seasonal and interannual changes of chemical characteristics and phytoplankton in a mountain lake of...

Int. Revue ges. Hydrobiol. 1 82 I 1997 I 1 I 15-31

NICO SALMASO AND FABIO DECET

IDipartimento di Biologia, Universitl di Padova, Via Trieste 75, 1-35121 Padova, Italy 2Presidio Multizonale di Prevenzione - ULSS 1, via S. Andrea 1 , 1-32100 Belluno, Italy

Seasonal and Interannual Changes of Chemical Characteristics and Phytoplankton in a Mountain Lake of the Eastern Italian Alps

(Lake Calaita, Trentino)

key words: mountain lakes, acidification, alkalinity, phytoplankton, seasonal dynamics

Abstract

Measurements at Lake, Calaita, a small mountain lake located at 1605 m. a.s.1. in a metamorphic catchment area in the south of the Trentino Region (Northern Italy), have revealed great seasonal variations in the chemical characteristics and phytoplankton community during the ice free period in 1992 and 1993. The acidity present in wet precipitations (H+, NH:) was neutralised within the drainage basin by mineral dissolution which led to an increase of basic cations and alkalinity in the runoff. The dilution during periods of higher discharge, e.g. in spring and autumn, resulted in low values of alkalinity (up to 60 peq 1-I), pH (mostly <6.7) and conductivity ( ~ 1 8 pS cm-I, 20 "C). In summer, the decrease in runoff caused higher alkalinity (>I 10 peq I-'), pH (6.9-7.4) and conductivity values (up to 30 pS cm-I). The phytoplankton showed a major development in summer (with biovolume values of up to 7000 mm3 m-j), two different taxa being dominant in 1992 (Oocystis cf. factutris) and 1993 (Synedrn sp.). Unpredictable climatic conditions have a strong influence on the physical stability of the lake, which makes it more difficult to explain the evolution of the phytoplankton community as opposed to deeper lakes.

1. Introduction

The study of high altitude Alpine lakes, initiated by the extensive work of TONOLLI and TONOLLI (1951), has been revived during the last decade (MOSELLO, 1984; GIUSSANI et al., 1986; MOSELLO et al., 1994) because of the serious threats posed by atmospheric depositions (PSENNER and CATALAN, 1994; PSENNER, 1994; STUMM and SCHNOOR, 1995). Beside descriptions of the chemical equilibria of these water bodies (MOSELLO et al., 1987; MOSEL- LO er al., 1992) recent investigations have been directed towards macroinvertebrate communities and fish (WATHNE, 1992), sediment composition, paleolimnology (WATHNE, 1992; CAMERON et al., 1993; MARCHETIO and LAMI, 1994) and phytoplankton (e.g. Ron, 1988; PUGNETTI et al., 1993).

Difficulties of access and subsequent logistical problems of high altitude lakes have hin- dered the establishment of regular study programmes, so little information is available about the temporal distribution of chemical, physical and biological parameters. This is an important gap in our knowledge because the major controlling environmental factors (e.g. atmospheric depositions, snowmelt, acid inputs) are typically characterised by strong seasonal patterns in such lakes.

Two sampling campaigns have been carried out during the ice free period (May-October 1992 and 1993) in the small, shallow Lake Calaita in the eastern Italian Alps. During periods of snowmelt and autumn rains pH and alkalinity values were significantly lower than those reported during the summer season. Such differences were accompanied by strong sea-

16 N. SALMASQ and F. DECET

Alto Adige

Trentino

W

\ i 8 . S. Martino .,..-.*gt*

di Castrozza I V . w!

i. Bovo / I

III

0 5 k m

Figure 1. Location of Lake Calaita.

sonal changes in phytoplankton composition. This paper analyses the changes in the chemistry and phytoplankton community evaluating critically the factors involved in the observed variations.

2. Study Site

Lake Calaita is located in the eastern Trentino Region, at the southern extremity of the Paneveggio-Pale di S. Martino Park at 1605 m above sea level (Fig. 1). The length and width of the lake are 450 and 200 m respectively and the surface area is about 0.05 km2 at maximum water level; maximum depth is about 3 m (TOMASI, 1962).

Lake Calaita is of glacial origin. It is underlain by a metamorphic basement of medium to fine grained schists and gneisses with albite nodules and potassic feldspars (LEONARDI, 1967). The southern part of the basin is obstructed by a terminal moraine. At times of low water level, the northern part develops into a swamp with grass tussocks emerging from the irregular lake bed. Part of the catchment is covered by coniferous forest. The slopes which delimit the lake

from the east are characterised mainly by thick layers of morainic rocks and detritic sediment. The catchment surface to lake ratio is approximately 35 : 1.

Spring snowmelt and autumn rains contribute the greatest proportion of the annual runoff. Data relating to the thickness of the snow mantle and atmospheric depositions recorded in two stations located near the study area are shown in Fig. 2. During summer, water levels are maintained by an artificial tributary, originating within the catchment, with a discharge of 3-5 1 c'. This tributary mainly drains water collected after summer rains and probably groundwater. Losses through the lake bottom take place in the area proximal to the moraine, but apart from these the lake has no visible outlet.

Chemical Characteristics and Phytoplankton in a Mountain Lake 17

0 1992 1993

Figure 2. Thickness of the snowpack and monthly precipitations measured at Malga Losch and Villa S. Andrea, at 13 km east and 14 km northeast of Lake Calaita, respectively.

Owing to the variability of runoff contributions from the catchment, from August to Sep- tember 1992 water levels decreased by about 1 m and then suddenly increased in October, at the start of the autumn rains. A similar increase was observed in October 1993. In 1993 spring water levels were lower due to a thinner snow mantle before ice thaw (Fig. 2) and remained at a similar level until the beginning of September.

3. Materials and Methods

Water samples for chemical and biological analysis were collected at the surface in proximity of the eastern edge of the lake in a site where the shores, rocky and free of vegetation, descend at sharp slopes. Owing to the low maximum depth (3 m) reached only at maximum water level, these samples are representative of the surface water column that, in the small L . Calaita, coincides with the greatest por- tion of the lake. Samplings were carried out between May and October for a total of nine sampling trips in 1992 and five in 1993.

Chlorides, nitrates and sulphates were determined by ionic chromatography (Dionex 200 I). Calcium, magnesium, sodium and potassium were measured by atomic absorption (Perkin Elmer 31 10). pH, conductivity (20 "C), alkalinity, ammonium, soluble reactive phosphorus (SRP), total phosphorus (TP) and silica were analysed using the standard procedures of the A. P.H. A. et al. (1989) as reported in detail in DECET ef al. (1995). In 1993 temperature measurements were available only for the months of July and August.

Chlorophyll a (corrected for pheopigments) was analysed by spectrophotometry after extraction in 90% acetone (LORENZEN. 1967; MARKER ef d., 1980). Phytoplankton samples were preserved in Lug01 solution with added acetic acid (SARACEN] and RUGGIU, 1974) and algal cells subsequently counted by inverted microscope (Zeiss IM35) following the technique described by LUND et al. (1958). The taxo- nomic classification adopted was that of BOLD and WYNNE (1985). Species identification was carried

18 N. SALMASO and F. DECET

Out following the manuals of the series SuJwasserJlora von Mitteleuropa founded by A. PASCHER and the text of BOURRELLY (1972). Some groups were identified using other specific keys: KOMAREK and F o ~ r (1983) for the Chlorococcales, HUBER-PESTALOPI (1955) for the Euglenophyceae and HUBER-PESTALOZZI and Fon' (1968) for the Cryptophyceae. Single cells were enumerated even in the case of colonial species. Biovolumes were computed after direct measurements of the cells and approxi- mation to geometrical shapes (ROT, 1981). Counts were carried out on both identified and unidentified fractions: the latter are represented almost exclusively by ultraplankton (small spherical and microflagellate cells with a maximum linear dimension <4 pm) and by a small number of minor flagellate cells with a volume <60 pm3. NO data on phytoplankton are available for the 22nd September 1992.

Several multivariate analysis techniques (cf. ORL6C1, 1978) were adopted for the treatment of the chemical and phytoplankton data. Individual samples were ordered by principal component analysis (PCA) calculated from the correlation matrix of the untransformed chemical variables. A cluster analysis (Euc- lidean distance, complete linkage method) was carried out on standardised variables to isolate groups of samples according to their chemical composition. The analysis of the phytoplankton data matrices were carried out by cluster analysis (complete linkage method) and nonmetric multidimensional scaling- NMDS (KRVSKAL and WISH, 1978; SALMASO, 1996); both techniques were applied to Bray and Curtis' dissimilarity matrices (BRAY and CURTIS, 1957) computed on density and biovolume data relative to identifiable algae (ultraplankton and small unidentified flagellates were excluded). Double square root transformation of the, original data was applied to reduce the weight of the most abundant species and those with high biovolume (FIELD et al., 1982; CLARKE and GREEN, 1988). Rare species found on one occasion only were not considered in the calculation.

4. Results

4. I . Physical and Chemical Characteristics

After ice thaw, the lake warmed up very quickly. In August the surface temperature rose to 17-20 "C (Fig. 3a).

Conductivity values were between 15-25 pS cm-' in 1992 and 17-30 pS cm-' in 1993. They showed an increase towards the end of the summer and a decrease following the autumn rains (Fig. 3 b).

Lake pH was always >6.0, but lowest during autumn and highest (7.0-7.4, Fig. 3c) in summer. The general pattern of seasonal variation of both pH and conductivity was similar between the two years. In 1993, however, values showed a faster increase than in 1992.

The concentration of total ions remained relatively low, i.e. 262-405 peq I-' and 316-539 peql-I, respectively, in 1992 and 1993. Bicarbonate was the dominant anion (66-151 peq 1-' in 1992 and 66-200 peq 1-' in 1993), sulphate was less important. Domi- nant cations were calcium and magnesium while sodium and potassium made up only 4% and 5%, respectively, of the total ion sum (Fig. 4).

The difference between the sum of anions and cations was always less than 10% of the total ionic concentration. The observed incongruences were probably due to the presence of ionic forms not included in the balance such as iron, manganese, organic anions and alumi- nium. These have been shown to be important elsewhere (COOK et al., 1986; HEMOND, 1990; 1994; PSENNER, 1987) and although low concentrations of Mn, Fe and A1 have been measured in this lake (DECET er al., 1995) no information is available so far about the presence of organic acids.

Increases in the total ions during the summer were caused by an increase in bicarbonates and calcium and magnesium. The concentration of these three ions in particular showed a highly significant correlation with conductivity (P < 0.01). The increase in total ions was more pronounced in 1993, in parallel to the observed increase in conductivity and PH.


p v 2o 2 5 /

a! 15- a . E g 10-

+ 5 - F?

0 M J J A S O

+ , 6.01 " " " " " M J J A S O

Figure 3. Changes in temperature (a), conductivity (b) and pH (c).

Nitrate concentrations were 9-52 pg NO3-N I-' and 6-97 pg N03-N I-' in 1992 and 1993 respectively. The highest values were observed during the spring period, however a peak value was also recorded in summer 1993. Nitrite and ammonium were always below detection limit (3 and 8 pg N 1-', for N-NO; and N-NH: respectively).

Spring and autumn total phosphorus concentrations were generally below 5 pg I-'; in summer this compound reach values over 30 pg I-' during both years of study. Orthophosphate concentrations were constantly below 3 pg I-'.

SiOz showed an increase during the summer and autumn months (up to 1.1 mg I-' in August 1992 and October 1993) but with a less regular pattern than phosphorus. A sample collected in August 1993, during a strong development of diatoms (see below), yielded values close to the limit of detection.


Figure 4. Ionic balance of Lake Calaita: concentration (top panel) and percent ionic composition (lower panel); ammonium which was always below detection limit, was not included in the

calculations.

4.2. Phytoplankton

Values of chlorophyll a, phytoplankton density and biovolume were generally highest during the summer months (Fig. 5). However, chlorophyll a reached relatively high levels, close to 18 pg 1-‘, in October 1993 (Fig. 5a).

The uluaplanktonic component contributed an important fraction of the overall phytoplankton density (Fig. 5b). In terms of biovolume however, this component was of less importance due to its modest size (Fig. 5c). The mean sample volume for the identified taxa (calculated as the ratio of biovolume to the number of cells for each sample) is comprised between 218 and 1294 pm3. The identified organisms belonged almost exclusively to the nannoplanktonic group (sensu KALE, 1972: linear dimensions <64 pm) with only 2 gene- ra larger than 6000 pm3, Peridinium and Staurastrum. The greatest roportion of the taxa

quent size was 66-524 pm3. Excluding the October 1993 sample, chlorophyll a significantly followed the general pat-

tern of phytoplankton total biovolume and determined fraction biovolume (? = 0.69 and 0.72 respectively; P c 0.01). A similar relationship holds for density, but only for the identified fraction (2 = 0.66; P < 0.01).

The largest number of organisms identified refer to Chlorophyceae (over 30 taxa), and Bacillariophyceae (13). The remaining groups (Crypto-, Chryso-, Dino- Eugleno- and Dino- phyceae) contribute between 4 and 6 taxa each. The most frequent tuxa, found in almost half of the samples, are represented by Ankistrodesmus fisiformis CORDA, Chlamydomonas sp., Monoraphidium griffithii (BERK.) KoM.-LEGN., Oocystis cf. lacustris CHODAT, Spondy- losium sp., Staurastrum spp., undetermined Tetrasporales, Mallomonas sp., Achnanthes sp.,

identified in the entire sampling series was in the size range >524 pm P (Fig. 6). the most fre-


" M J J A S O

" M J J A S O

Figure 5 . Changes in chlorophyll a concentration (a), density (b) and biovolume (c) of phytoplankton; for the density and biovolume data black symbols refer to the identified fraction only; the light grey

symbols refer to total phytoplankton, ultraplankton included.

Cyclotella sp., Synedra sp., Glenodiniurn sp., Cryptomonas sp. and Cryptomonas tetrapyre- noidosa SKUJA.

The changes in dominance between the 7 phytoplankton classes (Fig. 7) showed that the least common were Cyanophyceae (with density values of 0%-6.7% and biovolumes of 0%-0.7 %) and Euglenophyceae (0%-2.9 % and 0%-8.8 % respectively).The dominant classes in at least one sample, with close to 80% density or biovolume, were Chlorophyceae, Bacillariophyceae and Chrysophyceae. There were some interannual differences : the 1992 samples from July and August were dominated by Chlorophyceae (almost entirely Oocystis cf. lacustris, 3580-6270 cells ml-'; 1255-2195 mm3 m-3) while the 1993 samples were


0.5

0.4

$ 0.3 6)

E 0.2

0.1

0.0

Fr,

Figure 6. Frequency distribution of phytoplankton volumes (identified fraction and ultraplankton) monitored in the entire sampling series in Lake Calaita; frequency classes, reporting the superior limit, were determined on the basis of spherical volumes obtained from the sequence of diameters 3, 5, 10.

15, 20 and 25 (cf. LARSON, 1973).

100

75

50

25

0 100

75

50

25

0

1992 1993

Crypto. Dino. Bacillario. Chryso. Eugleno. Chloro. Cyano.

Figure 7. Changes in the percentage contribution of the determined phytoplankton classes to density (top panel) and biovolume (lower panel).


dominated by Bacillariophyceae (Synedra sp., 575- 16950 cells ml-' ; 170-5085 mm3 m-3) with a partial contribution of Chlorophyceae and Dinophyceae. Chrysophyceae and Crypto- phyceae were of relative importance only during the spring and autumn months.

5 . Discussion

5.1. Chemical Characteristics

The chemical characteristics of Lake Calaita can be related to those of other lakes on igneous or metamorphic catchments (see MOSELLO et al., 1994) whose ionic contents reflect a limited action of substrate erosion.

The seasonal change in ionic concentrations followed a comparable pattern in both years, but the increase was faster in 1993. To highlight the relationship between the different samples on the basis of their chemical composition, the individual sampling dates were ordered by PCA analysis (Fig. 8; Table 1); in the ordination graph, the four principal groups identified by means of cluster analysis have been enclosed by dotted lines. The first two principal components (Table 1) explain 47.3 % and 26.3 % of the total variance. The first component shows a significant positive correlation with calcium, magnesium, sodium and bicarbonates, and a negative correlation with H'. This component reflects the influence of weath- er in the catchment. Such processes are mediated by the amount, composition and residence time of runoff: They produce more consistent effects during the summer period (right hand side quadrant) than during spring and autumn (left hand side quadrant). The first component, given its positive correlation with the dominant ions, can be interpreted also as a concentration gradient: the first axis is positively correlated to conductivity ( r = 0.85) and to the total sum of ions ( r = 0.85). The second component is positively correlated to potassium, chloride and sulphate and may be interpreted as an axis related to the enrichment of

PCA Axis I

Figure 8. Ordination of the samples of Lake Calaita along two principal components on the basis of their chemical composition. The points corresponding to consecutive sampling units have been joined with a broken line. The four principal groups identified by means of cluster analysis carried out on the same variables utilised in PCA have been enclosed by dotted lines. Roman numbers refer to the sam-

pling months.


Table 1. Percentage of explained variance and correlation between the first two components and the original variables; significant correlations ( P S 0.01) are reported in bold.

PCA Axis I1 I ~ ___

Variance explained 47.3% 26.3%

ca2+ 0.82 0.37 Mg2+ 0.89 0.42 Na' 0.68 -0.40 K+ -0.5 1 0.78 H' -0.89 0.27

so;- 0.46 0.65 c1- -0.57 0.74 N 4 -0.10 0.24

HCOj 0.87 0.44

these ions by evaporative concentration or atmospheric deposition. The second axis distin- guishes the 1992 summer samples from those of 1993 and the spring samples from those of the autumn.

The four groups identified by the cluster analysis indicate comparable chemical characteristics of the spring and autumn samples in both years when grouped by season, but some difference ih composition between the summer groups in 1992 and 1993.

5.2. Chemical Characteristics in Relation to Runoff

The mean ionic composition of the four principal groups identified on the basis of the cluster analysis (cf. Fig. 8) is summarised in Fig. 9. To highlight the influence of runoff on the chemical characteristics of the lake, the figure includes average values of the ionic composition of the principal tributary of Lake Calaita based on data (reported in DECET et al., 1995) including samples representative of the spring, summer and autumn seasons; these data are characterised by low chemical variability (with pH and conductivity comprised in the range 7.15-7.46 and 31-49 pS cm-' at 20 "C respectively). Also included is the mean composition of the rain collected with a wet only deposition sampler in two of the closest mountain stations of the RIDEP (Rete Italiana per lo studio delle Deposizioni atmosferiche) network: Malga Gallina (1868 m, about 60km N N W ) between 1988 and 1991 and L. Sauris (1000 m, about 80 km NE) between 1990 and 1992 (MOSELLO, 1991 ; 1993a; 1993b; MOSELLO and MORSELLI, 1992a; 1992b). Taking into consideration the moderate variability of the mean values calculated for the two stations during the different years, these deposition data can be considered as typical of this alpine area and are comparable to other available data (basic cations, SO:-, NO$ CI-) obtained during a series of sampling expeditions to the Mount Cherz station (2000 m), about 30 km NE of Lake Calaita (DECET et al., 1995).

The higher ionic content and the different composition of the tributary in relation to the atmospheric depositions reflect transformations by biogeochemical processes occurring in the soils. In particular a net decrease in nitrogen (as NH:, with production of H+, and also as NO; implying a consumption of H') is observed as a result of active adsorption by plants and bacteria (cf. VAN MIEGROET, 1994) and a net increase in alkalinity and basic cations occurs as result of weathering processes in the soil. Chlorides (primarily of meteoric origin) behave conservatively while sulphates show a slight increase. Taking into account the levels of total acidity (H+, NH,') present in atmospheric depositions, the chemical composition of the tributary indicates a net production of alkalinity in the catchment. Such neutralisation of


400

7 200 d

6

g o

200

400

I

Con&-18 pH-5 26

I

Cond.-23 pH-7 17

I Cond.48 pH-7.27

I

Cond -16 pH-6.2S

H H’ 0 NH,’ E l K’

Mg” Na’

Ca2+ R NO,- 0 c1-

N HC0,- a so,”

Figure 9. Mean ionic balances of atmospheric deposition in two stations next to the examined area, of the main tributary and of the four groups of samples pointed out in Figure 8. The top panel shows also

the mean values of conductivity and pH relative to the different groups (explanation in the text).

acid depositions takes place also by runoff generated after snowmelt and by precipitation and its intensity depends on the contact time between the soil and the wet deposition; however, considering also the dispersed nature of these sources of runoff, the variability of their composition has not been considered in this study.

The lower values for the sum of ions measured in the lake in spring and autumn are related to the dilution caused by atmospheric deposition and melting of the snow cover (spring) and by atmospheric deposition alone (autumn); such processes are favoured by the small volume of the lake. The chemical composition of Lake Calaita in spring is estimated to be a consequence of the dilution of the artificial tributary (and of runoff of similar composition originating within the catchment) with the water produced by melting snow in a ratio of 1 :3 (DECET et al., 1995). In spring and in autumn the sum of ions, the conductivity and the concentration of the principal ions, show comparable values. The main difference between the two periods seems to be related to the higher concentration of chloride and potassium observed in the autumn samples. It can be assumed that this difference derives from changes in the nature of runoff in autumn and in spring.

Despite events of rapid runoff during periods of snowmelt or intense rain events (see Fig. 2) the lake does not show the classical symptoms of acute acidification. The data reported show that the effects are limited to a reduction of the alkalinity which, however, did not fall below 60 peq 1-’. The maximum difference between summer alkalinity values and those of spring or autumn equals 85 peq I-’ in 1992 and 134 peq 1-’ in 1993; such relatively large changes, however, indicate the importance of an adequate sampling frequency in assessing


the sensitivity of surface waters to the effects of acidification. Knowledge on seasonal variability appears to be crucial to understand the distribution of aquatic organisms in water of low ionic content (PSENNER and CATALAN, 1994).

The increase in ionic content and pH observed in the lake during summer is due to the decrease of atmospheric deposition, to the reduced velocity of the soil water fluxes (improv- ing mineralisation), to evaporative concentration and to inputs by the tributary ; the increase of pH values was enhanced by photosynthetic activity leading to C02 depletion. It is difficult to try and estimate the contribution of each factor to the concentration processes due to the lack of precise and continuous monitoring of the discharge of the tributary (and groundwater), of the incoming runoff and of losses from the lake basin. The increase in the sum of ions during the summer period (see also Fig. 8) is primarily due to the dominant ions (basic cations and bicarbonate). Taking into account also the high catchment surface to lake ratio and the small lake volume, a large part of the alkalinity seems to be produced in the catchment although the processes of internal production by the reduction of sulphates and nitrates (see KELLY et af., 1982; SCHINDLER et al., 1986; SCHINDLER, 1988; PSENNER, 1988) should not be excluded.

The seasonal pattern of nutrient concentrations in the lake is determined by the loads and concentrations of runoff, but also by internal utilisation by the photosynthetic communities and by bacterial mineralisation processes. Nitrate decrease in summer is related to assimila- tion by plant communities. Localised peaks of concentration could be explained by the impact of sudden rain events and leaching. Orthophosphate is readily taken up by the photosynthetic .communities; therefore, levels were constantly below the limit of detection. The increase in total phosphorus during summer appears instead to be related to high phosphorus concentrations in sediments and to its potential mobilisation by bacterial action and/or mixing. The decrease in silicate concentration is directly related to the appearance of diatoms (next section).

5.3. Changes in the Phytoplankton Community

The summer biovolume levels measured in Lake Calaita (particularly in 1993) are high compared to the studies of other mountain lakes (TILZER, 1973; LARSON, 1973; R o n , 1988; PUGNETTI et al., 1983; RECHE et al., 1994). These high values may be explained by the low mean depth of the lake implying rapid interchange between water and sediment favouring resuspension and mineralisation of organic matter. In deeper lakes sedimentation and burial in lake sediments prevail. The frequency distribution of the biovolume values, including values lower than the 8181 pm3 (see Fig. 6) , seems compatible with the dominance of nannoplanktonic forms considered characteristic of mountain lakes (PUGNETTI et al., 1993; PECHLANER, 1971). In deep lowland lakes, subject to lower yearly temperature ranges, the size range of phytoplankton may be greater than in L. Calaita and include broader volume- tric limits (e.g. in Lake Garda, 14-87114 pm3, cf. SALMASO, 1996).

Changes in composition during annual succession are better understood if the shifts in the algal community are interpreted at the species level with the identification of dominant assemblages.

The NMDS ordination of the individual samples on the basis of their phytoplankton composition (Fig. 10) allowed to identify the principal groups by cluster analysis. The configu- ration and groups were obtained on the basis of density values, but the biovolume data gave the same results.

Results from these analyses indicate 1) the presence of stable populations for longer or shorter periods; 2) the different evolution of the community in the two years. The ordination results clearly show that the starting situations in 1992 and 1993 were completely different and that the trajectories drawn by the two series followed opposite paths.


I

NMDS Axis I

Figure 10. Ordination of the sampling dates into two-dimensional configurations obtained by NMDS analysis on the basis of phytoplankton density. The points relative to consecutive sampling dates have been joined with a broken line. The principal groups identified by means of cluster analysis canied out on the same dissimiiarity matrix utilised in the NMDS have been enclosed by dotted lines (see section

3 for details). The roman numbers indicate the month of sampling.

The composition of the 5 algal groups highlighted in Fig. 10 is described in Table 2. For each group only species reaching more than 3% of the density and/or biovolume (of the total calculated for each group) were retained. Spring samples showed a higher diversity in relation to the summer ones. In May and June 1992 and May 1993, the community was characterised by the presence of virtually all algal classes (excluding Cyanophyceae and Eugle- nophyceae) but with different composition and specific relationships between the two years (cf. also Figs. 5 and 7).

During July and August 1992 there was an intensive development of the chlorophycean component alone (reaching density percentages between 92 % and 97 %) determined almost exclusively by Oocystis cf. lacustris. In October community structure and composition changed radically favouring in particular flagellate forms (Mallomonas sp., Chromulina spp., Cryptomonas sp.) and Sphaerocystis schroeteri.

Between July and October 1993 we observed a completely different situation with respect to the summer period of the previous year due to the intensive development of Synedra sp.. Low silica concentrations measured at the beginning of August (cf. 4.1.) correspond to the maximum level of development of this diatom. Other important taxa (with biovolume >5%) are represented by Peridinium umbonatum (absent in October), Staurastrum spp. and Cryp- tomonas sp.. The density and biovolume decrease observed during the last two sampling excursions (Figs. 5 b and 5c) is essentially due to a drastic reduction of Synedra sp.. This decrease is able to substantially modify dominance relationships between the different classes (Figs. 7) leaving the overall community structure unaltered (Fig. 10).

The differences observed in the development of the phytoplankton community during the two years of study are of a much greater amplitude than planktonic successions described in deeper low altitude lakes (e.g. LUND, 1954; REYNOLDS, 1984; SOMMER, 1987; SALMASO et al., 1995). It should be considered however that in shallow mountain lakes such as the Calaita, environmental conditions are more variable and the factors determining algal succession are more difficult to identify compared to deeper lakes (REYNOLDS, 1988; SALMASO,


Table 2. T a u ordered into the five groups pointed out in Fig. 10, with a percentage of density (%D) and/or biovolume (%B) superior to 3% of the total group. The number in

brackets, to the right of the months, indicates the mean number of tuxu per sample.

1992 1993

May-June (19)

Chlamydomonas sp. Dinobryon sertularia EHRENBERG Glenodinium sp. Tabellaria flocculosa (ROTH) KUIXNG Chroomonas sp. Cyclotella sp. Synedra sp. Phacotus sp. Cryptomonas sp. Peridinium sp. Trachelomonas globularis (AwER.) LEMM.

25.6 4.6 17.9 19.6 11.1 4.1 10.2 20.2

7.8 4.7 6.7 1.6 2.3 10.0 2.2 5.2 1.9 4.6 1.1 12.2 0.6 4.1

Chromulina spp. Rhodomonas minuta SKUJA Gymnodinium sp. Nirzschia sp. Synedra sp. Ankistrodesmus fusiformis CORDA Oocystis cf. lacustris CHODAT indeterminate Tetrasporales Monoraphidium grifithii (BERK.) KOM.-LEGN. Cryptomonas sp. Staurastrum spp.

%D %B

18.8 1.6 18.8 3.2 9.4 24.1 9.4 23.1 8.3 5.3 5.5 1.3 5.3 4.0 4.7 1.4 3.2 1.1

1.5 8.9 1.0 15.9

July-October (25) July-August (18) . %D %B

%D %B Synedra sp. 79.2 47.8

Oocystis cf. lacustris CHODAT 86.6 86.2 Peridinium umbonatum STE~N 2.1 19.0 Synedra sp. 1.0 4.4 Phacotus sp. 1.2 4.1 Cryptomonas sp. 0.5 3.2 Cryptomonas sp. 0.9 5.0

Staurastrum spp. 0.7 10.3 October (13)

%D %B

Mallomonas sp. 63.9 72.4 Chromulina sp. 15.4 1.3 Sphaerocystis schroeteri CHODAT 8.1 5.9 Merismopedia glauca 4.1 0.2 (EHRENB.) NAG. Cryptomonas sp. 1.3 13.8

1996). Smaller high altitude lakes are more subject to meteorological changes and to the variability of runoff and, consequently, differences in hydrology and chemical composition alter phytoplankton succession more dramatically.

6. Conclusions

The chemical composition of Lake Calaita is characterised by a low ionic content typical of lakes in siliceous catchments.

The acidity present in wet precipitation (H+, NH;) was neutralised within the drainage basin by weathering processes and ion exchange, with increase of basic cations and alkalinity in runoff. In comparison to summer periods, dilution processes at times of high discharge (snowmelt, rain events) in spring and autumn were important. As a consequence a decrease in alkalinity and pH occurred with a decrease of the sum of ions.


Three hydrological phases can be identified on the basis of loading dynamics and changes in chemical composition: 1) dilution due to snowmelt and wet precipitation in spring; 2) decrease in discharge and summer concentration processes (evaporation, less precipitation and longer water residence time in the soils); 3) dilution due to heavy autumn rains. The first and the third phase are characterised by patterns which occur every year (in particular the autumn phase) because dilution overrides all other factors. Differences between the two study years show that the second phase is more variable as it is highly dependent on the runoff and meteorological conditions (snowmelt runoff, irradiance levels, storms, tributary fluxes, groundwater fluxes).

The shallow depth and small volume make Lake Calaita particularly sensitive to the variations of these hydrological factors which then cause profound differences in phytoplankton composition and abundance. With some difference between the two years, the evolution of the community reflects the three hydrological phases. Maximal phytoplankton development occurs during the warmest months because of the lake’s morphometric characteristics, the ice free surface for six months per year and the high temperatures; immediately after the ice thaw and autumn rains the lake experiences low levels of density and biovolume; these changes are followed (more strictly in 1992) by a reorganisation of the community’s structure, but with different dominant species in the two study years.

In contrast with deeper lakes, the lack of regular and internal physical cycles (such as stra- tification) and the strong influence of climatic conditions and unpredictable meteorological events, makes it very difficult to predict the seasonal development of the phytoplankton community in-these type of lakes.

7. Acknowledgements

We want to express our thanks to DAVID M. HARPER of the University of Leicester and to two ano- nymous reviewers whose suggestions improved the typescript. We are grateful to NICOLA PACINI of EAWAG (CH) and to ROSARIO MOSELLO, DELIO RIXGIU and ALDO MARCHETTO of the C. N.R. - Isti- tuto Italian0 di Idrobiologia di Pallanza, for their helpful comments on an earlier version of the typescript. Thanks are also due to ETTORE SARTORI (Director of the Paneveggio-Pale di S. Martino Parc) for his logistic support in the field.

8. References

A. P. H. A., A. W. W. A. and W. P. C. F., 1989: Standard methods for the examination of water and wastewater. 17th ed. - American Public Health Association, Washington, D. C., USA.

BOLD, H. C. and M. J. WYNNE, 1985: Introduction to the algae. Structure and reproduction. 2nd ed. - Prentice-Hall, Inc., Englewood Cliffs, N.J, USA.

BOURRELLY, P., 1972: Les Algues d’eau douce. Initiation B la SystCmatique. I: 1-572. Les Algues Ver- tes. - N. BoubCe and Cie, Paris.

BRAY, J. R. and J. T. CURTIS, 1957: An ordination of the upland forest communities of southern Wis- konsin. - Ecol. Monogr. 27: 325-349.

CAMERON, N. G., N. L. ROSE, P. G. APPLEBY and L. J. FLOWER, 1993: The ALPE Project: Palaeolim- nology. - In: GIUSSANI, G. and C. CALLIERI (eds.): Strategies for lake ecosystems beyond 2000. - 5th International conference on the conservation and management of lakes. Stresa, 17th-21st May:

CLARKE, K. R. and R. H. GREEN, 1988: Statistical design and analysis for a “biological effects” study. - Marine Ecology 46: 213-226.

COOK, R. B., C. A. KELLY, D. W. SCHINDLER and M. A. TURNER, 1986: Mechanisms of hydrogen ion neutralization in an experimentally acidifed lake. - Limnol. Oceanogr. 31: 134-148.

DECET, F., R. BURIGO and N. SALMASO, 1995: Materiali per I’idrochimica di alcuni laghi del Parco Natu- rale di Paneveggio-Pale di S . Martino ed in particolare del L. CaIaita (Dolomiti, Alpi Orientali). - Boll. Chim. Igien. 46: 139-163.

401 -402.


FIELD, J. G., K. R. CLARKE and R. M. WARWICK, 1982: A practical strategy for analysing multispecies distribution patterns. - Mar. Ecol. Prog. Ser. 8: 37-52.

GIUSSANI, G.. R. DE BERNARDI, R. MOSELLO, I. ORIGGI and T. RUFFONI, 1986: Indagine limnologica sui laghi alpini d’alta quota. - Documenta 1st. ital. Idrobiol. 9: 1-415.

HEMOND, H. F., 1990: Acid-neutralizing capacity, alkalinity and acid-base status of natural waters con- taining organic acids. - Environ. Sci. Technol. 24: 1486-1489.

HEMOND, H. F., 1994: Role of organic acids in acidification of fresh waters. - In: STEINBERG, C. E. W. and R. F. WRIGHT (eds.): Acidification of freshwater ecosystems : implications for the future. - J. Wiley and Sons Ltd., England: 103-1 15.

HUBER-PESTALOZZI, G., 1955 : Euglenophyceen. - In: HUBER-PESTALOZZI, G.: Das Phytoplankton des SiiRwassers. Die Binnengewasser, Band XVI, 4. - E. Schweizerbart’sche Verlagsbuchhandlung,

HUBER-PESTALOZZI, G. and B. F o ~ , 1968: Cryptophyceae, Chloromonadophyceae. Dinophyceae. - In: HUBER-PESTALOZZI, G: Das Phytoplankton des SiiBwassers. Die Binnengewasser, Band XVI, 3(2). - E. Schweizerbart’sche Verlagsbuchhandlung. Stuttgart.

KALFF, J.. 1972: Net plankton and nanoplankton production and biomass in a north temperate zone lake. - Limnol. Oceanogr. 17: 712-720.

KELLY, C. A,, J. W. M. RUDD, R. B. COOK and D. W. SCHINDLER, 1982: The potential importance of bacterial processes in regulating rate of lake acidification. - Limnol. Oceanogr. 27: 868-882.

KOMAREK, J. and B. FOTT, 1983: Chlorophyceae (Griinalgen) Ordnung: Chlorococcales. - In: HUEIER- PESTALOUI, G.: Das Phytoplankton des SuSwassers. Die Binnengewbser, Band XVI, 7(1). - E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart.

KRUSKAL, J. B. and M. WISH, 1978: Multidimensional scaling. - Sage Publications, Beverly Hills and London.

LARSON, G.’L., 1973: A Limnology study of a high mountain lake in Mount Rainier National Park, Washington State; USA. - Arch. Hydrobiol. 72: 10-48.

LEONARDI, P., 1967: Le Dolomiti, Vol. 1. - CNR, Roma LORENZEN, C. J., 1967 : Determination of Chlorophyll and Pheo-Pigments: spectrophotometric equati-

ons. - Limnol. Oceanogr. 12: 343-346. LUND, J. W. G., 1954: The seasonal cycle of the plankton diatom, Melosiru italica (EHR.) KUTZ. subsp.

subarticu 0. MOLL. - J. Ecol. 42: 151-179. LUND, J. W. G., C. KIPLING and E. D. LE CREN, 1958: The inverted microscope method of estimating

algal numbers and the statistical basis of estimations by counting. - Hydrobiologia 11: 143-170. MARCHETTO, A. and A. LAMI, 1994: Reconstruction of pH by chrysophycean scales in some lakes of

the Southern Alps. - Hydrobiologia 274: 83-90. MARKER, A. F. H., E. A. NUSCH, H. RAY and B. RYEMANN, 1980: The measurement of photosynthetic

pigments in freshwaters and standardization of methods: conclusions and recommendations. - Arch. Hydrobiol. Beih. 14: 91-106.

MOSELLO, R., 1984: Hydrochemistry of high altitude alpine lakes. - Schweitz. Z. Hydrol. 46: 86-99. MOSELLO, R., 1991 : Situazione degli studi sulla chimica delle deposizioni atmosferiche umide nel 1988

in Italia. RIDEP 4. - Documenta 1st. ital. Idrobiol. 29: 1-41. MOSELLO, R., 1993a: Situazione degli studi sulla chimica delle deposizioni atmosferiche umide nel 1991

in Italia. RIDEP 11. - Documenta 1st. ital. Idrobiol. 42: 1-49. MOSELLO, R., 1993b : Situazione degli studi sulla chimica delle deposizioni atmosferiche umide nel 1992

in Italia. RIDEP 12. - Documenta 1st. ital. Idrobiol. 43: 1-45. MOSELLO, R. and L. MORSELLI, 1992a: Situazione degli studi sulla chimica delle deposizioni atmosfe-

riche umide nel 1989 in Italia. RIDEP 5; - Documenta 1st. ital. Idrobiol. 33: 1-34. MOSELLO, R. and L. MORSELLI, 1992b: Situazione degli studi sulla chimica delle deposizioni atmosfe-

riche umide nel 1990 in Italia. RIDEP 8. - Documenta 1st. ital. Idrobiol. 37: 1-48. MOSELLO, R., G. A. TARTARI and A. MARCHETO, 1987: Alterazione delle deposizioni atmosferiche ed

effetti sulle acque superficiali: la situazione dell’Italia nord occidentale. - Docurnenta 1st. ital. Idro- biol. 14: 1-18.

MOSELLO, R., A. BARBIERI, A. MARCHETO, R. PSENNER and D. TAIT, 1992: Research on the suscepti- bility of Alpine lakes to acidification. - Docurnenta 1st. ital. Idrobiol. 32: 23-30.

MOSELLO, R., A. BOGGERO, M. CARMINE, A. MARCHETTO, A. SASSI and G. A. TARTARI, 1994: Ricerche idrochimiche sui laghi delle valli Ossola e Sesia (Alpi Pennine e Lepontine). - Documenta 1st. ital. Idrobiol. 46: 1-436.

Stuttgart.


ORL~CI , L., 1978: Multivariate analysis in vegetation research. - W. Junk B. V. Publishers, Boston. PECHLANER, R., 1971 : Factors that control the production rate and biomass of phytoplankton in high-

mountain lakes. - Mitt. Internat. Verein. Limnol. 19: 125-145. PSENNER, R., 1987: Deposizioni acide in Austria ed effetti sulle acque dolci e sulla vegetazione. - Docu-

menta 1st. ital. Idrobiol. 14: 35-71. PSENNER, R, 1988: Alkalinity generation in a soft-water lake: watershed and in-lake processes. - Lim-

nol. Oceanogr. 33: 1463-1475. PSENNER, R., 1994: Environmental impacts on freshwaters: acidification as a global problem. - Sci.

Total Environ. 143: 53-61. PSENNER, R. and J. CATALAN, 1994: Chemical composition of lakes in crystalline basins: a combinati-

on of atmospheric deposition, geological background, biological activity and human action. - In: MARGALEF, R. (ed.): Limnology Now: a paradigm of planetary problems. - Elsevier Science B. V., Amsterdam: 255 -3 14.

PUGNETTI, A., M. MANCA, A. M. NOCENTINI, A. BOGGERO, R. BEITWEIT, M. BONARDI and P. CAMMA- RANO, 1993: Ecological aspects of two alpine lakes (Lakes Paione, Italy). - In: GIUSSANI, G. and C. CALLIERI (eds.): Strategies for lake ecosystems beyond 2000. - 5th International conference on the conservation and management of lakes. Stresa, 17th-21st May: 440-443.

RECHE, I., P. SANCHEZ-CASTILLO and P. CARRILLO, 1994: The principal regulating mechanism on the structure of the phytoplankton community in a high mountain lake. -Arch. Hydrobiol. 130: 163-178.

REYNOLDS, C. S., 1984: The ecology of freshwater phytoplankton. - Cambridge University Press, Cam- bridge.

REYNOLDS, C. S., 1988: The concept of ecological succession applied to seasonal periodicity of freshwater phytoplankton. - Verh. Internat. Verein. Limnol. 23: 683-691.

R o n , E., 1981 :-Some results from phytoplankton counting intercalibrations. - Schweiz. Z. Hydrol. 43:

Ron, E., 1988: Some aspects of the seasonal distribution of flagellates in mountain lakes. - Hydro- biologia 161: 159-170.

SALMASO, N., 1996. Seasonal variation in the composition and rate of change of the phytoplankton community in a deep subalpine lake (Lake Garda, Northern Italy). An application of nonmetric multidimensional scaling and cluster analysis. - Hydrobiologia 337: 49-68.

SALMASO, N., P. CORDELLA and M. MANFRIN, 1995: Seasonal changing rates of the phytoplankton community in Lake Garda. - Giorn. Bot. ital. 129(2): 153.

SARACENI, C. and D. RUGGIU, 1974: Techniques for sampling water and phytoplankton. - In: VOLLEN- WEIDER, R. A. (ed.): A manual on methods for measuring primary production in aquatic environments. - IBP Handbook 12, Blackwell, Oxford, pp. 5-7.

SCHINDLER, D. W., 1988: Confusion over the origin of alkalinity in lakes. - Limnol. Oceanogr. 33:

SCHINDLER, D. W., M. A. TURNER, M. P. STAINTON and G. A. LINSEY, 1986: Natural sources of acid neutralizing capacity in low alkalinity lakes of the Precambrian shield. - Science 232: 844-847.

SOMMER, U., 1987: Factors controlling the seasonal variation in phytoplankton species composition. A case study for a deep, nutrient rich lake. - Prog. Phyc. Res. 5: 123-178.

STUMM, W. and J. L. SCHNOOR, 1995: Atmospheric depositions: impact of acids on lakes. - In: LER- MAN, A,, D. IMBODEN and J. GAT (eds): Physics and chemistry of lakes. - Springer Verlag,

TILZER, M. M., 1973: Diurnal periodicity in the phytoplankton assemblage of a high mountain lake. - Limnol. Oceanogr. 18: 15-30.

TOMASI, G., 1962: Origine, distribuzione, catasto e bibliografia dei laghi del Trentino. - St. Trent. Sc. Nat. 39: 1-356.

TONOLLI, V. and L. TONOLLI, 1951 : Osservazioni sulla biologia ed ecologia di 170 popolamenti zoo- planctonici di laghi italiani di aka quota. - Mem. 1st. ital. Idrobiol. 6: 53-136.

VAN MIEGROET, H., 1994: The relative importance of sulfur and nitrogen compounds in the acidification of fresh water. - In: STEINBERG, C. E. W. and R. F. WRIGHT, (eds.): Acidification of freshwater ecosystems: implications for the future. - J. Wiley and Sons Ltd., England, pp. 33-49.

WATHNE, B. M., 1992: Acidification of mountain lakes: paleolimnology and ecology. The AL:PE Pro- ject. - Documenta 1st. ital. Idrobiol. 32: 7-22.

34-62. .

1637- 1640.

pp. 185-215.

Manuscript received June 28th. 1996; revised January 8th, 1997; accepted January 14th. 1997

seasonal and interannual changes of chemical characteristics and phytoplankton in a mountain lake of...

Documents