Abstract

Changes in Hickling Broad, since its creation in the 14th or 15thcenturies by the flooding of peat diggings, have been deducedfrom dating and analysis of a sediment core, historical informa-tion and current limnological studies . Until the 1930's there waslittle major change . Increased agricultural land fertilization ledto markedly increased organic sedimentation from the 1930'sonwards, due to increased growth of submerged macrophytes .Inorganic sedimentation increased concurrently as more power-ful pumps were installed to help drain the adjacent fens andmarshes .There was no evidence of increased plankton populations

during this phase, but epiphytic diatom populations increased . Inthe mid 196o's the current period of hypereutrophication began .Epiphytic diatom numbers increased markedly and in the early1970's the previous luxuriant macrophytes became sparse andthe water became turbid with phytoplankton. These changes areattributable mainly to increases in the size of a roost of migratoryblack headed gulls (Larus ridibundus L.) on the lake in autumnand winter .

Introduction

Hickling Broad is the largest (120 ha) of a group of morethan forty small and shallow (1-2 m) lakes in East Nor-folk, U .K. The lake basins were excavated from alkalinepeat deposits up to the fourteenth century (Lambert et al.,1960) when the peat workings became flooded and wereabandoned. The area, called Broadland (Fig . 1) comprisesthe valleys of fertile lowland rivers draining uplands ofchalk and glacial drift. A series of channels (dykes), cut inpast centuries to facilitate transport of marsh productssuch as reed and wildfowl, connects most of the lakes

Hickling Broad

CORE SRE-.

Heigham SoundCATFIELD DYKE

•

LAND DRAINAGE PUMP

Hydrobiologia vol . 6o, 1, pag. 23-32, 1978

THE ECOLOGICAL HISTORY OF A MEDIAEVAL MAN-MADE LAKE. HICKLING BROAD, NORFOLK,UNITED KINGDOM

Brian MOSS

School of Environmental Sciences, University of East Anglia, NORWICH, U.K .

Received November 4, 1977

Keywords : Palaeolimnology, eutrophication, diatoms, guanotrophy, Norfolk Broads

(Broads) with the rivers . Other, smaller, dykes were cutlater for land drainage . The catchments are generally veryfertile and many of the marsh and fen lands in the rivervalleys were drained for agricultural use first by windpumps (18th century onwards), later (early 20th century)by steam pumps and currently are drained by electricallypowered pumps .

Hickling Broad is a National Nature Reserve prized forits bird fauna and, until about 1972, for its submergedaquatic macrophyte flora and clear water . The flora in-cluded Utricularia spp, Myriophyllum spp, Zanichelliapalustris L ., Najas marina L ., Potamogeton pectinatusL., Ceratophyllum demersum L ., Hottonia palustris L.,

sHorsey Mere

O DYKED EA

2km

Dr. W. Junk b.v . Publishers - The Hague, The Netherlands

Fig . 1 . Location of Broadland and Hickling Broad .

NorthSea

123

Hippuris vulgaris L ., Fontinalis sp and several stonewortspecies in 1968 (Morgan, 1972) ; Ellis (1965) gives evidenceof Stratiotes aloides L. and Nuphar lutea (L.) Sm. also inthe Broad and refers to the Hickling charophytes as `therichest and most spectacular assemblage of differentkinds of these plants in Britain' . Currently the submergedmacrophyte flora is sparse and dominated by Potamoge-ton pectinatus. Hippuris vulgaris and Myriophyllum sppare common locally and only a few plants of Najas mari-na and Chara may be found. The water is turbid, there areabundant phytoplankton populations, and the total phos-phorus and phytoplankton chlorophyll a concentrationsexceed 200 µg F' in summer (Leah et al., in press; Hold-way et al., in press) . Levels prior to the early 1970's of theseparameters are likely to have been between io and 8o µgF' judging from measurements made in other Broadswhere macrophytes persist in quantity (Phillips, 1977 ;Holdway et al., in press) .

Coupled with these changes in Hickling Broad has beenan increase in the incidence of fish kills caused by thebrackish water, Haptophytan phytoplankter, Prymne-sium parvum Carter. Hickling Broad receives saltwaterby percolation from the nearby North Sea and its chlori-nity in recent years has averaged 7.7% of that of sea water .It has been suggested that the salinity of the Broad has in-creased since the installation of powerful electrical landdrainage pumps in the late 196o's . These are felt to exploita deeper, more saline water table than that previouslydrained . Evidence for an increase in salinity, however, isequivocal (Holdway et al., in press), because limnologicaldata for the Broad are scant prior to the 1970's .

Nutrient enrichment, changes in salinity, and even in-creased activity of recreational boats on the Broad haveall been suggested as responsible for the decline in sub-merged macrophytes or the Prymnesium-induced fishkills or both . It seemed that palaeolimnological investiga-tions might put the current situation into perspective .This paper discusses evidence from a dated sediment coretaken in Hickling Broad in 1975 .

Methods

Cores were taken from the relatively shallow sedimentlayer (about 20-50 cm, Lambert et al ., 1960) with openended, 6 cm diameter plastic pipes pushed into the peatand clay layers which underly the basin . The pipes hadbeen sliced longitudinally and rebound with waterprooftape, so that after removal of surface water the pipe could

24

be opened and the core sliced into i cm sections . Therewas probably some compression, particularly of theupper layers, and some surface fluid sediment was un-avoidably lost with the removal of the supernatant water .Sub-samples from each section were analysed for watercontent, and `organic content' by ignition at 500'C andprepared for diatom analysis as detailed in Osborne &Moss (1977) . The remaining sediment was dated by mea-surement of Pb-2,o levels at the Atomic Energy ResearchEstablishment, Harwell, U .K . An explanation of themethod is given in Osborne & Moss (i977) .

Results

Fig. 2 gives details of the fresh and ashed appearances ofa typical core, the age of the core layers and of the totalnumber of diatom frustules found per unit dry weight ofsediment . The total depth of lake sediment was about 30cm. The southern part of Hickling Broad, from which thiscore was taken is floored by a mixture of amorphous peatand clay, the latter deposited during a marine transgres-sion about a thousand years before the basin was ex-cavated . At 49 cm in the core the deposit was a grey claywith some intermixed fibrous peat . Between 49 and 34cm it was of clay with an increasing organic content whichwas overlain at 30-34 cm with a black peaty layer whichseems to mark the bottom of the excavated basin . Simi-lar deposits underly Heigham Sound, just to the south ofthe coring site (Lambert et al., 1960) .

Within the lake sediment ('nekron mud' of Lambert etal ., 1960) little differentiation in fresh appearance wasseen, the column being a uniform grey-brown colour .Ignited sediment, however, showed some markedchanges. From 30 cm to 9 cm the ash was rusty-brown incolour, as it was in the basin peat and clay layer. Between6 cm and 9 cm there was a rusty-grey transition layer toa surface grey layer, which, unlike the lower rusty layer, orthe deep grey clay layers, effervesced strongly with diluteacid and contained a high content of carbonates (marl) .The inorganic content of the lake sediment was relativelysteady and high between 30 cm and 15 cm, with sometendency to increase upwards . Between 14 and 6 cm itfell from 80-90% to 70-75% and then increased to its pre-vious highest level in the upper 6 cm . This pattern was re-flected in the water content of the sediment which in-creased slightly in the more organically rich layer between6 and 15 cm .

Diatoms were scarce in the clay and peat layers and in

0

5

I0

15

E 20

25CLW0 30

35

40

45

% dry wt. % ash-%water --%ignn loss Sed'n rate

oo 501

50 10 0 0 .5

1Fresh Ashed Total diatoms g d .w. '

0 500

Fig. 2 . Physical characteristics, sedimentation rates and diatom content of the core . Dates are deduced from Pb-21odatings.

the lower layers of lake sediment, though intact frustuleswere present in even the deepest samples . There was amodest increase in numbers beginning at about 17 cmwhich was sustained from 17 to 6 cm when a marked in-crease to the surface was recorded .

Fig. 3 gives data on the Pb-21o content of the sediment,from which a calibration curve was calculated to givedates inserted on Fig . 2 . Levels of Pb-21o declined fromthe surface downwards as expected from the decay of thisnaturally occurring isotope but fell to extremely low levels(less than o . t pCi g- ') between 19 and 20 cm and were un-detectable thereafter. Dates obtained were 1972 ± 6 for 2cm, 1955 ± 9 for 6 cm, 1962 ± 7 for 9 cm. Interpolationfrom the best fit curve gives a date of 1935 ± 9 for 15 cm.

Sediment cores obtained in Hickling Broad have all con-tained less than 50 cm of lake sediment, consistent withfindings of Lambert et al. In the sheltered bay where thepresent core was taken there are no rapid scouring cur-rents . The lowest lake sediment in the core is thus likely tobe that laid down when the basin was first flooded . Sedi-mentation rates, calculated from the datings, were thusvery low (Fig . 2) in the first five hundred years of the lake'sexistence if a nominal date of i 4oo AD is taken as the dateof flooding, and averaged 0 .028 cm yr' . In the 1930's or1940's the rate increased to 0 .25 cm yr ' and from the mid196o's onwards it increased to its current rate of about0.5 cm yr' . The later increase corresponded with the pre-dominance of marl in the core, the earlier one with

25

19721 bi Da~ E

* >%>%- -

Phase- III

-1955 *_ 9

-

KJ62+7-

1948Y Phase II

c1850

1760

30

1580 >,Phase I

U

a~CDCP

Pre- lakephase

to

v >.

'w. *~

26

24

0 8U

4--CIOCL 12

III.II1111111 11111

0 .1

1 .0

0 20 40 60Years before presentPb-210 p Ci g -1

Fig . 3 . Pb-21o content of the core, and calibration curve for sediment dating .

(f) Sedimentation ratedry matter inorganic rrIatter

organic matter total diatoms0 50 100 0

100

200

300 0

50

0

1000

2000

Fig. 4. Sedimentation rates of dry, inorganic and organic matter and of total diatoms . Figures are relative and werecalculated as indicated in text .

changes in the organic content of the core around 15 cm .Fig. 4 gives data on the relative rates of deposition of

dry, inorganic and organic matter and of diatom frustulesin the lake since its origin . These data are relative andwere derived by multiplication of the sedimentation rate(mm yr• ') at the time by, respectively, the percentage ofdry matter in fresh sediment, the percentage ash in driedsediment, the percentage loss on ignition at 500 c fromdried sediment, and the diatom content per unit dryweight, for each i cm slice of the core . They show marked

PHASE I I

PHASE I

increases in all parameters from the 1930's onwards, withfurther increases after the mid-i96o's in inorganic anddiatom deposition . Organic matter deposition thoughstill very high in the last decade, decreased compared withthat from the 1930's to the mid 196o's .

Details of the diatom flora are given in Figs . 5 and 6 . Aconvenient ecological classification of the diatom florasof the Norfolk Broads largely follows the usual taxono-mic scheme. Centrales in the area are generally plank-tonic, with the exception of certain Melosira spp which

If) sedimentation rateCentrales

Fragilariae Araphidi eae Monoraphidineae Biraphidineae500 10 0 0

50

100 0

5oo 0

50

0

0

25

Fig . 5 . Sedimentation rates (relative values) of major groups of diatoms in the core . Data for the pre-lake phase assumea similar sedimentation rate to that during phase i in the absence of definite dates for these layers .

were absent in Hickling Broad cores ; Araphidineae maybe planktonic or epiphytic, but although substantialpopulations of planktonic Synedra and Diatoma are cur-rently found in Hickling Broad (Leah et al., in press),most of the Araphidinean genera are epiphytic . Fragilariahas been treated separately because it is not a characteris-tic epiphytic genus on totally submerged macrophytes(Eminson, 1978) but is usually found in the loose massof diatoms and filamentous algae associated with thebases of the marginal emergent macrophytes (mostlyPhragmites australis (Cav.) Trin. ex Steud. and Typhaspp. and with the surface of marginal sediment . Mono-raphidineae are associated with the epiphytic flora on sub-merged macrophytes and Biraphidineae may be eitherfirmly attached to submerged macrophytes (Gomphone-

Phase II

Pre-lake phase

ma, Epithemia, Cymbella) or loosely associated withmacrophytes or sediment .

Centric diatoms increased in the core from about 4 cmupwards. There were occasional frustules found at 6 cmand 9 cm but not below. Of the two genera found Cyclo-tella predominated, only two frustules of Coscinodiscusbeing counted in the entire core. Chaetoceros occurs cur-rently in Hickling Broad (Leah et al ., in press) but the spe-cies are small and delicate and their frustules are unlikelyto be preserved intact . Chaetoceros was not found in thecore .

All other groups were much more abundant than wereCentrales and began to increase in the core at deeper levelsthan the latter-mostly between 15 and 17 cm . Also allshowed a steady increase up to about 6 cm, followed by

27

a marked increase from there to the surface . Examples ofthe more abundant species and genera are shown in Fig .6 . The increase in Araphidineae other than Fragilaria isaccounted for mostly by Raphoneis amphiceros varrhomboides Cl ., a benthic or epiphytic species and not byplanktonic Araphidineae . Although Synedra and Diato-ma are currently very abundant in early spring -in theplankton of the Broad, they were rare in the core . Bothhave thin, delicate frustules, easily broken, and there isevidence that currently substantial amounts of silicate arereleased from Hickling Broad sediment in summer (Leahet al., in press) . However, the complete absence of Diato-ma from the surface layers of the core suggests that disso-lution is not the entire explanation . It is much more likelythat the Synedra and Diatoma populations are a very re-

28

cent phenomenon (1-2 years prior to the taking of thecore) and that their remains were lost in the very fluid sur-face sediment which was unavoidably lost when super-natant water was poured from the core. Fluid surfacesediment was carefully sucked from the bottom of theBroad in August 1977 and found to contain abundant Sy-nedra and Diatoma .

Certain diatoms (Raphoneis amphiceros var rhom-boides, Campylodiscus echensis Ehr ., Navicula cinctaCl.) are considered to be indicative of brackish conditions(Hustedt, 1930) . They showed patterns in the core similarto the equally or more abundant diatoms found normallyin freshwaters (Fig . 6) . There was no evidence of a changeof community from less to more halophile in the core .The temporary floodings by sea water, which broke

(f) sedimentation rate

0 25 0 5000

500

2000

1000

500

Fig. 6 . Sedimentation rates (relative values) for Cyclotella kutzingiana Thr. and the most common diatoms in thecore -Fragilaria pinnata Ehr, F. brevistrata Grun ., Raphoneis amphiceros var rhomboides Cl., Cocconeis diminutaPant ., C. placentula (Ehr .), Achnanthes lanceolata var elliptica Cl, Campylodiscus echensis Ehr ., Navicula cincta Cl .,

Amphora ovalis Var. pediculus Kutz ., and Epithemia spp .

CYCLOTELLA

FRAGILARIA

COCCONEIS C . PLACENTULA40 KUTZINGII

BREVISTRIATA

DIMINUTA

U IFRAGILARIA PINNATA

RAPHONEIS AMPHICEROS VAR.

t 0 100 200 0

500

1100 0 2000 200D.

~__

Ir--0 10 --- is- - ---------1 f

------- ---------

20

30

40ACHNANTHESLANCEOLATA VAR .

NAVIO(JLACINCTA

AMPHORAOVALISIVAR

I CAMPYLODISCUS ECHENSIS EPITHEMIA SPP

through the sand hills which protect the area from theNorth Sea, in 1607, 1655, 1792, 18o5, 1897 and 1938, andthe upstream surge of seawater which occurred during astorm in 1953 (R. R . Clarke in Ellis, 1965) seem to havebeen too temporary for any evidence of them to have re-mained in the core .

Discussion

Three main phases can be distinguished in the history ofHickling Broad. The earliest and longest, phase I lastedfrom the origin of the basin, nominally taken as 1400A.D ., though perhaps up to a hundred years earlier orlater than this, until sometime just before or around thetime of the Second World War, nominally taken as1937. Phase II lasted from then until the mid or late 196o'sand Phase III is the contemporary phase .

Phase I was a long period of little change . Sedimenta-tion rates were very low (only about 3 cm in 100 years)with perhaps a small increase in mineral sedimentationfrom about the middle of the 18th century . Diatom re-mains in the core from this period were sparse and nonewere of planktonic species . The lake probably had verylow plankton populations, as noted by Gurney, as late as1949, and the macrophyte flora, at least towards the endof phase I seems to have been dominated by swards ofChara spp, with some Najas marina (Bennett, 1883 a, b,1884, 1910; Bennett & Salmon, 1903) . The low phase I se-dimentation rates are to be expected from the low plank-ton populations, the nature of the macrophyte commu-nity (Chara spp do not produce the bulky biomass oflarger macrophytes) and the hydrological regime of theBroad. Hickling Broad lies on a `blind arm' of the Hick-ling Broad/Heigham Sound/Horsey Mere complex,(Fig . 1) which has a large catchment area . Most drainagewater, however, moves from Horsey Mere to HeighamSound and out to the R . Thurne, by-passing HicklingBroad. In summer, little `new' water enters the system be-cause precipitation is exceeded by evaporation in thecatchment area and water mixes between Hickling Broadand Horsey Mere under the action of small daily tides(Leah et al ., in press, Holdway et al., in press) . HicklingBroad, despite its size, therefore has a small effectivecatchment area from which allochthonous sediment canbe provided . During much of phase I, although parts ofthe catchment would have been farmed (Ellis, 1965) therewould have been a much greater extent of undrainedmarsh and fen through which drainage water would pass

before entering the Broad . (The first drainage mills,directing water through channels to the Broads, date tothe start of the 18th century (R. R . Clarke in Ellis, 1965) ) .Fen and marsh, because they lay down peat, act to someextent as nutrient traps as well as sediment filters, and thismay have reduced the nutrient load applied to the lakeand thus the autochthonous as well as allochthonoussupply of sediment. A small lowland lake in Italy, theLago di Monterosi, with a comparably low catchmentarea to basin area ratio was found to have had an evenlower sedimentation rate (0 .5 cm per 100 yr) (Hutchinsonet al., 1970) .

Apart from a very slight increase in the rate of inorganicsedimentation from the mid 18th century to the end ofphase I there is little evidence that changes in the catch-ment area consequent on land drainage had much effecton the lake . There were major changes in phase II, how-ever. Sedimentation rates increased markedly, both ofinorganic and of organic matter and there was a steadyincrease in non-planktonic diatoms . Anecdotal reportsmostly concerning the latter part of the phase commenton the clear blue waters and on the abundant submergedmacrophytes (Gurney, 1948 ; Barry & Jermy, 1952; Ellis,1965; Cadbury, 1965; Morgan, 1972 ; Nature ConservancyCouncil, 1977) . Press reports drew attention to over-growth of the Broads by weed (Anon, 1949a; Arrow, 1949)and reported on trials of a mechanical weed cutter to clearareas for sailing in the Horsey Mere area (Anon, 1949b) .

This was clearly a phase of increased productivity andthe increases in epiphytic and bottom living diatomsrather than of plankton, as well as the anecdotal evidence,suggest an increased abundance of submerged aquaticplants compared with phase I . There may also have beenchanges in the balance of macrophyte species-Barry &Jermy (1952) comment on the competitive ability ofNgias marina compared with Chara species in the Broad .However, most of the species in phase II had been re-corded by Nicholson (1906), though there are no quanti-tative data . Increased macrophyte productivity explainsthe increased organic sedimentation, though not the in-creased sedimentation of inorganic matter, which wasnot of marl, but largely of clay and silt particles, whichhad, on ashing, the rusty-brown colour of the phase I sedi-ments. The increased inorganic sedimentation can be ex-plained by intensified land drainage and the increase inproductivity by leaching of nutrients from a catchmentwhich was increasingly being used for intensive agricul-ture .

Phase 2 began around the 193o's and showed its main

29

features by the 1940's (Anon, 1949a, b ; Arrow, 1949 ;Jennings & Lambert, 1949) . The use of agricultural fertil-izers increased markedly from the 1930's onwards andparticularly in the 1940's because of the need for increasedhome agricultural production during the War (see graphin Morgan, 1972) . Although phosphorus, in particular, isreadily adsorbed by soil particles, the export of phospho-rus from fertilized arable land is two or three and some-times four times greater than that from natural ecosys-tems (Omernik, 1976) . Land drainage water in the Nor-folk Broadland is sufficiently fertile, in the absence of ani-mal or human sewage or sewage effluent to encouragethe growth of abundant submerged aquatic macrophytes .The sites now richest in submerged aquatic macrophytes(Martham Broad, Upton Broad, Calthorpe Broad andmany field drainage dykes) receive land drainage waterbut little or no sewage or other effluents .

At the same time that land fertilization was increasingduring phase II, conversion of the wind pumps to morepowerful steam and diesel pumps was in progress to allowmore intensive drainage and conversion of previouspasture to arable land . This conversion started a little ear-lier and most pumps were modified between 1918 and1945 (R . R. Clarke in Ellis, 1965). The pump at HorseyStaithe (Fig . i), for example, was converted to steam inthe early 20th century and to diesel in the 1930's (Piersse-ne, 1976). The more vigorous pumps, draining the dykesmore rapidly than wind pumps, give less time for clayand silt particles to settle and thus increase the input of in-organic sediment to the waterway, thus increasing alloch-thonous sedimentation .

The third phase in Hickling Broad's history has beenone of hyper-eutrophication . Since the late 196o's therehas been a progressive diminution in submerged macro-phytes and increased turbidity of the water associated withdense phytoplankton crops (Leah et al., in press, Hold-way et al ., in press) . The supply of phosphorus necessaryto sustain the total phosphorus levels measured cannotbe entirely accounted for by the current contribution fromland drainage (Leah et al., in press), and appears to comefrom excretion by black-headed gulls (Larus ridibundusL .) . There has been a major increase in the numbers ofmigratory black-headed gulls which roost on the Broadin autumn and winter . These gulls breed in Holland,Belgium and particularly around the Baltic Sea (Anon,1975) and currently (1976) the maximum size of the roostat Hickling Broad is estimated as 250,000 birds (Leah etal., in press) . This represents a major increase since 1952(Hickling, 1954) when the peak numbers were around

30

25,000 and Hickling Broad was then one of the morepopulous roosts in the British Isles . In phase I also, al-though detailed counts do not exist, flocks of 10,ooo-20,000 were felt to be dramatically large enough for com-ment in 1921 (Riviere, 1921) . Migratory black-headedgull populations have increased markedly in Britain thiscentury and although the roost at Hickling Broad was notcounted, Hickling (1977) found a doubling in overallpopulation in the previous decade and that the largestroosts, which had had around to o birds in the 195o's, nowhad around 10 5 birds . The phosphorus loading suppliedcurrently by the gulls at Hickling Broad has been found toaccount for between 135 µg 1 - ' and 270 µg F' of the sum-mer concentration of total phosphorus (Leah et al., inpress) . If similar criteria are used for calculation the roostin phases I and II would have provided between 13 .5 and27 µg F', which is somewhat less than that provided byland drainage (up to 72 µg P F' at present, perhaps lessthan this in phase II) . The transition between phases IIand III can therefore be ascribed to the steady increase ingull populations during the 195o's and 6o's with perhapsa more marked increase in the 196o's .

Sedimentation rate of inorganic matter increasedduring phase III and that of organic matter decreasedslightly compared with that of phase II . The latter is at-tributable to a switch from high production of submergedmacrophytes, with deposition of refractory higher plantremains, to high phytoplankton production of which alesser proportion is refractory . The further increased sedi-mentation rate in phase III of inorganic matter is not at-tributable entirely to clay and silt particles pumped inwith the drainage water by large electrically operatedpumps which, coincidentally were installed in the late196o's, since the nature of the sediment changes to marlin phase III . The alkalinity and calcium levels of HicklingBroad are high (mean Ca" 248 mg 1-', mean alkalinity2 .44 mequiv . F'), and the dense phytoplankton crops insummer (over 200 µg 1 - ' chlorophyll a) undoubtedly in-duce sufficiently high pH late in the day to favour forma-tion of carbonate ions . pH's are normally well above 8 .0even in the mornings . With the high level of calciumpresent, the solubility product of CaCO3 must be readilyexceeded leading to marl deposition and the strong effer-vescence when acid is added to phase III sediments . Withgood growth of Chara, marl must have been formed alsoin phase I and II but since it is not prominent in the sedi-ments, much of it must have redissolved in the presum-ably rather less alkaline water . Gurney (1928) commentson the lack of deposition of marl anywhere in Broadlandat that time .

In the early years of phase III and the later years ofphase II, which include the transition zone between therusty-brown ashed sediment of phase I and early phase IIand the grey marl of phase III, some of the marl precipi-tation is attributable to photosynthesis by increasingcrops of epiphytic algae and the macrophytes themselves .Phillips et al . (1978) have hypothesized that eutrophica-tion first stimulates epiphytic and filamentous benthicalgal growth before that of phytoplankton . Evidencefrom Hickling Broad is consistent with this hypothesis .Epiphytic algae began to increase in phase II, planktononly in phase III, and even then came to dominate thesystem only in 1973 or 1974, the year before the core wastaken. Currently epiphyte populations in the lake are lowbecause of the paucity of submerged macrophytes, butsince the top centimetre or so of the core was too fluid tobe kept intact, the contemporary situation of low epiphyteand high phytoplankton diatom numbers is not re-presented and the top of the core records the peak in epi-phyte abundance just before the phytoplankton becamepredominant .

Part of the conservation value of the Norfolk Broadslies, or rather lay, in the wealth of their submerged aquaticmacrophytes. The loss of these from many areas hasbeen attributed to the effects of agricultural fertilization(Mason, 1976) but the evidence from this core, as well asfrom the abundance of macrophytes in dykes drainingthe land, suggests that macrophyte abundance has beenfavoured by agricultural fertilization . The causes of hyper-eutrophication of what was a naturally eutrophic systemseeem to lie in the entry of highly concentrated nutrientsin sewage effluent (Osborne & Moss, 1977) or in the caseof Hickling and its associated Broads, in gull excreta .

Acknowledgements

I am grateful to the School of Environmental SciencesConsolidated Research Fund for financing most of thecore dating and also to the Nature Conservancy Counciland Norfolk Naturalists Trust for contributions to thesecosts . Dr . G . L . Phillips in particular and other of my col-leagues provided useful discussion and Dr . A. M. O'Rior-dan drew my attention to valuable references .

References

Anon. 1949a . The Broads-Eastern Daily Press . 28 April 1949,P . 4 .

Anon . 1949b . Reclaiming weeded areas of the Broads for sailing .Eastern Daily Press. 28 April 1949 . 3 .

Anon. 1975 . Black headed gull recoveries . Norfolk Bird andMammal Report 24 : 28-29.

Arrow, J . 1949 . The Broads as a National Park . Archit . Rev . 106 :87-100.

Barry, D . H . & Jermy, A. C . 1952 . Observations on Najas marinaI . Trans . Norf . Norw . Nat . Soc . 17 : 294- 297 .

Bennett, A. 1883a. On Naias marina as a British plant. J . Bot . 21 :353-355 .

Bennett, A . 1883b . Chara tomentosa L . in Norfolk . Trans . Norf.Norw. Nat . Soc . 3 : 568-570 .

Bennett, A . 1884 . Plants new to Norfolk with notes on other spe-cies . Trans . Norf. Norw . Nat. Soc . 3 : 633-636 .

Bennett, A . 1910 . Naias marina L. and Chara stelligera Bauer asNorfolk plants. Trans . Norf. Norw . Nat . Soc . 9 : 47- 50.

Bennett, A . & Salmon, C. E . 1903 . Norfolk notes . J . Bot . 41 : 202-204.

Cadbury, J . C . 1965 . Hickling Broad National Nature Reserve .Norf. Nat . Trust Ann . Rep . 39 : 22-27 .

Ellis, E . A . 1965 . The Broads . Collins, London .Eminson, D . F . 1978 . A comparison of diatom epiphytes, theirdiversity and density, attached to Myriophyllum spicatumin Norfolk dykes and Broads . Br . Phycol. J . 13 : 57 - 64 .

Gurney, R. 1928 . The fresh-water Crustacea of Norfolk . Trans .Norf. Norw . Nat . Soc. 12 : 550-581 .

Gurney, R . 1948 . Notes on frogbit (Hydrocharis) and hairweed(Potamogeton pectinatus) . Trans . Norf. Norw . Nat . Soc. 16 :381 -383

Gurney, R . 1949 . Aquatic life in the Norfolk Broads . New Natura-list 6 : 24-25 .

Hickling, R . A . O . 1954 . The wintering of gulls in Britain . BirdStudy 1 : 129-147 .

Hickling, R . A. O . 1977 . Inland wintering of gulls in England andWales, 1973 . Bird Study 24 : 79 - 88 .

Holdway, P . A ., Watson, R . A . & Moss, B . 1978 . Aspects of theecology of Prymnesium parvum (Haptophyta) and waterchemistry in the Norfolk Broads . Freshwat . Biol . (in press) .

Hustedt, F . 1930. Die Susswasser-flora Mitteleuropas . to . Bacil-lariophyta (Diatomae) . Gustav Fischer . Jena .

Hutchinson, G . E ., Bonatti, E ., Cowgill, U . M ., Goulden, C . E .,Leventhal, E . A ., Mallett, M . E., Margaritora, F., Patrick, R .,Racek, A., Roback, S . A ., Stella, E ., Ward-Perkins, J . B . &Wellman, T . R . 1970 . lanula : an account of the history anddevelopment of the Lago di Monterosi, Latium, Italy . Trans .Amer. Phil . Soc . 60 : 1-178 .

Jennings, J . N. & Lambert, J . M . 1949. The shrinkage of theBroads. New Naturalist 6 : 26-27 .

Lambert, J . M ., Jennings, J . N ., Smith, C . T ., Green, C . & Hut-chinson, J . N . 1960 . The making of the Broads : a reconsidera-tion of their origin in the light of new evidence . Royal Geo-graphical Society Memoir 3 : 1-242 .

Leah, R . T ., Moss, B . & Forrest, D . E . In press . Experiments withlarge enclosures in a fertile, shallow, brackish lake, HicklingBroad, Norfolk, United Kingdom . Int . Rev . ges . Hydrobiol .

Mason, C . F . 1976 . Broadland . In : Norfolk Naturalists Trust,Nature in Norfolk. Jarrold, Norwich, 79-89 .

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Morgan, N . C . 1972 . Problems of the conservation of freshwaterecosystems. Symp . zool . Soc . Lond . 29 : 135- 154 .

Nature Conservancy Council . 1977 . Nature Conservation Review11 . Cambridge University Press, Cambridge .

Nicholson, W . A . 1go6 . A preliminary sketch of the bionomicalbotany of Sutton and the Ant district . Trans . Norf. Norw . Nat .Soc . 8 : 265-289 .

Omernik, J . 1976 . The influence of land use on stream nutrientlevels . U .S . Environmental Protection Agency Report EPA -6oo/3-76-o14, 1-105 .

Osborne, P . L . & Moss, B . 1977 . Palaeolimnology and trends inthe phosphorus and iron budgets of an old man-made lake,Barton Broad, Norfolk . Freshwat . Biol. 7 : 213 - 233

Phillips, G . L . 1977 . The mineral nutrient levels in three NorfolkBroads differing in trophic status, and an annual mineral con-tent budget for one of them . J . Ecol . 65 : 447-474 .

Phillips, G . L ., Eminson, D . F . & Moss, B . 1978 . A mechanismto account for macrophyte decline in progressively eutrophi-cated freshwaters Aquat . Bot . 4 : 103-126 .

Pierssene, A . 1976 . Water transport in Norfolk . Norfolk Muse-ums Service, Norwich .

Rviere, B . B . 1921 . The autumn movements of gulls on the Nor-folk coast. Trans. Norf . Norw . Nat . Soc . 11 : 104-127 .

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