Hydrobiologia 269/270 : 453-462, 1993 .H. van Dam (ed.), Twelfth International Diatom Symposium .© 1993 Kluwer Academic Publishers. Printed in Belgium .

Effects of nutrient (N, P, C) enrichment upon the littoral diatomcommunity of an oligotrophic high-mountain lake

Pius Niederhauser & Ferdinand SchanzInstitut of Plant Biology, Limnological Station, University of Zurich, Seestr. 187, CH-8802 Kilchberg,Switzerland

Key words: diatoms, nutrient enrichment, artificial substrate, high-mountain lake

Abstract

The effects of nutrient additions upon the epilithic diatom communities and the algal standing crop wereinvestigated in the oligotrophic, softwater Lake Piccolo Naret, situated in the Swiss alps. Nutrient-diffusing flower pot substrates were filled with either N (0 .15 mol NaNO 3), P (0.0 15 mol Na2HPO4) orC (0.15 mol NaHCO 3 ) or combinations of them . Twenty-five pots representing eight treatments wereplaced into the lake in July 1991 and sampled after 42 days of exposure .

On the surfaces of all pots containing P we measured higher algal biomasses as on the control pots .The chlorophyll-a maximum of 12 .9 .tg cm - 2 was obtained on NPC pots (0.47 pg cm - 2 on control pots) .On pots with P, NP or NPC supply high amounts of green algae were detected, also reflected in anincreased chl-b/chl-c ratio related to the controls which showed algal communities dominated by dia-toms .

The diatom communities on the control pots as well as on the pots with N, P and NP had a struc-ture similar to the epilithic community in Lake Piccolo Naret (dominance identity > 58 %) . However,the community structures of the diatoms from pots with C addition (C, NC, PC and NPC) differedconsiderably. This is discussed in view of the cell densities of dominant diatom species . For furthercomparisons the results of two additional high-mountain lakes are used . By means of a cluster analy-sis it could be shown that epilithic diatom communities were considerably influenced by C addition,while N and P supply were of minor importance .

Introduction

Benthic algal communities in softwater lakes aresensitive to environmental changes as acidifica-tion or eutrophication (Stevenson et al ., 1985 ;Fairchild & Sherman, 1992) . Inorganic phorpho-rus and inorganic nitrogen are mostly consideredto be limiting factors for the growth of periphytonalgae (Stockner & Armstrong, 1971 ; Marcus,1980 ; Fairchild & Lowe, 1984 ; Fairchild et al.,1985). However, several investigators have shown

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that the concentration of inorganic carbon is alsoof some importance for the biomass developmentand the species composition of periphytic algalcommunities in oligotrophic lakes (Sheldon &Boylen, 1975; Turner et al ., 1987, 1991 ; Fairchild& Everett, 1988; Fairchild et al ., 1989 ; Fairchild& Sherman, 1990, 1992) . Moss (1973) found thatthere is a relationship between the followingratios of inorganic carbon ion concentrationsand the species composition : CO2/HCO3 andHCO3 /C03 - . As these ratios depend on the pH

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values there is in turn also a possibility to explainthe influence of pH changes on the periphytoncommunities .

Diatoms are widely used to assess the lakewater quality since they are often good indicatorsfor pH, salinity or nutrient conditions (Smol,1987; Charles et al., 1989). Samples taken fromsediment layers allow us to reconstruct the de-velopments of the above mentioned environmen-tal factors in the past (Dixit et al., 1992). Theseecological investigations are all based on earlierprojects characterizing simultaneously the diatomcommunities of water systems and their abioticcharacteristics . The results are used to get an in-sight into the dependance of diatom communitieson environmental conditions . However, observedchanges in environmental conditions might be thecause of a biological alteration but there are al-ways numerous physical, chemical and biologicalfactors which are not considered in an ecologicalproject. Furthermore we know little about thedirect effects of environmental factors on diatomcommunities. Therefore, the above mentionedempirical procedure can lead to severe misinter-pretations. Experimental work should be done toelucidate the direct relationships between envi-ronmental factors and the composition of diatomcommunities. The in situ method of Fairchild &Lowe (1984) uses artificial substrates which con-tinuously release some nutrients . The procedureneeds small amounts of material and can there-fore be applied also in alpine regions which haveto be visited on foot . After a certain time of ex-posure the resulting biofilms on the artificial sub-strates are compared with those of the nearbylittoral benthos . In most of the investigations littleattention was paid to the diatom communities(Fairchild & Lowe, 1984 ; Fairchild et al ., 1985 ;Fairchild & Everett, 1988; Fairchild et al., 1989 ;Fairchild & Sherman, 1990, 1992) .

The study presented herein should show theinfluence of inorganic carbon, phosphorus andnitrogen alone or in combination on the epilithicdiatom communities of an oligotrophic, softwaterlake of the high-mountain area of Switzerland .We were especially interested in the effects of anincreased nitrate concentration on the diatom

community, because in the alpine region the ni-trogen component of the wet deposition (34-65µeq 1 - t ) is considerably higher than in remoteareas in Northern European countries (e.g. Nor-way, 14-41 µeq 1 -1 ; Wathne et al., 1990). To getan idea of the ecological significance of the resultsobserved we looked further for the natural con-ditions of alpine lakes showing similar diatomcommunities as on the artifical substrates .

Study area

The artificial substrates were exposed in LakePiccolo Naret (Cristallina area, southern Switzer-land, Fig . 1). The lake has a surface area of 2 .3ha, a maximum depth of 4 .0 meters and is situ-ated at 2348 meters a.s .l . The drainage area ischaracterized by cristalline rocks (gneiss) and ascarce vegetation . Cows and goats pasture therefor a period of three to four weeks in the summermonths .

Fig. 1 . Geographical situation of the Cristallina area in Swit-zerland (upper left hand side) and map with the Lakes Pic-colo Naret (1), Val Sabbia (2) and Naret . Altitudes in metersa . s . 1 .

The diatom communities of Lake Val Sabbia(Fig. 1) and Lake M in the Macun region wereconsidered for comparisons with the experimen-tal results. Lake Val Sabbia has a surface area of1.7 ha and a maximum depth of 8 .7 meters ; itsdrainage area consists of cristalline rocks andlimestone (Bilndnerschiefer) . Lake M (2550meters a.s .l.) lies in south-eastern Switzerlandwithin the Macun region which was described indetail by Schanz (1984) . It has maximum depthof 1 .3 meters and a surface area of 0 .2 ha ; thereare only cristalline rocks in the drainage area .Near lake M a military refuge is periodically oc-cupied. It might be, that nutrients and acid neu-tralizing substances are released from the refugeinfluencing the chemistry of lake M .

All three lakes are usually covered with ice fromNovember to June. The chemical and physicalparameters of the three water bodies are given inTable 1 .

Materials and methods

In the field experiment we used nutrient-diffusingclay flower pot substrates (Fairchild & Lowe,1984). In preparation for the fieldwork we firstestimated nutrient flux rates from the pots in thelaboratory .

Nutrient release experiment

Clay flower pots (d = 8.6 cm) were exposed to 0.5n HCl for one day, hold in deionized water foranother two days to minimize the nutrient con-tent of the clay pots. The hole in the bottom of thepots was closed by a rubber plug ; the pots werethen filled with one of the following solutions :

(1) Controls : deionized water, but no agar (fourreplicates, n = 4); to check for N and/or Pcontamination of the pots themselves ;

(2) Carbon treatment (C) : 300 ml of an agar so-lution (2%, w/v) with 0 .15 mol NaHCO 3 perpot (n = 3) ;Nitrogen and phosphorus treatment (NP) :300 ml of an agar solution (2%, v/w) with0.15 mol NaNO 3 and 0.015 mol Na2HPO4per pot (n = 3) .

Each pot was closed at the top using a petri dish(PD) and sealed up by silicon material . We putthem individually in a 3-liter plastic beaker (PB)filled up with 2 liters of deionized water and cov-ered them tightly to minimize gas exchange . Theplastic beakers were kept at room temperature(20 ° C + / - 2 ° C) for 29 days and the water waschanged every 24 h. Nitrate (UV-method; Ameri-can Public Health Association, 1975), Phosphate(ascorbic acid method ; DEV, 1981) and alkalin-

(3)

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Table 1 . Chemical and physical characteristics of Lake Piccolo Naret (L1), Lake Val Sabbia (L2) of the Cristallina region andLake M (L3) of the Macun area . Dates for L1 and L2 from July 23, August 13 and September 3, 1991 and dates for L3 fromAugust 30, 1988 .

Lake Piccolo Naret Lake Val Sabbia Lake M

July Aug . Sep . July Aug. Sep . Aug.

Conductivity (µS cm -1 ) 9 12 14 19 18 21 40pH 6.6 6 .4 6 .6 7 .0 6 .7 7 .1 7 .0Chlorophyll-a (µg 1 - ') 0 .1 0 .3 1 .9 0 .1 1 .1 1 .8 -Alkalinity (µeq 1 - ') 46 57 81 145 140 152 201Total P (µmol 1 - ') 0 .06 0.13 0 .32 0 .19 0 .16 0 .22 -P043- (µmol l _ ') <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05N03- ( µmol1 -1 ) 11 18 9 4 5 2 6S04 2- (µmoll - ') 17 17 26 30 33 34 21NH4 '(µmoll - ') <0.3 1 1 < 0 .3 2 < 0 .3 1

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ity (endpoint titration; DEV, 1981) were mea-sured after 1, 2, 3, 6, 9, 12, 15, 23, and 29 days .

In situ experiments

On July 23, 1991, 25 pots were anchored in thelittoral zone of Lake Piccolo Naret at a depth of0.4 to 0.6 m. The following factorial design wasused in the experiment (for the agar and the nu-trient concentrations, see the nutrient release ex-periment) : nitrogen addition (abbreviated withN), phosphorus (P), inorganic carbon (C), NP,NC, PC and NPC . There were triplicates of eachexperimental setup . An additional 4 pots werefilled with deionized water to get controls as de-scribed by Fairchild et al . (1989). We did not addagar because of its high purity no or only smallamounts of substances would be released, whichwould have no influence on algal growth . Thedistance between the pots was almost one meter .

The pots were sampled after 42 days. A 500-mlbeaker was placed over each pot, enclosing thepot surface (175 cm2) and 230 ml of surroundingwater. The algal periphyton was removed using atoothbrush and added to the surrounding water .Sample volume was adjusted to 400 ml withdeionized water. From the vigorously shaken sus-pension we used 100 ml for preparation of dia-toms, 50 ml for inspections of the living algalmaterial and 50 ml for chlorophyll-a preparation .Three stones were taken out of the littoral zoneand the algal periphyton removed from an area of9.1 cm2 each (Douglas, 1958) was used for chlo-rophyll determination (Jeffrey & Humphry, 1975)and diatom analysis . The diatom samples werecleaned with H 2 S04 (Straub, 1981), and the ma-terial was embedded in Naphrax . Five hundredvalves were counted (Leitz Diaplan microscope ;objective PL Fluotar 100, magnification 1000 x,N.A. 1 .32). Krammer & Lange-Bertalot (1986-1991) and Flower & Jones (1989) were used fortaxonomy. The similarities between the replicateswere characterized by the dominance identities(Renkonen, 1938) .

Comparisons of means (chlorophylls, cell den-sities of some diatom species) has been done by

the t-test (Linder & Berchtold, 1979 ; level of sig-nificance P<0 .05).0.05). We used the programMULVA-4 (Wildi & Orloci, 1990) for clusteranalysis (no transformation of data, chord dis-tance as similarity measure, hierarchical classifi-cation according to the minimum-variance-analy-sis). Only those species with relative abundance>_ 1 % were included in the cluster analysis (49taxa). The absolute distance values calculated bythe program are of minor importance for inter-pretation purposes ; therefore, all results are re-lated to the maximum value .

Analysis of lake water

We determined the alkalinity by the method ofGran-plot (Stumm & Morgan, 1981) . The con-ductivity was measured at 20 'C . The othermethods of water analysis are given by Schanz(1987).

Results

Nutrient release experiment

The control pots released 1 .1 µmol d - 'N03 -N (s = 0.21) and 1 .1 µmol d -' P04 - -P(s = 0 .13) within the first 24 hours of exposition .The C pots released 2 .7 µmol d - ' N03 -N(s = 0 .31) and 12.1 µmol d -' P04 - -P (s = 0.66)within the first 24 hours and after six days 0 .0µmol d -' N03 -N (s = 0.00) and 1.1 µmol d - `PO4 - -P (s = 0 .17). These rates of release are verysmall compared with the diffusion rates of thepots with additions of N and P (Fig. 2) and thuscan be neglected . The diffusion rates of bicarbon-ate, nitrate and phosphate decreased consider-ably from the first to the second day and subse-quently followed an exponential decreaseresulting in a linear semi-logarithmic relationship .The calculated diffusion rates after 42 days (du-ration of the field experiment) were 750 µmol d -'for C, 200 for N and 140 for P. Since these dataare based on release rates in a static containerusing pots without biological cover one would

100 -

10

0 .1

000

4

AP

0

10

20

30

dayFig . 2 . Daily rate of N, P and C release from pots contain-ing 0.15 mol NaNO3, 0.015 mol Na 2HPO4 or 0.15 molNaHCO3 . Means of three parallel experiments, standard de-viation in percent of the mean N < 15 %, C < 7 % and P < 13 %(except day 1 of the measurements : 25%) .

suspect that they would be different in the natu-ral environment where more water movement oc-curs (increasing the diffusion rate) and the biofilmon the pot is getting denser with time (decreasingthe diffusion rate). Therefore, it seems to be ex-tremely difficult to determine the diffusion ratesunder natural conditions .

In situ experiment

In Lake Piccolo Naret the control pots had lessbiomass of algal periphyton (measured as µg chl-acm - 2 ) than the littoral stones (Fig . 3A). The algalbiomasses on the pots with the additions of onlyN (z = 0.59 pg chl-a cm - 2, s=0.11) or C(x = 0.45, s = 0 .04) did not differ significantly fromthe controls (z = 0 .47, s = 0 .05), whilst the addi-tion of both N and C resulted in a increasedbiomass (z =0 .91, s=0 .04) . All pots containingphosphorus (P, NP, PC, NPC) had a significantlyhigher biomass on their surface than the controls .Most biomass occurred on the NPC pots with amean of 12.2 jig chl-a CM-2 (s = 2 .44) .

The microscopical inspection of the algal per-iphyton showed that only Bacillariophyceae,

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Fig. 3 . Fig . A. Biomass as chlorophyll-a per cm2 of pot sur-face . Fig . B . Ratios of chlorophyll-b/chlorophyll-c at the endof the experiments . Values presented : means ± standard de-viations (n = 3, except for controls n = 4). L1 = Lake PiccoloNaret, Cont . = Control, with deionized water, N = 0 .15 molNaHNO3 , P = 0 .015 mol Na 2HPO4 , C = 0.15 mol NaHCO 3and NP, NC, PC and NPC nutrient combinations using theconcentrations indicated .

Chlorophyceae and Conjugatophyceae were ofsome quantitative importance. The ratios of chl-b/chl-c of the periphyton material from the potsurfaces and of those from the littoral stones arepresented in Fig . 3B . We expect low values whenthe diatoms dominate the algal community . In theexperiment presented here we calculated ratios of0 .7 for the control pots and 0 .8 for the stoneswhich point to a more or less equal dominance ofdiatoms . The other pots showed higher chl-b/chl-c-ratios (Fig . 3B). The high algal biomasses onthe P, NP and NPC pots were caused by green-algae (Oedogonium sp ., Spirogyra sp ., Zygnemasp. and Scenedesmus sp.) .

Figure 4 presents the cell densities on the potsurfaces at the end of the experiment for fourdiatom species to demonstrate their different sen-sibilities for the nutrient additions . This is thecause for alterations in the community structuresobserved. Compared with the controls significanthigher cell densities resulted in Achnanthes minut-issima Kiitz . when bicarbonate was added(Fig . 4). As to the controls the cell densities ofCymbella minuta Hilse ex Rabenh. were also in-creased on pots with bicarbonate supply ; thehighest cell density of 8 .14 cells cm -2(s = 2 .14) occurred on the PC pots . Gomphonemaparvulum (Kiltz.) Ktitz . and Nitzschia gracilisHantzsch showed higher cell densities on potswith PC or NPC than on the controls . Achnanthesbioretii Germain, Cymbella silesiaca Bleisch, Fragi-

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32 .5

21 .5

Achnanthes minutissima

14.

U3 Gomphonema parvulum

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ZFig. 4 . Cell densities (per cm 2 of pot surface) at the end of theexperiments for the eight treatments. Values presented, asFig . 3 . For Cont., N, P, C, NP, NC, PC, and NPC see Fig . 3 .

lana capucina var . gracilis (ostrup) Hustedt andNitzschia hantzschiana Rabenh. also showedmaximum cell densities on PC pots .

Relative abundance values of the most impor-tant species are shown in Fig . 5 . The species com-position on the littoral stones of Lake PiccoloNaret (Lake 1), Lake Val Sabbia (Lake 2) andLake M of the Macun area (Lake 3) are presentedin the same figure as well . The diatom communi-ties on pots with identical nutrient additions wereof high structural similarity (dominance identity66-90%). The communities on the control potsas well as those with N, P and NP additions arecharacterized by the species Achnanthes curtis-sima Carter, A. minutissima, A . scotica Flower &Jones, A. subatomoides (Hustedt) Lange-Bertalotand Cymbella minuta Hilse. The sum of these spe-cies reached a proportion between 44% (P pots)and 58% (N pots) of the total number of indi-viduals. The periphyton of Lake Piccolo Naret isalso characterized by the above mentioned fivespecies . A . minutissima was the most abundantspecies on C and on NC pots ; the species with thesecond highest abundance was C. minuta . Theperiphyton of Lake Val Sabbia was dominated byA . minutissima ; besides Cymbella microcephalaGrun . and Denticula tenuis Kiitz. were of someimportance. These two species has been found onthe experimental pots with relative abundances aslow as S 0.2 % . C. minuta was the most common

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20

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® Nitzschia hantzschiana

® Nitzschia gracilis

Gomphonema parvulum

Cymbella minuta

Fig . 5 . Relative abundance of diatom species with relativeabundance > 8% on the experimental pots and on stones inthe littoral zones of Lake Piccolo Naret (Li), Val Sabbia (L2)and Lake M (L3). Values = means (n = 3, except for controlsn = 4) . For Cont., N, P, C, NP, NC, PC, and NPC see Fig. 3 .

•

2

•

1

species on PC pots, whereas A . minutissimareached a relative abundancy of 11 %, which wasin the same range as Nitzschia hantzschiana (9%)and N. gracilis (11 %) . The diatom periphyton onstones of the Lake M was characterized by highproportions of C. minuta (23%) and N. hantz-schiana (11 jo ) a result similar to that from the PCpots in Lake Piccolo Naret. However, Fragilariapinnata Ehrenb . was with 22% much more com-mon in Lake M than on the experimental pots .Gomphonema parvulum was the most abundantspecies on NPC pots ; this species did not exceeda proportion of 4 % in any of the alpine waterbodies investigated .

The diatom communities from all 25 pots andthose from the littoral zones of the alpine lakes

0 0CL Z

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Achnanthes subatomoides

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Achnanthes scotica

0 Achnanthes minutissima

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Achnanthes curtissima

were grouped according to their similarities bymeans of a cluster analysis (Fig . 6) . It is evidentthat all experimental setups with bicarbonate ad-ditions (Fig . 6, groups 2, 3 and 4) could be sepa-rated from those without bicarbonate (group 1) .Group 1 consists of the littoral periphyton com-munities of Lake Piccolo Naret (Lake 1) and thoseof the pots containing N, P and NP . The diatomcommunities on the C and NC pots as well asthose from Lake Val Sabbia (Lake 2) form group2. Group 3 contains the communities of the NPCpots . The diatom communities of PC pots andthose from the littoral stones of the Macun LakeM (Lake 3) are associated together in group 4 .

Discussion

The C:N:P-ratio in algal biomasses is 106 :16 :1with an C :N-ratio of 6 .6 (Redfield, 1978) . Bicar-bonate concentrations (= Alk + [ H + ]) of 46, 57and 81 µeq 1 - ' were measured in the pelagial ofLake Piccolo Naret. Considering conductivityand temperature the bicarbonate represents be-

0CC

PNPPN

P N

NCont .NCont .Cont .Cont .Lake INLake ICNCC

CNLake 2Lake 2Lake 2CPN

W CPNCPN

AVERAGE WITHIN - GROUP DISPERSION AS PERCENT OF TOTALB

CP•

CP

A

CPLake 3 i

o Lake 3Lake 3

Fig. 6. Cluster analysis based on the diatom communities onpots and on stones in the littoral zone of three lakes . Lake 1= Piccolo Naret, Lake 2 = Val Sabbia, Lake 3 = M (Macunarea) . For Cont., N, P, C, NP, NC, PC, and NPC see Fig . 3 .

459

•

P

N•

C

tween 51 and 62% of the Total Inorganic Carbon(TIC) at pH values in the range of 6.4 to 6 .6(Hatter, 1990) ; we calculated the following TICconcentrations : 77, 112 and 130 µeq 1 - 1 . At thesame sampling data the concentrations of inor-ganic nitrogen ([NO3 -N] + [NH4 -N]) were 11,19 and 10 µeq 1 - ' . These results led to C :N-ratiosof 7.0, 5 .9 and 13.0 which are nearby the criticalratio of 6.6 mentioned above. Therefore we as-sume that the algal growth in Lake Piccolo Naretwas limited not only by phosphates (which werenever detectable during the investigation period)but also by carbon and nitrogen . This could alsobe demonstrated by the in situ experiments show-ing a positive relationship between the biomassdevelopment and the phosphorus supply (Fig . 3) ;however, if phosphorus is offered in addition withcarbon and nitrogen, a further increase of bio-mass can be observed .

Fairchild & Everett (1988) did similar experi-ments in the oligotrophic, softwater Lake La-cawac as we did; they used the same substrateand the same concentrations of nitrogen andphosphorus. Carbon was added as glucose (0 .0 15or 0.15 mol) or as bicarbonate (0 .58 molNaHCO3 ) . Additions of nitrogen, phosphorusand carbon led to the highest algal yields . Carbonalone or carbon together with phosphorus had noeffect on the algal biomass when compared withthe controls, whilst carbon together with nitrogenpromoted the algal growth . Fairchild & Sherman(1992) found that the epilithic algal standing cropwas strongly related to the lake alkalinity . Nand/or P supply enhanced the chlorophyll-a con-centration typically only in lakes with an alkalin-ity greater than 100 µeq 1 - ' . This results supportour findings that the concentration of inorganiccarbon can be of great importance for the algaldevelopment in oligotrophic, softwater lakes .However, in Lake Piccolo Naret the phosphatewas of greater importance for the algal biomassthan it was in the above mentioned studies .

Diatom communities on pots with identical nu-trient supply (replicates) showed dominanceidentities (DI) of 66-90% which demonstratetheir close similarity (Engelberg, 1987) . The com-munities on the control pots were in turn very

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similar to the periphyton on the littoral stones inLake Piccolo Naret (DI > 60 % ). We thereforeconclude that on littoral stones the periphytoncommunities are more or less identical comparedwith those on pot surfaces with similar experi-mental conditions .

The growth of Achnanthes minutissima wasconsiderably influenced by inorganic carbon . Thehighest cell densities of the species were measuredin algal covers on pots enriched exclusively withinorganic carbon . Salden (1978) describes that A .minutissima prefers pH values between 7.2 and7.8 at high N :P-ratios. Round (1990) reported aconsiderable increase in the density of A . minut-issima after the liming of three acidified lakes ; itwas then the dominant species . The same resultwas found by Fairchild & Sherman (1990) . Theyobserved a further increase of the cell density ofthe species on artificial substrates enriched withNC or NPC exposed in a limed lake . We assumethat A. minutissima develops well under nutrient-poor conditions and increased concentrations ofinorganic carbon. This was indeed the case inLake Val Sabbia (Table 1) where the species wasfound with a relative abundance of 39% . In thethree lakes investigated by Round (1990) the den-sities of Cymbella minuta were markedly increasedafter the liming procedure . In our experiments wedetermined increased cell densities of C. minutaon pots with carbon enrichment compared withthose on control pots . Gomphonema parvulumgrew best on pots with NPC addition whichshowed voluminous biofilms on their surfaces .The dominance of G . parvulum on this pots isprobably caused by the ability of the species togrow on stalks in the third dimension. This is anadvantage compared with species living exclu-sively adherent . According to Salden (1978) thespecies prefers eutrophic water bodies . Nitzschiagracilis showed the highest cell densities on PCpots; it is typical for eutrophic waters with a lowpH value (Salden, 1978) .

Cluster analysis (Fig. 6) has many advantagesfor discussing changes of diatom communitystructures as shown in Fig . 5 . The results of thisanalysis show that the pots with carbon supplycan be clearly separated from those without ad-

ditional carbon. This may indicate the high im-portance of inorganic carbon on the diatom com-munities in oligotrophic, softwater lakes . Charles(1985) investigated the diatom communities in thesurface sediments of 34 lakes with pH values be-tween 4.5 and 7 .8 ; the species compositions weremainly influenced by the pH values and the alka-linities of the lake waters and less by the trophicsituation . The buffer capacity of the water at pHvalues below 8 .2 is mainly determined by its bi-carbonate concentration . If the alkalinity ischanged there are also alterations in the pH val-ues as well as in the components of the ratioCOZ:HCO3 :CO2 - (Stumm & Morgan, 1981) .The following possibilities can be discussed as tobe causes of pH influences on the composition ofthe diatom communities in water bodies: Thecommunity changes might be (i) due to the con-centration of protons, (ii) due to the alkalinity or(iii) due to other factors related to the pH as thetoxicity of aluminium, the uptake of phosphate orthe calcium concentration (Smith, 1990) . In ourstudy the first two points must be taken into con-sideration . The supply of bicarbonate in experi-mental pots must have increased the pH on theirsurface. However, we have no possibilities to sep-arate the influences of the proton concentrationon diatoms from the influences of the bicarbon-ate concentration . Because bicarbonate is a directinorganic carbon source for algal cells we sup-pose that it is more likely to be responsible for thecommunity changes than the protons . Their in-fluence on the algal growth is not obvious .We found the same diatom communities on

pots with nitrogen supply as on the controls andon stones in the littoral of Lake Piccolo Naret .Additional nitrogen together with phosphorus orcarbon did not lead to changes in the communitystructure compared with the treatments withphosphorus or carbon alone . It is therefore as-sumed that a future increase of the nitrogen inputinto the lake by precipitation does not alter thediatom community of the lake .

Comparing the diatom communities on the ex-perimental pots with those on the littoral stonesin Lake Val Sabbia and Lake M of the Macunregion we expected a better interpretation of the

experimental data : The experimental setups withcarbon alone or carbon and nitrogen showedsimilar diatom communities as the periphyton onthe stones in Lake Val Sabbia which is charac-terized by relatively high alkalinity 140-152 µeq1 - ' and low concentrations of POI - -P (< 0 .05µmol 1 - ') and N03 -N (2-5 µmol 1 - ') . The dia-tom communities on the experimental pots withphosphorus and carbon supply showed high simi-larities to those on stones in Lake M of the Macunregion (alkalinity: 200 µeq 1 - ') . There were highperiphyton biomasses and dense mats of greenalgae in Lake M ; it can be assumed that a higheramount of phosphorus was available in that lakethan in the others investigated, although no phos-phate could be detected .

We could not find conditions in anyone of thehigh-mountain lakes investigated which weresimilar to those in the experimental pots with ni-trogen, phosphorus and carbon supply .

Conclusions

Our experiment in the Lake Piccolo Naret showedthat an increase of phosphorus input would led toan increase in the epilithic algal biomass, althoughit was supposed from experiments reported in theliterature the structure of the diatom communitywould stay stable . However, an increase in thenitrogen content of the precipitation (HNO 3 , NH4 ) in the alpine region would have no influenceon the biomass nor on the community structureof diatoms . Associated with the nitrogen input byprecipitation is an increase in the acid input . Theinfluence on the alkalinity of the lake water wouldhave consequences on the epilithic diatom com-munities .

Acknowledgements

We thank U . Durst, S . Flock, P . Jann, M . Keller,M. Koss, E. Seger and T . Zwimpfer for their helpin the field and H. Maag for his technical support .Mr. Tomasin from the fisheries and hunting de-partment of the Canton Tessin kindly informed

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the fishery officers about the planned activities .The research was supported by the Swiss Na-tional Foundation, Grant 31-27375 .89 .

References

American Public Health Association, 1975. Standard Meth-ods for the examination of water and wastewater . 14th edn .American Public Health Association, Washington D . C.,1193 pp.

Charles, D . F ., 1985 . Relationships between surface sedimentdiatom assemblages and lakewater characteristics in Ad-irondack lakes . Ecology 66 : 994-1011 .

Charles, D . F ., R. W . Batterbee, I . Renberg, H . van Dam &J. P . Smol, 1989 . Paleoecological analysis of lake acidifi-cation trends in North America and Europe using diatomsand chrysophytes . In S . A. Norton, S . E . Lindberg & S . L .Page (eds), Acidic precipitation . Soil, aquatic processes,and lake acidification, Vol . 4 . Springer, N .Y ., 293 pp .

DEV, 1981. Deutsche Einheitsverfahren zur Wasser-, Ab-wasser- and Schlamm-Untersuchung . Verlag Chemie,Weinheim .

Dixit, S . S ., J . P . Smol, J . C. Kingston & D . F . Charles, 1992 .Diatoms : Powerful indicators of environmental change .Environ . Sci. Technol . 26: 22-32 .

Douglas, B ., 1958 . The ecology of the attached diatoms andother algae in a small stony stream . J . Ecol . 46: 295-322 .

Engelberg, K ., 1987. Die Diatomeen-Zonosen in einem Mit-telgebirgsbach and die Abgrenzung jahreszeitlicher Aspe-kte mit Hilfe der Dominanz-Identitat . Arch . Hydrobiol .110: 217-236 .

Fairchild, G . W. & A . C . Everett, 1988 . Effects of nutrient (N,P, C) enrichment upon periphyton standing crop, speciescomposition and primary production in an oligotrophicsoftwater lake . Freshwat . Biol. 19 : 57-70 .

Fairchild, G . W. & R. L. Lowe, 1984 . Artificial substrateswhich release nutrients : Effects on periphyton and inverte-brate succession . Hydrobiologia 114 : 29-37 .

Fairchild, G . W., R . L. Lowe & W. B. Richardson, 1985 .Algal periphyton growth on nutrient-diffusing substrates :An in situ bioassay . Ecology 66 : 465-472 .

Fairchild, G . W . & J . W. Sherman, 1990 . Effects of liming onnutrient limitation of epilithic algae in an acid lake . Wat . AirSoil Pollut . 52: 133-147 .

Fairchild, G . W. & J . W. Sherman, 1992. Linkage betweenepilithic algal growth and water column nutrients in soft-water lakes . Can . J . Fish . Aquat. Sci . 49: 1641-1649 .

Fairchild, G . W ., J . W. Sherman & F . W. Acker, 1989 . Ef-fects of nutrient (N, P, C) enrichment, grazing and depthupon littoral periphyton of a softwater lake . Hydrobiologia173: 69-83 .

Flower, R . J . & V . J . Jones, 1989. Taxonomic descriptionsand occurences of new Achnanthes taxa in acid lakes in theU. K. Diatom Res . 4 : 227-239 .

462

Hotter, L. A., 1990 . Wasser and Wasseruntersuchung, 4thedn. Otto Salle Verlag, Frankfurt am Main, 511 pp .

Jeffrey, S . W. & G . F. Humpfrey, 1975 . New spectrophoto-metric equations for determining chlorophylls a, b, cl andc2 in higher plants, algae and natural phytoplankton. Bio-chem. Physiol. Pflanzen 167 : 191-194 .

Krammer, K. & H. Lange-Bertalot, 1986-1991 . Bacillari-ophyceae : SOsswasserflora von Mitteleuropa, 2 (1-4) . Fis-cher Verlag, Stuttgart, 4 Vols .

Linder, A. & W. Berchtold, 1979 . Elementare statistischeMethoden. Birkhauser Verlag, Basel, 248 pp .

Marcus, M. D., 1980. Periphytic community response tochronic nutrient enrichment by a reservoir discharge . Ecol-ogy 61 : 387-399 .

Moss, B., 1973 . The influence of environmental factors on thedistribution of freshwater algae : An experimental study. IIThe role of pH and the carbondioxide-bicarbonate system .J . Ecol . 61 : 157-177 .

Redfield, A . C ., 1958 . The biological control of chemical fac-tors in the environment. Am . Scient. 46: 205-221 .

Renkonen, 0 ., 1938 . Statistische-bkologische Untersuchun-gen Ober die terrestrische Kaferwelt der finnischen Bruch-moore. Ann. Zool . Soc . Zool . Bot . Fenn. Vanamo 6 .1 :1-231 .

Round, F. E ., 1990 . The effect of liming on the benthic dia-tom populations in three upland Welsh lakes . Diatom Res .5 : 129-140 .

Salden, N ., 1978. Beitrage zur Oekologie der Diatomeen (Ba-cillariophyceae) des SUsswassers . Dechania-Beih . 22 :1-238 .

Schanz, F., 1984 . Chemical and algological characteristics offive high mountain lakes near the Swiss National Park .Verh . int . Ver. Limnol . 22: 1066-1070 .

Schanz, F., 1987 . Beurteilung des Einflusses von sauren Nie-derschlagen auf das Macun-Gebiet int Unterengadin(Schweiz). Verh . Ges . Oekol . 15: 249-255 .

Sheldon, R. B. & C. W. Boylen, 1975 . Factors affecting the

contribution by epiphytic algae to the primary productionof an oligotrophic freshwater lake . Appl . Microbiol . 30 :657-667 .

Smith, M. A., 1990 . The ecophysiology of epilithic diatomcommunities of acid lakes in Galloway, southwest Scot-land . Phil . Trans. R. Soc . Lond . B 327: 251-256 .

Smol, J . P ., 1987. Methods in quarternary ecology 1 . Fresh-water algae . Geosci . Can . 14: 208-217 .

Stevenson, R . J ., R . Singer, D . A. Roberts & C. W. Boylen,1985. Patterns of epipelic algal abundance with depth,trophic status and acidity in poorly buffered New Hamp-shire lakes. Can . J . Fish . aquat . Sci . 42: 1501-1512 .

Stockner, J . G . & F. A . J . Armstrong, 1971 . Periphyton of theExperimental Lakes Area, Northwestern Ontario . J . FishRes. Bd Can . 28 : 215-229.

Straub, F ., 1981 . Utilisation des menbranes filtrantes en teflondans la preparation des diatomees epilithique. Comptesrendus du 2e colloque de I'ADLAF . Cryptogamie. Algolo-gie 2 : 153 .

Stumm, W. & J . J. Morgan, 1981 . Aquatic chemistry. 2ndedn. Wiley & Sons, New York, 780 pp .

Turner, M . A ., M . B . Jackson, L . D . Findlay, R . W. Graham,E. R. DeBruyn & E. M. Vandermeer, 1987 . Early responsesof periphyton to experimental lake acidification. Can. J.Fish . aquat . Sci . 44: 135-149 .

Turner, M . A ., E. T. Howell, M. Summerby, R . H . Hesslein,D. L . Findlay & M . B . Jackson, 1991 . Changes in epilithonand epiphyton associated with experimental acidification ofa lake to pH 5 . Limnol . Oceanogr . 36: 1390-1405 .

Wathne, B . M ., R . Mosello, A . Henriksen & A . Marchetto,1990 . Comparison of the chemical characteristics of moun-tain lakes in North and South Europe . In M . Johanessen,R. Mosello & H . Barth (eds), Acidification processes inremote mountain lakes . E. Guyot SA, Brussels: 41-58 .

Wildi, O . & L . Orlbci, 1990 . Numerical exploration of com-munity patterns . SPB Academic Publishing, The Hague,124 pp .