VERTICAL DISTRIBUTION AND SEASONAL CHANGES IN THE NUMBER OF BACTERIOPLANKTON IN THE WATER OF LAKE HAŃCZA, PARTICULARLY IN THE PERIOD OF RESERVOIR SUMMER STRATIFICATION

Anna Gotkowska-Płachta, Stanisław Niewolak, Ewa Korzeniewska

ABSTRACT

The paper presents the results of the study on seasonal changes in the number of bacterioplankton, its vertical distribution and morphological composition in the waters of the deepest Polish lake, Lake Hańcza (108.5 m deep). The research was performed from 1997 till 2000. Water samples were collected at a research station placed at a location where the lake depth was maximum, at monthly intervals from May to October. Water was sampled along the following depth profile: from 0-1 cm layer, at 0.3 m and at the depth of 1m, 2m, 5m, 10m, deeper down at 10m intervals towards the bottom. Bacteriological analyses were completed with measurements of selected physico-chemical parameters such as temperature, pH, oxygen, nitrogen and phosphorus contents. Obtained results with regards both to the number of bacterioplankton and its morphological structure, namely 68% for cylindrical rods forms, and physico-chemical assays confirmed the purity of Lake Hańcza. The vertical distribution of bacterioplankton

Key words: Bacterioplankton, lake, physico-chemical parameters..

INTRODUCTION

In natural, undisturbed water ecosystems it is natural to find stable groups of interrelated microorganisms. Any imbalance introduced into such systems by external factors triggers immediate reaction of the microorganisms. Qualitative and quantitative fluctuations in bacterioplankton biomass provide a very sensitive indicator of changes occurring in reservoirs [24]. The major parameters regulating the number and distribution of planktonic bacteria biomass are availability of organic nutrient substrates [11,16], particularly in oligotrophic reservoirs, predation that is most pronounced in eutrophic waters [8,22], concentration of mineral compounds, pH [18], and temperature [23]. In lake waters the number of bacterioplankton is also affected by inorganic nitrogen, phosphorus, and organic carbon content, as well as by the bacteria-phytoplankton correlation [27]. Usually, the highest number of bacterioplankton is reported when phytoplankton produces nutrients, namely in the spring, late summer, and early autumn [4,10]. The type of lake is also of significant importance. A complex analysis of lakes with different trophy levels confirmed that the number of bacteria in oligotrophic lakes is considerably lower than in contaminated eutrophic ones [14,16]. Quantitative distribution of micro-organisms is also determined by natural and climatic conditions, as well as by the season. It applies particularly to lakes of mild climates, where thermal layers occur and are inhabited by populations of various microorganisms. Both vertical and spatial distribution in lake waters is of significant importance for determining the current state of a lake and forecasting the direction of changes occurring therein.

The aim of the study was to determine vertical distribution, seasonal changes in the number of bacterioplankton and its qualitative composition in the waters of Lake Hańcza.

MATERIAL AND METHODS

Lake Hańcza. Lake Hańcza is the deepest (108.5 m deep) gutter reservoir not only in Poland but also in the central part of European Depression. It has been recorded on the list of the purest lakes in Poland [2]. The lake surface area accounts for 311.4 ha. It has a volume of 120 364,100 m³, while the maximum length and width correspond to 4525 m and 1175 m, respectively (see Table 1 for detailed morphometric and limnologic data). The reservoir is characterised by high banks of very steep slopes reaching the depth of 10 m. The lake is located far away from any industrial plants or buildings. It does not provide any water intakes. Direct sources of water pollution [2] have not been reported. High thickness of hypolimnion compared to epilimnion was reported for lake Hańcza. The water is greenish-blue and transparent up to 9 m [29]. The lake is known for high oxygen balance in the deeper layers. Due to its extraordinary natural, geographical, geologi cal, and limnological value, the lake Hańcza has been established a protected reserve in 1963.

Samples Collection. Water samples were collected at monthly intervals from May to October from 1997 till 2000. The samples were collected at the research station located at the deepest site of the lake (108.5 m, station 1 in Fig. 1). Collecting water samples followed a regular vertical pattern: 0-1 cm layer, the depth of 0.3 m, 1m, 2m, 5m, 10m, and finally at systematic 10m intervals down to the bottom. Demersal samples were collected ca. 20-30 cm above the bottom. Surface water samples (0-1 cm) were collected directly to sterile glass containers (V=300 cm³) with a ground cork. The samples from deeper layers were collected with a Ruttner apparatus and placed in the glass containers of same type. The apparatus had to be used due to necessity of parallel water sampling for chemical analyses. While collecting water samples water temperature measurements with a mercurial thermometer fixed to a Ruttner apparatus were performed, as well as oxygen saturation, pH and visibility by means of a Secchi disk determined. The location of the measurement-control station had been determined as approximation of geographical position measurements by means of ScoutMaster GPS. After collecting the samples were transported in containers kept at 4-6°C to the laboratory where they were immediately analysed. The interval between collecting the samples and performing their analyses never lasted longer than 12 h.

Microbiological analyses. Microbiological analyses consisted in determination of the number of planktonic bacteria by direct counting under a microscope. Membrane SYNPOR filters with pore diameter of 0.6 µm (Chemapol, Prague, Czech Republic) were used for the analyses. Microscopic samples on the membrane filters were prepared according to the technique described by Rodina (1968). Bacteria on the filters were counted in 30 visual fields of the microscope, at 12.5ˇ100 magnification. All morphological forms were counted separately. The results were then recalculated into 1 cm³ of water. The total number of planktonic bacteria in the investigated samples was calculated according to the formula described by Rodina (1968), i.e.:

where, the letters stand for:
S –filter working surface (µm²);
s – surface of visual field of a microscope (µm²);
N – mean number of bacterial cells in 1 visual field;
V – volume of water filtered (cm³).

Physicochemical assays of Lake Hańcza water included determination of the following compound and elements content: ammonium nitrogen (mgN-NH₄/dm³), nitrite nitrogen (mgN-NO₂/dm³), nitrate nitrogen (mgN-NO₃/dm³), organic nitrogen (mgNorg/dm³), total nitrogen (mgNtot/dm³), mineral phosphorus (mgP-PO₄/dm³), organic phosphorus (mgPorg/dm³), total phosphorus (mgPtot/dm³), oxygen (mgO₂/dm³), and last but not least water pH value. The measurements, compliant with Polish standards, were performed at the Institute of Environmental Protection, Suwałki, and at the Department of Water and Waste Technology of the University of Warmia and Mazury in Olsztyn.

RESULTS

Vertical and seasonal distribution of bacterioplankton (Tables 2,3, Figs. 2,3). In 1997, in Lake Hańcza pelagial waters (at station 1), the number of bacterioplankton ranged from 1.44ˇ10⁶ cells/1cm³ in May at the depth of 108 m up to 8.44ˇ10⁶ cells/1cm³ in July at the depth of 5 m. On average, for the whole aquifer, the lowest number of planktonic bacteria was reported for September, while the highest for July and August and amounted to 2.984ˇ10⁶, 4.212ˇ10⁶ and 4.151ˇ10⁶ of cells/1 cm³, respectively.

During summer stratification, a characteristic vertical distribution of bacterioplankton was observed, characterised by higher numbers identified respectively in the upper and lower layer of epilimnion where it was smaller than 8.00ˇ10⁷, and 5.5ˇ10⁷ cells/1 cm³. With depth, rapid changes in the number of such bacteria were observed within the hypolimnion layer (10-100 m), with a descending tendency at the bottom.

In the years 1998-2000 the number of planktonic bacteria varied from 4.34ˇ10⁵ of cells/1 cm³ to 6.835ˇ10⁶ of cells/1 cm³. Their lowest number was reported in August 1999 at the depth of 0.3 m, while the highest one was found the same month at the depth of 1 m. On average, the lowest number of these bacteria was reported in 1999, while the highest one in 1998 when it amounted to 1.959ˇ10⁶ and 3.136ˇ10⁶ of cells/1 cm³, respectively.

In the period of summer stratification, the maximum number of bacterioplankton was noted at a depth of 1 m in 1998 and 1999 (6022 and 6835 of cells/1cm³, respectively), and in the hypolimnion layer at 70 m in 2000 (3468 of cells/1cm³). From May to September 1998, the mean number of planktonic bacteria slightly varied from 3.034ˇ10⁶ to 3.330ˇ10⁶ cells/1 cm³. A higher fall in their number below 2713 cells/1 cm³ was reported not sooner than in October. In 1999, the lowest mean number of bacterioplankton in the entire aquifer was observed in July and the highest in August (9.58ˇ10⁵ and 3.087ˇ10⁶ cells/1 cm³); while in 2000 the respective values were reported in May (lowest) and in July and September (highest) when they reached 1.658ˇ10⁶,4.086ˇ10⁶ and 4.425ˇ10⁶ cells/1 cm³, respectively. In the years 1998 and 1999 surface water (0-1 cm) were us ually less abundant in these bacteria, while at the depths of 03-1.0 and 2.0 m their number increased several times. In 2000, a vertical stratification of bacterioplankton was more of a micro-zonal character, though the number of these bacteria was also lower in the surface water than at the depth of 0.3 m. Only in July their number was higher in the layer of surface water. Occasionally, a higher number of bacterioplankton was reported in the near bottom waters.

Qualitative composition of bacterioplankton (Table 3). Qualitative composition of bacterioplankton was determined in the years 1998-2000. Three morphological forms of such micro-organisms were identified: cocci, rods, and bacilli. Cylindrical rods forms prevailed and constituted from 33 % to 68 % of the total bacterioplankton number in July 1999 and August 1999, respectively. The cocci ranged from 29 % to 65 % (August 1999 and July 1999), while the bacilli – from 1 % to 7-8 % of the total planktonic bacteria number in June 1999 and October 2000 and in September and October 1999, respectively. Neither a correlation was found between the number of particular morphological forms of these bacteria and the season, nor any regularity in their vertical concentration was observed in particular months of the experimental period. Occasionally, a higher number of bacilli was reported in near bottom water.

Thermal-oxygen relations (Figs. 2, 3). In the summers of the experimental period, a clear thermal stratification of the lake was observed. In August of 1997, 1998 and 2000, warm epilimnion reached 5 m, and in 1999 it went down to 7 m. The temperature fluctuated in this water layer and the following values were recorded in middle-August of the 3 consecutive years 1997, 1998, 1999: 21.6-19.6°C (22^nd August), 18.1°C (14^th August), and 21.6-21.3°C (13^th August). Below a metalimnion layer of a thickness of several meters and mean temperature gradient of ca. 3°C/m occurred. In the cold hypolimnion temperature decreased gradually from 8.3°C, 8.0°C, and 9.5°C at the respective depths of 12 m , 11 m and 10 m (1997 and 1998, 1999 and 2000), to 4.3°C in the near bottom water. Spring circulation was reported to begin in April or May, while autumn circulation in September, October or November . Measurements of oxygen saturation at the depth of 50 m yielded 6.5 mg and 16.1 mg O₂/dm³ in October 2000 and spring 1999, respectively. In spring and autumn the oxygen saturation was approximately equal at all analysed depths, with a descending tendency at the bottom at (depth of 107 m (Figs. 2,3). During summer stagnation, the oxygen saturation in epilimnion did not fall below 8.5 mg/dm³. Moreover, a higher concentration of this element equal to 12.5/dm³ and 16.9 mg/dm³, dated to 14^th August 1998 and 13^th August 1999, respectively, was reported in the lower part of thermal jump, i.e. in the water layer from 10 m depth.

The content of hydrogen ions (Table 4). The reaction of Lake Hańcza water was neutral or slightly alkaline and its pH ranged from 7.35 to 8.70. A higher pH in the upper layers was reported. Values of this parameter decreased with depth.

The content of ammonia, nitrite, nitrate, and total nitrogen (Table 4). The content of ammonia nitrogen changed within the values limited by the detectable limit, i.e. less than 0.001 mg N-NH₄/dm³, and the value of 0.040 mg N-NH₄/dm³. Such values were found in most of samples in all experimental seasons of 1999 and 2000, and in spring for 108-m deep water, respectively. The content of nitrite nitrogen ranged from the detectable threshold value of 0.001 mg N-NO₂/dm³ recorded in the 10-m water layer in October 1999, regardless the depths in autumn, and in water at the depth of 50 and 108 m in August 2000, to the maximum of 0.092 mg N-NO₂/dm³ measured in water at 10 m depth in August 1999. The content of nitrate nitrogen varied from 0.004 mg N-NO₃/dm³ in 10 m deep water, in August 2000, up to 0.352 mg N-NO₃/dm³ at the depth of 108 m in spring 2000, whereas the total nitrogen content changed between 0.248 mg N_tot/dm³ at a depth of 0.3 m (April 1999) to 1.168 mg N_tot /dm³ at the depth of 108 m in summer 1999. In spring and winter 1999 and 2000, higher concentrations of ammonia nitrogen were found in the near bottom water, while in summer – at 5 m depth. The contents of nitrite and nitrate nitrogen in the examined samples of Lake Hańcza waters demonstrated regularity neither in the vertical, nor in the seasonal distribution, though the total nitrogen content tended to increase with depth.

The content of mineral and total phosphorus (Table 4). Over the research period the mineral phosphorus content ranged from 0.001 mg P-PO₄/dm³ (108 m depth in spring) up to 0.046 mg P-PO₄/dm³ in August 1999. The content of organic phosphorus varied between 0.012 mg P_org/dm³ at the depth of 108 m in autumn 2000, up to 0.068 mg P_org/dm³ at the 10 m depth in August 2000. Total phosphorus content varied between 0.027 mg P_tot/dm³ found at 0.3 and 50 m depth in spring 1999, to 0.089 P_og/dm³ at 10 m depth in August 1999. Slightly higher concentrations of the examined forms of phosphorus were reported regularly for demersal layers of water, and occasionally, depending on the season and year of the study, also for the surface layers.

Statistical analysis of the results (Table 5) of bacterioplankton and water temperature points to a high or even very high correlation for the samples collected in 1997 (r_xy=0.549) and 1998 (r_xy=0.857). A weak positive correlation was also observed between the number of planktonic bacteria and oxygen saturation in the waters of Lake Hańcza during summer lake stratification in 1996 (r_xy= 0.228) and 1998 (r_xy= 0.184). What is more, a negative correlation was found between the number of bacterioplankton and depth, namely r_xy= -0.545, r_xy= -0.792 and r_xy=-0.362 in 1997, 1998, and 1999, respectively. It may be explained by lower oxygen saturation, lower temperature and usually lower content of easily-available organic matter inhibiting development of micro-organisms in the deeper layers of water.

DISCUSSION

The obtained results of physico-chemical analyses for Lake Hańcza waters proved that all the investigated water samples meet the standards specified in the Directive of Minister for Environmental Protection, Natural Resources and Forestry dated November 5, 1999 (on classification of waters and requirements to be met by waste introduced into water and soil regarding N-NH₄, N-NO₂, N-NO₃, N mineral phosphorus, total phosphorus and oxygen contents) and correspond to I class purity waters. The number of bacterioplankton differed highly both with regards to water and individual months of the experimental period.

The total number of planktonic bacteria in Lake Hańcza, namely 4.34ˇ10⁵-8.44ˇ10⁶ cells/1 cm³, was found to be slightly lower in comparison to other lakes of a similar trophic status [9,11,17,28]. Throughout the experimental period (1997-2000) a remarkable variability was observed in the number of such bacteria occurring in the lake; it seems typical for pure waters as other research supports [6,17]. A significant impact on the number, structure and variability of bacterioplankton has been ascribed to the content and availability of organic matter [3,8,21,27]. In the pure, protected Lake Hańcza, surface flows from the basin or products of other micro-organism metabolism provide the main source of biogenes. In lakes with a low trophy, algae and associated numerous zooplankton [13] are considered to be the main reservoir of organic matter [19], especially in the blooming period [7,21]. Bacteriocytic organisms [8,14,16] can be also responsible fo r modifying the number of planktonic bacteria.

The enumerated factors may, to a high extent, evoke an influence on considerable quantitative and qualitative diversity of bacterioplankton reported for particular research seasons, as well as its vertical distribution in the water of Lake Hańcza. Usually, throughout the experimental period, lower numbers of bacterioplankton were reported in the surface water layer (0-0.3 m) that could be related to bactericidal activity of UV radiation [14,17]. In the period of summer stratification of the lake, higher concentrations of these bacteria were usually noted at depths of 2, 5 or 10 m. An increase in the planktonic bacteria content observed in the lower layer of warm trophogenic zone, is typical for lake stratification due to an increase in water density gradient, water temperature, and accumulation of organic matter in this part of a lake. Świątecki [25] has reported that with a temperature increased by 10°C, the number of planktonic bacteria doubles, on average. Higher number of bacteriopl ankton in summer months at specific depths and the whole capacity of Lake Hańcza may also be related to development and atrophy of phyto- and zooplankton that provides nutrients to bacterioplankton [12,17]. Well-documented decrease in bacterioplankton number down the depth of Lake Hańcza, is characteristic for oligotrophic lakes [9]. Similar regularity was reported by Niewolak and Kaczor [17] in oligotrophic Lake Wukśniki, where such a phenomenon was observed and explained by the lack of higher concentration of detritus in these water layers. In other lakes [9,15], higher concentrations of bacteria are usually reported in the over-sediment layer of water. In Lake Hańcza, the phenomenon was reported to occur a few times, namely in June 1997, June and September 1999, and May and September 2000.

In the experimental seasons, a higher number of bacterioplankton was reported in spring, which most likely resulted from the surface flows to the lake, and in summer that was attributed to the increased availability of organic matter content due to development and atrophy of other groups of organisms. The distinctive decrease in the number of planktonic bacteria in autumn can be explained by decreasing temperature and content of available organic matter, and/or by the pressure of bacteriocytic organisms [8,12,13,24]. Factors that determine the number of bacterioplankton also affect its morphological composition. The size of bacteria may provide a good indicator for the state of an ecosystem [1]. Lake Hańcza is dominated by a form of cylindrical rods which constitute from 33% to 68% of total bacterioplankton. A similar structure of these bacteria occurs in other Mazurian Lakeland [25,26,8]. The percentage of cocci, ranging between 29 and 69%, was also of significant importance. According to Godlewska-Lipowa [5], spherical forms of bacteria dominate in lakes with a low trophy, and the number of cylindrical bacteria tends to increase with an increased trophy. Lakes were less abundant in cylindrical forms of bacilli, which demonstrated insignificant concentration (lower than 8%) only at the bottom of the aquifer.

CONCLUSIONS

1. The results of physico-chemical analyses proved purity of the waters of Lake Hańcza.

2. The determined number of bacterioplankton, as well as its morphological structure, namely prevailing content of cylindrical rod forms, also support water purity in Lake Hańcza.

3. Over the experimental period vertical distribution of planktonic bacteria usually demonstated micro-zonal character indicating a ‘focal’ distribution of organic and mineral matter.

4. Higher number of bacterioplankton was reported in surface or near bottom water of the lake, which may be attributed to organic matter of auto- or allochtonic origin accumulating in the zones.

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