Hydrobiologia 441: 93–106, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Carbon flow in the littoral food web of an oligotrophic lake

Mark R. James1, Ian Hawes2, Mark Weatherhead2, Carmen Stanger2 & Max Gibbs1

National Institute of Water and Atmospheric Research Ltd.;1P.O. Box 11 115, Hamilton, New Zealand(Fax: +64-3348-5548);2P.O. Box 8602, Riccarton, Christchurch, New Zealand

Received 20 July 1999; in revised form 26 July 2000; accepted 25 September 2000

Key words:stable isotopes, carbon flow, littoral foodwebs, lakes

Abstract

Benthic food web dynamics and carbon flow were examined in the littoral zone of Lake Coleridge, a large deepoligotrophic lake, using radioactive and stable isotope techniques in conjunction with analyses of stomach contentsof the fauna. We specifically address two hypotheses: (1) that macrophytes only contribute to the carbon flowto higher trophic levels when they have decayed; and (2) that epiphytic algae is the major source of carbon formacroinvertebrates, and thus fish, with only minor contributions from phytoplankton or terrestrial sources. Epi-phytic diatoms were a major component of the stomach contents of the gastropod snailPotamopyrgus antipodarum,and of chironomids. Animal remains were also common in the diet of some chironomids, while amorphous organicmatter predominated in the stomachs of oligochaetes. A variety of epiphytic algal taxa was found in trichopteran lar-vae. Feeding rate ofP. antipodarummeasured with radioactive tracers increased by 10× on decayed macrophytes(Elodea)compared with live material, while feeding rates on characean algae increased by a factor of 3 whendecayed material was presented. However, assimilation rates were less than 20% on decayed material comparedwith 48–52% on live material. Potential carbon sources were easily distinguished based on theirδ13C values,although isotopic ratios showed significant variation among sites. Epiphytic algae showed less variation amongsites than macrophytes and were depleted by 4–5‰ compared with macrophytes. Detrital material, organic matterin the sediments and plankton were significantly depleted inδ13C relative to macrophytes and slightly depletedrelative to epiphytic algae. Most macroinvertebrate taxa showed a similar pattern among sites to macrophytes andepiphytic algae.P. antipodarumand chironomids were slightly enriched compared with epiphytic algae. Ratios forthe common bully (Gobiomorphus cotidianus) were generally consistent with a diet dominated by chironomids,while there was some evidence for terrestrial inputs for koaro (Galaxias brevipinnis) and juvenile brown trout.Epiphytic algae appear to underpin much of the production in the littoral zone of this oligotrophic lake, withtrichopteran and chironomid larvae mediating carbon flows from algae to fish. Macrophytes do not make a majorcontribution directly to carbon flow to higher trophic levels even when decayed. The lack of a direct link betweenmacrophytes and higher trophic levels is due to the faunal composition, including a lack of large herbivores.

Introduction

Macrophytes generally contribute the greatest biomassto the organic carbon pool in the littoral zones oflakes and potentially provide a valuable food sourcefor aquatic macroinvertebrates. However, since thecontroversial statement by Shelford (1918) that "Onecould probably remove all the larger plants and sub-stitute glass structures of the same form and surface

texture without generally affecting the immediate foodweb", there has been considerable debate on the roleof macrophytes in the carbon flow of lakes. Despitesubsequent comprehensive reviews, including that byGaevskaya (1966) which listed some 620 species ofanimals that graze on live macrophytes, a number ofrecent studies have found little evidence of a directlink between macrophytes and higher trophic levels(Howard-Williams, 1979; Hamilton et al., 1992; Fors-

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berg et al., 1993; Hecky & Hesslein, 1995; Keough etal., 1996; Toetz, 1997).

Although live macrophytes may not contribute dir-ectly to carbon flow to higher trophic levels, organicmaterial originating from decaying macrophytes canprovide an important carbon source for detritivorousmacroinvertebrates. In one of the few attempts toquantify these interactions, Kornijów et al. (1995)showed experimentally that decomposed macrophytesare consumed and digested more than fresh macro-phytes, and this applied to both filamentous algae andvascular plants. Similar observations were made bySuren & Lake (1989). In another study, Kornijów(1996) demonstrated that although macrophytes suchas Elodea may contribute<10% to the diet of themacroinvertebrate community, up to 23.6% of themacrophyte biomass was consumed during the grow-ing season. Thus, he concluded that herbivory onrooted macrophytes was potentially as significant infreshwater as is herbivory in terrestrial habitats.

Macrophytes in the littoral zone of lakes providea large, three-dimensional habitat for the growth ofepiphytic algae. These algae in some cases contrib-ute >50% of primary production (Kairesalo et al.,1989) and it has been postulated that epiphytic algaemay provide the major source of food for herbivorousmacroinvertebrates in the littoral zone of lakes (Hawes& Schwarz, 1996). Dissolved organic carbon (DOC)derived from algal exudates has also been implicatedas a major source of carbon for heterotrophic fungiand bacteria which form the organic layer on hardsubstrates and provide a source of energy for grazers(Rounick & Winterbourn, 1983). The contribution ofDOC and decayed organic matter to carbon flow in thelittoral zone, however, remains equivocal.

Analyses of stomach contents of macroinverteb-rates have been used in numerous studies to determinepredator–prey interactions and develop foodweb mod-els. A major drawback with stomach content analysesis that they do not distinguish between material whichis assimilated and that which is indigestible, neitherdo they characterise amorphous detritus, which is fre-quently the most common item. Use of stable isotopeshas provided significant new insights into trophic link-ages in aquatic food webs. A major advantage ofthis technique is that the stable isotope ratios in con-sumers’ tissues integrate carbon sources over a longtime period, and more importantly, can measure whatis actually assimilated into body tissue.

The ratio of13C–12C in C assimilated by aquaticautotrophs varies depending on the dissolved inor-

ganic carbon (DIC) source. During photosynthesis, therelative proportions of carbon isotopes incorporatedinto plant tissues depends on whether plants utiliseC3, C4 or CAM photosynthetic pathways, as well asthe availability of DIC which can be dependent onboundary-layer diffusion effects at the plant cell walls(Hecky & Hesslein, 1995). Once carbon is fixed asorganic matter in autotrophs, the isotope ratio is passedon to consumers with only slight enrichment in theheavier isotope (13C) at each trophic transfer (Rounick& Hicks, 1985; Peterson & Fry, 1987).

Most of the early work with stable isotopes focusedon the relative importance of allochthonousversusautochthonous carbon sources for invertebrates (Rau,1980; Rounick et al., 1982). More recently, the tech-nique has been used to examine potential food sourceswithin aquatic habitats. Studies in Lake Superior byKeough et al. (1996), for example, have examinedthe relative importance of phytoplankton, benthic al-gae and macrophytes to carbon flows. They concludedthat there was no link in terms of carbon flow betweenthe abundant macrophyte biomass and fish, althoughin a later study (Keough et al., 1998) they suggestedthat decomposing macrophytes may make a signific-ant contribution to the DIC pool through respirationassociated with macrophyte decomposition. However,these whole lake stable isotope studies are relativelyrecent and have only been applied to a few trop-ical African, temperate Canadian lakes (Hecky &Hesslein, 1995; Keough et al., 1996, 1998) and SouthAmerican flood plains (Hamilton et al., 1992; Fors-berg et al., 1993). In the case of the study of carbonsources in the Amazon, Forsberg et al. (1992) foundphytoplankton were far more important to commercialfisheries than first thought.

New Zealand lakes are generally characterised bya depauperate macroinvertebrate fauna with low di-versity (James et al., 1998) and poor representationby insect groups such as shredders (Winterbourn &Lewis, 1975). The trichopteranTriplectides ceph-alotes (Walker) and aquatic lepidopteranHygraulanitens(Butler) are considered to be the only aquaticmacroinvertebrates which eat live vascular plants(Winterbourn & Lewis, 1975) and there are no herb-ivorous fish in most New Zealand lakes. We have pre-viously shown that the gastropodPotamopyrgus an-tipodarum(Gray), which dominates the macroinver-tebrate community in the littoral zones of oligotrophicNew Zealand lakes, feeds extensively on epiphytic al-gae (James et al., 2000). Preliminary observations onstomach contents of a range of macroinvertebrates in

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New Zealand lakes found a predominance of amorph-ous detritus of unknown origin.P. antipodarumcanbe common in the stomachs of some salmonid andnative fish but the calorific value of this prey itemhas been questioned (McCarter, 1986) and they havea hard shell and operculum, so whether organic matterfrom these gastropods is assimilated by fish is equallyunknown.

In the present study, we have used radioactiveand stable isotope techniques in conjunction with ana-lyses of stomach contents to examine carbon flowin the benthic food web of the littoral zone in LakeColeridge, a large oligotrophic lake. We specificallyaddress two hypotheses: (1) that macrophytes onlycontribute to the carbon flow to higher trophic levelswhen they have decayed; and (2) that epiphytic algaeis the major source of carbon for macroinvertebrates,and thus indirectly for fish with only minor contribu-tions from phytoplankton or terrestrial sources. Wehave then used results from this study to construct afood web and identify carbon flows and the possiblefate of organic material.

Study site

This study was carried out in Lake Coleridge (43◦20′ S, 171◦ 30′ E) a large, deep glacial lake in theCanterbury high country region of the Southern Alps,South Island, New Zealand. The lake is 17.8 km long,3.4 km wide and has a maximum depth of 200 m.Planktonic production in the lake is low and it hasbeen postulated that secondary production in the lit-toral zone underpins biological production through tofish (James et al., 1998). The littoral region consistsof three zones: a ‘shallow zone’ (water depth<5 m)devoid of macrophytes with a cobble/gravel substrate,low macroinvertebrate abundance and a communitydominated by Trichoptera and chironomid larvae; a‘mixed macrophyte zone’ at depths of 5–10 m dom-inated by characeans, withIsoetes alpinus(Kirk),Potamogeton cheesemaniiA.Brenn andMyriophyllumtriphyllumOrchard and extensive ‘characean meadowzone’ from 10 to 30 m water depth. Both zones 2 and3 have a macroinvertebrate community dominated bygastropods, chironomids and oligochaetes (Schwarz &Hawes, 1997; James et al., 1998).

The epiphytic algae are dominated by diatoms,particularlyEunotia pectinalisandAchnanthes minu-tissima. Adnate taxa predominate at depths greaterthan 5 m with erect taxa including chlorophytes and

Figure 1. Location map showing littoral sites (•) where samplesof periphyton, macrophytes, macroinvertebrates and fish were col-lected and the open water site (∗) where plankton samples werecollected.

stalked diatoms contributing to increased diversity atdepths shallower than 5 m. Hawes & Schwarz (1996)found maximum biomass of epiphytic algae was at10–15 m but maximum production was at 5 m.

The fish community consists of introduced sal-monids (brown troutSalmo trutta(L.), rainbow troutOncorhynchus mykiss(Richardson), quinnat salmonOncorhynchus tshawytscha(Walbaum)), and two nat-ive fish, the small common bully (Gobiomorphuscotidianus(McDowall)) and the koaro (Galaxias bre-vipinnis(Gunther)).

Four sampling sites were established in the lake,two close to the major inflows and two at differentdistances from inflows (Figure 1).

Methods

Stomach analyses

Benthic macroinvertebrates were collected fromamongst the macrophyte beds at 5 m, at each site inMarch, 1998. Samples were preserved immediatelyand sorted and identified back in the laboratory. Fore-guts of insect larvae, gastropods and oligochaetes were

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dissected out, mounted on slides in Karo Light CornSyrup with 4% formaldehyde, and viewed on a LeicaDMLB microscope at 200× magnification. For eachslide (contents of up to four individuals), 10 fieldsof view were randomly selected and the number ofsquares occupied by different items were counted. Thecategories used were: animal, cyanobacteria, filament-ous, pennate and centric diatoms, fungi, filamentousgreen algae, green unicells, inorganic matter, uniden-tified organic matter, vascular plant fragments andfilamentous yellow-green algae.

Decay experiments

To determine whetherP. antipodarum, the mostabundant macroinvertebrate in Lake Coleridge, in-gests and assimilates decayed macrophytes preferen-tially over fresh macrophytes, we used radioactiveisotopes as tracers based on the methods developedby Kornijów et al. (1995).P. antipodarumand mac-rophyte material were collected from Site 4 (Fig. 1) inthe littoral zone and the macrophyte tissue thoroughlywashed with distilled water. To prepare labelled food,sections of new growth ofChara and Elodea wereincubated with Na H14 CO3 for 3 days under simu-lated natural light conditions. New growth tips werechosen as they were mostly devoid of epiphytic al-gae and thus provide ‘clean’ macrophyte samples forfeeding experiments. The labelled plant material wasthoroughly washed with distilled water then rinsedwith 0.1 N HCl solution to remove any radioactivematerial adhering to the plants. One subsample of eachmacrophyte taxon was frozen for 7 days and then leftin the dark at room temperature for 3 months for themacrophytes to decay. Another subsample was imme-diately transferred to 500 ml of fresh lake water forgrazing experiments.

Thirty P. antipodarum,which had been maintainedin tanks with macrophytes from the lake, were addedto each beaker and allowed to feed for 30 min on ‘hot’14C-labelled macrophyte tissue in an incubator at am-bient lake temperatures (10–12◦C). At the end of thefeeding period three replicates of five individuals werekilled by transferring to boiling water and the anim-als were then frozen at−20 ◦C. The other 15 snailswere thoroughly rinsed in distilled water and trans-ferred to ‘cold’ macrophyte tissue, which had not beenlabelled with14C, and left for a further 4 h to void anyradioactive labelled material in their stomachs. Prelim-inary experiments had shown this to be a suitable time(James et al., 2000).

To estimate the radioactivity taken up by the mac-rophytes, subsamples of the plant tissue were driedat 60◦C for 48 h, ground and 1 mg was transferredto scintillation vials. The uptake of radioactively la-belled material byP. antipodarumwas determinedafter thawing and careful removal of tissue from shells.P. antipodarumtissue was dried at 60◦C for 48 hrs,dry weight measured and transferred in groups of fiveto scintillation vials.P. antipodarumand ground mac-rophyte tissue were digested in vials by the additionof 0.2 ml of NCS (Amersham) tissue solubiliser, andincubated in a water bath for 4 h at 60◦C. Ten ml ofHisafe (Wallac) cocktail was added and radioactivityassayed on a LKB1217 Rackbeta scintillation counter.Counts were corrected for quenching and background.Blanks were prepared from snails and plants whichhad not been exposed to H14CO3.

Calculations

Grazing rate (G) was calculated as:

G= (dpm per mg animal tissue before egestion− blank)

(dpm perµg plant tissue− blank)× 1

t,

(1)

whereG is µg (mg animal DW)−1 h−1, t is incuba-tion time in hrs and dpm is radioactivity expressed asdecays per minute.

Assimilation efficiency (A) was calculated as:

A= (dpm per mg animal tissue after egestion)

(dpm per mg animal tissue before egestion)× 100. (2)

Statistical analyses were carried out using ANOVAwith Duncan’s Post-hoc testing with STATISTICA forWindows.

Stable isotopes

Surface sediments, macrophyte tissue and macroin-vertebrates were collected around the edge of LakeColeridge from a water depth of 5 m at four sites,plankton from a central open water site (Fig. 1), andjuvenile fish from shallow depths at selected sites.

Macrophytes and macroinvertebrates were col-lected from each site with diver- operated Wiscon-sin grabs. Single composite samples were collectedin March 1997 and three replicates at each site inDecember 1997. Macrophytes were sorted, thoroughlywashed to remove epiphytic algae, and new growthtips were cut off and freeze-dried and ground to a

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powder. Detrital material (mostly terrestrially derivedtwigs and leaves) was separated at two sites wherethere was sufficient material and treated as for mac-rophytes. Macroinvertebrates were sorted into groupsand left overnight in filtered lake water (WhatmanGF/C) to allow complete gut evacuation. Gastropodswere removed from their shells and trichopteran larvaefrom cases prior to freeze-drying. Composite and rep-licate samples of up to 10 individuals of each taxa werefreeze-dried separately and ground into a powder.

To collect clean epiphytic algal samples fromwithin characean beds, we used plastic strips to mimiccharophytes. While these are not strictly epiphytes,they are referred to as such from here on to indicatethat they were collected amongst dense macrophytecommunities and to distinguish them from epilithon(algae, bacteria and fungi on rocks) and planktonicalgal samples. Strips, 2 mm diameter and 30 mm long,were deployed in December, 1997, amongst the mac-rophytes at 5 m water depth at Sites 2, 3 and 4 andadditionally at 10, 15, 20 and 25 m depth at Site 4.No epiphytic algae samples were collected at Site 1because there were no macrophytes present. At the endof 2 months, the strips were retrieved by divers andthe epiphytic algae on three replicates of four stripswere scraped into vials. The slurry was well mixed,filtered onto precombusted Whatman GF/F filters andfreeze-dried. Rock scrapings (epilithon) were also col-lected from Site 2, but in shallow water and treated asfor epiphytic algae. Sediments were dried overnightat 60◦C, washed with 1 N HCl to remove inorganiccarbon, dried for 4 h and freeze-dried as for the othermaterial.

Replicate plankton samples (mostly phytoplank-ton) were collected from a mid-lake site by filteringone litre onto precombusted Whatman GF/F filters,which were then freeze-dried as for epiphytic algae.

Juvenile brown trout and adult koaro and commonbullies were collected for stable isotope analyses atselected sites by electric fishing. Muscle tissue wasremoved and freeze-dried as for macroinvertebrates.Isotopic ratios in muscle tissue has been found to beintermediate for a range of tissues and is easier tocollect (Rounick & Hicks, 1985).

Stable isotopes were analysed by mass spectro-metry on a Finnigan MAT Delta C Continuous FlowMass Spectrometer. Results were reported as the rel-ative difference in parts per thousand (‰) between theisotope ratio of the sample and that of the international

C reference standard (Pee Dee Belemnite) as follows:

δ13C=[

13C/12Csample13C/12CPDB

− 1

]× 103. (3)

Analytical precision was<0.1‰.

Results

Stomach analyses

Stomach content analyses were made for six categor-ies of macroinvertebrate:P. antipodarum, Trichoptera,oligochaetes and the chironomidsProcladius,Ortho-cladiinae and Chironominae. Because the numbers ofanimals in some groups were low, data were pooledfor all sites. Organic matter, diatoms and animal tissuedominated the diet of Chironominae, whereas diat-oms (86%), and organic and inorganic matter (mostlysilt), were the most prominent components in the dietof Orthocladiinae (Fig. 2). Diatoms also dominatedthe diet of the gastropod,P. antipodarum,along withorganic and inorganic detritus and there were smallamounts of both vascular plants and yellow-green al-gae. In contrast, 90% of the stomach contents of thechironomidProcladiussp. consisted of animal tissue(legs, claws etc.). The majority of the stomach con-tents of oligochaetes consisted of amorphous organicmatter while those of trichopteran larvae (mostlyHud-sonema) was mainly diatoms and yellow-green fila-mentous algae. Amongst the diatoms, pennate formswere overwhelmingly dominant with very few centricsand filamentous diatoms seen in any stomach contents.

Grazing rates

Feeding and assimilation rates forP. antipodarumsnails feeding on fresh macrophytes were low andnot significantly different between the two macrophytetaxa (ANOVA, P<0.05). The mean and standard er-ror for ingestion were 0.92±0.01 and 1.39±0.27µg(mg P. antipodarumDW)−1 h−1 and for assimilatedorganic material 0.44±0.11 and 0.73±0.14µg (mgP. antipodarumDW)−1 h−1 for Elodea and Chararespectively (Figure 3). There were clear differencesbetween feeding rates on fresh versus decayed macro-phytes for both macrophyte taxa (ANOVA,P<0.05).The mean ingestion rate when feeding on decayedElodeacompared with fresh material increased by anorder of magnitude from 0.92 to 8.75µg (mgP. anti-podarumDW)−1 h−1 andCharaby a factor of 3 from1.39 to 4.70µg (mgP. antipodarumDW)−1 h−1.

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Figure 2. Percentage of different foods found in the stomach contents of macroinvertebrate taxa.

Surprisingly, the amount of organic material as-similated intoP. antipodarumtissue was not signi-ficantly different for either macrophyte taxon for liveversus decayed material. Assimilation efficiency aver-aged only 20% forP. antipodarumfeeding on decayedChara and 10% on decayedElodeacompared with48–52% on live material.

Epiphytic algae composition

The composition of epiphytic algae which colonisedthe plastic strips incubated in Lake Coleridge is sum-

marised in Table 1. The community on all strips wasdominated by diatoms,Achnanthidiumsp. contribut-ing 39.7–75.5% andEunotia pectinalis7.7–38.7% tototal abundance. The contribution of the latter speciesincreased with depth at Site 4 and was most abund-ant at 20 and 25 m.Achnanthidiumis much smallerthanE. pectinalisand in terms of biovolume,E. pec-tinalis was dominant at all depths. Species diversityat 5 m was greatest at Site 3 with a diverse diatomcommunity and three species of adnate diatoms. Epi-lithon on rocks from Site 2 also exhibited a diverse

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Table 1. Percentage composition of epiphytic algae, by cell numbers, which colonised plastic strips in Lake Coleridge.Epilithon was only collected at Site 2

Species Site-depth Epilithon

2–5 m 3–5 m 4–5 m 4–10 m 4–15 m 4–20 m 4–25 m

(%) (%) (%) (%) (%) (%) (%)

Diatoms

Achnanthessp. 0 12 1 0 1 4 4 10

Achnanthidiumsp. 65 48 73 63 76 40 49

Achnathes laevis 0 9 0 0 0 0 1

Cocconeis placentula 4 2 0 6 1 1 1 1

Colonial greens 0 0 0 0 0 1 1

Epithemia sorex 9 1 3 4 1 3 1 1

Eunotia pectinalis 11 9 8 12 15 39 35

Gomphoneis 0 0 1 0 0 0 0

Gomphonema intricatum 9 10 15 12 7 7 7 20

Gomphonemasp. (large) 0 0 0 0 0 0 0 3

Melosirasp. 0 2 0 0 0 0 1

Navicula rhynchocephala 0 1 0 0 0 2 1 1

Nitzschia linearis 0 4 0 0 0 0 1 23

Protodermasp. 0 1 0 2 1 2 0

Synedrasp. 0 1 1 0 0 1 1 25

Tabulariasp. 0 1 0 0 0 1 0 9

Other

Bulbochaetesp. 1 1 0 1 0 1 0

Mougoetiasp. 0 0 0 0 0 1 0

Cyanobacteria

Phormidiumsp. 1 0 0 0 0 0 0

Leptolyngbyasp. 1 0 0 0 0 0 0

autotrophic community consisting of similar taxa tothose colonising the strips, with the addition of the cy-anobacteriaPhormidiumand Leptolyngbyaalthoughthey contributed<1% to total taxa.

Carbon isotopic ratios

Different primary producers were easily distinguishedbased on theirδ13C values (Table 2, Fig. 4). Macro-phytes had highδ13C signatures which ranged from amean of−17.2‰ to−12.1‰ in December 1997 and−18.9‰ to−14.9‰ in March 1997. Although therewas only a single sample collected from each site inMarch, the range and pattern across sites was similarto December with characeans from Site 1 the mostenriched (−14.9‰) and those from Site 3 the leastenriched (−18.9‰).Elodeawas only present at Site4 and the isotopic ratio was the same forChara fromthat site.

Figure 3. Grazing and assimilation rates for the gastropodP. an-tipodarum feeding on radioactively labelled epiphytic algae andfresh and decayed macrophytes (Elodea canadensisandCharaspp.)Error bars are± 1 SE. (Epiphyton results are from James et al.,1998).

Isotopic ratios of epiphytic algae showed less vari-ation among sites than macrophytes with lower ratios

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Table 2. Mean and range ofδ13C values for each trophic component at each site sampled inLake Coleridge in December 1998. Values in brackets are single values for March 1997

Food web group Abbreviation Site Mean δ13C range

Sediments Se 1 −20.5 −23.2 to−20.6

2 −24.2 −26.9 to−22.9

3 −23.8 −24.3 to−23.5

4 −23.2 −23.9 to−22.9

Epilithon El 2 −22.7 −23.8 to−21.9

Detritus De 3 −24.7

4 −27.4 (−28.5) −27.7 to−27.2

Plankton Pl mid-lake −29.4 −29.8 to−29.0

Epiphytic algae Ep 2 5 m −19.9 −20.1 to−19.8

3 5 m −22.0 −22.7 to−21.0

4 5 m −22.6 −23.6 to−21.7

4 10 m −20.9 −21.4 to−20.5

15 m −22.8 −23.7 to−21.6

20 m −20.8 −23.8 to−18.8

25 m −23.3 −24.4 to−22.0

MacrophytesChara Ma 1 −12.1 (−14.9) −12.8 to−11.8

2 −15.1 (−16.5) −16.1 to−14.3

3 −17.2 (−18.9) −17.8 to−16.7

4 −16.2 (−16.4) −17.0 to−15.5

Elodea 4 −16.4 −17.1 to−16.1

MacroinvertebratesOligochaetes 3 −22.5 −23.4 to−21.6

P .antipodarum 1 −14.2 (−17.8) −17.8 to−12.2

2 −15.7 (−16.2) −16.9 to−14.8

3 −19.5 (−19.6) −19.9 to−18.9

4 −20.7 (−19.3) −21.1 to−20.4

Chironomids 1 −13.8 −14.7 to−12.4

2 −20.8 −21.3 to−20.6

3 −21.8 −22.3 to−21.4

4 −22.4 −22.5 to−22.3

Trichoptera 1 −17.0 −17.5 to−16.5

2 −18.9 (−18.6) −19.1 to−18.7

3 −19.7 (−15.6) −20.0 to−19.5

4 −20.8 (−18.7) −20.8

FishBullies 2 −21.5 −22.9 to−19.5

3 −18.1 −19.4 to−17.6

4 −20.0 −20.7 to−19.5

Koaro 3 −23.8 −24.2 to−23.4

4 −20.8 −22.5 to−19.4

Brown trout 2 −23.4 −23.6 to−23.0

4 −21.7

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and means ranging from−22.6 to−19.9‰. Epiphyticalgae samples were depleted by 4–5‰ compared withmacrophytes on which they were collected. The dif-ferences between epiphytic algae and macrophyteswithin sites were significant atP<0.05 (ANOVA).There was no evidence of changes in isotopic ratiosof epiphytic algae with depth (note that depth profileswere only collected at Site 4). The epilithon at Site2 was slightly depleted inδ13C relative to epiphyticalgae from the same site. Detrital material, consistingprimarily of twigs and leaves from terrestrial vegeta-tion, was only collected at Sites 3 and 4 and the meanδ13C was relatively low at−24.7‰ and−27.4‰,respectively.

Organic material in the sediments at the base ofthe macrophyte beds was significantly depleted inδ13C relative to macrophytes and slightly depleted(1.4–4.3‰) relative to epiphytic algae. This couldbe attributed to a contribution from sedimentation ofplanktonic material (mostly phytoplankton) which ex-hibited aδ13C of −29.4‰ in samples from the openwater.

Most macroinvertebrate taxa exhibited a similarpattern to macrophytes and epiphytic algae withgreatest enrichment at Site 1 and a trend of decreas-ing δ13C between Sites 2 and 4. Oligochaetes wereonly found in high numbers at Site 3 and were slightlydepleted inδ13C relative to other taxa, potentiallyreflecting a similar depletion observed for organic ma-terial in sediments. The snailP. antipodarumexhibitedδ13C ratios of−20.7 to−14.2‰, i.e. 1.9–4.2‰ en-riched compared with epiphytic algae. The chironom-ids analysed forδ13C were dominated byChironomuszealandicusand orthoclads, and theδ13C ranged from−22.4 to−13.8‰, which is slightly more depletedin δ13C thanP. antipodarumcollected from the samesites. The signatures for chironomids however, didshow a similar trend among sites toP. antipodarumwith greatest enrichment at Site 1. Trichoptera in thesamples analysed were mostlyHudsonemaspp. andthey exhibited similar signatures toP. antipodarumwithin and among sites.

Most of the fish collected for stable isotope ana-lyses were collected in shallow areas amongst cobblesubstrates and low growing macrophyte communities.There was no obvious pattern in intersite variability.Ratios for the common bully ranged from a mean of−21.5‰ at Site 1 to−18.1‰ at Site 3. Koaro and ju-venile brown trout both exhibited more enrichedδ13Csignatures at Site 4 cf. Site 3 and Site 1 respectively.

There was no significant difference inδ13C betweenthe three fish species collected at the same site.

Discussion

Stable isotopes

Stable isotopes of carbon have been used very effect-ively to describe trophic interactions in a range ofaquatic ecosystems. The method relies on potentialfood sources possessing distinctly different signaturesand a good understanding of fractionation which oc-curs at each trophic interaction. The data for Lake Col-eridge clearly showed that macrophytes were consist-ently enriched by>5‰ within sites relative to othersources of carbon in the littoral zone. Detritus of ter-restrial origin and phytoplankton (−27.4 and−29.4‰respectively) were also sufficiently depleted inδ13Cto distinguish them from macrophytes and epiphyticalgae. Isotopic fractionation with each trophic transferis difficult to quantify accurately except in well con-trolled laboratory experiments but various studies havedemonstrated that animal tissue becomes enriched inδ13C by 1–2‰ relative to their food sources (De Niro& Epstein, 1978; Rounick & Hicks, 1985).

Much of the recent debate regarding the utilisationof this method to identify general patterns in carbonpathways (France, 1995, 1996; Doucett et al., 1996)has focussed on temporal and spatial variability inδ13C signatures. This variability was demonstrated foraquatic plants by Boon & Bunn (1994) who cautionedagainst the use of single samples from one site or time.They foundδ13C values for epiphytes could span up to10‰ units in different billabongs. An important find-ing of the present study is that even within a single lakesystem theδ13C of primary producers in the littoralzone can span 6‰.

There was some variation between seasons (sum-mer and autumn) in our study, but the variability wasminor compared with these spatial differences. Thisvariability is most likely due to dissolved inorganiccarbon (DIC) sources. Unfortunately, we did not col-lect DIC samples forδ13C analyses, but assuming thephotosynthetic fractionation is the same for the macro-phyte species at each site, then the DIC source must beenriched with13C at Site 1. The site is close to a majorriver input with high episodic inputs of suspended sed-iment which tend to plunge in summer (Schallenberget al., 1999). Whether this alters theδ13C level of at-mospheric CO2 in equilibrium with water is unknown,

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but one of the few mechanisms which produces en-richedδ13C signatures of such a magnitude in primaryproducers is a shift to bicarbonate (HCO3) utilisationas opposed to atmospheric CO2. This has been demon-strated for regions with large riverine inputs (Osmondet al., 1981).δ13C signatures could also be expectedto change with depth. Hecky & Hesslein (1995) hypo-thesised that greater boundary layer thickness wouldbe expected at greater depth which would result in en-richedδ13C signatures, but surprisingly there was nosignificant difference inδ13C signatures of epiphyticalgae with depth in Lake Coleridge.

This is the first study of stable isotopes for lakebiota in New Zealand. A similar study in streams byRounick et al. (1982) found that macrophytes, such asElodeaandMyriophyllum, exhibitedδ13C signaturesof −31.1 to−21.8‰, epilithon averaged−33.2‰,P. antipodarum−34.5‰, chironomids−29.2‰ andtrichopteran larvae ranged from−35.8 to−29.3‰.All theseδ13C signatures are depleted compared withbiota in Lake Coleridge. Osmond et al. (1981) foundthat more negativeδ13C values were associated withplants in fast flowing water compared with sluggishwaters due to differential boundary layer diffusionand HCO3

−uptake. Theδ13C signatures in biota fromLake Coleridge are at the upper end of the reportedrange for C3 macrophytes, but this can be accountedfor by boundary layer diffusion resistance restrict-ing the supply of DIC to photosynthetic organismsin lentic waters. Where DIC concentrations at theplant/water interface are limiting, fractionation willbe reduced.δ13C signatures in Lake Coleridge weresimilar to those found for temperate Canadian lakeswhere Hecky & Hesslein (1995) observedδ13C signa-tures of−20.1 to−19.4‰ for epiphyton and−12.4 to−11.4‰ for the macrophytePotamogeton. Theδ13Cfor DIC in the Canadian lakes were−4 to−3‰, con-siderably enriched compared with the−12.1‰ foundby Rounick et al. (1982) in New Zealand streams.

Food web interactions

We hypothesised that macrophytes only contribute tocarbon flow to higher levels after decomposition andthat epiphytic algae is the primary source of carbonfor higher trophic levels. Interpretation of food webinteractions has largely been hampered by the use oftechniques such as gut analyses which cannot char-acterise amorphous organic matter. In our study, forexample, up to 90% of stomach contents of oligo-chaetes and 70% of those of Chironomidae could

not be identified. However, given this proviso, res-ults from stable isotope and stomach analyses for arange of macroinvertebrate taxa indicate a clear de-pendence on epiphytic algae as a source of carbondespite the high biomass of macrophytes. This is alsoconsistent with macroinvertebrate stomach analyseswhere macrophyte material was only a minor con-stituent, if present at all. Our findings that there isno direct trophic link between macrophytes and fishis consistent with most studies of temperate lake eco-systems (Hecky & Hesslein, 1995; Keough et al.,1996). However, the relative importance of plank-tonic algae differs markedly in our study. Keoughet al. (1996) suggested that phytoplankton were thedominant source of carbon in coastal food webs ofoligotrophic Lake Superior in mid to late summer andthat although attached algae (epiphytic) may accountfor significant portions of algal biomass in more eu-trophic Great Lakes they did not play a significant rolein primary productivity or carbon flow in Lake Super-ior. Lake Coleridge is also an oligotrophic lake yet theepiphytic algae appear to be the dominant source ofcarbon, at least for herbivorous macroinvertebrates.

Our interpretations depend on the assumption thatthe δ13C signatures of apical macrophyte sectionsand epiphytic algae grown on strips intimately po-sitioned among macrophytes, represented the overallsignatures of macrophytes and epiphytic algae. In-cubation of plastic strips for 2 months within densestands of macrophytes will have minimized any dif-ferences between DIC supply from the water columnto strip algae and true epiphytes. The possibility ex-ists that recycling of carbon within macrophyte tissuesbetween host plants and epiphytes could change thisenrichment, though we have been unable to test thisyet. However, utilisation of respired macrophyte car-bon, depleted inδ13C by photosynthetic epiphytes,would tend to increase the difference between thetwo primary producers by increasing depletion withinepiphytes.

The gastropodP. antipodarumand trichopteranlarvae were enriched on average by 0.2–4‰ relat-ive to epiphytic algae. Laboratory experiments havedemonstrated enrichments of only 1–2‰ per trophicinteraction (DeNiro & Epstein 1978). There are atleast two possible explanations for an enrichment of4‰. The first is within site variability of the signa-ture in epiphytic algae. However, at Site 2 theδ13C ofepiphytic algae varied by only 0.3‰ compared withP. antipodarumwhich varied by 2.1‰. The second,and most likely explanation, is thatP. antipodarum

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was feeding on a mixture of food sources includingepiphytic algae, bacteria and fungi. This is supportedby the stomach analyses where 62% of the contentsin P. antipodarumwere diatoms and 32% amorphousorganic material. Bacteria and fungi could be slightlyenriched compared with epiphytic algae dependingon whether they were utilising DOC originating fromepiphytic algae or macrophyte tissue.

Oligochaetes and chironomids exhibited a smallerdepletion inδ13C relative to epiphytic algae thanP.antipodarumor trichopteran larvae indicating a po-tential contribution by phytoplankton to their foodsource. The contribution required for phytoplanktoncan be estimated using a relative proportional method.At Site 2 for example, if we assume a meanδ13Cfor chironomids of−20.8‰, epiphytic algae−19.9‰and phytoplankton−29.4‰, and a 1‰ enrichmentthrough trophic level, then epiphytic algae must havecontributed around 80% to their carbon source. Sim-ilar proportions would apply to oligochaetes at Site 3.It is also likely that some of these animals could beingesting organic matter from the sediments (−23.8 to−24.2‰), but this is likely to have been derived fromsedimenting algae (phytoplankton and epiphyton).

An alternative hypothesis is that consumers reliedon a mixture of macrophytes (−17.2‰ to−12.1‰)and phytoplankton (−29.4‰). If this were the casethen macrophytes would have to be the major com-ponent of the diet. At Site 3, for example, macro-phytes would have to contribute 80% and 65% of thediet for P. antipodarum(−19.5‰) and chironomids(−21.8‰), respectively. This is highly unlikely in thelight of findings from our other experiments.

These results then are consistent with the hypo-thesis that it is epiphytic algae which underpins littoralproductivity, and based on the stable isotope data,none of the macroinvertebrates rely primarily on liv-ing macrophytes. Stomach contents often consist ofan amorphous mass of detritus so do decaying mac-rophytes provide an indirect source of carbon formacroinvertebrates? Unfortunately, there have beenfew studies of changes in isotopic composition duringdecomposition. Keough et al. (1998) demonstrated ex-perimentally that theδ13C of DIC in containers withdetritus declined by an average of 11‰ over 25 dayscompared with controls with no detritus. The remain-ing detritus, however, is likely to have changed verylittle as there is little fractionation of carbon isotopesduring decomposition of organic matter (Peterson &Fry, 1987). Thus, if detritus in the littoral zone of alake originated from macrophytes, detrivores would be

expected to have a slightly enrichedδ13C signature.Clearly, this was not the case in Lake Coleridge asall macroinvertebrates exhibited ratios which were sig-nificantly depleted compared with macrophytes at thesame sites. The depleted levels in macroinvertebratesare consistent with our decomposition experimentswhich demonstrated that althoughP. antipodarumin-gested up to 8.8µg (mg snail DW)−1 h−1 of decayedElodeatissue, an order of magnitude greater than livematerial, only 10% of this was actually assimilated.This suggests that even though nutritional value mayincrease with decomposition (Kornijów, 1996) andthere is a loss of anti-gustatory compounds (Suren& Lake, 1989), the presence of lignins and toughcell walls still inhibit the assimilation of this organicmaterial.

Feeding rates forP. antipodarumon periphytic al-gae derived from James et al. (2000), also in LakeColeridge, are compared with feeding on macrophytetissue in Figure 3. Both experiments used the sameexperimental protocols and were conducted at similartemperatures. This comparison clearly demonstratesthe preference for epiphytic algae particularly in termsof assimilated carbon. In terms of daily ration forP.antipodarum, this represents an assimilation of 29%of its body weight when feeding on epiphytic algae(derived from James et al., 1998), 2% on live mac-rophytes and 5% on decayed macrophytes. Kornijów(1996) suggested that herbivory on rooted macro-phytes was as significant in freshwaters as terrestrialhabitats and macroinvertebrates consumed on average8% of macrophyte biomass in European lakes. In LakeColeridge, biomass ofP. antipodarumreached 12.5 gDW m−2 (James et al., 1998), but even at this biomass,they would ingest less than 0.2% of the macrophytebiomass per day.

Most of the detrital material at Site 4 consistedof twigs and leaves in various states of decay. Thismaterial would have originated from the shorelinevegetation which consisted of mixed broadleaf angio-sperms and kanuka and reached the water’s edge. Theorganic detrital material at this site exhibitedδ13C ra-tios very close to−27‰, which is characteristic ofterrestrial vegetation in New Zealand (Rounick et al.,1982) and like phytoplankton does not appear to makea significant contribution to carbon flow in the littoralzone of Lake Coleridge.

Most fish found in New Zealand lakes are selectivefeeders with diet composition varying seasonally andspatially depending on food supply available. Juven-ile brown trout collected in the shallow zone of Lake

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Figure 4. Range of isotopic ratios for different components of the food web from samples collected in the littoral zone of Lake Coleridge.Trophic levels are (1) primary producers, (2) macroinvertebrates and (3) fish. The label is placed at the mean value and the solid line indicatesthe range ofδ13C values for each level. Sediments and detritus are placed with primary producers for this exercise. Note that theδ13C valuefor a food will increase, i.e move to the right by 1–2‰ in the consumer relative to its diet.

Figure 5. Schematic diagram showing the food web for the littoral zone of Lake Coleridge based on stable isotope analyses, radioactive tracerexperiments and stomach contents. Numbers in the boxes are meanδ13C values for Site 4, numbers in brackets represent biomass (dry weight)and the strength of the lines represents the importance for the next trophic level.

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Coleridge at Site 4 exhibitedδ13C signatures consist-ent with a mixed diet of chironomids and trichopteranlarvae. At Site 2, theδ13C signatures for trout weredepleted in13C compared to the macroinvertebrateswhich would have been available. A possible explan-ation is that they were feeding on insects of terrestrialorigin either entering the lake as drift from riversnearby or falling into the lake around the edge. Ter-restrial and riverine insects generally show a markeddepletion inδ13C relative to lake biota, as was dis-cussed earlier. Similarly koaro caught at Site 3 alsoshowed a depletion inδ13C relative to macroinver-tebrates and could have been feeding, at least partly,on terrestrial insects.δ13C signatures for the commonbully Gobiomorphus cotidianus, were generally con-sistent with a diet dominated by chironomids and a2–3‰ enrichment (Sites 3 and 4). The contributionof P. antipodarumto fish energetics remains some-what equivocal withδ13C signatures either similar to,or slightly depleted compared withP. antipodarum.Snails can be an important component of the diet ofadult rainbow trout and have been observed in thestomachs of juvenile salmonids and bullies (Graynothet al., 1993). However, juvenile salmonids generallyselect for chironomid larvae, Hydroptilidae andHud-sonemaspp. and against molluscs and oligochaetes(Graynoth, NIWA, pers. comm). In the case ofP.antipodarum, this is probably because they have anoperculum and harder shell than other gastropod taxa(McCarter, 1986).

Carbon flow

The application of stable isotopes has given us newinsights into the food web dynamics in the littoralzone of Lake Coleridge. We have combined these res-ults with those from radioactive tracer experimentsand analyses of stomach contents (Graynoth et al.,1993; Graynoth, NIWA, pers. comm) to postulate afood web in terms of carbon flow (Fig. 5). Macro-phytes contributed over 99% of the biomass at theprimary production level while gastropods, dominatedby P. antipodarum, dominated the primary consumerlevel (88%). From our experiments and observations,macrophytes and gastropods dominate the biomass be-cause there is little top down control and their biomassis limited more by physical factors such as light andwave activity, and food supply, respectively. Furtherevidence thatP. antipodarumis controlled by bottomup factors are the vertical profiles of periphyton pro-duction, macrophyte biomass andP. antipodarumdis-

tribution. All these variables displayed a remarkablysimilar depth distribution which in turn was similarto Kd for light attenuation (James et al., 2000). Oli-gochaetes are rarely observed in stomach contents offish, and likeP. antipodarumcould be considered asink in terms of carbon flow, although it is possiblecarbon from oligochaetes could be assimilated morerapidly than fromP. antipodarum.

Epiphytic algae appear to underpin much of theproduction in the littoral zone of this oligotrophic lakewhile trichopteran and chironomid larvae mediate car-bon flows through to fish. This contrasts markedlywith a number of other studies where phytoplank-ton have been suggested as the major driving force(Keough et al., 1996). A feature of Lake Coleridgeand other South Island, New Zealand lakes is the lackof large herbivores such as freshwater crayfish andherbivorous fish.

In conclusion, it could be argued that Shelford’s(1918) early ideas on the role of macrophytes holdtrue for lakes like Lake Coleridge, but this is largelydue to the absence of large obligate herbivores. Itis this lack of large herbivores and selection againstmacroinvertebrates such asP. antipodarumby predat-ors that contributes to the abundant macroinvertebrateand macrophyte communities (James et al., 1998) andsome of the deepest growing macrophyte communitiesworldwide (DeWinton et al., 1991; De Winton, 1994).What happens to the large biomass of macrophytesremains a mystery. Some material will be transpor-ted down the steep sides of lakes such as Coleridgethrough physical processes (Schwarz et al., 1999).However, in general, under stable conditions, bio-mass of characean macrophytes varies little over anannual period, with apical growth balanced by basaldecay (Schwarz et al., 1999). Further work is requiredto determine whether macrophytes provide a signi-ficant source of DIC for primary producers throughbasal decomposition and respiration and the role ofmacrophytes in DOC cycling.

Acknowledgements

We thank Eric Graynoth for helpful comments on thedraft manuscript and an anonymous referee. The workwas funded by the New Zealand Foundation for Re-search, Science and Technology (Contract CO1816).

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