Studies on Some Limnological Variables in Subtropical Lakes of the Pokhara Valley, Nepal*

Jpn. J. Limnol., 49, 2, 71-86, 1988.

Masami NAKANISHI, Makoto M. WATANABE, Akira TERASHIMA, Yoshihiko SAKO, Toshifumi KONDA, Keshab SHRESTHA, Hemanta R. BHANDARY and Yuzaburo ISHIDA

Abstract

Some limnological variables were investigated and analyzed in Phewa, Begnas and Rupa Lakes of Nepal in the late-monsoon and dry seasons. From the data on the levels of TN, TP, Chl. a and phytoplankton primary productivity, Phewa Lake seemed to be meso-eutrophic, and Begnas and Rupa Lakes to be eutrophic. The gradient of the regression line of PC on Chl. a ranged from 42.1 to 71.4 and suggested that phytoplankton in the investigated lakes had a moderately high or high relative growth rate. In comparing the C : N : P : ratios of the seston with REUFIELDRatio and other data on phytoplankton chemical ratio, all the lakes appeared to be P-limited. Variation of species diversity of phytoplankton was great between both the seasons in each lake and tended to depend mainly on the share of the predominant species. Microcystis aeruginosa, in total cell number in Phewa and Begnas Lakes, and on that of Tabellaria fenestrata in Rupa Lake. The similarity index of phytoplankton indicated that phytoplankton in Phewa Lake were similar to those in Begnas Lake but not to those in Rupa Lake.

Key words : trophic status, C : N : P ratios, diversity, similarity

waters. At present, however, limnological data on I. Introduction Nepalese lakes seem to be poor for comparative Nepal borders India on the south and Tibet on discussion of the properties of lakes located in the north. The distance between the Terai region different altitudes or climates, though they have on the border of India and the Himalaya Range on been accumulated through some investigations on the border of Tibet is only about 200 km. The high altitude lakes (4,500-5,600 m above the sea) in Terai region is ca. 100 m above the sea and nearly the Khumbu Himal Region (LOFFLER, 1969), on tropic in climate. On the other hand, the Lake Rara (2,990 m) in the far west Nepal (FERRO, Himalaya Range with 6,000-8,000 m above the sea 1978/79 ; OKINO and SAioII, 1986) and on low is freezing. Therefore, the climate in Nepal is altitude lakes (600-750 m) in the Pokhara Valley extremely varied from south to north with eleva- (NICKEL, 1973 ; FERROand SwAR, 1978 ; SWARand tion, as well as by the existence of the monsoon and FERNANIX),1979, 1980 ; KATO and HAYASKI,1980 ; dry seasons. The precipitation is brought about in FERR0, 1981/82). summer by the effect of monsoon. This phe- From the published data on physical and chemi- nomenon is observed mainly in the southern face of cal variables like water temperature, Secchi disc the Himalaya and Mahabharat Ranges. reading, electric conductivity, pH and dissolved Nepal is dotted with small lakes from subtropical oxygen, it has become clear that the lakes of the to freezing zone. Thus, as pointed out by HICKEL Pokhara Valley are monomictic or incompletely (1973), Nepal is a favorable area for comparative monomictic and mostly anoxic in hypolimnion studies on the influence of altitude and climate on during the thermal stratification period. How- the biological, chemical and physical properties of ever, there is no information on diagnosis for

* Contribution from the Otsu Hydrobiological Station, Kyoto University, No. 317 (Foreigen Language Series) 72 NAKANISHI, WATANABE, TERASHIMA, SAKO, KONDA, SHRESTHA, BHANDARY and ISHIDA

Fig. 1 Maps showing the location of Phewa, Begnas and Rupa Lakes in Nepal.

trophic status of the lakes and phytoplankton Begnas Lake and at Station 1 (4 m) or 14 (3 m) in growth because of the little data on dissolved Rupa Lake. The following variables were meas- nutrients, particulate carbon, nitrogen and phos- ured vertically at each station : water temperature, phorus, and chlorophyll a concentrations. Further, pH, light attenuation into water, dissolved oxygen from the biological aspect, there is no analytical (DO), Chl. a, PC, PN, dissolved total nitrogen study on community structure except for data on (DTN), dissolved total phosphorus (DTP), total some characteristics of composition and seaso phosphorus (TP) and cations. In addition, water nality of phytoplankton, zooplankton and fish. samples for measurement of daily primary produc- In the present study, with a view to elucidate tion and observation of phytoplankton composition general features including trophic status of the were collected. All the samples were taken from lakes and growth condition of phytoplankton, the various depths with a Van-Dorn water sampler. dissolved nutrients, particulate carbon, nitrogen Water temperature was measured with a normal and phosphorus, chlorophyll a and phytoplankton alcoholic thermometer, pH by the colorimetric primary productivity were measured in Phewa, method and light attenuation into water with an Begnas and Rupa Lakes of the Pokhara Valley underwater photometer (Tokyokoden, ANA-200). (Fig. 1) in the latemonsoon (September 1984) and In November December 1982, water temperature, dry (November December 1982) seasons. In pH and DO were measured with a water checker addition, phytoplankton structure was analyzed (Horiba U/7). Samples for Chl. a, PC and PN using indices of species diversity and community analyses were obtained by filtration through a similarity. Whatman GF/C glass fiber filter precombusted at 450°C, 2hr. The samples on the filters were desic- 2. Material and methods cated in a sealed plastic container with silica gel Water temperature, pH, Secchi disc reading and under dark condition. The filtrates through the concentrations of chlorophyll a (Chl. a ), particu- glass fiber filter were subjected to determination of late carbon (PC) and particulate nitrogen (PN) in DTN and DTP. Samples for determination of surface waters were surveyed horizontally at 17-18, total inorganic carbon (TIC) poured into 10 ml 20 and 19 stations in Phewa, Begnas and Rupa glass bottles with 0.05 ml neutral formalin and were Lakes, respectively (Fig. 2). An intensive survey kept in a 500 ml plastic bottle filled with water was carried out at the deepest Station 11 (ca. 20 m (OKINO,personal communication). Algal samples deep) in Phewa Lake, at Staions A and B (5 m) in were fixed with glutaraldehyde solution and Limnological Variables in Subtropical Lakes in Nepal 73

cell number of sample. The similarity index (CA) between algal communities was calculated by using Mofzrsrra's (1959) formula derived from SIMI'soN's measure of diversity (A), ni (ni - 1) A= N (N - 1)

where N and ni are the total cell number in sample and cell number of each species, respectively.

where A, and A2, N, and Nz are A and N of the community I and II, and n, i and nz i are the cell number of the i-th species found in the community I and II, respectively.

3. Results

3-1. Horizontal distribution of some variables The variations in some variables among stations in the respective lakes are compared for the two seasons investigated (Fig. 3). In the latemonsoon season (September 1934), the pH value in surface waters varied from 6.2 to 8.4 among stations in Fig. 2 Maps showing sampling stations and isother Phewa Lake, 6.4 to 6.9 in Begnas Lake, and 7.0 to 7. mic lines in Phewa (A), Begnas (B), and Rupa 6 in Rupa Lake. A pH value higher than 8.0 was (C) Lakes. Shaded area in Phewa Lake indicates macrophyte zone. The maps were observed only in Phewa Lake except for Stations 1, drawn on the basis of bathymetric maps by 2 and 3 (6.2-7.0) . In the dry season (November - FeRK() and SwaIK (1978). December 1982), it was in the range of 5.5-7.1, 6.6-6. 9 and 6.6-7.5 in Phewa, Begnas and Rupa Lakes, preserved. All the chemical and biological sam- respectively. As a whole, the pH value in Phewa ples were flown to Japan for the analyses. Chloro- and Rupa Lakes tended to be higher in the late phyl a concentration was determined by the UNfaco monsoon season than in the dry one, while in method (UNesco, 1969), and PC and PN with a Begnas lake it was not different between the CHN analyzer (Yatiaco, MT 2). DTN, DTP and seasons. TP were determined with an autoanalyzer The range of surface water temperature was (Technicon, AA II) and TIC with an infrared 24.4-27.8°C in the latemonsoon season and 18.5-2L analyzer (Horiba-AIA 21). Cations were meas- 9 in the dry one in Phewa lake, being 28.2-31.0°C ured by the ICP emission spectrochemical method. and 19.1-21.1`C in Begnas Lake and 27.0-27.1°C and The daily primary production at various depths 17.8-19.1°C in Rupa Lake, respectively. Taking was measured by the in situ13C technique (Hale et diurnal change in surface water temperature into cl., 1983), twice in the daytime from sunrise to consideration, we noted that the local variation in around noon and from around noon to sunset. Algal water temperature was not significant in each lake cells were counted microscopically using a hae- except for the area around Station 1 with the matocytometer. lowest temperature in Phewa Lake. As in FERR0 The species diversity of phytoplankton derived (1981/82) , the surface water temperature changed from Sh[ANNoti'S formula was used here. seasonally with the range of 15.2-28.2,15.1-30.3 and 15.2-30.2°C in Phewa, Begnas and Rupa Lakes, respectively. Thus, the water temperature during where H' is the diversity of bits•cell-'•ml-', and ni the investigated period in the latemonsoon season and N are cell number of i-th species and the total seemed to be nearly equal to the maximum and in 74 NAKANISHI, WATANABE, TERASHIMA, SAKO, KONDA, SHRESTHA, BHANDARY and ISHIDA

Fig. 3 Comparison of distribution of pH, water temperature (W. T.), Secchi disc transparency and Chl. a concentration in surface waters in Phewa, Begnas and Rupa Lakes between the latemonsoon (September 1984) and the dry (November December 1982) seasons. Shaded area, the late -monsoon ; open area , the dry. the dry season to be slightly higher than the not so great as in the latemonsoon one except for minimum. Rupa Lake. The range was 7.0-10.5, 7.4-13.8 and The range of Secchi disc reading or transparency 9.6-29.9 mg•m- in Phewa, Begnas and Rupa Lakes, was 1.3-3.4 m in the latemonsoon season and 2.0- respectively. Except for a few stations (Stations 2.6 m in the dry one in Phewa Lake, 1.0-2.0 and 1.6- 2, 3 and 7 in Phewa Lake and Station 19 in Begnas 2.3 m in Begnas Lake and 1.9-2.4 and 1.4-2.1 m in Lake) with very high Chl. a concentration in the Rupa Lake, respectively. With the exception of latemonsoon season, the Chl. a level in surface the values at a few stations, the level tended to waters of Phewa and Rupa Lakes tended to be decrease in the following order : Phewa, Rupa and lower in the latemonsoon season than in the dry Begnas in the latemonsoon season and Phewa, one, whereas in Begnas Lake it was not so different Begnas and Pupa Lakes in the dry one. in either season (Fig. 3). Chlorophyl a concentration of surface waters in PC and PN concentrations in the latemonsoon the latemonsoon season varied largely among season were in the ranges of 460-4,650 mg C•m-3 stations in each lake, with ranges of 4.0-61.9, 7.5-40. and 60-600 mg N • m_3 in Phewa Lake, 1,070-3,33,320 8 and 8.9-21.4 mg•m- in Phewa, Begnas and Rupa mg C•m-3 and 150-410 mg N•m-, in Begnas Lake Lakes, respectively. In the dry season, the vari- and 730-1,230 mg C•m-~ and 110-220 mg N•m-3 in ation of Chl. a concentration among stations was Rupa Lake. Exceptionally high concentrations of Limnological Variables in Subtropical Lakes in Nepal 75

PC and PN were observed at Station 3 with the and/or detritus changes delicately from season to highest Chl. a concentration in Phewa Lake and at season. As a whole, however, the average C : N Station 19 in Begnas Lake. Their concentrations ratio (by weight) of the particulate matter tended in the dry season were in the ranges of 760-1,470 mg to be relatively constant between both the seasons C•m-3 and 100-230 mg N•m_3 in Phewa Lake, in each lake : 7.1 (range, 6.1-7.7) in the late 1,130-1,710 mg C•m-3 and 170-300 mg N•m-3 in monsoon season and 7.5(6.4 3.9) in the dry one in Begnas Lake and 650-1,740 mg C•m-3 and 110-290 Phewa Lake, 7.2 (6.1 7.9) and 6.0(5.6-6.3) in mg N•m-3 in Rupa Lake. On the linear regres- Begnas Lake and 6.6(5.7 8.2) and 6.2(5.5 6.6) in sions of PC and PN on Chl. a, the correlation Rupa Lake, respectively. coefficient was high (p<0.01) except in the dry TN (DTN + PN) and TP concentrations at season in Phewa Lake (Fig. 4). This means that several stations in the latemonsoon and dry sea- the variations of PC and PN depend mainly on the sons are shown in Table 1. TN concentration was Chl. a concentration or the phytoplankton standing very high in the dry season as compared with that crop in each lake. in the latemonsoon season in any lake, whereas As to the relationship between PC and PN, the TP concentration did not differ in either season. regression line of PC on PN is shown in Figure 5. The high concentration of TN in the dry season The correlation coefficient of PC on PN in any was due to high DTN concentration. regression line was highly significant (p<0.01). Judging from variations of the above mentioned The gradient of regression lines was significantly variables among stations, the lakes are not always different between both the seasons in Phewa and homogeneous in relation to Fi RI o's (1981/32) Begnas Lakes. This fact suggests that relative prediction. content in chemical components of phytoplankton

Fig. 4 Relationship between PC or PN and Chl. a in surface waters of the three lakes in the latemonsoon (solid circle) and dry (open circle) seasons. Solid and broken lines are the linear regression lines of PC or PN on Chl, a in the latemonsoon and dry seasons, respectively . All the lines were significant at P<0.01. 76 NAKANISHI, WATANABE, TERASHIMA, SAKO, KONDA, SHRESTHA, BHANDARY and ISIIIDA

Fig. 5 Relationship between PC and PN in surface waters of the three lakes in both the seasons . Symbols are the same as those in Figure 4.

Table 1. TN, DTN, TP and DTP concentrations in surface waters of Phewa, Begnas and Rupa Lakes.

3-2. Vertical distribution of some variables observed in stratified temperate lakes during Vertical variations of some variables are given in summer (PARSONS et at., 1984). Vertical profiles of Figure 6 and Table 2. PC and PN are somewhat different from that of Phewa Lake Chl. a. Though PC and PN showed one peak at 5 Water temperature in the latemonsoon season m depth with the Chl. a maximum, they tended to (25 September 1984) dropped at a depth of 5 m, and increase again at 20 m depth without any striking a moderate thermocline was formed (Fig. 6). The decrease at 10 and 15 m depths. PC : Chl. a (100- pH value also decreased at 5 m depth. The Chl. a 107) and PN : Chl. a (14-15) ratios in the layer concentration, on the other hand, showed a peak at above 2.5 m depth decreased to 55 and 7 at 5 m 5 m depth and decreased sharply in the layers below depth, respectively. They increased remarkably in 10 m depth. This pattern is similar to that the layers below 10 m depth and ranged from 272 Limnological Variables in Subtropical Lakes in Nepal 77

Fig. 6 Vertical profiles of water temperature (W . T.), pH, dissolved oxygen" (DO), Chl. a, PC and PN at Station 11 in Phewa Lake (A), at Station B in Begnas Lake (B) and at Station 1 or 14 in Rupa Lake (C). Solid and broken lines are the latemonsoon and the dry seasons, respectively . Bar in the Figure shows the range of diurnal changes of each variable .

and 50 in each ratio at 10 m depth to 630 and 92 at condition. The DTN concentration increased with 20 m depth. These facts suggest that a fair an extremely high gradient towards deeper layers amount of particulate matter consisting mainly of (Table 2). The noticeable increase in DTN in the substances other than phytoplankton remained in deeper layers seemed to be closely related to an the layers below 10 m depth. It is also noteworthy ammonium -N accumulation. On the other hand, that, despite the extraordinarily high PC : Chl. a there was no observable increase in the DTP and PN : Chl. a ratios below 10 m depth , the PC concentration in these layers, perhaps due to the PN ratio was not so different vertically or rather originally poor storage of P in this lake. low, ranging from 5.4 to 6.8 (cf. 7.2-7.4 in the layers The average equivalent percentage composition above 5 m depth). of the cations was Na 18.4, K 3.6, Mg 27.1 and Ca We could not obtain data on the vertical profile 50.9 in the upper layer (0-5m) and Na 12.7, K 3.5, of DO in the latemonsoon season because of a Mg 17.0 and Ca 62.9 in the lower layer (15 and 20 water checker mishap. In regard to the vertical m). The proportion of Na tended to decrease and variations of cations, the iron (Fe) and manganese that of Ca to increase in the deeper layer. This (Mn) concentrations increased noticeably with tendency indicates higher salinity in the lower layer depth (Table 2). This fact suggests that layers (HLTcI{INsoN, 1957). The Sr : Ca ratio by atomic below 15 m depth were mostly anaerobic or reduc- proportions did not change significantly in the tive. In fact, there was a strong H2S odor at whole water column, ranging from 1.7: 1,000 to 2.5: depths of 15 and 20 m at sampling time. It was 1,000. This ratio corresponded with that of soft expected that ammonium -N and dissolved phos- water in Sweden (Hurc}fiNsox, 1957). phate-P would be released from sediments and In the dry season (30 November 1982) , lake water accumulated in the deeper layers under reductive at Station 11 was under homoiothermic condition 78 NAKANISHI, WATANABE, TERASHIMA, SAKO, KONDA, SHRESTHA, BHANDARY and ISHIDA

Table 2. Vertical changes in TN, DTN, TP, DTP and cations at Station 11 in Phewa Lake (25 September 1984), at Station B in Begnas Lake (9 September 1984) and at Station 1 in Rupa Lake (16 September 1984). Figures in parentheses are data on 30 November 14 December 1982.

Phewa Lake

Begnas Lake Rupa Lake Limnological Variables in Subtropical Lakes in Nepal 79 and vertical profiles of pH, DO and concentrations pattern similar to the water temperature, showing a of Chl. a, PC, PN, DTN, TP and DTP were mostly tendency to decrease with the depth. The Chl. a, uniform (Fig. 6 and Table 2). There were no PC, PN, DTP and DTN were nearly uniform in significant differences in variations of the PC : Chl. their vertical profiles (Fig. 6 and Table 2). The a, PN : Chl. a and PC : PN ratios, averaging 111, 16 equivalent percentage composition of the cations in and 7.2, respectively. surface water at Stn. 1 was similar to that at Stn. Begnas and Rupa Lakes B in Begnas Lake : Na 29.7, K 2.6, Mg 29.7 and Ca In the latemonsoon season (9 and 10 September 37.9. The Sr : Ca ratio was 2.0 : 1,000. 1984), the depth gradient of water temperature at In the dry season (8-14 December 1982), water Stations A and B in Begnas Lake was slight and temperature in both lakes was mostly uniform in variable due to diurnal change in the water the vertical profile, whereas the pH and DO temperature of upper layer. The pH value showed evidenced a clear depth gradient with higher value nearly uniform distribution with no diurnal change. and concentration in the upper layer even though The Chl. a, PC and PN concentrations tended to they fluctuated considerably within a day probably increase with depth (Fig. 6), while DTN and DTP due to phytoplankton photosynthesis (Fig. 6). were vertically uniform. In cations, only Mn Taking diurnal changes into account, the vertical tended to increase considerably in the deeper profiles of Chl. a, PC and PN at Stations A and B layers. The Fe concentration, having shown in Begnas Lake were nearly uniform, but at Station vertically easy grade, was much higher than in the 14 in Rupa Lake they showed a depth gradient with upper layer at Station 11 in Phewa Lake (Table 2). high concentrations in the deeper layer. The depth gradient of Mn and high concentration 3-3. Daily productivity of phytoplankton of Fe indicates that the lake water below 2.5 m was The daily primary production rate of phytop- at least temporally reductive. The equivalent lankton was measured by the in situ 13C technique percentage composition of the cations at Station B at Station 11 in Phewa Lake, at Stations A and B was Na 29.5, K 2.5, Mg 29.5 and Ca 38.7 and not in Begnas Lake and at Station 1 or 14 in Rupa different in the vertical distribution. This percen- Lake. The daily primary production was not so tage composition was a little different from that at different in each lake between September 1984 and Station 11 in Phewa Lake: The Na proportion was November December 1982 (Table 3), though it was higher at Station B than at Station 11 while the Ca expected to be higher in September than in proportion was lower. The Sr : Ca ratio at Station November December because of the higher water B was quite similar to that at Station 11, ranging temperature and light intensity in the latemonsoon from 1.6 : 1,000 to 2.7 : 1,000. season. As with FERRO (1981/82), the daily gross The depth gradient of water temperature was not primary production in Phewa and Begnas Lakes distinct at Station 1 in Rupa Lake on 16 September tended to be relatively constant from August to 1984, and the pH value varied vertically with a January.

Table 3. Comparison of daily primary production of phytoplankton and light attenuation coefficient between the latemonsoon (September 1984) and the dry (November -December 1982) seasons . 80 NAKANISHI, WATANABE, TERASHIMA, SAKO, KONDA, SHRESTHA, BHANDARY and ISHwA

Daily primary production depends mainly not To compare the phytoplankton structure bet- only on water temperature and light intensity but ween the two seasons or among the lakes, indices of also on phytoplankton biomass and TIC concentra- species diversity and community similarity were tion. Phytoplankton standing stock expressed as calculated. Table 4 gives the diversity index of all Chl. a amount in the photosynthetically active layer species, green algal species and diatom species in at the stations in September 1984 was lower than in each lake. The diversity index of all species was November December 1982 (Fig. 6). Moreover, the very different in terms of the season in either lake. TIC concentration was about half as much as in The index values in Phewa Lake in September 1984 November December. Thus, the daily primary and in Begnas Lake in December 1982 were lowered production of phytoplankton in the latemonsoon due to the high cell numbers (4.5 x 104 and 1.3 x 104 would be enhanced by higher water temperature cells•ml-' in Phewa and Begnas Lakes, respec- and light intensity, but depressed by lower phyto- tively) of M. aeruginosa, while the value in Rupa plankton biomass and TIC concentration. As a Lake in December 1982 was lowered by the high cell result, no significant difference in the daily primary number (2.1 x 104) of T , fenestrata. The index value production between the two seasons was observed. of green algae was high in all lakes through the 3-4. Phytoplankton structure seasons and ranged from 2.50 to 3.58. The value of The species composition of phytoplankton was diatoms was lower than that of green algae and surveyed qualitatively and quantitatively at Sta- fluctuated fairly between the seasons. Rupa Lake tion 11 in Phewa Lake, at Station B in Begnas Lake showed very low values in the two seasons due to and at Station 1 or 14 in Rupa Lake in both seasons. the high cell numbers of T. fenestrata. The numbers of algal species which could be To examine the similarity of phytoplankton detected were 34 in the latemonsoon season and 41 between the lakes and between the seasons, an in the dry one in Phewa Lake, 26 and 55 in Begnas index of similarity (C i) was calculated. As shown Lake, and 28 and 32 in Rupa Lake, respectively. in Table 5, the value of C was very high between They were similar to those listed by HIcKEI. (1973). Phewa and Begnas Lakes (0.955 and 0.835 in the Of the qualitative composition, green algae were latemonsoon and dry seasons, respectively), but most abundant in the species number through both very low and nearly zero between Phewa and Rupa seasons : 23, 36 and 26 spp. were the sum for both Lakes or between Begnas and Rupa Lakes. This seasons in Phewa, Begnas and Rupa Lakes, respec- result suggests that Phewa and Begnas Lakes were tively, followed by diatoms (11, 9 and 6 spp. in the nearly similar communities but Rupa Lake was respective lakes). From the viewpoint of average composed of different communities than the other cell numbers of the samples collected from the two lakes. In terms of similarities of green and depths of 0, 1 and 2.5 m, the blue green alga, diatom communities relatively rich in species Microcvstis aeruginosa (103-104 cells' ml-') , was numbers, the values of C~ in these communities in dominant in Phewa and Begnas Lakes in both Phewa and Begnas Lakes were not always high and seasons and the diatom, Tabellaria fenestrata (103 fluctuated greatly from 0.128 to 0.835 through the -104 cells•ml-') in Rupa Lake . The cell numbers two seasons. The C values of these communities of other species were at most 102 cells•ml-'. between Phewa and Rupa Lakes or between Begnas

Table 4. Species diversity (H') of phytoplankton in Phewa, Begnas and Rupa Lakes.

* Percentage of predominant species to total cell number of all species, green algae or diatoms. Limnological Variables in Subtropical Lakes in Nepal 81 and Rupa Lakes were extremely low in the two 4-l. Trophic status seasons. It may be concluded that Rupa Lake was YosIllyR~lza(1937) showed the standard of TN and very different from Phewa and Begnas Lakes not TP concentrations as one indicator for classifica only in the whole phytoplankton but also in green tion of the trophic status of harmonic lakes and and diatom communities. The Ca value for both defined lakes, with the concentrations above 0.2 mg seasons in each lake was very high (Table 6), N • 1-' and 0.02 mg P• 1-' as eutrophic and below indicating that phytoplankton in the latemonsoon them as oligotrophic. In surface waters of Phewa season were similar to those in the dry one in each Lake, TN and TI concentrations were nearly equal lake. to YosI!!yn iz.v's standard or rather low except TN concentration in the dry season (Table 1). In 4. Discussion Begnas and Rupa Lakes, TN and TP were above We investigated the trophic status of the lakes 0.25 mg N • 1 ' and 0,02 mg P• 1 ' through the two under study on the basis of the data on TN, TP, seasons, respectively. According to Y )1 IIyI[R v's Chl. a and phytoplankton productivity. The definition, it is hard to say that Phewa Lake is phytoplankton growth conditions in the lakes were eutrophic, though it can be said that Begnas and also discussed on the basis of the data on PC : Chl. Rupa Lakes are eutrophic. a, PN : Chl. a and PC : PN : PP ratios, and some Aii c. (1973) summarized that chlorophyll characteristics of phytoplankton structure in concentration in euphotic layer was 0,05-1,1-15 and connection with indices of species diversity and 17-140 mg•m_3 in Japanese oligotrophic, meso- community similarity. trophic and eutrophic lakes, respectively. Further,

Table 5. Values of similarity index (C,) of whole phytoplankton, green algal and diatom communities between Phewa and Begnas Lakes, Phewa and Rupa Lakes, and Begnas and Rupa Lakes .

Table 6, Values of the similarity index (Ci) of whole phytoplankton, green algal and diatom communities between the latemonsoon and the dry seasons in each lake . 82 NAKANISHI, WATANABE, TERASHIMA, SAKO, HONDA, SHRESTHA, BHANDARY and ISHIDA

ICHIMURA(cited by ARUGA,1973) showed that gross ratio is high (100-150) at a low relative growth rate primary productivity on a fine day was 0.03-0.10, and low (25-50) at a high relative growth rate. The 0.05-0.27 and 0.18-1.72 g C• m-2•d -' in the respective gradient of the regression lines of PC on Chl. a lakes mentioned. The Chl. a concentration in ranged from 42.1 in December of Rupa Lake to 71.4 surface waters of the investigated lakes varied in September of Phewa Lake (Fig. 4) . As largely among stations and reached both the compared with the C : Chl ratio of phytoplankton at mesotrophic and eutrophic levels with the excep- low relative growth rate shown by GOLDMAN (1980) , tion of Phewa (7.0-10.5 mg•mLL3) and Begnas(7.4- the values of the gradient were fairly low. 4n the 13.8) Lakes in the dry season (Fig. 3). Thus, in assumption that the gradient reflects approxi- terms of Chl. a concentration, the trophic status of mately the C : Chl. a ratio of phytoplankton, the C : all the lakes under study is locally different and Chl. a ratio of phytoplankton in the investigated mesotrophic to eutrophic. The daily primary lakes seems to be consitent with moderately high or production rate of phytoplankton at the deepest high relative growth rate. station in Phewa, Begnas and Rupa Lakes (Table 4-3. C : N : P ratios of particulate matter 3) corresponded with the value obtained in the C : N : P ratios of particulate matter in the eutrophic lakes of Japan. respective lakes were calculated from data on PC, From a summary of the data on TN, TP and Chl. PN and PP (TP-DTP) concentrations in surface a concentrations and primary productivity for waters measured at several stations. The range of diagnosis of trophic status, Phewa Lake seems to C : N : P ratios (by weight) through the seasons be eutrophic or rather meso-eutrophic, while was 98:15:1- 287:33:1.108:18:1 -152:21: 1 Begnas and Rupa Lakes appear to be eutrophic, and 72 : 11: 1-108 : 17 : 1 in Phewa, Begnas and despite certain inconsitencies among the variables. Rupa Lakes, respectively. Variation of the ratios 4-2. PC : Chi. a and PN : Chl. a ratios in each lake was significant among stations rather PC : Chl. a and PN : Chl. a ratios in surface than between the seasons. The REDFIELD Ratio on waters of the investigated lakes were calculated on the basis of algal tissue analysis is about 42C: 7N the basis of the data shown in Figure 4. The PC : 1P by weight (cited by REYNOLDS, 1984 and H Alzxis, Chl. a and PN : Chl. a ratios varied greatly from 1986) . Recently, VOLLENWEIDERand HARRIS (cited station to station through the investigated seasons by HARRIS, 1986) showed Ca. 64C : 9N : 1P as the and in the ranges of 40-171 and 6-25, respectively. more representative phytoplankton chemical ratio. In general, the ratios tended to be high at stations And from the viewpoint of physiological indicators with low Chl. a concentration. HARRIS (1986) of nutrients deficiency in algae, TEZUKA (1985) pointed out that variations of C : Chl, N : Chl and summarized ranges of N : C, P : C and N : P ratios, P : Chl ratios in particulate matter depend on algal differentiating N and P deficiency on the basis of biomass expressed as chlorophyll : at low algal data by HEALEY and 11ENDZEI. (1979,1980), who used biomass the ratios are high at 250, 50 and 5, respec- N : C of phytoplankton as an indicator for N tively, and at high algal biomass they decrease to deficiency and P : C and N : P ratios for P ca. 40, 8 and 0.3. This suggests that at low algal deficiency. The C : N, C : P and N : P ratios of biomass the contribution of detrital material other phytoplankton under no N- or P-deficient condi- than phytoplnkton to PC, PN and PP becomes tion, as calculated from the data summarized by relatively high, and at high algal biomass relatively TE7UKA (1985), were <7, <50 and <10, respec- less. Thus, variations of the PC : Chl. a and PN tively. These values are quite similar to those Chl. a ratios observed among stations seem to have derived from the RI•;DEIELDRatio and the ratio by been mainly brought about by a difference in the VoLLENv~EIDER and HARRIS. As mentioned before, relative contribution of phytoplankton to PC and the C : N ratio of particulate matter in the present PN. Here, however, we should keep in mind that lakes was relatively constant and similar to that variations of these ratios depend on the physiolog- derived from the REDFIELD Ratio and the other ical state and the species of phytoplankton as well phytoplankton chemical ratio reported by the as the algal biomass. above authors. On the other hand, C : P and N : P According to GOLDMAN(1980), the C : Chl ratio ratios in the lakes were more variable among in algal cell varies greatly with the difference in the stations than the C : N ratio and very high as relative growth rates of phytoplankton ; the C : Chl compared with the ratios calculated from the Limnological Variables in Subtropical Lakes in Nepal 83

phytoplankton chemical ratio. Thus, the C : N : P ratios obtained here indicate that N is sufficient for phytoplankton growth but P is too low for their further growth in all the lakes. From the high C : P and N : P ratios, however, one cannot absolutely conclude that phytoplankton growth in the lakes is limited by P. This is because the minimum P requirement is very different among algae and more variable than that of N (HARRIS, 1986) ; also, these ratios in particulate matter which is composed not only of phytoplankton but also of other substances do not necessarily agree with the algal chemical ratio. 4-4. Phytoplankton diversity and similarity The index of species diversity derived from Si1ANNON's formula is influenced by species richness and equitability or relative abundance of species Fig. 7 Relationship between species diversity (11") of (SA<

The C value between the two seasons was very tudes, it will be necessary to study the lake high in each lake. This fact suggests that there metabolism and phytoplankton in particular. are no great changes in the relative abundance of More accumulation of comparable variables in the predominant species and/or numbers of common Nepal Himalayan lakes is also required. species between the latemonsoon and dry seasons in the subtropical lakes. Acknowledgments There are some questions for application of the We are grateful to the Director, Prof. R. L. species diversity index and similarity index to SHRESTHA and ex chief Prof. P. N. MISIIRA, analysis of phytoplankton structure. It is a Natural History Museum, Tribhuvan Univer- question whether all species forming natural sity, and Prof. H. KADoI A, Kinki University, for phytoplankton can be indentified and counted. their support for this survey. We also extend NAUATA (1986) reported that the picophytoplankton thanks to Mr. D. B. SwAR, the Fishery Develop- (<3 ,um) which play an important role in an ment Centre, Pokhara, Nepal, Mr. M. WADA, ecosystem occur abundantly at a given time of the Japan Oversea Cooperative Volunteer and year in natural waters. And as pointed out by Rio!) Messers. MITHO TAMANC, NIMA SHEIZI'A and (1983), HEyVES et at. (1984) and NA(;ArA (1986), it DORGEESIIERPA for their help in the field survey. seems to be very difficult to identify and count We also thank Dr. T. OKINO, Suwa Hydrobio- these picophytoplankters by normal microscopic logical Station, Shinshu University, Dr. Y. techniques. In natural waters where picophyto- WATANABE,Tokyo Metropolitan University, Dr. plankters are abundant, one cannot neglect their T. MIYAZAKI,Tsukuba University, and Drs. M. populations for phytoplankton structure analysis. AIZAKI and Y. NOJIRI, National Institute for In the present study, these phytoplankters may Environmental Studies, for chemical analyses. have been missed. We appreciate the valuable comments on the Another question is how to define the individual manuscript by members of the Otsu Hydrobio- in phytoplankton. Here, the individual was logical Station, Kyoto University. defined as a cell. Taking the vital function of This study was supported by a Grant in Aid algae into consideration, however, it may be for Overseas Scientific and Survey from the unsuitable for some algal species to define the Ministry of Education, Science and Culture of individual unit as a cell : unit of vital function of Japan (Project Nos. 59041076 and 60043042). some algae may be colonial or filamentous form but not the cell. If we define the individual in 摘要

SHANNON'S formula and MORISITA as a vital urlit, ネパールの亜熱帯湖沼における the diversity index of phytoplankton as bits•cell 陸水学的諸量に関する研究 and the similarity index may have provided less ネパールの亜熱帯3湖沼一フェワ,ベグナス,ルバ than correct information on phytoplankton struc- 湖一においてこれまでに報告のない陸水学的諸量に関 ture. する調査研究を1984年9月(モンスーン後期)と1982 In comparison with the representative phyto- 年11-12月(乾期)に行なった。 plankton chemical ratio, C : P and N : P ratios in 1)全窒素,全リン,クロロフィル α 濃度および植 particulate matter were very high in the investiga- 物プランクトンの日基礎生産量からフェワ湖は中一富 ted lakes. This suggests that phosphorus behavior 栄養的,ベグナスとルパ湖は富栄養的であると判断さ in the lakes is an important subject for further れた。 study on the metabolism in the subtropical lakes of 2)セストン中のC:N:P比をREI)FIELI)Ratio等 Nepal. Despite the lack of distinct differences in と比較すると,いずれの湖沼もC:P,N:P比が著し the physical and chemical conditions among the く高く植物プランクトンにとってP制限型であるよう lakes, the composition of phytoplankton in Rupa に思われた。 Lake was very different from that in Phewa and 3)植物プランクトンの種の多様性および湖沼間の Begnas Lakes. Analytical study on factors regula- 植物プランクトンの類似度を検討した。多様性指数は ting species composition in lakes of the Pokhara 種類数に対する依存性は弱く,細胞数が他種に比べ著 Valley is thus needed. しく多かったル1ガcπ)cys然磁㎎加,sα(フェワ,ベグナ To compare the limnological characteristics ス湖),7淘 ∂611α加ル麗 ∫!履α(ノレパ湖)の細胞密度に among Nepalese lakes located in different alti- 依存し変動した。類似度指数は両期ともフェワ湖とべ LimnologicalVariablesinSubtropicalLakesinNepal 85

グナス湖間で高い値(0.853～0.955)を示したがフェ lake of Pokhara Valley, Nepal. Int. Revu ges.

ワ湖とルパ湖間およびベグナス湖とルパ湖間では著し Hydrobiol., 58: 659-672.

く低い値(0.001～0.012)を示した。このことはフェ HUTCHINSON, G. E. (1957) : A Treatise on Limnology,

ワ湖とべグナス湖間の植物プランクトンは比較的似て vol. 1, Geography, Physics and Chemistry. John

いるが,ルパ湖のそれは他の2湖とはまったく異なる Wiley & Son inc., New York.

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SWAR,D. B. and C. H. FERNANDO(1980) Some 教室,〒606京都市左京区北白川追分町;近田俊文, studies on the ecology of limnetic crustacean 国立予防衛生研究所細菌部,〒141東京都品川区上大 zooplankton in lakes, Begnas and Rupa, 崎240-35;MasamiNAKANISHI,OtsuHydrobiolo- Pokhara Valley, Nepal. Hydrobiologia, 70: gical Station, Kyoto University, Shimosakamoto, 235-245. Otsu, Shiga-ken, 520-01, Japan ; Makoto M. TEZUKA, Y. (1985) : C : N : P ratios of seston in WATANABE,National Institute for Environmental Lake Biwa as indicators of nutrient deficiency Studies, Onogawa, Tsukuba, Ibaraki-ken, 305 ; in phytoplankton and decomposition process of Akira TERASHIMA,Department of Zoology, Kyoto hypolimnetic particulate matter. Jpn. J. University, Kitashirakawa, Sakyo-ku, Kyoto, 606: Limnol., 46 : 239-246. Yoshihiko SAKO and Yuzaburo ISHIDA,Department UNESCO(1969) : Determination of Photosynthetic of Fisheries, Kyoto University, Kitashirakawa, Pigments in Sea Water. Monographs on Sakyo-ku, Kyoto, 606; Toshifumi KONDA,Depart- Oceanographic Methodology, UNESCO,Paris. ment of Bacteriology, National Institute of health, YOSHIMURA,S. (1937) : Koshogaku (Limnologv), Kami-osaki, Shinagawa-ku, Tokyo, 141 ; Keshab Sanseido, Tokyo. SHRESTHAand Hemanta, R. BHANDARY,Natural History Museum, Tribhuvan University, Swayam- (著者:中西正己,京都大学大津臨湖実験所,〒 bhu, Kathmandu, Nepal) 520-01大津市下阪本4-1-23;渡辺信,国立公害研究所,〒305つくば市小野川16-2;寺島彰,京都 Received : 19 November 1987 大学理学部動物学教室,〒606京都市左京区北白川追 Accepted : 12 January 1988 分町;左子芳彦・石田祐三郎,京都大学農学部水産学