periphyton investigations in humex lake skjervatjern in 1992

Pergamon Environment International, Vol. 20, No. 3, pp. 321-328, 1994

Copyright @1994 Elsevier Science Ltd Printed in the USA. All rights roaox'ved

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PERIPHYTON INVESTIGATIONS IN HUMEX LAKE SKJERVATJERN IN 1992

E.-A. Lindstr~m Norwegian Institute for Water Research, N-0808 Oslo, Norway

EI 9307-189 M (Received 12 July 1993; accepted 16 February 1994)

During the first year of artificial acidification of the dystrophic Lake Skjervatjern with a combination of sulphurie acid and ammonium nitrate, extensive growth of filamentous green algae occurred in the acidified half, Therefore, investigations studying the effect of acidified humic water on periphyton started early 1992. Quantitative measurements in the two lake-halves (acidified-A, control-B) were carried out, using small clay flower pots as growth substrates. The pots were placed in the two lake halves at 0.5 and 2 m depth. During the first three months, accumulated periphyton, measured as dry weight and chlorophyll a, was 50-600% higher in the acidified basin A than in B. Chlorophyll a per unit dry weight was higher in A than B, indicating higher photosynthetic activity in the acidified half. C/N-ratios in freeze-dried material from basin A were 10-11, indicating little or no N-deficiency during the first three months of the experiment at both 0.5 and 2 m depth in the acidified half. In the control basin B, C/N-ratios between 15 and 19 indicated N-deficiency during the first three months of the experiment at 0.5 m depth. At 2 m, there was no growth in basin B for the first 56 d of the experiment. After three months, periphyton accrual at 2 m had approximately the same chlorophyll a per unit dry weight and C/N-ratios as in basin A. This indicated that conditions in basin B at 2 m depth changes from strong growth limitations early in the summer to high photosynthetic activity and little or no N-limitation later in the year. The bluegreen alga Hapalosiphon fontinalis (Ag.)Born. is known to fix elemental nitrogen. It proliferated in the control basin, but was scarce in the acidified half. This supported the assumption of nitrogen limitation in the control basin.

INTRODUCTION

Several investigations have reported that attached algae become increasingly important in the primary production of lakes undergoing ac id i f ica t ion (Schindler and Turner 1982; Stokes 1986). A char- acteristic feature is changes in the littoral zone, often introducing pronounced proliferations of filamentous green algae from the family Zygnemataceae (Grahn et al. 1974; Hendrey and Vertucci 1980; Howell et al. 1990). The filamentous green algae are generally attached to benthic substrata; in acidified lakes however, the alga often form loose flocs, mats, or

clouds that float just above the lake bottom or in open water (Howell et al. 1990). This later form called meta-phyton is neither benthic nor planktonic (Wetzel 1983). The development of metaphyton can be quite extensive and can cover fish nets with a thick green coat within 24 h (LindstrCm 1993). Although periphyton (attached algae) and metaphyton (loose flocs of algae) can be a striking feature in lakes undergoing acidification, there is scarce knowledge about their causes, consequences to lake biota, and contribution to the metabolism of the lake.

321

322 E.-A. Lindstrlam

Most studies on artificial acidification have been done in neutral lakes of fairly low humic content (Turner et al. 1991). The effect of acidification on periphyton in humic and naturally acidic lake has not been studied.

During the first year of artificial acidification of one basin of the humic Lake Skjervatjern (Gjessing 1992), extensive growth of filamentous green algae occurred. Therefore, investigations studying the effect of acidified humic water on periphyton started early in 1992. The main purpose was to improve knowledge about factors enhancing and limiting periphyton growth in acidified humic water. Both carbon (organic and inorganic) and transparency were considered to be important variables that needed further studies.

Recent investigations indicate that the amount of dissolved inorganic carbon (DIC) is one of the main factors regulating the productivity of periphyton in acidified lakes (Sheldon and Boylen 1975; Howell et al. 1990; Turner et al. 1991; Fairchild and Sherman 1992). With acidification, HCO 3 diminishes as a component of the inorganic carbon-system, from 79% at pH 7 to less than 4% at pH <5. In acidified lakes, the concentration of DIC is largely deter- mined by the solubility of CO 2. Photosynthesis seems frequently to be C-limited; and nitrogen and phosphorus may be of secondary importance. Turner et al. (1987) suggested that filamentous green algae from the family of Zygnematacea, known to proliferate in acidified lakes, are efficient users of the limited amounts of DIC availabe in acidified lakes. Therefore, Zygnematacean algae could be an important con- stituent of the periphyton in Lake Skjervatjern if DIC is a limiting nutrient. The acidification of Lake Skjer- vatjern was done with a combination of sulphuric acid and ammonium nitrate (Gjessing 1992). The addition of the nutrients ammonium and nitrate may also have a direct impact on the periphyton. The studies of Fair- child and Sherman (1992) in acidified water suggests inorganoc nitrogen to be a colimiting factor together with DIC.

In humic water, light generally becomes growth limiting towards depth. Increased transparency is reported to be associated with acidification of humic water (Yan 1983; Gjessing 1992) and is suggested to have some effect on enhanced periphyton growth in deeper zones after acidification (Turner et al. 1991). Other factors such as reduced grazing due to reduced or exterminated populations of invertebrates have also been reported to give high accrual of periphyton in acidified water (Stokes 1986).

METHODS AND MATERIALS

Details on the HUMEX Lake Skjervatjern and the experimental design is given by Gjessing (1992). The lake (2.4 ha) is dystrophic with a natural low pH. The catchment (8.9 ha) lies on granitic bedrock and is covered by a rather thin mat of organic soil. In the fall of 1988, the lake was divided in two by a plastic curain. Two years later, in October 1990, artificial acidification with sulpuhric acid and ammonium nitrate was started in one half (A). The other half (B) was kept as a control. Both the A-basin and its catchment were sprinkled with acids.

Methods used for quantitative periphyton studies vary and are often adjusted to the circumstances. To find a method, that gives a realistic picture of the periphyton community and implies factors enhancing and limiting periphyton growth, can be difficult. The method chosen for this study was developed by Fairchild and coworkers (Fairchild and Lowe 1984; Fairchild and Everett 1988; Fairchild and Sherman 1990). Minor changes in the experimental design are described below.

Small clay flower pots (volume 0.3 L) attached upside-down to a wooden rack were used as growth substrates. Each pot was attached with a stainless steel metal bolt that protruded through the upper part of the pot (the bottom). With this experimental design, an agar solution containing nutrient salts was filled inside the pots and allowed to diffuse through the walls, thereby, slowly adding nutrients to the periphyton. The first year of studies was carried out without nutrient- containing agar.

The initial intention was to determine gross production and community respiration, by measuring changes in oxygen concentration in closed chambers placed on top of the pots. Chambers made of clear plexiglass and sealed off at the base with a rubber O-ring were used. Chambers were placed over each pot in situ. To measure respiration rates, black chambers were used. Unfortunately, the chambers did not seal off the pots completely and were, therefore, unsuitable for measurements of changes in oxygen concentration. Further adjustments of the experimental design are obvious- ly needed.

Samples were collected by placing one plexiglass- chamber over each pot before disconnecting them from the wooden rack. Periphyton was removed from the substrates with a hard toothbrush and knife, and sampled into a jar together with the water enclosed by the chamber. The samples were frozen and freeze- dried. Measurements of dry weight, chlorophyll a, elemental carbon, nitrogen, and phosphorus were done on freeze-dried material. Chlorophyll a was extracted

Effect of acidified humic water on periphyton 323

3

2.5

2

1.5

1

0.5

0

June 18th

Chl a ~9/cm2

Aug. 4th

0.5m 2m 0.5m 2m

Fig. 1. Mean chlorophyll a accrual on clay flower pots. Basin A and B, Lake Skjervatjern 1992.

in methanol and analysed according to Norwegian Standard 4767 (1983). Elemental carbon and nitrogen were analysed on elementanalyser Carlo ERBA 1106. Separate samples to determine species composition and relative abundance of the organisms were collected and preserved in 2% formaldehyde. To determine relative abundance of algae, replicate fractions were taken out and counted under microscope. At least 300 cells or fragments (of algae filaments) were counted in each sample. Similar samples were taken from the littoral zone of the two basins (A and B) through the growth season.

Two racks of 36 pots each were placed in both acidified and control basin. One rack was positioned in the littoral zone at 0.5 m depth and the other in open water attached to a buoy at 2 m depth. The racks were placed in the lake on 23 April, 1992. Three replicate samples of the periphyton pots were collected on June 18th (56 d exposure), August 4th (103 d exposure) and September 29th (157 d exposure).

RESULTS

Periphyton standing crop

During the first three months of exposure, periphyton accrual on the pots measured as chlorophyll a per cm 2 was 50-600% higher in the acidified than in the control basin (Fig. 1). In the control basin, chlorophyll a was close to 0.5 p.g/cm 2 on average. This is consistent with the findings of Fairchild and Sherman (1992). They used similar clay flower pots as substrates and found that standing crop accrual on control substrata measured as chlorophyll a averaged 0.8 I.tg/cm 2 in 12 softwater lakes of low nutrient content. In the

acidified basin, chlorophyll a accrual on August 4th averaged 3 and 1.2 Ixg/cm 2 at 0.5 m and 2 m depths, respectively. This indicates better growth conditions in the acidif ied basin A, than would be expected inother sof twater lakes of low nutrient content (Fairchild and Sherman 1992). At 2 m in basin B, there were probably several factors limiting growth for the first 56 days of the experiment, while no growth occurred at all (Fig. 1). (Preliminary observations indicate similar growth limitations at 2 m depth in the control basin in June 1993.)

Chlorophyll a per unit dry weight was in general higher in basin A than B (Fig. 2). This indicates higher biomass specific photosynthetic activity in the acidified basin than in the control. On August 4th, samples from 2 m depth had similar chlorophyll a per dry weight in both the acidified and control basin, implying that growth conditions in the deeper parts of the two basins became more similar during the growth season. Periphyton standing crop (measured as chlorophyll a/cm 2) was however still higher in the acidified half (Fig. 1). This is probably a result of high accrual of periphyton in basin A prior to August 4th. High dry-weight values compared to chlorophyll a at 0.5 m in June could be a result of high content of debris on these pots, that were located in the littoral zone. Therefore, they were transported out to open water in June and attached to a buoy close to where the 2 m pots were hanging.

C/N ratios in periphyton

Elemental C and N in freeze-dried material gave C/N-ratios between 10 and 18 by weight on average (Fig. 3). C/N-ratios can be quite variable. Ratios

324 E.-A. Lindstr0m

3 I 2.5

2

1,5

1 - -

°"l 0

June 1 8 t h

chl a/dry weight

A u g . 4 t h

L, , / , , ! 0.5m 2m O.5m 2m

E2B

Fig. 2. Mean chlorophyll a/dry weight on clay flower pots. Basin A and B, Lake Skjervatjern 1992.

20 - -

IS q-

1 6 +

14 fi-

12 -F

10 q-

I

O . 5 m

Junu 1 8 t h

2m

C/N ratio

O,5m

Aug. 4 t h

2 m

Fig. 3. Mean C/N ratios in freeze-dried material from clay flower pots. Basin A and B0 Lake Skjervatjern 1992.

between 5 and 13 are reported to be within normal variation range (Reynolds 1984). C/N-ratios in freeze- dried material from basin A were 10-11, indicating little or no nitrogen deficiency at both 0.5 and 2 m depth in the acidified half. In the control basin B, C/N ratios between 15 and 19 indicate N-deficiency during the first three months of the experiment at 0.5 m depth (Fig. 3).

Periphyton community structure

Table 1 gives mean frequency of dominant algal species measured as percentage of total cell numbers. Only small differences in algal species composition were observed in the two basins. A common feature within all algae groups was a shift between basin A

and B in time of maximum development. Most species seemed to develop first in basin A.

Only one bluegreen alga, Hapalosiphonfontinalis, was abundant on the clay flower pots. Hapalosiphon is known to fix elemental nitrogen (Kaushik 1987). It proliferated in the control basin, but was scarce in the acidified half (Fig. 4). This might be a result of less bioavailable nitrogen in the control basin than in the acidified basin.

Filamentous green algae dominated in both the acidified and the control basin. The dominant species belonged to the family Zygnemataceae and are known to proliferate in acidified lakes in Northern United States and Eastern Canada (Wei et al. 1989; Howell et al. 1990; France et al. 1992). The most frequent


Table 1. Frequency of dominant algae on clay flower pots. Basin A and B, Lake Skjervatjern 1992. Frequency is given in five inter- vals (5: >30%; 4: 30-15%; 3: 15-7%; 2: 7-3%; 1: <1%).

Organism

Cyanophyceae Hapalosiphon fontinalis Chlorophyceae Filamentous forms: Binuclearia tectorum

Mougeotia a (8-10u)

Mougeotia a/b (17-20u) Oedogonium a (6-10u) Zygogonium sp. (12-14u)

Zygogonium cf. tunetanum Other green algae: Botryococcus braunii Micractinium kuetzingii Bacillarlophyceae

Frustulia rhomboides v. Navicula subtilisima Tabellaria binalis

Tabellaria quadriseptata

Basin A

June 18th August 4th Sept. 29th

0.5m 2m 0.5m 2m 0.5m 2m

0 0 0 1 0 1

2 2 3 3 4 4 4 4 0 1 2 2 2 3 3 3 0 0 1 0 0 2 0 1

1 1 3 2

0 0 0 0

1 2 I 2 2 2 0 1 0 0 1 I 2 2 1 2

0 0 5 5 3 2

June 18th 0.5m 2m 0.5m

3 3 N 0

0 0

1 G 2

0 I~ 0

Basin B

August 4th Sept. 29th 2m 0.5m 2m

2 3 2

2 2 2 0 3 3 0 W 2 1 0 1

H

3 3 1

0 0 0

1 2 3 0 1 0 1 0 1 1 1 2

0 1 0 2 4 3 0 1 0 2 2 1 0 3 3 0 2 1

3 3 4 4

2 3 0 1

2 2 2 2 0 0 2 1 0 0 1 0 1 0 2 2

species were Mougeoi ta a (7-9u) and Zygogonium cf. tuenetanum ( Fig. 4). They are both reported to be major species of filamentous green algae in acidified softwater lakes (Wei et al. 1989; Howell et al. 1990; France et al. 1992). They dominated in basin A most of the growth season, while they did not grow until late summer in basin B. This indicates that the two basins provided more similar growth conditions in late summer than early in the growth season. Except for slightly higher occurrence of Binuclearia tectorum and Mougeot ia a/b in basin A, pronounced differences in community composition of the filamentous green algae were not observed (Table 1). Binuclearia is quite common in Norwegian lakes of low pH and low nutrient content (Lindstrem 1992). So far, it has not been reported with high abundance in acidified lakes elsewhere.

Frustulia rhomboides var. saxonica was the most frequent diatom species (Table 1). Frustulia is known to grow in humic water and has a pH optimum around 5.1 (Berge 1985). In 1992, it was slightly more common in the control basin than in the acidifid half. This might be a result of changes in the humic substances or in pH in the acidified half, that would provide

more unfavourable growth conditions for Frustulia. The other diatoms of quantitative importance have pH optima from 4.7 to 4.8 (Table 1). They seemed to be slightly more common in the acidified basin.

D I S C U S S I O N

Periphyton standing crop and productivity

The 50 to 600% higher accrual of periphyton in the acidified basin than in the control, and the 50 to 300% higher accrual than measured in similar experi- ments in 12 lakes of low nutrient content (Fairchild and Sherman 1992) are probably the result of enhanced growth conditions in basin A. Generally, higher chlorophyll a per unit dry weight does also indicate higher photosynthetic activity in the acidified half. Therefore, an eventual reduction in invertebrate grazing due to acidification is not likely to be the only reason for high accrual of periphyton in the acidified basin. This is consistent with Fairchild and Sherman (1992) who found small differences in grazer effects on periphyton substrates from lakes repre- senting a wide range in alkalinity.

326 E.-A. Lindstrem

Hapalosiphon fontinalis (Ag.)Braun

June 181h Aug. 4th Sept. 29th

0.Sin 2m O.Sm 2m

' I

0.5m

i,i 2m

5 7- ~ Meugeotia a (Israelson 1949)

June 18th Aug. 4th Sept

4 +

3 -t-

1 + :

0.5m 2m 0.5m 2m 0.5m 2m

I i

3 +

2 +

Zygogonium cf. tunetanum Gautier-Lievre

June 16th Aug. 4 th Sept. 291h

I +

0.5m 2m 0.5m 2m 0.5m 2m

Fig. 4. Frequency of the N-fixing bluegreen alga Hapalosiphon fontinalis and two filamentous green algae, Mougeotia and Zygogomium. Frequency measured as % of total cell number (5: >30%° 4: 30-15%, 3: 15-7%, 2: 7-3%, 1: <3%. Qualitative samples from clay flower pots.


Bioavailable carbon

Turner and coworkers (1991) found the greatest stimulation by inorganic C to appear in acidified lakes containing the highest densities of filamentous green algae from the family of Zygnemataceae. This finding is consistent with the hypothesis that the Zygnematacean algae are efficient users of the limited amounts of DIC available in acidified lakes (Turner et al. 1987). High abundance of Zygnematacean algae in both the A and B basin could suggest that C-limitation occur in both basins. The high content of organic carbon in the humic Lake Skjervatjern might not be easily bioavailable. Further investigations are needed to assess this possibility. Further investigations are also required to see if the hypothesis of a C- limitation can be applied to humic waters in general.

Availability of other nutrients

The difference in periphyton standing crop and shift in time of maximum occurrence of Zygnematacean algae indicate that factors in addition to DIC regulate the periphyton development in the two basins. The Redfield ratio of 106C:16N: 1P approximates the relative nutrient content of planktonic algae growing under nutrient-sufficient conditions (Redfield 1958). However, a range between 5.7 and 14 is considered to be normal (Reynolds 1984). Therefore, C/N-ratios of 10-12 in basin A suggest little or no N-limitation in the acidified half. In the control basin B, C/N- ratios between 15 and 19 during the first three months of the experiment at 0.5 m depth indicate N-limitation (Fig. 3). This assumption is supported by the proliferation of the nitrogen-fixing bluegreen alga Hapalosiphonfontinalis in the control basin (Fig. 4). Hapalosiphon was scarce in the acidified half. This is in accordance with the chemical observations in the lake. After 18 months of treatment with ammonium nitrate (and sulphuric acid), a gradual in- crease of inorganic and organic nitrogen in the A-basin was reported (Gjessing 1992).

In the B-basin, at 2 m depth, there could be a number of factors limiting periphyton growth for the first 56 d of the experiment, since no growth occurred at all (Fig. 1). After three months, a marked change in growth conditions in the control basin seemed to have taken place. Accumulated periphyton at 2 m depth had approximately the same chlorophyll a per unit dry weight as in the acidified basin. C/N-ratios averaged 10 in both basins and indicated little N-limitation. The Zygnematacean algae previously scarce in the control basin, were now proliferating in both halves. A possible explanation for this change can be nutrient pulses released from the bottom (or

littoral zone) of the control basin (Peters and Cat- taneo 1984; Howell et al. 1990).

Based on the first year of studies in the HUMEX Lake Skjervatjern, only tentative conclusions can be drawn. Proliferation of Zygnematacean algae, known to occur in C-limited water, might be caused by C-limitation in both the acidified and naturally acidic basins. This suggests that the high content of organic carbon, normally present in humic water, is not readi- ly bioavailable. In the control basin, additional factors seem to be growth limiting. High C/N-ratios and high occurrence of nitrogen-fixing alga indicate N-limitation. In the acidified half, the N-limitation was most likely overcome by the nitrogen added with artificial precipitation. The nitrogen added does probably en- hance periphyton growth to levels above normal in nutrient-poor water. Further studies with nutrient dif- fusing substrates may give more conclusive knowledge about factors enhancing and limiting periphyton growth in Lake Skjervatjern.

Acknowlegment - - This work was sponsored by The Norwegian Institute for Water Research, by the Commission of European Communities (STEP-CT90-0112 and NV5V-CT92-0142) and by the Norwegian Research Council (NTNF). In addition, there is a con- siderable contribution from a number of institutions in Canada, Europe, and USA.

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periphyton investigations in humex lake skjervatjern in 1992

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