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Hydrobiologia 470: 23–30, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 23 Aquatic food web dynamics on a floodplain in the Okavango delta, Botswana P. Høberg 1 , M. Lindholm 1 , L. Ramberg 2 & D.O. Hessen 1,1 University of Oslo, Department of Biology, P.O. Box 1027 Blindern, 0316 Oslo, Norway 2 Harry Oppenheimer Okavango Research Centre, University of Botswana, P/Bag 285, Maun, Botswana Corresponding author. E-mail: [email protected] Received 1 May 2001; in revised form 30 October 2001; accepted 15 November 2001 Key words: Okavango, flood dynamics, nutrients, plankton, fish, food web, floodplain, planktivory, detritus Abstract The study presents the succession of food web compartments during the annual flood of a floodplain of the great inland delta, Okavango, Botswana, and emphasizes how the floodplains serve as key recruitment areas for fish. By onset of the flood, the rather nutrient poor water from the main river becomes strongly enriched by inorganic nutrients and organic debris as it flushes over the savannah. At the chosen site, peak concentrations of nitrogen and phosphorus were 4 mg and 560 µgl 1 , respectively. The high concentrations of nutrients yielded an immediate burst in primary production and phytoplankton standing stock, being almost 300 µgCl 1 d 1 and 24 µg Chl. a l 1 respectively at the maximum. When submersed by water, there was apparently a succession in hatching of zooplankton resting eggs present as a ‘egg-bank’. Peak concentrations of zooplankton reached 10 mg DW l 1 , mostly of cladocerans Moina and Ceriodaphnia, with extreme near-shore concentrations of 90 g DW l 1 . The zooplankton was probably nutritionally subsidized by detritus. During the flood a number of fish species moved in and spawned in the shallow waters. The fish fry fed on a variety of zooplankton species, before the water level again receded two months later. Thus the annual flood creates a highly dynamic shallow water system with high productivity that supports fish recruitment in the delta. The study also emphasizes the strong links between aquatic and terrestrial production in the delta. Introduction Delta areas, where annual flooding provides water, nutrients and organic carbon to adjacent banks and land areas, are among the worlds’ most productive ecosystems. For inland deltas or perennial wetlands, the annual flood may feed floodplains and temporal water bodies that may serve as important habitats for biodiversity and production (Kalk et al., 1979; Wel- comme, 1979; Dumont, 1992). In these areas there may be a ‘payback’ from terrestrial to aquatic systems via release of nutrients from soil, detritus and faeces (McLachlan, 1971; Thornton, 1986; Talling, 1992), and organic carbon from soil and debris (Shepherd, 1976; Engle & Melack, 1993). One of the world’s major inland deltas is the Okavango delta that cov- ers 25 000 km 2 of the north-western part of Botswana. The water dynamics are governed by the annual flood that is generated by the seasonal rainfalls in the An- gola highlands (November–March). The water masses move slowly down the Okavango river, ending 4–7 months later (May–October) in the semi-arid regions of the Kalahari. Here the river vanishes in the large inland delta, where vast areas are temporarily flooded by shallow water. Such floodplain areas probably represent key hab- itats for the water-land ecotons, and are of profound importance for the fish production (Welcomme, 1979). Different fish-species, particularly catfish, barbs and different tilapias, are common in floodplains, that probably serve as spawning and recruitment areas. In total, 80 different species of fish have been recorded in the delta (Merron, 1991), and annual fish pro- duction has been roughly estimated to some 10 000

Aquatic food web dynamics on a floodplain in the Okavango delta, Botswana

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Hydrobiologia 470: 23–30, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

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Aquatic food web dynamics on a floodplain in the Okavango delta,Botswana

P. Høberg1, M. Lindholm1, L. Ramberg2 & D.O. Hessen1,∗1University of Oslo, Department of Biology, P.O. Box 1027 Blindern, 0316 Oslo, Norway2Harry Oppenheimer Okavango Research Centre, University of Botswana, P/Bag 285, Maun, Botswana∗Corresponding author. E-mail: [email protected]

Received 1 May 2001; in revised form 30 October 2001; accepted 15 November 2001

Key words: Okavango, flood dynamics, nutrients, plankton, fish, food web, floodplain, planktivory, detritus

Abstract

The study presents the succession of food web compartments during the annual flood of a floodplain of the greatinland delta, Okavango, Botswana, and emphasizes how the floodplains serve as key recruitment areas for fish.By onset of the flood, the rather nutrient poor water from the main river becomes strongly enriched by inorganicnutrients and organic debris as it flushes over the savannah. At the chosen site, peak concentrations of nitrogen andphosphorus were 4 mg and 560 µg l−1, respectively. The high concentrations of nutrients yielded an immediateburst in primary production and phytoplankton standing stock, being almost 300 µg C l−1 d−1 and 24 µg Chl.a l−1 respectively at the maximum. When submersed by water, there was apparently a succession in hatching ofzooplankton resting eggs present as a ‘egg-bank’. Peak concentrations of zooplankton reached 10 mg DW l−1,mostly of cladocerans Moina and Ceriodaphnia, with extreme near-shore concentrations of ∼90 g DW l−1. Thezooplankton was probably nutritionally subsidized by detritus. During the flood a number of fish species movedin and spawned in the shallow waters. The fish fry fed on a variety of zooplankton species, before the water levelagain receded two months later. Thus the annual flood creates a highly dynamic shallow water system with highproductivity that supports fish recruitment in the delta. The study also emphasizes the strong links between aquaticand terrestrial production in the delta.

Introduction

Delta areas, where annual flooding provides water,nutrients and organic carbon to adjacent banks andland areas, are among the worlds’ most productiveecosystems. For inland deltas or perennial wetlands,the annual flood may feed floodplains and temporalwater bodies that may serve as important habitats forbiodiversity and production (Kalk et al., 1979; Wel-comme, 1979; Dumont, 1992). In these areas theremay be a ‘payback’ from terrestrial to aquatic systemsvia release of nutrients from soil, detritus and faeces(McLachlan, 1971; Thornton, 1986; Talling, 1992),and organic carbon from soil and debris (Shepherd,1976; Engle & Melack, 1993). One of the world’smajor inland deltas is the Okavango delta that cov-ers 25 000 km2 of the north-western part of Botswana.

The water dynamics are governed by the annual floodthat is generated by the seasonal rainfalls in the An-gola highlands (November–March). The water massesmove slowly down the Okavango river, ending 4–7months later (May–October) in the semi-arid regionsof the Kalahari. Here the river vanishes in the largeinland delta, where vast areas are temporarily floodedby shallow water.

Such floodplain areas probably represent key hab-itats for the water-land ecotons, and are of profoundimportance for the fish production (Welcomme, 1979).Different fish-species, particularly catfish, barbs anddifferent tilapias, are common in floodplains, thatprobably serve as spawning and recruitment areas. Intotal, 80 different species of fish have been recordedin the delta (Merron, 1991), and annual fish pro-duction has been roughly estimated to some 10 000

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tonnes, with inter-annual fluctuations that seem correl-ated with the flooding regime (Merron, 1991). Kolding(1996) estimated an average total fish biomass of 50kg ha−1. The fish production serves as an importantfood source for other food web compartments suchas water-birds, crocodiles, otters – and humans. Byreceding flood the savannah turns into highly product-ive grasslands that support dense and diverse stocks ofmammalian grazers, notably ungulates, and their pred-ators. Vice versa, when the rather nutrient-poor waterfrom the main river system (Cronberg et al., 1996)re-enters the floodplains, it may be enriched withdissolved nutrients that may boost autotrophic andsubsequently heterotrophic production (McLachlan,1971; Sheperd, 1976).

There is great inter-annual variability in the floodsize and velocity. To this add a number of anthropo-genic factors that severely could impact the annualflood and thus the entire ecosystem. For understand-ing the overall role of the flood and the floodplainsfor ecosystem productivity and fish yield, intimateknowledge of the flow of nutrients and energy throughthe lower food web compartments is a prerequis-ite. In the present study we attempted to resolvethe basic bottom-up food web dynamics from inor-ganic nutrients to primary production, zooplankton toplanktivorous fish fry on a floodplain.

Materials and methods

The studied floodplain is located in Moremi GameReserve, on the southwestern side of Chief Island. Itconsists of an approximately 1000 m long and 50–300 m wide ground depression, probably a part ofan old river channel. The area is connected to – andfed by – the Boro river, one of the three main riverchannels through the delta. The central part of thefloodplain is dominated by tall stands of sedge andreed, which in dryer areas change to open grassland,dominated by different Poaceae. The plain is surroun-ded by dry forest savannah, dominated by salt-tolerantpalms (Hyphaene petersiana), thicket, and groves ofdifferent acacias (Acacia spp.). Before the arrival ofthe flood the floodplain was covered by thick layersof dry sedge stands and grassy detritus, produced afterthe previous flood season.

Sampling was performed in the period June toSeptember 1998. The maximum extension of thefloodplain was 15 ha (max depth 100 cm). Extensiveflooding started on June 10, and reached maximum

water level on 25 June, before again slowly reced-ing through August. In the beginning of Septemberthe plain dried up, and turned to a rich grassland,intensively grazed by different ungulates. Samplingwas performed during the whole flood season, fromJune 16, to August 20. Seasonal measurements werecarried out on a weekly basis, at a fixed point onthe central part of the floodplain, and at fixed time(10:00). For zooplankton samples we defined a furthersampling point in shallow (<25 cm depth) water, closeto the shoreline. Due to climate and the shallow waterbody, great diurnal variance was probable for severalparameters. Three diurnal measurements (on a 4-h fre-quency) were carried out to investigate these changes(on 19 June, 24 July and 19 August). The follow-ing variables were examined: oxygen, temperature,pH, total nitrogen, total phosphorus, calcium, silica,chlorophyll a, primary production, zooplankton (qual-itatively and quantitatively) and fish (qualitatively andquantitatively). To test for the representativity of thesampling location, a transect with five sampling pointswas drawn from the inlet close to Boro river, andacross the floodplain to the innermost part. Transectmeasurements were carried out on 18 and 27 June.The results indicated that the ordinary values wererepresentative for the system in general.

Oxygen, temperature, and pH were measured insitu: oxygen and temperature with an electrode (Met-tler Toledo MO 128), and pH with EDT RE 357pH-meter. Due to the fact that the water level wastoo shallow for the use of an ordinary water collector(Ruttner sampler), all water samples were carried outusing a 10-l bucket. All samples were transported tothe Oppenheimer Research Centre in Maun and storedcold until air-transport to University of Oslo for ana-lysis. As storage before analysis may have obscured afractionated analysis of nutrients, chemical parameterswere analyzed on total pools only, basically followingGolterman et al. (1978). Total nitrogen was analyzedby the use of Flow Injection Analyzer 5020 afterpotassium-peroxydisulfate digestion, total phosphoruswas analyzed spectrophotometrically by the standardammonium-molybdate method after persulfate diges-tion, while calcium (mg l−1) was analyzed using aSpectr AA-10 Varian flame-spectrometer. Silica (mg(SiO2) – Si l−1) was analyzed spectrophotometricallyafter complexation with molybdate.

Samples for chlorophyll a were filtered through aMillipore GS 0.22-µm filter, and the filters were driedand kept cold. In the laboratory the filtrate was ex-tracted using 100% methanol, and the absorbance was

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measured at a wavelength of 665 nm, using a TurnerDesign fluorometer. Pelagic primary production wasmeasured four times in June, using the 14C-method(Vollenweider, 1969). 150 ml glass and dark bottleswith [14C] bicarbonate were placed at 45 cm depthand close to the bottom (ca 80 cm depth), and incub-ated in triplicate for 3 hours from noon. Due to theshallow depths, we here report this as an average oftwo depths. Incubation was terminated by filtering onGF/F-filters that were put in 20-ml scintillation vials.Ultima Gold was used as scintillation fluid, and count-ing was carried out on a Packard 1500 Tri-Carb LiquidScintillation Analyzer.

For zooplankton analysis, a 5-l sub-sample wasfiltered through a 95-µm filter and fixed with acidlugol. To determine the effect of the sampling methodon the results, parallel measurements with Ruttner col-lector and bucket were performed when water depthwas sufficient. There were no significant differencesbetween the two methods. Zooplankton was identi-fied and counted in counting trays, using a Leica MZ8 stereomicroscope. Zooplankton biomass was estim-ated from published length-to-dry weight regressions(Dumont et al. 1975). For species not covered in thisreview, we applied conversion factors for similar sizedand shaped taxa. Rotifers were abundant at periods,but was not included in this study since the main focuswas devoted the potential role of zooplankton in fishnutrition.

Qualitative fish samples were carried out in Juneusing standard sampling gill nets, and later handnet for fish fry in particular. In August quantitativefish fry biomass was estimated, using a circular netring, 133 cm diameter confining a cylindrical volume.Sampling was performed at 10 different locations onthe floodplain, and all the fry within the confinedspace was caught and fixed, and the water volume wascalculated for each sampling location.

Results

In June the water covered increasing areas of the sa-vannah, gradually turning it into shallow lakes andfloodplains. The flood in the river itself culminatedon 14 June, whereas maximum water level on thefloodplain was reached on 25 June. Through Julythe flood receded, and in August the water inflowwas negligible, with dynamics governed by internalfactors.

Figure 1. Seasonal development of total nitrogen and total phos-phorus concentrations in the central part of the floodplain.

The floodplain showed marked differences in bothbiotic and abiotic parameters through the flood season.High concentrations of both nitrogen and phosphoruswere measured during rising flood in June, with3.1 mg l−1 total N and 466 µg l−1 total P, respectivelyfor the central part of the floodplain (Figure 1). Thetwo transects revealed a moderate decrease in concen-tration for both nutrients over the floodplain. Highestvalues were found close to the river (3 and 4 mg l−1

total N, and 510 and 560 µg l−1 total P, respectively,on the two different measuring days), and lowest onthe innermost parts of the system (2.5 and 3.5 mgl−1 total N, and 420 and 480 µg l−1 total P respect-ively, on the two measuring days). Through July andAugust the concentration of N decreased gradually toless than half of the initial concentration, whereas theP-content was reduced to < 150 µg l−1. The N:P ra-tios (by weight) through the season showed a slightincrease from 6.8 (June) to 11.7 (August). There wasa positive correlation between total N and total P (r2=0.91), suggesting the same cause for depletion of thesenutrients.

Although the concentration of phosphorus did sup-port a high primary production and high standingstocks of phytoplankton biomass, this was not reflec-ted in the oxygen concentrations. Even at noon verylow oxygen levels were measured, especially at thefirst part of the flood. In late June and early July,only 38 and 22% saturation was found, respectively,rising to 54% at July 24. The system displayed strongdiurnal variations in oxygen concentrations however,from 14% at sunrise to 90% saturation at sunset atthe maximum. The general low saturation reflected thedominance of heterotrophic processes, obviously dueto microbial breakdown of large amounts of terrestrial

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Figure 2. Chlorophyll a and primary production during the peakproduction during the early flood. Primary production is based onthe average of two depths, and give on an area basis.

detritus (dry sedge and grass) that became prone tomicrobial breakdown when the area was flooded. pHand calcium concentrations did not show significantvariations. Concentration of Si dropped from 11.2 mgl−1 to 2.1 mg l−1 during the first weeks, thereafterstabilizing around 1-2 mg l−1 throughout rest of theflood.

Samples for chlorophyll a and primary productionwere only taken in June (Fig. 2). Chlorophyll a con-tent increased rapidly during the first week (from 16 to22 June) from 2.6 to 23.5 µg l−1, thereafter recedingto 10 µg l−1. Primary production was measured fourtimes, and displayed a pattern corresponding to that ofchlorophyll, starting with 63 µg C l−1 day−1 at theonset of the flood, thereafter increasing to 264 µg Cl−1 day−1 within a week (25 June) before a gradualdecrease to 82 µg C l−1 day−1 by the end of themonth.

The zooplankton biomass also experienced strongfluctuations over the flooding season, with a peak bio-mass approaching 10 mg DW l−1in mid June, anda strong decline from July towards almost extinctionin mid August and then a recovery to 0.1 mg DWl−1 in late August (Fig. 3). A high number of clado-ceran species were identified during the flood, withAlona affinis, Ceriodaphnia quadrangula, Chydorussp., Daphnia laevis, Macrothtrix sp., Moina micruraand Simocephalus vetulus as the most common spe-cies, but also Bosmina longirostris, Ceriodaphniadubia, C. rotunda and Diaphanosoma sp. occurredregularly. Copepods and Ostracods were common inperiods, but were not further identified.

The zooplankton showed a pronounced speciessuccession during the flood. By rising flood in mid

Figure 3. Log zooplankton biomass concentration over the season.

June the system was totally dominated by Moinamicrura (805 ind l−1 on 17 June), to a lesser de-gree also by Ceriodaphnia spp. (60 ind l−1 on June17), which together made a total mass of 6700 µgDW l−1. Extreme concentrations were observed alongthe shoreline, which during the first measuring weekslowly moved inland. This zone showed a 10–30 cmbroad girdle, where a mass of M. micrura built aridge, which even rose above the water surface. Meas-urements from this zone showed ∼90 g DW l−1

zooplankton biomass – it was even possible to noticethe characteristic smell of crustaceans. Nine days later,as the flood already had culminated, the zooplanktonbiomass showed no unusual concentrations along theshoreline.

The Ceriodaphnia population peaked at the endof June, but the density dropped more gradually thanMoina. Medio July the water column still containedalmost 500 ind. l−1 close to the shoreline. Both spe-cies were of minute size, on average 0.65 mm (Moina)and 0.48 mm (Ceriodaphnia). A later peak was dom-inated by Daphnia laevis, starting in the beginning ofJuly, culminating after 2 weeks (114 ind l−1) and thendecreasing to <10 ind l−1 later through the season.The species showed increasingly cyclomorphosis sim-ultaneously with increasing presence of juvenile fish.The flood season ended with a burst of small (0.23–0.35 mm) Chydoridae, dominated by Chydorus sp.(probably C. sphaericus), reaching 311 ind l−1 at theend of the flood. The other species mentioned wereonly sporadically observed in the samples.

A simple test was performed to check for the pres-ence and hatching of resting eggs from the sediment,since the rapid burst of zooplankton could indicate acrucial role of such a seed-bank for recruitment and

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succession. Sediment samples from September (afterended flood) contained high numbers of resting eggs(up to 46 ephippia g−1 of soil). Tests by adding wa-ter to dry soil caused an almost immediate burst ofa diverse zooplankton community, and interestinglythe hatching succession followed a similar pattern tothat recorded from the field samples, with an im-mediate hatching of Moina (over the first 5–8 days),followed by Ceriodaphnia. Single Simocephalus vetu-lus and Daphnia laevis were also found, but withoutany clear pattern of occurrence. Chydorus sp. was thelast species in this succession.

In June several different fish species followed therising flood into the area. Only qualitative sampleswere carried out in this phase of the flood cycle.Tilapias (Oreochromis andersonii, Tilapia rendalli, T.sparrmanni), catfish (Clarias gariepinus) and differ-ent barbs (Barbus barnardi, B. bifrenatus) were thedominant groups. During July and August juvenilescould be observed with increasing frequency in dif-ferent parts of the floodplain. A quantitative estimatecarried out in August indicated a total biomass of 0.3 gDW fish fry m−3. All captured fry were <5.0 cm.

Gut analysis of fish fry was carried out for 180 in-dividuals, all of which had a total body length between7.5 and 79 mm. Gut content was categorized into fourdifferent groups: detritus/unidentified; algae/pollengrains; insects; and zooplankton. The fraction of de-tritus, algae and pollen grains increased with bodylength, whereas the fraction of zooplankton showed anopposite trend. Zooplankton content related to body-length showed a significant correlation. Fry <20 mmcontained a high fraction of zooplankton, whereas lar-ger individuals apparently switched to herbaceous ordetritus dominated diets (Fig. 4).

The selectivity index of Ivlev (1961) was used toidentify possible dietary preferences for the four dom-inant cladoceran species (Alona affinis, Ceriodaphniaspp., Macrothrix sp. and Simocephalus vetulus) fromsix different samples of fish fry (Fig. 5). All six series,covering a range in fish size and species, showed apreference for Macrotrix sp., whereas the other threecladocerans were all negatively selected for. Some-what surprisingly, pronounced negative preferencewas found for the largest cladoceran in the system(S. vetulus). This may be due to the conspicuoustransparency of this species.

At the end of the flood season, as the water bodywas reduced to a few isolated pools, hardly any fishfry were observed. Obviously they migrate out of

Figure 4. Percent of zooplankton in the diet related to total lengthof fish fry.

the floodplain into the main river system before theconnection with the river is closed.

Discussion

The studies strongly underpinned the major impact ofthe flood on the aquatic productivity in the delta. Thewater from the main river is characterized by moderatelevels of nutrients (by raising flood 0.9 mg l−1 totalN, 44 µg l−1 total P; cf. Cronberg et al., 1996) andthus low primary production. When the water entersthe floodplain it becomes strongly enriched by nutri-ents, with peak values exceeding 3 mg l−1 of totalN and 0.5 mg l−1 total P. Three different sources fornutrients come into account: soil (Thornton 1986), de-tritus (Howard-Williams 1979) and mammalian feces(McLachlan, 1971). Probably all three are of import-ance in this system, but our data did not allow for aseparation between these sources. As commonly re-ported from African aquatic systems, nitrogen turnsout to be the limiting mineral nutrient for primaryproduction (Beadle, 1981; Thornton, 1986), judgedfrom the prevailingly very low N:P ratios. The nu-trient support gives rise to an almost immediate burstof phytoplankton, as revealed by a high primary pro-duction and high biomasses of phytoplankton. Thefloodplain is also enriched by detritus from decayedmacrophytes and terrestrial plants that become ac-cessible for microbial breakdown when submersed.These processes give rise to a vigorous productionof autotrophs (both phytoplankton, periphyton andaquatic macrophytes) that support dense populationsof zooplankton, notably cladocerans.

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Figure 5. Ivlev index of selectivity of four key species of Cladocera (Alona affinis, Ceriodaphnia spp., Macrothrix sp. and Simocephalusvetulus) from 6 different samples of fish: (1)=Brycinus lateralis 7 ind./average size 21 mm; (2)= Oreochromis andersonii 8 ind./10 mm; (3)=Tilapia sp. 10 ind./15 mm; (4)= Tilapia sp. 8 ind./13 mm; (5)= Tilapia sp. 9 ind./53 mm; (6)= Tilapia sp. 6 ind./15 mm.

Peak biomass of zooplankton approached 90 gDW l−1 along the shoreline, and although this trulyextreme biomass was not representative of the totalsystem, it points to a very productive and dynamicsystem, in support of McLachlan (1974). Such an ex-treme swarming property is also previously observedfor Moina (cf. Johnson & Chua, 1973; Ratzlaff, 1974),yet the causality for this behavior is not settled. Max-imum primary production in June was 293 µg C l−1

day−1, or 264 µg C l−1 day−1averaged over the twodepths. By assuming 45% C of DW in Cladocera(Andersen & Hessen, 1991), zooplankton carbon con-tent on the same day exceeded 3 mg C l−1. Settingbasic metabolic requirements for respiration alone to25% of C mass per day (Lampert, 1987), this meansthat the zooplankton population would need a min-imum of 754 µg C l−1 day−1 just for maintenance,which far exceeds the primary production of 293 µgC l−1day−1. Thus, although phytoplankton probablywas an important source of carbon for the zooplank-ton, the population was obviously strongly subsidizedby detrital particles and bacteria, especially during theperiod of rising flood in June.

Also the low oxygen concentrations suggest thatheterotrophic production exceeded the primary pro-duction in this system, and support the view that zo-oplankton production is fuelled from various sourcesof C, and that perhaps bacteria and detritus constitutethe major sources of C. This is in line with studiesfrom other systems strongly influenced by alloch-thonous matter. Grobbelaar & Toerien (1985) foundallochtonous detritus and bacteria to be a significantnutrient source for zooplankton in shallow impound-

ments in South Africa. In the hypertrophic Hart-beespoort Dam (RSA), seasonal bursts of Moina andCeriodaphnia were associated with autumnal break-down of autochthonous detritus and high bacterialproduction (Allanson et al., 1990). Nyirenda (1975)has demonstrated that M. micrura from Lake Chilwa(Malawi) in culture was able to breed and survivesolely on a bacterial diet. Kalk (1979) found that burstof Moina in Lake Chilwa coincided with high con-centrations of terrestrial detritus and bacteria, madeavailable by rising water level. In discussing the ap-parently widespread detritivorous feeding behaviouramong tropical cladocerans, she relates their wellknown minute size (Fernando, 1994) not exclusivelyas an adaptation to strong fish predation pressure, butalso to small sized food particles like bacteria. Hessenet al. (1990) estimated by radiolabelling of differentfood sources that detritus was the main source of bodycarbon for all zooplankton species in a humic lake, andthat also heterotrophic bacteria contributed more tobody C than did phytoplankton for most zooplanktonspecies. The share of detritus, bacteria and phyto-plankton in the diet of the various species will dependon feeding mode and filtering abilities of the variousspecies.

The almost immediate biomass build-up in thecladoceran populations suggests a considerable bankof resting eggs, since drift or migration of zooplank-ton from the main river are insufficient to account forthe explosive biomass increase. The hatching time ofMoina micrura seems to be particularly short (Kalk,1979), as stated in the experiments mentioned above.Rzoska (1961) observed M. micrura in temporary

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pools in the Sudan within 2 days of the first rain. Thehigh number of resting eggs is consistent with obser-vations from corresponding ecosystems (el Moghraby,1977), and strongly support the view of the eggbankas a main determinant of zooplankton population re-sponses (Hairston et al., 1995; Cousyn & de Meester,1998). The finding that hatching succession also is amajor factor for the succession of species over the sea-son is at variance with the findings of Hairston et al.(2000). They found that all dominant species duringsummer succession in lake systems of New York Stateemerged from the sediment egg bank during spring.

The third trophic level in this dynamic food chainis dominated by fish, yet also invertebrates like corix-ids did for periods feed intensively on the zooplankton.While various reports focus on the contribution of zo-oplankton to fish diet in man-made reservoirs in south-ern Africa, little is known about the dynamics of nat-ural systems (Allanson et al., 1990; Fernando, 1994).There is, however, a general view that zooplanktonavailability and fish breeding times in the tropics arelinked. The sites where this occurs are floodplainsand littoral of lakes (Lowe-McConnell, 1975; Wel-comme, 1979; Fernando, 1994). A variety of fishes(Barbus sp., Brycinus sp., Clarias sp., Oreochromissp. and Tilapia sp.) migrate into the shallow poolsand floodplains in the Okavango delta for spawning,and the fry are released when zooplankton food areabundant. The immediate drop in zooplankton bio-mass simultaneously with hatching of fish fry on thefloodplain could probably be accredited both bottom-up and top-down mechanisms, i.e., a reduced biomassof detritus and phytoplankton and an increasing pred-ation pressure from fish. Furse et al. (1979) found thatzooplankton accounted for 48–70% of the gut con-tent in fish fry of Lake Chilwa (Malawi), with barbs,catfish and tilapia as dominating species. Balarin(1979) found that Oreochromis andersonii changedfrom planktivory to herbivory at 70 mm body-length.Lazzaro (1991) showed that Tilapia rendalli >60 mmchanged to herbivory. In Lake George (Uganda) Oreo-chromis nilotica was zooplanktivorous at body-lengths<55 mm, thereafter changing to a diet of bluegreenalgae (Moriarty & Moriarty, 1973). Robotham (1990)found that Oreochromis leucosticus >17 mm changedfrom planktivory to herbivory in Lake Naivasha.

These findings are consistent with our results,and emphasise the importance of a protein-rich dietfor fast-growing juveniles, even for fish-species thatswitch to herbivory or detrivory early in life his-tory (as cichlids do). It is reasonable to believe that

the zooplankton production on the floodplains in theOkavango delta is of significant importance for thebreeding success for many dominant fish-species inthe delta (cf. Fernando, 1994). The moderate fishdensity on the floodplain in 1998 may be related to themodest flood this year, which on the other side maycorrespond to the high zooplankton concentration.The years 1997 and 1999 both showed a consider-ably higher flood velocity, and this was accompan-ied by high fish densities and moderate zooplanktonconcentration.

The data thus strongly support the importance ofthese flooded areas as a source for production andfish recruitment for the delta. It also points to thestrong links between the terrestrial and aquatic eco-systems of the Okavango delta. The flooded areas arekey habitats for terrestrial plant production, ungulatesand terrestrial mammals in general, and the animalwastes could even be instrumental for the loading ofnutrients fuelling the primary production (McLachlan,1971; Thornton, 1986). Thus the interplay betweenthe terrestrial and aquatic food-webs are probablyinstrumental for overall productivity in the delta.

Aknowledgements

This study was financed by grant no. 119729/730 fromthe Norwegian Reserach Council to D. Hessen. We aremost indebted to our colleagues at the Harry Oppen-heimer Okavango Research Center for collaboration.

References

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Andersen, T. & D. O. Hessen, 1991. Carbon, nitrogen and phos-phorus content in common crustacean zooplankton species.Limnol. Oceanogr. 36: 807–814.

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