Bass, J.A.B.

Pinder, L.C.V.

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( 1-; i; /f70Institute ofFreshwaterEcologyNatural Environment Research CounciláSTRATEGIC ECOSYSTEM STUDIES OF LARGESLOW FLOWING, LOWLAND RIVERSProgress Report: April 1990 to October 1990LCVPinder &JABBassINSTITUTE OF FRESHWATER ECOLOGYEastern Rivers LaboratoryMonks Wood Experimental StationAbbots RiptonHuntingdonCambs PEI7 2L5Project Leader L C V PinderReport Date: October 1990Report to NRAIFE Report Number ERG/T04052b1/3TFS Project Number T04052b1This is an umpublished report and should not be citedwithout permission, which should be sought through theDirector of The Institute of Freshwater Ecology in the firstinstance.The Institute of Freshwater ecology is part of theTerrestrial and Freshwater Sciences Directorate of theNatural Environmnet Research Council.á1. IntroductionThis interim report summarises some of the data relating tozooplankton and epiphytic invertebrates and their macrophytesubstrata that were obtained from the Great Ouse during 1969 and1990. Data on algal periphyton and phytoplankton, as well aslarval and juvenile fish and their diets have been obtained overa similar period. Research on turbidity, its causes and impacton growth of algae and macrophytes is the subject of a separatereport. The programme of research on fish populationsis not funded by NRA, but the important conclusions from thisstudy, in so far as they are relevant to the 2 NRA funded (orpart funded) projects will be incorporated into the final reporton "Strategic ecosystem studies" which is due in April 1991.The preceding progress report in this series, which covered theperiod from October, 1989 to April 1990, presented some of thedata arising from the 1989 sampling programme. During 1989samples of zooplankton were taken from 5 sites. Three of thesewere on the main river, above and below Godmanchester lock andfurther downstream close to the "Pike and Eel" public house nearNeedingworth. The fourth site was on Lees Brook, a back channelclose to the upstream Godmanchester main river site, and thefifth was the "Pike and Eel" marina at Needingworth. Samples ofthe fauna associated with leaves of the yellow water lily, Nupharlutea were also taken at all sites except for the marina. In 1990sampling of N.lutea has been confined to Lees Brook and theadjacent main river, while plankton sampling has continued atthese 2 sites and in the marina.2. ResultsThe following are examples, only, of the data that are beingaccumulated and which will be reported on fully in the finalreport on this project which is due in April of next year. Thatreport will also relate the results of this study to those of theother 2 main projects being carried out concurrently.Surveys of channel profiles, bottom deposits and macrophytedistribution at several sites on the Great Ouse will be reportedin some detail in the final report of the related project onturbidity and plant growth in the river. However some of theresults of these surveys are relevant in a broader ecologicalcontext and are therefore briefly reported below.In the Great Ouse the main channel is too deep and the water tooturbid to permit much primary production to take place on mostof the river bed where there is insufficient light for growth ofeither macrophytes or attached algae. Consequently, higher plantsthat are able to erow in marginal and other shallower areas arepotentially very important substrata for the development ofattached algal populations and invertebrates. They are also ofimportance in providing refugia for fish, especially larval andjuvenile fish, from high current velocities. However, because ofthe needs of land drainage and navigation, growth of macrophytesis often very restricted by the shape of the channelised riverbed and by dredging and weed—cutting operations.Mapping of macrophyte distribution was carried out by meansof a series of transects on 5 occasions between September, 19882and September 1990, on 3 main river reaches and 3 side—channelreaches. In the main river transects were spaced at 20 mintervals and in side channels at 10 m intervals. Presence ofmacrophyte species was recorded at 1 m intervals along eachtransect.In terms of the extent of river bed occupied, Nuphar lutea,the Yellow water lily, is the most important species ofmacrophyte in the Great Ouse system and it is also common inother large lowland rivers. This plant produces 2 types of leaf;rather thick, rigid, floating leaves and thin, delicate submergedleaves. In the Great Ouse, although the floating leaves are mostconspicuous it is the submerged leaves that provide the greatestsurface area for the development of periphyton. In situationswhere visibility was restricted by the depth and / or turbidityof the water the presence or absence of submerged leaves wasdetermined by using a grapnel.The growing season of N. lutea extends from April to Octoberand area of cover was assessed on 4 sampling dates from September1988 to September 1990 (Figure 1) The late summer, early autumndates represent the period of maximum development of this plant.These indicate an increase in the area covered by N. lutea fromabout 5% in 1988 to approaching 10% in 1990. It seems likely thatthe severe floods of the winter of 1987/88 dislodged a proportionof the rhizomes of this plant with a gradual expansion in arearesulting from the much more stable conditions that pertainedduring the ensuing very dry years.The percentage cover of Elodea in the main river atGodmanchester was assessed over the same period of time (Figure32). This species did not occur at all in Lees Brook where thereare no suitable shallow areas for it to develop. However, anextensive shallow area of the main river near Godmanchester wasextensively occupied by this and a number of other submergedmacrophyte species in 1989. Up to 30% of the river area in thisparticular reach was occupied by Elodea. Such an extensiveshallow region is unusual in the middle reaches of the main GreatOuse, although some of the shallower side streams, such asCooke's Backwater also have the potential to support an extensiveand diverse submerged flora. The lack of Elodea in 1988 isprobably attributable to the relatively high current velocitiesduring the spring and preceding winter which would haveextensively scoured the soft marginal sediments in which thisspecies has its roots. The mild winter and low discharge regimeof the following year, coupled with the relatively clear waterthat prevailed over much of 1989, would on the other hand havefavoured the development of submerged species. The absence of anyextensive development of submerged plants in 1990 is morepuzzling, but is probably related to the much more turbidconditions in the spring and early summer, caused by highphytoplankton numbers that persisted for a much longer time thanwas the case in 1989.Large numbers of larval and juvenile fish were associated withElodea during the period when it was abundant and the lack ofthis plant and other submerged plants that provide abundantshelter for both young fish and their food organisms is likelyto be an important factor influencing early fish survival.42—During 1989 the fauna of N. lutea was investigated at the 4 sitesdescribed above. Differences between sites were relatively slightand in 1990 only Lees Brook and the adjacent main river wereinvestigated.Leaf surface area varied substantially over the year and therewere also significant differences in leaf size between sites.This may be seen by reference to Figures 3 and 4 whichrespectively show the mean area of sampled leaves during 1989 forLees Brook and the adjacent Main River. Whereas, in the mainriver there was a steady decline in mean leaf area over thesampling period, the mean leaf area in Lees Brook tended toincrease during the early part of the growing season and declinedthereafter.The principal components of the fauna of Nuphar leaves areCopepoda, Cladocera and Chironomidae, all of which are importantconstituents of the diet of juvenile cyprinids. Rot fera are alsocommon but were not encountered in most of the samples that havebeen analysed since only the fauna reta ned by a sieve of 250 umhas been examined so far. Other groups of invertebrates thatoccur more sporadically or in low numbers include Gastropoda,Naididae and Hydra.As an illustration of the data that will be presented fullyin the final report Figures 5 and 6 respectively compare thenumbers of Cladocera and chironomid larvae associated withN./utea during 1989 and 1990 in the main River Ouse atGodmanchester.In 1989 similarly low numbers of Cladocera were recorded on5N. lutea at all of the sites that were investigated. The muchhigher densities in 1990 were also evident in Lees Brook. Asimilar contrast between the 2 years was also shown in thenumbers of Copepoda on this plant, but, as Figure 6 indicateschironomid numbers were similar in the 2 years.Both the main river at Gedmanchester and Lees Brook had peaknumbers of Cladocera in early summer and in autumn of 1989whereas in 1990 there was an extended period, during whichnumbers were 1 or 2 orders of magnitude greater than in theprevious year. Although numbers were also higher in the planktonthese were of different species. Bosmina and Daphnia predominatedin the plankton samples while on the lily leaves the dominantgenera were Chydorus and Pleuroxus.The peak in numbers of chironomids in 1989 was recorded betweenJune and mid-July, in the main river and Lees Brook respectively.These were followed in the autumn by a further rise in numbersin both situations. In the following year there were 2 welldefined peaks in numbers in both the main river and Lees Brook,during weeks at the end of May and end of June respectively.Numbers were generally higher in the main river than in LeesBrook.The zooplankton is numerically dominated by Rotifera, withrelatively low numbers of Copepoda and Cladocera. The low numberof Rotifera in 1989 is in marked contrast to the relatively highdensities that persisted for much of the spring and early summerof 1990 (Figure 7). The low densities shown in Figure 7 wererepeated at all of the river sites.6In terms of weather, 1989 and 1990 were very similar. Both werewarm and dry with discharge continuously low from early in 1989through to the summer of 1990. The marked differences between the2 years in populations of certain plants and animals was nottherefore expected.Rotifers are the dominant item in the diet of recentlyhatched roach. During the early days after hatching only verysmall organisms are capable of being ingested and it is possiblethat the availability of rotifers at this critical time is animportant factor influencing their early growth and survival.Investigations on other larval cyprinids, carried out inhatcheries has indicated that maximum intake of rotifers onlyoccurs at densities greater than about 1500 l. At densitiesless than this consumption rapidly declines. In 1989 densitiesonly exceeded this, apparently critical, level for a very shortperiod. This coincided with the time when roach were hatching butbefore the appearance of young bream. In 1990 densities ofrotifers were above this critical density for most of the latespring and early summer. It is interesting to note that in 1989numbers of young bream were found with empty guts, perhapsreflecting a scarcity of suitable food items.Dietary studies of young roach in the Great Ouse alsoindicate a shift in feeding behaviour later in the summer frompredominantly plankton feeding to grazing of aufwuchs, withrotifers becoming less important and Cladocera, Copepoda andother invertebrates becoming correspondingly more important. Themuch greater abundance of Copepoda and Cladocera on N.lutea in71990 may also have aided survival during the transitional periodbetween reliance on very small planktonic animals and progressionto feeding on macro—invertebrates such as Chironomidae. Iffeeding conditions were limiting in 1989 it would be predictedthat 1990 will have been a better year for cyprinid, especiallybream, recruitment in the Great Ouse than 1989 in spite of bothbeing very warm years with little disruption of populationsthrough periodically high current velocities.8— da5 --• tie; A2c779.tiM1;444,40724.",j7tS..t. C:4 ;:r •14 •M1PHARCOWL(1988-1990)MOORS21100E%NupharGrowingSeason:Apri1-Oct,Pith aFigure 1Proport on of river bed in the Great Ouse at Godmanchester andin Lees Brook covered by Nuphar lutea in September 1988, 1989 &1990.El_CIIDECI COVER . C19813-1990>RIVER OUSE. GOUttANCHESTER42ot12 Mayxt4 88 JAN 89 JAM 90 DEC 90El ociea i n.91 Season Cti3 r- i 1-0e .rj main [I] leesFigure 2Proportion of river bed in the Great Ouse at GodmanchesterOccupied by Elodea in 1988, 1989 & 1990.LEAFAREA.1AKNS ( Kay13-Mov4 )Mm lbaacatasits 4 becatIlNitFigure 3Mean surface area of sampled Nuphar lutea leaves from Lees Brook,with 95% confidence limits.Godnanchester(Hain)Lein131101;3)!DUNI: 10 31 11 11 C 11Weekno,( Kay13-Kov4)190911PamFigure 4Mean surface area of sampled Nuphar lutea leaves from the mainriver at Godmanchester, with 95% confidence limits.- -* 01/0— . 1910It11- 11•••;2 gmFigure 5Mean density of Cladocera on N.lutealeaves from the main riverat Godmanchester in 1989 and 1990.x- -X 191190-0 1190SOOI n 5 0Figure 630 0X--"tIrMean density of Chironomidae on N.lutea leaves from the mainriver at Godmanchester in 1989 and 1990.. ••Figure 7Mean number of Rotifera per litre of river water from the mainriver at Godmanchester in 1989 and 1990.A NaturalEnvironmentResearchCouncil

Strategic ecosystem studies of large slow flowing lowland rivers. Progress report: April - October 1990

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Strategic ecosystem studies of large slow flowing lowland rivers. Progress report: April - October 1990

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