

STATISTICS

Results were tested for normality and, following transformation ðlog ðnþ 1ÞÞ where necessary, wereanalysed by repeated measures analysis of variance, followed by Tukey tests to investigate differencesamong treatments. Bonferroni corrections were made. Zero values were replaced by a small numberappropriate to the unit being tested. Percentage values were arcsine transformed. Data obtained from thefinal harvest and to investigate relationships among the seasonally obtained data were examined usingregression.



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RESULTS

Table 1 summarizes significant differences (repeated measures ANOVA) in water chemistry. The salinitytreatments (expressed as chloride concentrations) were maintained as intended and are referred to as ‘low’,‘moderate’, ‘sub-present’ (just below current values) and ‘high’. Increasing salinity was associated withsteady declines in pH and nitrate and increase in total phosphorus (TP). For TP, the two lower salinitytreatments were not different, but there was an increase in the sub-present treatment and a more markedincrease at the highest salinity. There were significant effects on alkalinity but with no regular pattern.Soluble reactive phosphorus and total N decreased significantly with salinity, but not regularly.

Values for phytoplankton chlorophyll a are shown in Figure 3 in relation to a plot of total phosphorusconcentrations. Salinity was closely correlated with total phosphorus (r2 ¼ 0:32; p50:001; n ¼ 92; for the

Table 1. Summary of effects of salinity treatments on water chemistry in a mesocosm experiment simulating conditions in HicklingBroad, Norfolk. Values are means� SD across dates for 12 replicates of each salinity treatment on each date. Summer 2004, n ¼ 96;winter, n ¼ 48; summer 2005, n ¼ 121; overall, n ¼ 265: Asterisks indicate significant differences (two-way repeated measuresANOVA), *p50:05,**p50:01;***p50:001: Letters indicate significant differences among treatments at p50:05: Values with shared

letters across rows are not significantly different

Determinand Period Low salinity Moderate salinity Sub-present salinity High salinity

Chloride Summer 04*** 552� 76 a 1158� 134 b 1661� 199 c 2518� 324 d(mgL�1) Winter*** 391� 116 a 874� 97 b 1302� 68 c 1987� 148 d

Summer 05*** 630� 96 a 1250� 129 b 1800� 176 c 2838� 217 dOverall*** 565� 123 a 1161� 183 b 1678� 299 c 2601� 402 d

pH Summer 04* 9.7� 0.4 a 9.6� 0.5 a 9.7� 0.4 a 9.5� 0.3 b(log units) Winter*** 8.8� 0.5 a 8.7� 0.5 b 8.3� 0.6 c 7.8� 0.6 d

Summer 05*** 10.1� 0.7 a 9.8� 0.7 b 9.6� 0.8 c 9.4� 0.9 dOverall*** 9.7� 0.8 a 9.5� 0.8 b 9.4� 0.9 c 9.1� 1.0 d

Alkalinity Summer 04** 1.3� 0.27a 1.2� 0.19 b 1.2� 0.24 b 1.4� 0.28 a(mequiv L�1) Winter*** 1.6� 0.19 a 1.4� 0.1 b 1.7� 0.3 a 2.4� 0.5 c

Summer 05*** 2.1� 0.26 a 1.7� 0.16 b 1.8� 0.17 b 2.3� 0.39 cOverall*** 1.7� 0.43 a 1.5� 0.31 b 1.6� 0.38 b 2.0� 0.61 c

Nitrate-N Summer 04*** 3.3� 1.1 a 3.2� 1.0 a 3.1� 1.2 a 2.5� 1.3 b(mgL�1) Winter*** 3.0� 1.0 a 3.2� 1.1a 2.4� 1.4 b 1.6� 1.3 c

Summer 05*** 5.0� 1.0 a 4.9� 0.9 a 3.2� 1.7 b 2.4� 1.8 cOverall*** 4.1� 1.4 a 4.1� 1.3 a 3.1� 1.5 b 2.3� 1.5 c

Total N Summer 04** 4.5� 1.1 a 4.6� 1.1 a 4.8� 1.2 b 4.3� 1.4 c(mgL�1) WinterNS 3.9� 1.1a 4.2� 1.2 b 3.8� 1.5 a 3.7� 1.5 c

Summer 05*** 7.0� 1.1a 7.2� 1.2 b 5.6� 1.5 c 6.3� 1.8 dOverall*** 5.7� 1.7 a 6.0� 1.7 b 5.1� 1.5 c 5.2� 1.9 c

SRP Summer 04*** 2.7� 1.6 a 1.1� 0.8 b 1.2� 1.0 b 0.9� 0.8 c(mgL�1) Winter* 2.7� 1.2 a 2.6� 2.1 a 1.7� 0.8 b 4.3� 1.7 c

Summer 05 NS 8.0� 7.0 a 6.8� 5.0 b 3.9� 1.5 c 5.4� 2.4 dOverall*** 5.1� 5.4 a 4.0� 4.3 b 2.6� 1.7 c 3.7� 2.7 b

Total P Summer 04** 36� 6.2 a 33� 5.0 a 42� 4.4 b 38� 8.0 b(mgL�1) Winter*** 19� 2.4 a 18� 3.0 a 35� 4.0 b 87� 24 c

Summer 05*** 59� 40 a 63� 35 a 70� 21 b 154� 53 cOverall*** 43� 31 a 44� 30 a 53� 21 b 104� 65 c

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whole period; r2 ¼ 0:51; p50:0001; n ¼ 60; for the period following 16 November 2004, which marked thesenescence of the 2004 crop). Salinity and phytoplankton chlorophyll a were significantly related (r2 ¼ 0:27;p50:0001; n ¼ 92; r2 ¼ 0:45; p50:0001; n ¼ 60; respectively) and chlorophyll a was very stronglycorrelated with total P (r2 ¼ 0:84; p50:0001; n ¼ 92; r2 ¼ 0:81; p50:0001; n ¼ 60; respectively).Chlorophyll a was not significantly correlated with other chemical variables. There was an inverserelationship (r2 ¼ 0:15; p50:0001; n ¼ 92) between nitrate–N and chloride, and a weak inverse relationship(r2 ¼ 0:034; p50:1; n ¼ 92) between chloride and pH, but pH was correlated with neither chlorophyll norTP. Periphyton chlorophyll a was not significantly affected by salinity (repeated measures ANOVA,p ¼ 0:165).

Macrophytes were estimated regularly as PVI. The final harvest gave an opportunity to calibrate thisagainst the biomass (g d.w. per tank). This was done for the nearest date (16 August 2005) on which PVI

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Figure 3. Changes in total phosphorus and phytoplankton chlorophyll a in relation to salinity treatment in a mesocosm experiment onthe ecosystem of Hickling Broad. Light dotted line, low salinity; light dashed line, moderate salinity; heavy dashed line, sub-present

salinity; heavy line, high salinity.



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had been estimated. For data obtained for the most common species (C. hispida, C. intermedia,M. spicatum, P. pectinatus and filamentous algae, the relationship was PVI ð%Þ ¼ 0:04 biomassþ 4:6;(r2 ¼ 0:47; n ¼ 240; p50:0001) and for total PVI it was PVI ð%Þ ¼ 0:05 biomassþ 10:5 (r2 ¼ 0:49; n ¼ 48;p50:0001).

There was a strong effect of salinity on macrophyte total PVI (Figure 4). Macrophytes establishedsteadily during the first season but major differences appeared in the tanks over winter and were carriedforward into more prominent differences in the growth season of 2005. Repeated measures ANOVA(Table 2) showed highly significant differences in total PVI in both years and also over the whole period. In2004, filamentous algae, C. hispida, M. spicatum, Potamogeton berchtoldii Fieber and L. trisulca decreasedwith salinity. In 2005 more prominent effects were shown, with filamentous algae, C. globularis,C. intermedia, total charophytes, C. demersum, E. canadensis, H. vulgaris, M. spicatum, Ranunculuscircinatus and L. trisulca all declining, N. marina established from seed in the sediment added in only onetank, at the highest salinity, and was the only species apparently to be favoured by salinity. Some speciesbecame much less abundant in the second year when the current dominants in Hickling Broad(C. intermedia, M. spicatum, P. pectinatus) became more abundant. Chara aspera appeared unplanned asdid P. berchtoldii.

The final harvest confirmed the effects of salinity on plant abundance. There were significant inverserelationships between salinity and macrophyte species richness, filamentous algal dry weight and totalmacrophyte dry weight (Table 3). There was a significant inverse relationship between filamentous algal dryweight and that of rhizoidous/rooted (charophytes plus vascular) plants. Macrophyte species richness in thesecond year, when the communities had fully established, declined with salinity (10.5� S.D. 1.1,7.9� 1.07, 7.9� 1.1, 6.7� 0.48 ðn ¼ 48Þ species per mesocosm, respectively, at low, moderate, sub-presentand high levels), but remained steady with time, while Shannon–Weaver diversity also decreasedsignificantly with salinity (respectively 2.17� 0.017, 2.07� 0.14, 2.1� 0.31, 1.89� 0.21 binary units perindividual ðn ¼ 48ÞÞ and also decreased significantly with time ðp50:001Þ:

Zooplankton communities included Daphnia magna, Daphnia hyalina/longispina, Daphnia cucullata Sars,Daphnia curvirostris Eylmann, Ceriodaphnia quadrangula (O.F. Mull.), Bosmina longirostris O.F. Mull,

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Figure 4. Total plant abundance in relation to salinity in a mesocosm experiment on the Hickling Broad ecosystem. Dotted line, lowsalinity; light continuous line, moderate salinity; heavy dashed line, sub-present salinity; heavy line, high salinity.

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Chydorus sphaericus (O.F. Mull.), Alona rectangula Sars, E. affinis, Eurytemora velox (Lillj.) and cyclopoidcopepods in 2004. In addition Daphnia pulex (De Geer), Simocephalus sp. and Alonella nana (Baird) werenoted in 2005. Biomass of the major groups and total crustacean zooplankton are shown in Table 4. In2004, D. magna dominated the daphnid biomass and neither its biomass nor total daphnid biomass wasinfluenced by salinity. Biomass of other Daphnia species (largely D. hyalina/longispina) and of otherCladocera (largely C. sphaericus) were significantly reduced above moderate salinity. Calanoid copepods(largely E. affinis) increased with salinity, though total copepod biomass was not influenced. Totalcrustacean zooplankton biomass was not influenced by salinity. In the presence of normal fish biomass (upto 18 g fresh weight m�2), D. magna, other daphnids and total daphnids were markedly reduced by salinityabove low levels, and other cladocerans and total cladocerans steadily declined with increasing salinity.Copepods tended to increase in biomass with salinity.

Compared between years as surrogates for predation intensity, biomass at particular salinities mostlydeclined and never increased at normal fish densities compared with low fish density. Figure 5 shows thestrong influence of the combined effects of fish and salinity in determining the composition of the

Table 2. Effects of salinity treatments on macrophyte percentage volume infested (PVI) in two years of an experiment carried out inmesocosms with Hickling Broad sediment and water. Actual salinities are shown in Table 1. Differences are not significant (repeatedmeasures ANOVA) except where indicated by asterisks: *p50:05;**p50:01;***p50:001: Letters then indicate (Tukey HSD tests)

differences among treatments. tr, trace

Year 2004 2005

Salinity Low Moderate Sub-present High Low Moderate Sub-present High

Filamentous algae 8.8 a* 6.8 a 5.1 b 4.8 b 24.3 a* 18.1a 6.7 a 1.01 bEnteromorpha sp. 0.6 0.3 0.5 1.2 0.34 0.2 0.6 0.03Chara aspera Detharch ex Willd. 0.06 0.07 0.07 0.05 0 tr tr 0Chara globularis 0.7 0.4 0.5 0.7 0.07 a* 0.008 b 0.001 b 0 cChara hispida 3.4 a* 4.8 b 4.0 b 3.6 a 5.1 4.6 5.6 3.4Chara intermedia 7.3 7.6 9.9 7.9 14 a** 12 a 12.1 a 6.8 bTotal charophytes 11.5 12.7 14.5 12.3 19.2 a* 16.6 a 17.7 a 10.2 bCeratophyllum demersum 0.4 0.4 0.2 0.3 0.15 a 0.01 b tr c 0 cElodea canadensis 0.7 0.02 0.01 0.015 0.8 a** 0 b tr b tr bHippuris vulgaris 0.2 0.15 0.2 0.2 0.06 a* 0 b 0.003 b tr bMyriophyllum spicatum 2.5 a** 2.3 a 0.9 b 1.2 b 7.1 a* 14.8 b 6.3 a 2.9 aNaias marina 0 0 0 0.02 0 0 0 0Potamogeton berchtoldii 0.12a* 0.06 b 0.04 b 0.02 c 0 0 0 0Potamogeton pectinatus 3.35 2 1.2 2.5 14.5 4.6 4 9.5Ranunculus circinatus 0.2 0.02 0.1 0.1 0.02 a tr b 0 b 0 bLemna trisulca 0.6 a* 0.06 a 0.04 b 0.02 c 0.1 a 0.001 b 0.001 b tr bTotal 28.9 a*** 25.0 a 22.8 b 22.6 c 66.5 a*** 54.3 b 35.3 c 23.6 d

Table 3. Relationships between salinity and final macrophyte harvest in mesocosms simulating conditions in Hickling Broad.Relationships between salinity and dry weights of all individual species were not significant at p50:05: Total d.w. includes filamentous

algae, charophytes and vascular plants. Rooted/rhizoidous macrophytes include charophytes and vascular plants

x y Slope Constant r2 n p

Chloride No. of species �0.0011 6.5 0.237 48 50.01Chloride Filamentous algal d.w. �0.234 767 0.212 48 50.01Chloride Rooted/rhizoidous d.w. nsChloride Total macrophyte d.w. �0.302 1773 0.206 48 50.01Rooted/rhizoidous d.w. Filamentous algal d.w. �0.963 1280 0.293 48 50.01



DOI: 10.1002/aqc

Table4.Effectsofsalinityandfish

onbiomass

ofcrustaceanzooplanktonin

amesocosm

experim

entsimulatingconditionsin

HicklingBroad.Values

(inmgdry

weight

L�1)are

overallmeans�

SD

fortreatm

ents.Statisticalsignificance

basedonrepeatedmeasuresANOVA

(n¼

48)in

2004(lowfish)and96in

2005(norm

alfish).Letters

showssignificance

ofvalues.Letters

intheseries

a,b,c,d

indicate

differencesamongsalinitytreatm

ents

within

fish

treatm

ents.Values

notsharinglettersdiffer

witha

probabilityof50.05.Letters

e,fsimilarlydenote

differencesbetweenfish

treatm

ents

forpaired

salinitytreatm

ents

Fish

Salinity

Daphnia

magna

Other

daphnids

Total

daphnids

Other

Cladocera

Total

Cladocera

Calanoid

copepods

Cyclopoid

copepods

Nauplii

Total

copepods

Total

crustacean

zooplankton

Low

Low

145�

172ae

10.4�

6.82ae

e156�

175ae

15.4�

14.1

ae

171�

162ae

69�

133ae

56.4�

32.1

ae

163�

107ae

288�

167ae

459�

302ae

Mod

123�

163ae

12.0�

26.4

ae

136�

165ae

21.5�

22.1

ae

158�

150ae

63.5�

131ae

63.5�

42ae

186�

95.6

ae

314�

173ae

472�

302ae

Sub-pr.

129�

209ae

0.16�

0.29be

129�

209ae

8.41�

11.7

be

138�

204ae

142�

275be

47.6�

27.7

ae

162�

123ae

351�

268ae

489�

464ae

High

169�

56.9

ae

0.23�

0.09be

169�

56.5

ae

10.0�

2.30be

171�

47.2

ae

208�

94.3

be

37.7�

14.0

be

142�

27.8

ae

310�

58.9

ae

476�

17.6

ae

Norm

al

Low

36.4�

63.5

af

5.58�

10.6

af

42.0�

65.7

af

102�

59.4

af

144�

99.4

ae

5.00�

3.55af

18.7�

8.55af

36.1�

12.0

af

59.8�

20.3

af

204�

111af

Mod

0.26�

0.52bf

0.03�

0.05bf

0.29�

0.52bf

60.7�

82.3

bf

61.0�

82.3

bf

2.15�

2.35af

27.5�

26.9

af

26.2�

19.6

af

55.8�

41.8

af

117�

121af

Sub-pr.

0.04�

0.11bf

0.02�

0.04bf

0.03�

0.04bf

17.0�

16.43cf

17.0�

16.5

cf4.83�

4.38af

10.1�

12.0

af

54.3�

37.0

af

69.2�

48.8

af

86.2�

49.0

af

High

5.63�

15.9

cf0bf

5.63�

15.9

cf5.93�

6.30df

11.6�

15.9

df

33.4�

42.7

bf

115�

228bf

127�

116be

276�

231be

287�

240bf

T. BARKER ET AL.232


DOI: 10.1002/aqc

zooplankton community. Cladocera declined greatly and copepods increased in proportion as salinitiesincreased, whilst biomass overall declined in the presence of fish (Table 4). In laboratory experiments(Figure 6), D. hyalina ceased to grow between salinities of 520 and 1025mgClL�1 while D. magna increasedits population at 1520mgClL�1 but declined at 2025mgCl L�1. Fish biomass data were available from2004, when only males were added, and from the final harvest in 2005. A regression of these data againstmean zooplankton biomass (Figure 7) from the four salinity treatments in each year gave a highlysignificant inverse relationship (r2 ¼ 0:88; p50:001; n ¼ 8).

There were strong effects of fish but relatively few effects of salinity on macroinvertebrates (Table 5). Thehighest salinity decreased the number of Gastropoda and Ephemeroptera when only low populations of fishwere present, but not at normal fish biomass. Numbers of Asellus aquaticus L. steadily declined with salinityin both years. Chironomid, dipteran, oligochaete and zygopteran numbers, and taxon richness were alluninfluenced by salinity. Numbers of Gammarus duebenii Lilljeborg increased with salinity in both years.Numbers in the presence of normal fish populations were almost invariably lower than when fishpopulations were low.

DISCUSSION

All experiments have their limitations. This one was conceived particularly to help solve a significantproblem in a lake where considerable social conflict had occurred among different groups of peopleconcerned with it. Consequently, sediment and water from the lake itself were used instead of cheaper,locally available surrogates. Nevertheless some aspects of the lake } the presence of a complete fishcommunity with piscivores, for example } could not be reproduced without loss of experimental facilityand replication. The first year of this experiment was not strictly intended to be an experimental year, as

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

L M SP H L+F M+F SP+F H+F

Daphnids Non-daphnid Cladocera Copepods

Perc

enta

ge o

f to

tal zoopla

nkto

n

b

iom

ass

Salinity and fish treatments

Figure 5. Distribution of zooplankton biomass in the two years of a mesocosm experiment on the Hickling Broad ecosystem. L, M, SPand H stand for low, moderate, sub-present and high salinities, respectively, and +F for the presence of a normal fish biomass.



DOI: 10.1002/aqc

inevitably time was needed for the plant communities to establish. The experiment proper began in thesecond year after the systems had fully established. Nonetheless the plants established very rapidly in thefirst year and although the maximum PVI values recorded were greater in the second year, conclusions canbe drawn from the first summer. Strictly speaking, the effects of low versus normal populations of fish in thetwo years are statistically not comparable. However, the general effects of fish in shallow lake systems arenow so well documented that commonsense interpretations can be made. With these strictures, the effects ofsalinity on the system are summarized in Figure 8.

All mesocosms received the same phosphorus loading, which was greater than that currently estimatedfor Hickling Broad. The highest salinity level was associated with marked increase in the total phosphorus

-2

-1.5

-1

-0.5

0

0.5

20 520 1020 1520 2020 2520

Chloride (mg L-1)

Daphnia magna Daphnia hyalina

Net

popula

tion c

hange (

per

day)

Figure 6. Intrinsic rate of increase (per day) for clones of Daphnia hyalina and Daphnia magna exposed to a range of salinities,expressed as chloride concentration (mgL�1).

y = -7.3325x + 479.43

R2 = 0.8871

0

100

200

300

400

500

600

0 10 20 30 40 50 60

Fish biomass (g per tank)

Tota

l zoopla

nkto

n b

iom

ass (

mg L

–1)

Figure 7. Relationship between fish biomass (fresh weight) and total zooplankton biomass per tank in a mesocosm experiment on theHickling Broad ecosystem.

T. BARKER ET AL.234


DOI: 10.1002/aqc

Table 5. Effects of salinity on populations of macroinvertebrates associated with plastic netting substrata in a mesocosm experiment.Values are means per standard strip with SD, n ¼ 60 for low fish, n ¼ 72 for normal fish. Shared letters in the series a–d across rowsindicate no significant difference at p50:05 in salinity treatment (repeated measures ANOVA). Shared letters between paired fish

treatments similarly indicate no effect at p50:05

Salinity Low Moderate Sub-present High

Chironomidae Low fish 44.5� 28.7 ae 64.7� 37.7 ae 40.3� 16.8 ae 46.1� 31.1 aeNormal fish 5.3� 4.9 af 2.45� 0.87 af 5.05� 4.4 af 11.6� 13.2 af

Diptera Low fish 44.9� 29.1 ae 63.9� 34.2 ae 41.2� 16.8 ae 46.6� 31.1 aeNormal fish 12.1� 5.9 af 9.77� 7.7 af 10.57� 3.82 af 15.5� 13.2 af

Gastropoda Low fish 8.09� 2.4 ae 6.5� 3.1 ae 6.2� 1.2 ae 3.8� 1.3 beNormal fish 1.3� 1.1 af 1.2� 0.8 af 0.7� 0.2 af 2.6� 1.1 af

Gammarus duebeni Low fish 0.1� 0.06 ae 0.2� 0.3 ae 0.4� 0.4 ae 1.4� 1.9 beNormal fish 0.2� 0.1 ae 6.0� 3.3 bf 7.6� 4.0 bf 90.2� 54.8 cf

Asellus aquaticus Low fish 3.4� 2.1 ae 0.4� 0.4 be 0.8� 0.5 be 0.3� 0.2 beNormal fish 3.5� 3.8 ae 0.1� 0.1 be 0.8� 0.5 be 0.04� 0.1 cf

Hirudineae Low fish 2.0� 2.3 ae 0.1� 0.1 be 0.02� 0.04 ce 0.1� 0.2 beNormal fish 1.0� 1.4 af 0.01� 0.03 bf 0.02� 0.04 be 0 cf

Oligochaeta Low fish 0.3� 0.4 ae 0.1� 0.1 ae 0.1� 0.1 ae 0.05� 0.07 aeNormal fish 0.2� 0.2 ae 0.3� 0.3 ae 0.1� 0.3 ae 0.1� 0.1 ae

Ephemeroptera Low fish 0.3� 0.3 ae 1.0� 1.0 be 0.8� 0.7 be 1.2� 2.3 beNormal fish 0.07� 0.1 af 0.01� 0.03 af 0.1� 0.3 af 0.04� 0.1 af

Zygoptera Low fish 0.03� 0.04 ae 0 ae 0.02� 0.04 ae 0 aeNormal fish 0 ae 0.05� 0.06 af 0.01� 0.03 ae 0.1� 0.2 af

Total taxa Low fish 5.2� 0.6 ae 4.9� 0.5 ae 4.7� 0.8 ae 4.8� 1.5 aeNormal fish 3.2� 1.1 af 3.7� 0.9 af 3.2� 0.6 af 3.8� 0.5 af

Increasing salinity

Increasing

mobilization of

phosphorus from

sediment

Removal of

Cladocera

Increased

phytoplankton

chlorophyll a

Low grazer

potential

Reduced light

availability

Reduction in

plant growth

Reduced

species

richnessMacrophyte

decline

Potentialincrease in

periphyton and

algal growth

Increase in

salt-tolerant

periphyton and

algaegrazers

filamentous

filamentous

Figure 8. Summary of main relationships linking salinity to plant performance in a mesocosm experiment on the Hickling Broadecosystem.



DOI: 10.1002/aqc

concentration in the water and there was a significant relationship between salinity and TP. The highavailability of P compared with N in estuarine waters has been explained by reduction of sulphate(Blomqvist et al., 2004), which is very abundant in waters made saline by sea water. Reduction of sulphateproduces sulphide ions that readily precipitate ferrous iron. With removal of iron, phosphate is able tomove freely from the sediment into the overlying water. This seems a plausible explanation for themechanism of phosphorus release in the mesocosms. The significant inverse relationships between salinityand nitrate and pH are consistent with this. Reduced redox would lead to increased denitrification and toincreased carbon dioxide, thus reducing pH. A background of nitrogen loading was given to the tanks,which (as part of a separate experiment) exceeded the current loading on the lake. Increased phosphoruswas thus reflected in increased algal populations, measured as chlorophyll a as it is in field data (Hoare,2002; Bales et al., 2003). This effect began in the winter between the two years of the experiment and grazingby zooplankters was unable to prevent it. This was not unexpected in the winter, when zooplanktonpopulations grow slowly at low temperatures, but grazing was not effective at the highest salinity level inparticular, because of loss of virtually all Cladocera especially in the presence of normal fish biomass.

At lower salinity values, where, at low fish densities, daphnids, including the very large Daphnia magna,developed, the water remained clear in the first year. It was less clear in the second year when fish reducedzooplankton biomass and therefore grazing potential. With increasing salinities, daphnids then othercladocerans were removed, leaving a community dominated by copepods at the highest salinity values.Laboratory experiments confirmed that salinity directly affected the survival of daphnids. In the absence offish, Daphnia magna could reproduce at salinities up to at least 1520mgCl L�1 but in the presence of fish,D. magna could not survive and other daphnids were found only at the lowest salinity. The anomalouspresence of D. magna (Figure 5) in the highest salinity in the second year arises because in two of the 12replicates, fish did not reproduce and their biomass was very low.

High chlorophyll a implies reduced light penetration and a lower potential for submerged plant growth(Bales et al., 1993). Reduction in plant biomass in Hickling Broad in the past has been associated withincreased algal turbidity. The light climate at the plant surface is also influenced by periphyton, which inturn is determined both by nutrient availability and grazing by invertebrates (Jones et al., 2002). Salinity didnot influence periphyton chlorophyll a. However, it did have some effects on the invertebrates likely tograze periphyton. In particular, Gammarus duebeni became very abundant at the highest salinity, anddominated the invertebrate community. It is possible that salinity, through increased phosphorus supply,increased the potential for periphyton growth but that this potential was not expressed because of acounterbalancing salinity-induced increase in grazing. One effect of this may have been the significantreduction of filamentous algae with salinity, for gammarids will eat algal filaments (Moore, 1975, 1997). Animportant role in periphyton grazing has been attributed to Neomysis integer in Hickling Broad in the past(Irvine et al., 1993). Neomysis did not grow in the mesocosms but its role may have been replaced byGammarus duebeni.

In addition to direct effects of salinity, manifested through phosphorus availability, and effects on thezooplankton community, there may have been a direct effect on submerged plant species richness. It is notpossible, however, to separate this from effects on light availability of TP and grazing. The generalreduction in macrophyte PVI by salinity suggests that the salinity mechanism was manifested in a largelynonspecific way through phytoplankton shading rather than by specific physiological effects, but thereduction in species richness may partly belie this.

Submerged plant communities are vulnerable because of their subordinate position in the water columnto phytoplankton and periphyton. On the other hand they are resilient and in other experiments, impacts ofnutrients, fish and temperature increase have not displaced plant dominance of mesocosm systems (McKeeet al., 2003). Submerged plant populations do disappear, however, and the 1970s decline in Hickling Broadwas rapid and dramatic. Many influences may determine whether such a community thrives or declines,and in some situations small changes in one or another may tip the balance and initiate a decline. Tributyl

T. BARKER ET AL.236


DOI: 10.1002/aqc

tin (TBT) compounds are present in recent sediments of Hickling Broad and on the basis of correlation in asediment core are suggested by Sayer et al. (2006) to be implicated in loss of plants through poisoning ofperiphyton grazers.

The rise in salinity in Hickling Broad from the 1960s onwards appears to be a more likely mechanism,however, on the basis of experimental evidence and the innocuous use of the same presumably TBT-contaminated sediments in the mesocosms. Salinities since the 1960s have been around 1800mgClL�1 inwinter and 2000mgClL�1 in summer when some evaporative concentration occurs in a basin that is notvigorously flushed. Such salinities are not incompatible with clear water and rich macrophyte (includingcharophyte) communities, for in 1998 and 1999 such conditions prevailed. At these salinities, however, itappears that small changes in other factors, such as favourable fish recruitment and its influence on grazers, oreven small impacts of toxins such as antifouling paint or herbicide leakage, might return the system to turbidity.In the 1980s and 1990s their impacts were clearly ineffective for the plant populations then steadily recovered.

Following the peak of plant development in 1998 and 1999 and the major conflicts between sailing andconservation as the plant biomass impeded the progress of boats, there has been a major and irregulardecline in the plant community. The recent decline is very perplexing given the steady increase in area ofcharophyte lawns during the 1990s. Both charophytes and vascular plants showed signs of die-backreminiscent of chemical toxicity in 2000 though analyses of the water for herbicides some weeks later(Environment Agency, pers. comm.) showed no traces.

In the meantime, the requirements of the European Union Habitats Directive and the listing of HicklingBroad as a Special Area of Conservation (SAC) have required the designation of conservation targets(Eaton, 1999). These include inter alia: a diverse macrophyte community, dominated by Chara andpondweeds; at least five species of charophyte among other specified vascular plants; at least 60 ha ofcharophyte lawns and 92% coverage by macrophytes; chlorophyll a between 15 and 50 mgL�1; and totalphosphorus concentration below 50 mgL�1. These targets mean that measures must be taken to reinstatethe clear water and charophyte communities in sufficient vigour that they will withstand the substantialcutting needed to permit competitive sailing. Boating is economically important in the area. The currentexperiment is the most convincing evidence yet that reduction in salinity may be the best strategy. It willreduce phosphorus concentrations, which appear to be the major driver and, if sufficient, may alsosubstantially increase grazing on the phytoplankton. Figure 9 gives a variety of options for doing this.

The salt is pumped in from low-lying land close to the sea between Hickling Broad and the coast. A mapin Pallis (1911) indicates that the groundwater salinities were about the same as they are now, yet the valuesin the surface waters of Horsey Mere and Hickling Broad were much lower (Gurney, 1911), suggesting thatit is the increased pumping of the water that is the core problem. This saline water is injected into HicklingBroad from Heigham Sound (Figure 1) on the rising physical tide generated in the estuary. One approach isto use a barrier or lock to prevent this water entering Hickling Broad. This is impracticable, however,because it would interfere with navigation and there is a legal right to free navigation on tidal waters in theUK, dating back many centuries. The navigation problems posed by the plants in 1998/99 suggest that alock or dam would be strongly contested. The solution then is to remove the salinity at source and there arethree options for this.

The first is to divert all the pumped drainage water direct to the sea, without need for any change in landuse in the farmed levels. This would have a high immediate capital cost in re-siting of pumps and excavationof new channels. It would also mean that the physical tide would bring water from lower down the riverinto Hickling Broad. This water would be derived from the River Bure, where phosphorus concentrationsare higher than in the saline pumped drainage water currently entering Hickling Broad and likewise fromCatfield Dyke, which enters the Broad on its inland margin, but which is also nutrient-rich. Salinities,however, would fall to very low values capable of supporting daphnid communities and extensive grazing.

The second option would be to encourage farmers in the pumped levels to join the current voluntary ESAscheme (now called High Level Environmental Stewardship) and convert their current arable to pasture



DOI: 10.1002/aqc

with higher water tables and hence less need for pumping. There are several tiers to the scheme withpayments increasing broadly in line with height of the water table. There would be no capital costs, butincreased running costs, payable by Government, perhaps with local enhancement, because the landownersconcerned, despite opportunities over nearly two decades, have declined to enter the scheme. The degree towhich salinity could be reduced is unpredictable, but the experimental results indicate that a quite modestreduction in salinity, perhaps by 20% to around 1600–1800mgClL�1 is likely to have a substantial effecton TP and chlorophyll and hence on the potential for plant growth. In general the effects of the sub-presentsalinity treatment were not greatly different from the lower salinities.

A third option may perhaps be combined with the second. Salinity concentrations in the levels are notuniform (Figure 1). The highest value in Figure 1 comes from an area known as the Hempstead Marshes,and Driscoll (1984), in a more detailed survey, also pinpoints this area as a major source of salt. If thedrains in this area were to be isolated, it might be possible to use small pumps to divert this water directly tothe sea, and effect salinity reduction without major alteration to the hydrology of the system or change infarming practice. Such an approach may also offer greater predictability of the salinity reduction thanmight otherwise be attained.

Whatever the final strategy, Hickling Broad can be restored to the scenario envisaged in its designation asan SAC. At the same time this will inevitably bring about major potential conflict with sailors. Stabilizationof the plant communities at a lower salinity offers a solution that might allow cutting of sailing boat racing-circuits among plants, such as C. intermedia, which are rare but by no means inherently delicate, asindicated by their vigorous growth in these experiments.

Reduction in salinity

Severe, by diversion of

pumped water directly to

the sea or use of most

stringent Environmentally

Sensitive Area

arrangements

Moderate by

raising water

tables by use of

Environmentally

Sensitive Area

arrangements

Moderate by

targeted diversion

of the most saline

water to the sea

Reduction to

<<1000 mg Cl L-1

Reduction to approx.

1600 mg Cl L-1

Major reduction in total P

and chlorophyll a by

nutrient limitation and

survival of daphnids

chlorophyll a,aided by

increased copepod grazing

Very diverse,permanent

plant community, rich in

charophytes capable of

withstanding cutting

Less diverse plant community,

rich in charophytes, with high

chance of permanency and

ability to withstand cutting

Sufficient reduction in TP and

Figure 9. Pathways and consequences of reducing salinity in the Hickling Broad ecosystem.

T. BARKER ET AL.238


DOI: 10.1002/aqc

There is an added and contradictory dimension. The Upper Thurne area was formerly an estuary and theRiver Thurne originally flowed in the opposite direction to that at present, discharging to the sea close toMartham Broad (Figure 1). The Broads in the area were probably excavated at least partly as clay pits inestuarine sediments and partly as peat pits at their distal ends. There may also have been some riverengineering by the mediaeval monastic houses to divert flows (Moss, 2001) and this would have reduced theerosive pressure on the former outlet and meant that it became permanent. The sand dune barrier thatseparates the Upper Thurne basin from the sea, however, is extremely thin and low and the basin has beenflooded with sea water several times in the past 200 years when storms have breached it and underminedartificial sea defences, the last major breach being in 1953.

At present, sea levels are rising absolutely, and the land is sinking owing to geological adjustment to theretreat of the last glaciers, so that net rise in sea level is comparatively high. Expectations are that the duneline will eventually breach again and that the basin will return to being an estuary, obliterating the presentfreshwater features. This may occur soon or equally may be several decades away. Policy in the area is toattempt to restore the current fresh and brackish water features of the system and abide by the targets forHickling Broad set in its designation as an SAC under the Habitats Directive. However, there is anargument, under the more recent Water Framework Directive that ‘good ecological status’, to which thearea must be restored, should mean active or passive conversion to its natural estuarine status, particularlysince this appears likely to happen anyway.

Finally, this experiment offers the first demonstration of the effects of a switch mechanism underexperimentally controlled conditions. This is important in two respects. First, there is some controversyabout whether nutrient increase alone can act to switch clear water conditions to turbid conditions.Inherent in alternative states theory is the concept that the states can exist over similar backgroundenvironmental conditions, which include nutrient loading. If the two states can be altered by nutrientloading alone, they are not true alternatives. In this experiment, a switch (salinity) was demonstratedirrespective of nutrient loading, which was held constant. Second, it is fashionable to value science thatcontains generality over that which embodies only the specifics of case studies. Such an attitude is perverse,however, in that generality can only emerge from the availability of specific studies. The case of HicklingBroad has lessons that can be applied more widely but its real value is as a specific case. In derivinggenerality, there is a loss of information, a sort of intellectual entropy, and, although it is always possible tomove from the specific to the general, it is, in contrast, very difficult to move back. For the management ofmany sites there can be no substitute for detailed work that takes into account the myriad details that makeevery site a specific problem.

ACKNOWLEDGEMENTS

This project was supported by Grant NER/A/S/2002/00759 from the UK Natural Environment Research Council. Weare grateful to English Nature, the Norfolk Wildlife Trust and the Broads Authority for their cooperation and toMartin Dade of Amis Piling for a solution to the problem that the sediment was ‘too thick to pump and too thin to dig’.

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