Hydrobiologia 185 : 249-257, 1989.© 1989 Kluwer Academic Publishers . Printed in Belgium .

Predation, sediment stability and food availability as determinants ofthe benthic invertebrate fauna in two shallow lakes

Brian Moss* & Mark TimmsSchool of Environmental Sciences, University of East Anglia, Norwich, UK (*present address : Dept. ofEnvironmental and Evolutionary Biology, The University, Liverpool, UK)

Received 21 December 1987 ; in revised form 21 September 1988 ; accepted 20 November 1988

Key words: predation, sedimentary benthos, shallow lakes

Abstract

The sedimentary benthos of a series of shallow, eutrophicated lakes, the Norfolk Broads is, in general,low not only in number of species but unexpectedly in number of individuals . In two of the lakes, HudsonsBay and Hoveton Great Broad, chironomids and oligochaetes dominated the fauna . Hudsons Bay hasan extensive stand of water lilies (Nuphar lutea) ; Hoveton Great Broad does not . There were significantrelationships between number of chironomids and of Potamothrix hammoniensis with organic content ofthe sediments, but these were due not to food availability but to the structure imparted to the otherwisefluid sediment by the organic matter . Sediment stabilised in plastic bowls developed much largerpopulations of oligochaetes than found in the unrestricted sediment. Protection of the community fromfish predation resulted in a further major increase in numbers . Sediment stability and predation ratherthan food supply were the major determinants of these benthic populations .

Introduction

The community of benthic invertebrates in lakeshas been used as an indicator of the fertility of thelake and has even been used as a basis for lakeclassification (Thienemann, 1925 ; Brinkhurst,1974). In general the productivity of the com-munity and its total biomass might be expected toincrease with increasing production in the over-lying water from which the food supply of thebenthos is derived (Lellak, 1965) .

However, other factors can upset such a simplerelationship . In multiple regression analyses relat-ing benthic standing stock or productivity todepth, area and total dissolved solids (a crudemeasure of fertility) of N . American lakes, dis-solved solids accounted for up to only 30 % of the

249

variability (Brinkhurst, 1974) . Johnson &Brinkhurst (1971) showed that in highlyeutrophicated waters, the quality of the foodmight decrease giving a decline in benthic produc-tion from a peak at intermediate stages of lakefertility . The refractory nature of the organicmatter derived from blue-green algae andallochthonous sources was implicated in this .

Furthermore, the nature of the benthic habitatmay also be important . Most macroinvertebratescannot grow under anaerobic conditions and indeep, stratified lakes, low hypolimnion tempera-tures may also restrict growth (Jonasson, 1978 ;Reynoldson, 1987). The structure of the sub-stratum is important also. Amorphous mudwould not support growth of a chironomidspecies in the middle of a shallow African lake but

250

the animal would grow adjacent to a Typhaswamp fringing the lake . Detritus from the swampplants provided materials used by the chironomidto construct burrows (McLachlan, 1974) . Thesedimentary benthos within beds of submergedaquatic plants is also generally much richer thanthat of bare mud. This may reflect the nature ofthe food supply or the provision of additionalniches by the roots and rhizomes and detritus ofthe plants .

Predation by fish may also be important . It hasbeen studied in plant beds where the plant-associated invertebrate community may be greatlychanged by the introduction of fish (Macan, 1966 ;Gilinsky, 1984 ; Hershey, 1985 ; Hall et al., 1970 ;Crowder & Cooper, 1982) and in weedy fishponds (Hayne & Ball, 1956) but the effects ofpredation on the benthic communities of baresediment have been studied much less frequently(Lellak, 1957 ; Hruska, 1961 ; Kajak, 1966) .

Overall some studies have found major effectsof fish (Kajak, 1966 ; Hall et al., 1970; Crowder &Cooper, 1982 ; Hershey, 1985) and others verylittle (Thorp, 1986 ; Thorp & Bergy, 1981 a, b) . Sihet al., (1985) have reviewed field experiments onthis and related topics .

This paper investigates the factors likely todetermine the numbers of macroinvertebrates inthe bare mud bottoms of a set of shallow lakes, theNorfolk Broads, in eastern England (Moss,1983). These lakes have been greatly eutrophi-cated in the past fifty years and an ecosystem oncedominated by submerged aquatic plants has beenlargely replaced by one dominated by phyto-plankton. Despite high production in the water,the benthic invertebrate fauna appeared extremelysparse when casual samples were taken. Thediversity of the community also was very low(Mason & Bryant, 1975). These two features can-not readily be explained by deoxygenation or thequality of the food supply because the system isriverine and well flushed . The overlying water isalways well oxygenated and palatable diatomsand flagellates dominate the phytoplankton formost of the growing season . The sediment is richin marl and amorphous, however, and the lack ofdiversity in Broads now lacking plants compared

with the few that still retain them may be relatedto a lack of structure . Artificial plastic plantsplaced in the water readily become colonised witha greater variety of animals than found in baresediment (Wortley, 1974) . However the scarcityof individuals in the apparently food-rich sedi-ment is not readily explained . It is the subject ofthis paper, which tests the hypothesis that the lackof structure in the sediment rather than the qualityof the food supply or the influence of fish preda-tion is responsible for the low numbers observed .

The study site

The Norfolk Broads (52 0 30'N, 1 0 30'E) areancient man-made lakes excavated as peat pitsbetween the ninth and fourteenth centuries andsubsequently flooded by rising water tables . Theylie amid extensive wetland vegetation (dominatedby Phragmites australis (Cav) Trin. ex Steud. orAlnus glutinosa L.) on the floors of five rivervalleys which ultimately converge in a commonestuary in the N . Sea. The lakes are flat-bottomedand of uniform depth. Since the fourteenthcentury about 1-1 .5 m of sediment have beendeposited in most of them, overlain by usually1-1 .5 m of water . The two Broads chosen for thisstudy were Hoveton Great Broad and HudsonsBay (Fig. 2). They are linked with one anotherand with the adjacent River Bure ; closeness to thesea means that there are small (5-10 cm) dailyfreshwater tides which promote mixing of thewater in the Broads with the effluent-rich riverwater. Fish may move freely between the Broadsand river and include bream (Abramis brama L.)and roach (Rutilus rutilis L.) . Total phosphorusconcentrations average 171 ±71 yg l - ' and phy-toplankton chlorophyll a concentrations are fre-quently between 100 and 200 pg 1 - ' . Centric dia-toms (Stephanodiscus hantzschii Grus., Cyclotellameneghiniana Kutz., Melosira spp) dominate thephytoplankton with moderate populations ofgreen algae and of blue green algae in late summer(Timms & Moss, 1984).

The two Broads differ in one major respect .Hudsons Bay retains an extensive stand of water

lilies (largely Nuphar lutea L. and with someNymphaea alba L.) whilst only a few patches oflilies remain in Hoveton Great Broad. Palaeo-limnological studies have shown that both Broadshad extensive stands of submerged plants untilthe 1950s (Moss, 1988) . Now submerged plants(other than the lily petioles) are virtually absent .Studies on the chemistry of the adjacent riverwater include Moss et al. (1984) and on the rela-tionship between phytoplankton and zooplank-ton, Timms & Moss (1984).

Methods

Samples of sediment were taken every two weeksfrom Hoveton Great Broad and Hudsons Baybetween April 1979 and March 1980 . A Petersendredge, which sampled an area of 730 cm 2 to adepth of about 15 cm in the fluid mud was usedand two random grabfuls constituted one sample .Three such samples were taken on each occasionfrom an area of about 2500 m2 in the bare mudcentres of each Broad . A synoptic survey usingfour sampling stations in Hudsons Bay and elevenin Hickling Broad was carried out on 11 February1980 using the same technique .

The samples were placed in polyethylene bagsand returned to the laboratory . The entire samplewas sieved through a screen of mesh size 0.63 mmand all macroinvertebrates picked out, examinedin a white tray, identified and counted live . Themesh size used is at the upper end of those usedin current studies (Jonasson, 1958 ; Bradt & Berg,1987 ; Lazim & Learner, 1986) but examination ofthe material washed through on several occasionsshowed that very few animals passed through it .Microscopic examination of oligochaetes foridentification was carried out on specimenscleared with lactophenol . Only the predominantChironomus plumosus L. was identified among thechironomid larvae . Consequently invertebratesfrom about sixty litres of sediment were sortedfrom each Broad on each occasion . Though thisstill gave high variability among samples in thisvery patchy community (Brinkhurst, 1974) it con-stitutes a compromise between time available and

25 1

statistical desirability that is at least as favourablydisposed to the latter as in most similar studies .The coefficient of variation (S .D . as% of themean) for a series of replicated samples was±90°/ .The organic content and carbonate content of

sediment taken during the synoptic survey wereanalysed gravimetrically after heating overnight at550 ° C and 950 ° C respectively in a mufflefurnace .

In May 1980, square polyethylene bowls of area860 cm2 annd depth 15 cm were three-quarters-filled with surface sediment which had been sievedfree of macroinvertebrates and allowed to re-settle . Ten such bowls were prepared for eachBroad with the sediment from that Broad in them .To each bowl was added 43 adult oligochaetes(approximately 1 :1 Limnodrilus hoffmeisteri andPotamothrix hammoniensis) . Five of the bowlswere covered with nylon netting of mesh size1 .5 mm stretched across their tops to excludevertebrate predators and five bowls were not soprotected . The bowls were weighted with stones,buried in the sediment so that their edges did notprotrude, and placed in the centre of the Broadsin the area from which the sediment was taken . Asmall buoy attached to a line was used to marktheir position . The bowls were recovered inOctober 1980, after being left undisturbed forabout five months . All the sediment was sievedand the number of oligochaetes determined .

Results

Community composition and seasonal changes

Eight taxa were recorded from Hoveton GreatBroad and fifteen from Hudsons Bay (Table 1) inthe seasonal sampling . In both Broads only threetaxa - the oligochaetes Limnodrilus hoffmeisteriand Potamothrix hammoniensis and chironomidlarvae, were consistently found and relativelyabundant .

Seasonal changes in numbers for the three rela-tively abundant taxa are shown in Fig. 1 .Chironomid larval numbers tended to be highest

252

Table 1 . Benthic macroinvertebrates recorded from Hoveton Great Broad and Hudsons Bay in nineteen sampling occasionsspaced over one year (1979/80) .

in autumn and winter, perhaps reflecting emer-gence and egg laying in summer and there weresignificantly more animals per unit area inHudsons Bay than in Hoveton Great Broad(Mann Whitney U test P < 0 .001). There was noparticular seasonal pattern for the two majoroligochaete species but again numbers were sig-nificantly greater (P < 0 .001) in Hudsons Baythan in Hoveton Great Broad .

Synoptic distribution

Figure 2 shows the distribution of chironomidlarvae and of oligochaetes on February 11, 1980in relation to sediment organic and carbonatecontents. The organic content was significantlyhigher (U test P < 0.05) in Hudsons Bay(z = 19.86 % S .D. ± 3 .1 %) than in HovetonGreat Broad (z = 15 .98 % S.D . ± 2.1 %). A

sample taken in a channel connecting HovetonGreat Broad and the river, and colonised by waterlilies, had a high organic content (23 .5%). Car-bonate content of the sediment (as CaCO 3 ) wasconversely significantly higher in Hoveton GreatBroad (53.1% ± 5 .6%) than in Hudsons Bay(44.6% ± 8.4%). The pattern of higher densitiesof both chironomid larvae and oligochaetes in thesediment of Hudsons Bay was clear, with highnumbers also in the lily-colonised dyke (station15) in Hoveton Great Broad .

Densities of chironomids were significantlycorrelated (data from both Broads) directly withsediment organic content (n = 15, r = 0.65,P < 0.01) and inversely with carbonate content(n = 15, r = - 0.65, P < 0.01). Oligochaete den-sities were also significantly inversely correlatedwith carbonate content (n = 15, r = - 0.7 1,P < 0.01) but although there was a direct relation-ship with organic content it was less significant

Hudsons Bay Hoveton Great Broad

Maximumpopulation(m

-2 )

Number ofoccasionsrecorded

Maximumpopulation(m -2 )

Number ofoccasionsrecorded

OligochaetesLimnodrilus hoffmeisteri Claparede 943 19 275 18Potamothrix hammoniensis (Michaelsen) 1465 19 160 18Potamothrix bavaricus (Oschmann) 5 1 5 1

InsectsChironomid larvae 558 19 298 19Ceratopogonidae 32 15 0 0Chaoborus sp 9 1 0 0Sialis lutaria Linn . 5 1 5 1Nematoda 137 9 5 2Hydracarina 18 8 18 1

LeechesHelobdella stagnalis (L.) 64 8 18 1Glossiphonia heteroclitus (Linn .) 9 1 0 0Piscicola geometrica (L.) 9 1 0 0

Bivalve molluscsAnodonta cygnaea (L .) 9 1 0 0

CrustaceansAsellus aquaticus (L .) 5 1 0 0Gammarus lacustris Sars 9 1 0 0

600

400

200

0

1000

0A

- -i '~,

•- •- 4

l-a----

Potamothnx HC' B

May 'June I July August ISept'berIOctober INovembrL-cem"anuary kbruaryIMarch

Fig. 1 . Seasonal distribution of numbers per m 2 of oligochaetes (lower) and chironomid larvae (upper) in Hoveton Great Broad(HGB) and Hudsons Bay (HUD).

than the others (n = 15, r = 0 .45, P > 0 .05 < 0.1) .The oligochaete relationships were conditionedlargely by Potamothnx hammoniensis (organicmatter, r = 0.63, P < 0.01, carbonate, r = - 0.7,P < 0.01) for insignificant relationships werefound in either case for Limnodrilus hoffmeisteri .

Exclosure experiment

Results of the exclosure experiment are shown inTable 2. There were major increases in numbersin all treatments, with covered bowls at the end ofthe experiment containing about two and a half

•

Chironomid larvae

-0

0•

o/H G B

0-

I

I

I0ligochaete worms

0

253

0- -

HUD °

times as many anmals as open ones . Numbers ofsmall animals were significantly greater (t test,P < 0.01) in the covered bowls than in the open

Table 2 . Results of an experiment in which the change inoligochaete populations over five months was measured insediment unprotected and protected from fish predation intwo Broads. Values are numbers of oligochaetes per m 2 .

I I

Hoveton Great Broad Hudsons Bay

Initial density(animals

m -2) 500 500Open bowls 5070 ± S.D. 1460 4450 ± S .D. 1240Covered bowls 11900 + S.D. 3080 12000 + S .D. 3650

254

1000E

A 500Ez 0

Organic (°/°)

Ca CO3 (°/°)

\ Oligochaetes

C h i ronomi ds

ones in both Broads but numbers in open bowlsdid not differ significantly between Broads, nordid those in covered bowls . Numbers recordedeven in open bowls were substantially greaterthan those ever found in the routine sampling ofthe two Broads . In addition, several taxa werefound associated with the bowls and the nettingin Hoveton Great Broad which had not beenrecorded in the routine sampling . These were :Asellus aquaticus, Piscicola geometrica, Dendro-coelum lacteum (Mull), Polycelis tenuis Ijima,Gammarus duebeni Lilljeborg, Eiseniella tetraedra(Savigny) and larvae of Limnephilidae (Tri-choptera) .

River BureFig. 2 . Distribution of oligochaetes, chironomid larvae, organic matter and calcium carbonate in the sediment of Hudsons Bayand Hoveton Great Broad in February 1980 . Means and range of the three samples taken at each station are shown forinvertebrate numbers . Stippling shows presence of lily beds during the previous summer . Values to the right of the histograms

are organic matter (upper) and calcium carbonate (lower) as percentages of dry weight .

Discussion

The benthic invertebrate populations in bothHoveton Great Broad and Hudsons Bay are loweven for the Norfolk Broads system, which overallhas restricted populations compared with otherlakes (Table 3) . All the benthic communities ofthe present day Norfolk Broads are dominated bytubificids and chironomids but a survey by Masonand Bryant (1975) revealed up to 27 taxa inBroads with abundant plants or harder substratacompared with the 8 found by us and 4 by Masonand Bryant in Hoveton Great Broad .

Three factors may explain the paucity in num-

05 km

Table 3 . Populations of oligochaetes and chironomids found in a range of lakes. Values are maximum populations recorded whereavailable .

bers of the benthic fauna in Hoveton Great Broadand Hudsons Bay : food availability; habitatstructure ; and predation . There is evidence for theimportance of the second and third but not thefirst of these .

Food availability is unlikely to determine thesize of the populations because populations ofoligochaetes which developed in the sediment inthe open bowls were much greater than those everfound in sampling of the Broad . Kajak (1966)found a similar increase in woodden cages enclos-ing sediment. Food sources in the sediment couldthus support much greater populations than are

Presence orabsence ofplants ator nearsample site

+

Reference

This studyThis study

-

Mason (1977)•

Mason (1977)

25 5

•

R.T. Leah & B . Moss un-published data

•

R.T. Leah & B . Moss un-published data

Lindegaard & Jonasson(1979)Deevey (1941)

Henson (1954)Mayer et al . (1978)Harr et al . (1980)

Thienemann (1925)-

Thienemann (1925)

presently found. There were nevertheless, rela-tionships between chironomid and oligochaetenumbers and sediment organic content whichmight suggest a dependence on availability oforganic matter for food. However organic mattermay also give a structure to the sediment whichfacilitates tube-building by animals or simply, byits fibrous nature, makes it firmer. The sedimentis relatively amorphous and fluid . Its surface iseasily disturbed by wind or the movement of fish .The inverse relationships between oligochaeteand chironomid populations and sediment car-bonate content is consistent with this. The car-

Lake Populationmeasure

Oligochaetes(m -2 )

Chironomids(m -2)

(a) NorfolkBroads

Hudsons Bay max 2151 558Hoveton Great max 353 298

BroadAlderfen Broad max 22500 704Upton Broad max 903 590

(Chironomus tentans)130.000

Martham SouthBroad

isolatedsampling 2000

(Tanytarsus)

6200

Hickling Broad max 222 1096

(b) Other lakesMyvatn, Iceland mean 6845 63260

Linsley Pond, max 23000U.S.A .

Cayuga, U .S.A . max 9800Chautaqua, U .S.A. max 13000 5000Canadarago, U .S.A . max 12000 1250

(c) GeneralClassification

Oligotrophic range 300-1000Eutrophic range 2000-10000

256

bonate is in the form of a fine precipitate producedby intense photosynthesis by the phytoplanktonin the overlying calcareous water and the greaterthe carbonate content the smoother the sediment .Carbonate content is independent of organic con-tent for there is a third component to the sediment- inorganic siliceous material, so the relationshipcannot be explained simply as the reciprocal ofthat with organic matter .

A greater stability to the sediment was probablygiven by placing it in bowls which restricted theeffects of disturbance on it and this perhapsexplains the increase in populations in the openbowls by over fourteenfold (over the maximumfound in the Broad) in Hoveton Great Broad,compared with only 2.1 fold in the more organicand structured sediment of Hudsons Bay .

The bowls themselves clearly provided a habi-tat structure for animals which were otherwise notfound by routine sampling in the Broad (see alsoKajak, 1966). Provision of artificial substrata asplant substitutes has been shown to increasegreatly the diversity of benthic invertebrates in theBroads (Wortley, 1974) but the plant-associatedbenthos should not be confused with the sedimen-tary benthos. We are here concerned with thelatter .

Protection from predation in the covered bowlsgave similar further increases in population by 2 .7fold in Hudsons Bay and 2 .3 fold in HovetonGreat Broad over the values found in open bowls .This contrasts with the differential effectsbetween the Broads of simply placing the sedi-ment in open bowls and adds credence to theargument that sediment stability is particularlyimportant .

There is much evidence for intense predationby fish on zooplankton in the Broads, evidencedby communities dominated by rotifers, smallCladocera and copepods, rather than by largeCladocera (Leah, 1978 ; Moss, 1983), but theimpact of benthic predation is less well estab-lished ; Loss of the aquatic plants througheutrophication has been associated with a declin-ing diversity of fish community and a failure ofroach to live much beyond 3 or 4 years (Mosset al., 1979). This is attributed to a shortage of

large plant-associated invertebrates as food forthe larger fish. This might suggest a transfer ofpredation pressure by adult fish from the plant-beds to the only available large food items in thesedimentary benthos . Bream, in particular, com-monly forage in the sediment and with roach formthe bulk of the fish community .

Conclusions

We conclude therefore that food availability isunlikely to determine the small size of the benthicinvertebrate populations in the Broads and thatsediment stability, as imparted by pieces of plant-derived organic matter is the most important fac-tor, followed by the impact of fish predation,where organic fragments impart some structure tothe sediment. The impact of predation may haveincreased as a result of loss of the invertebratecommunities associated with submerged plantsfrom these lakes as they have become hyper-eutrophicated .

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