- Email: [email protected]
Accumulation of freshwater red tide in a dam reservoir
~
Pergamon
Z. pp. 211-218. 1998. C 1998 IAWQ. PubJilhcd by Elsevier Science lid
WDr. Sci. T«II. Vol. 37, No.
Prillled in GrelII Britain.
PIT: 50273-1223(98)00026-2
0273-1223/98 SI9'OO + 0-00
ACCUMULAnON OF FRESHWATER RED TIDE IN A DAM RESERVOIR Masato Yamada*, Yoshiro 000** and Isao Somiya*** • Department 0/ Waste M(JJIQgement Engineering. National Institute 0/ Public Health, 4-6·1 ShirokDnedai, Minato-/al, Tokyo lOB. Japan •• Department 0/ Environmental &: Civil Engineering, Institute 0/ Environmental and Science &: Technology. The University o/Ohlyama. 2·1-1 Tsushif1Ul-naJca. Okayama 700, Japan ••• Department ofEnvironmental Engineering, Graduate School ofEngineering, Kyoto University. Yoshidahon11Ulchi. Salcyo·ku, Kyoto 606'()J. Japan
ABSTRAcr
n.e distribution and biomass of Peridinium bipes f. DCculratum were observed in the autumns of 1991, 1992 and 1994 at the head of the Shorenji Dam reservoir (Mie prefecture, Japan). Summarizing observations and some other information, we discussed processes related to the accumulation of Peridinium biomass. In the study area, biomass of Peridinium increased at a similar rate to growth day by day. and roughly 213 of the daily maximum biomass fluctuated in a given day. The horizontal 'import' and 'export' of Peridinium biomass by flows mainly change the biomass of red tide at the upstream end of the reservoir. Flow control in the reservoir. as well as growth control. will also be important in suppressing freshwater red tide. C 1998 IAWQ. Published by Elsevier Science Ud KEYWORDS Freshwater red tide; Peridinium bipes; dam reservoir; accumulation mechanism; transport. INTRODucnON
In Japan, the freshwater red-tide, which mainly consists of dinoflagellate Peridinium, has been observed at many dam reservoirs. This algal bloom tums the water surface brownish and reduces the aesthetic value of the lake environment. Therefore, this phenomenon is an important problem in reservoir management. Freshwater red tide is usually observed in the upstream end of reservoirs and is persistent throughout the year (Kagawa et al., 1984; Hata 1991). In thermally stratified reservoirs. circulative upstream flows in the upper layer result from the plunging of river water into the lower layer. On the other hand, Peridinium carries photosynthesis and swims toward light with two flagella due to the positive phototaxis. Under these hydraulic and ecological conditions, a Peridinium population growing on the surface of storage water accumulates near the upstream end of a reservoir (Hata 1991; Nakamoto 1991). The purpose of this study is to estimate the mechanism and characteristics of freshwater red tide in reservoirs by field observation and to provide a key concept for the control of its accumulation. 211
212
M. YAMADA el al.
METHODS Freshwater red tide dominated by Peridinium bipes f. occultatum were observed in Shorenji Dam Reservoir (Mie Prefecture, Japan), which has a total storage volume and a storage area of 27,200,000 m 3 and 1.04 km2• respectively. A dense bloom was observed mainly in two upstream ends. In 1991, 1992 and 1994, daily and diurnal variations of red tide biomass were observed at one of the upstream ends. Stations of these observations in 1991·1992 and 1994 are shown in Figures 1 and Figure 2, respectively. The biomass and water temperature data were measured vertically in the morning (9:00) and the afternoon (14:00) in 1991 and 1992, and for 30 hours (14:00 to 20:00) at 2-hour intervals in 1994. Water samples were taken with Van Don samplers (3L and 1.5L) and the P. bipes cells were counted under a light microscope. In 1994, samples were also taken with a 20 Ilm mesh plankton net (dia. 30 cm) hauled from a bottom to a surface. AJditionally, in 1994, the growing rate of P. bipes was estimated by monitoring a time-course change of P. bipes cells in the enclosures, which is l-L glass flask enclosed in the surface water and floating on the surface. The vertical temperature profile was measured with a thermometer (Handy Thermo DP-21, EIWA Electoric Instruments Co.). In 1994, flow velocities were also measured with an electromagnetic velocimeter (VM-20L and VNT-2-50-08PS, KENEK) whose sensor was hung and fixed by ropes from Kochi bridge.
NL..f-
Figure \. Observation Points in 1991's and 1992's survey.
Kochi Bridge 45m 15m
~
!
85m
•
a!i
15m
.,
~ -ll~:~~ ~1:~~·~:J-·-·I~~··:·-· b: .• . L···6.... ; 16.5rr
D
15m
0 ...
c
i
14m
34n,
·······0·
A
• Van Don sampler & plankton net • F10w meter o plll1kton net only .. EncIOlu....
Figure 2. Observation Points in 1994's survey.
Accumulation of freshwater red tide
213
RESULTS AND DISCUSSION Status of freshwater red tide at an upstream end Qf the ShQrenii Dam reservQir
Daily variation of P. bipes. Figure 3 shows the daily variatiQns of the mean biQmass within the area of stations (Fig. I) from [-25m] tQ [+275m] and depth from 0 tQ 3 min 1991 and 1992. During this period, the inflow rate was relatively stable and ranged from 3.5 tQ 5.9 m 3/s in 1991, and 1.5 to 4.6 m 3/s in 1992. The mean biQmass of P. bipes increased day by day, and the 11-like specific rates of them were estimated at 0.13 /day in 1991 and O.06/day in 1992.
Figure 3. Daily variation of P. bipes biomass in the study area in 1991 and 1992.
Flow condition in the upstream end. Figure 4 shows vertical prQfiles of temperature and flow velocity in 1994. In this figure, positive and negative values of velocity indicate down· and upstream flow, respectively. Temperature prQfiles show that the water column at the upstream end of the reservoir was thermally stratified. Since the downstream flow almost appeared under depth of I m, it would originate from a plunged inflow. On the other hand, a contour flow occurred in the upper layer. Comparing profiles of temperature and flow, the direction of flow reversed at the layer with temperature between 17 and 18°C.
Temp. rC) 15 1617 '81m
o.
Velocity (cm/s)
~~
Temp ('C)
~5
::::
1617 18 197
Velocity
Velocity
(cm/s) Temp. rC) (cm/s)
~~
:::: ;:'5 161718 '97
~~
:::: ;:
OS
jl. -5
1.5
d'2.0 2.5 3..0 0.0
~~~~+~H=i;M=i=+:ii;M=;;4t:rf1M;:;:;:44
0.5 61.0
:g 1.5 c!j2.0 '" 2.S
u."".£...l"""'J....--.......~""'''''''I~'-Io....l..ol
3.0~L...L..L..u .........~..
Figure 4. Vertical profiles of temperalure and flow velocily in 1994.
Diurnal variation of P. bipes. Figure S shows CQntQur diagrams Qf cell numbers of P. bipes and temperature in the vertical section of the line M (Fig. 2) in 1994. The diagrams for temperature show a weak thermocline which declined from the upstream side. In comparison with Figure 4, the up- and downstream flQW would
214
M. YAMADA et al.
occur above and below this thermocline, respectively. In the diagrams of cell number at 14:00. 6:00. and 12:00 (daytime), there was an area of dense accumulation of P. bipes (greater than 103.4 cellslmL) at the surface of the upstream side and contour lines followed the thermocline under the surface. On the other hand, in the diagrams at 18:00 and 0:00 (night), the area greater than 10 3 cellslmL extended along the thermocline to the lower layer. and the surface discoloration disappeared. Furthermore, the area greater than 103.4 cellslmL appeared in the downstream side from 18:00 to 0:00 on October 27. Point
Point
lfT-lT_--TB--.:;ADr-,:C_-r:-:>
4
Figure S. Contour diagrams of cell number of P. bipes and temperature in study &rea in 1994.
Figure 6 shows the diurnal variation of the mean biomass within the whole study area in 1994. The biomass of P. bipes was changed in approximately a 24-hour cycle. The amplitude of variation was about 109 cellslm 2 and indicates that about 213 of the daily maximum biomass fluctuated and 1/3 of it remained in the study area.
5x1d'"T"""---.--......----...-o--r-...
l
-~--
4x1oB
~.;"
11';;;' 3x1d'
_. ~
:;
0_
--.0;....
"" 1\
--
,,/ I
I
~ 2X1oB-I-'-".:~·o1----.lI--+
~
I
I
1x1oB-l---+-----!I--O+---+-l 18
0
6 12 Time (hour)
18
Figure 6. Diurnal fluctuation of P. bipes biomass in 1994.
Accumulation mechanism Contour diagrams (Figure 5) show the spatial movement and/or transportation of P. bipes in the upstream end. Summarizing information in the literature (Kagawa et al., 1984; Hata, 1991; Nakamoto, 1991) and our observation, we present our concept for the accumulation mechanism of Peridinium near the upstream end in Figure 7. In this concept, the reach close to an upstream end can be longitudinally divided into the 'accumulation area', where red tide can be observed in the daytime. and the 'dispersion area' which is defined as the area of dispersing Peridinuim at night The upstream reach can also be vertically divided into the
Accumulation of freshwater red tide
21.5
'upper layer' which flows upstream, and the 'under layer' which flows downstream. Within this reach, processes related to the vertical movement of red tide's biomass would be the 'vertical migration' of Peridinium and the 'entrainment' of water, while processes related to the horizontal movement of such biomass would be ~e 'import' and the 'export' by flows. Furthermore, the 'inflow' and 'outflow' are an input and output, respectively, of the defined reach of an upstream end. The biomass of Peridinium could be varied by certain biological processes such as growth, death, grazing, en-/excystment, which have not been clearly understood. Thus, these are summarized as the process of 'growth' in the present study.
Upper Layer
Under
~umulation / Area - , , - - - -
;
Layer
Outflow
Figul'C 7. Scheme of accumulation mechanism and its processes.
Biomass chan~e in the accumulation area A dense discoloration of the red tide (grater than 103.4 cell/mL of P. bipes) was usually observed in the study area (Figures I and 2). We consider this area as the 'accumulation area' in Figure 7. Figure 6 shows specific growing rate of 0.13 and 0.06 /day of Peridinium biomass under the relatively stable flow, while O.IS/day was also estimated with enclosures used in 1994's survey. These values are nearly to the specific growth rate of P. bipes in the culture at 1O-25°C, 0.10-0.15 /day (Nishibori et al., 1991). However, the diurnal fluctuation of Peridinium biomass (Figure 6) is much greater than the growth of P. bipes. Since P. bipes was seldom contained in the river water, the 'inflow' is likely to have little effect on the diurnal fluctuation. Thus, Peridinium biomass should be fluctuated by the 'import' and the 'export'. To confirm this reasoning, we estimate the transport by counter flows in the cross section of line C. The transport rate (cells/s) is defined as a net biomass transport in/out through a cross section of the accumulation area per second and is calculated by vertically adding up the transport fluxes (cells/m 2/s), which are multiplying mean cell numbers, flow velocities and depths at each 0.5 m layer of the section. Positive and negative values of them indicate the 'export' and the 'import', respectively. Table I compares diurnal change of the observed and predicted Peridinium biomass. The prediction of biomass was performed by subtracting the transport rate from the observed biomass at each time. The predicted variation of biomass is almost the same as the observed one (r2::0.72) and exhibits a diurnal cycle. This result shows that the Peridinium biomass mainly fluctuates by the horizontal transport of biomass and supports our reasoning. Since the diurnal variation of flow velocity was negligible (Figure 4), the amount of transport will almost depend upon the vertical distribution of P. bipes.
216
M. YAMADA d al.
Table I. Comparison between observed and predicted biomass
Time
Observed Biomass
Transport Rate
Ptedicted Biomass
average·· obs. 10· cells·s" 10· cells·s"
IOu cells
18:00
IOu cells. 3.92
20:00
1.81
6.11
22:00
156
2.41
0:00
2.09
-0.16
2:00
1.41
5.41
4:00
1.43
13.6
6:00
2.65
3.69
8:00
2.21
-lSJ
10:00
2..55
2.08
12:00
3.10
-8.01
14:00
3.15
·6.30
16:00
2.84
8.13
18:00
2..52
3.45
6.11·" 432 0.86 2.33 9.51 8.65 1.06 0.26 3.00 .1.19
0.92 5.79 2.21
0.91 20:00 1.59 i • Vol1l%l\e of estimateduea is 7915 m •• A.verage ofobs. 'lUlIeS in the upper end lower liM. ••• Use 20:00 data only
1.43 1.25 2.03 1.31 0.15 2.03 2.14 2.53 3.32 4.27 2.17 2.11 1.43
An attempt to Control the freshwater red tjde As discussed previously the transport processes by flows have a greater effect upon the variation of red tide's biomass than growth. Therefore, the control of flow, especially the upstream flow, is more likely to be effective for the control of freshwater red tide in the upstream end. Ideally, this flow control should be performed to prevent the import of organisms to the accumulation area, to enhance their export from that area, and ultimately to wash organisms out to the downstream reach where they will never return to the 'accumulation area'. Methods having potential to realize these objectives are, for example, the discharge control of a reservoir, the mixing of lake water by aeration or circulation, and the setting the barrier devices in the upstream end. These methods, however, have been applied to treat the lake water quality problem, such as algal blooms or low DO in eutrophic lakes. We made one attempt to re-evaluate the further use of these methods for the red tide control. Figure 8 shows our concept of fences for the control of red tide accumulation. Just after the 1992 survey, we experimentally installed three fences (made by vinyl and hung to I m of depth) at intervals of ISO m in the upstream end and observed the biomass variation of P. bipes in the same manner as described earlier. Figure 9 shows the daily change of Peridinium biomass before and after the installing of fences. This result suggests that the increase of biomass slacked after the installing of fences, but its overall effect was inconclusive at that time.
Accumulation of freshwater red tide
217
I Segmentation of acelumulaled organisms I Fence
Figure g. Concept of fences for freshwater red tide control.
1 5x10'0 , - - - - . . . - - - - - - - - - - - , ~ Installing lances
N'
~
~ 1.0x10'0
~
IS.OX108
co
---+-.--I----l--• o : _.
. ._-----.. . .
----;----.1 ~:~+'--'.<---_
0
O.Ox10 0
7
14
21
28
35 42 49 56 63
Day Figure 9. Biomass change installed before and after fences.
CONCLUSIONS We performed field observation of the freshwater red tide in the upstream end of Shorenji Dam Reservoir in 199), 1992 and 1994. The results exhibit the distribution patterns of P. bipes which vertically migrated by phototaxis, and vertically entrained at the plunging point, and horizontally moved upstream and downstream each day. The biomass of P. bipes increased at a similar rate as growth day by day, but it diurnally fluctuated with the amplitude of'lJ3 of the daily maximum biomass. Moreover, this diurnal fluctuation is much greater than the growth of P. bipes. We defined the areas where red tide is accumulated and dispersed, and consider the processes related to these variations as 'vertical migration' and 'entrainment'; 'imporl' and 'export'; 'outflow'; and 'growth', The transport rates estimated by the vertical profiles of P. bipes and flow velocity successfully explain that the diurnal fluctuation of P. bipes biomass mainly depended on the horizontal transport of biomass. This evidence suggests that the control of flow is more likely to be effective for the control of freshwater red tide in the upstream end. To control the freshwater red tide, we need further study of the re-evaluation of the current methods for water quality improvement in the aspects of flow control. Additionally, the clarification of the ecological status of red tide, such as en-/excystment. preylpredator relationship, etc.• is also necessary for further understanding of the phenomenon. ACKNOWLEDGMENT This material is supported by Special Coordination Funds for Promoting Science and Technology of the Science and Technology Agency, Japan. We thank all those in The Management Office of Kizu River and Shorenji Dam Reservoir of Water Resources Development Public Co., and in Water Resources Environment Technology Center for their assistance with our observation.
218
M. YAMAnA ~I aI.
REFERENCES Kagawa, H.. !seri. Y. and Ito. T. (1984). Environmental Conditions at the head of a teservoir where freshwater ted tide of Peridimum occurs-In the case of the lshitegawa Dam Reservoir-. SuishitsllbdDJa, K~roJ:y ... 7. 37S-383. (in Japanese) Hala, S (J 991). The Fresh-water Red Tide in Nagase Reservoir (Kochi Ptef.). Suishirsuodaku K~roJ:yu, 14. 293-297. (in Japanese) Nakamoto. N. (1991). On the freshwater Red Tide in Reservoir Kanna·lto. SuishitsllOdaJcM K~roJ:yu. 14. 281-285. (in Japanese) Nishibori. N.• Nishijima, T.• Onoda and Y.• HalA, Y. (1991). Effect of Lightlnltnsity. Temperature. pH. and Nitrogenous Nutrient on the Growth of Peridinium bipes fo. llCCultalum. Nippon SwJsan Gakkaishi. 57. 1729- 1735. (in Japanese)
Pergamon
Z. pp. 211-218. 1998. C 1998 IAWQ. PubJilhcd by Elsevier Science lid
WDr. Sci. T«II. Vol. 37, No.
Prillled in GrelII Britain.
PIT: 50273-1223(98)00026-2
0273-1223/98 SI9'OO + 0-00
ACCUMULAnON OF FRESHWATER RED TIDE IN A DAM RESERVOIR Masato Yamada*, Yoshiro 000** and Isao Somiya*** • Department 0/ Waste M(JJIQgement Engineering. National Institute 0/ Public Health, 4-6·1 ShirokDnedai, Minato-/al, Tokyo lOB. Japan •• Department 0/ Environmental &: Civil Engineering, Institute 0/ Environmental and Science &: Technology. The University o/Ohlyama. 2·1-1 Tsushif1Ul-naJca. Okayama 700, Japan ••• Department ofEnvironmental Engineering, Graduate School ofEngineering, Kyoto University. Yoshidahon11Ulchi. Salcyo·ku, Kyoto 606'()J. Japan
ABSTRAcr
n.e distribution and biomass of Peridinium bipes f. DCculratum were observed in the autumns of 1991, 1992 and 1994 at the head of the Shorenji Dam reservoir (Mie prefecture, Japan). Summarizing observations and some other information, we discussed processes related to the accumulation of Peridinium biomass. In the study area, biomass of Peridinium increased at a similar rate to growth day by day. and roughly 213 of the daily maximum biomass fluctuated in a given day. The horizontal 'import' and 'export' of Peridinium biomass by flows mainly change the biomass of red tide at the upstream end of the reservoir. Flow control in the reservoir. as well as growth control. will also be important in suppressing freshwater red tide. C 1998 IAWQ. Published by Elsevier Science Ud KEYWORDS Freshwater red tide; Peridinium bipes; dam reservoir; accumulation mechanism; transport. INTRODucnON
In Japan, the freshwater red-tide, which mainly consists of dinoflagellate Peridinium, has been observed at many dam reservoirs. This algal bloom tums the water surface brownish and reduces the aesthetic value of the lake environment. Therefore, this phenomenon is an important problem in reservoir management. Freshwater red tide is usually observed in the upstream end of reservoirs and is persistent throughout the year (Kagawa et al., 1984; Hata 1991). In thermally stratified reservoirs. circulative upstream flows in the upper layer result from the plunging of river water into the lower layer. On the other hand, Peridinium carries photosynthesis and swims toward light with two flagella due to the positive phototaxis. Under these hydraulic and ecological conditions, a Peridinium population growing on the surface of storage water accumulates near the upstream end of a reservoir (Hata 1991; Nakamoto 1991). The purpose of this study is to estimate the mechanism and characteristics of freshwater red tide in reservoirs by field observation and to provide a key concept for the control of its accumulation. 211
212
M. YAMADA el al.
METHODS Freshwater red tide dominated by Peridinium bipes f. occultatum were observed in Shorenji Dam Reservoir (Mie Prefecture, Japan), which has a total storage volume and a storage area of 27,200,000 m 3 and 1.04 km2• respectively. A dense bloom was observed mainly in two upstream ends. In 1991, 1992 and 1994, daily and diurnal variations of red tide biomass were observed at one of the upstream ends. Stations of these observations in 1991·1992 and 1994 are shown in Figures 1 and Figure 2, respectively. The biomass and water temperature data were measured vertically in the morning (9:00) and the afternoon (14:00) in 1991 and 1992, and for 30 hours (14:00 to 20:00) at 2-hour intervals in 1994. Water samples were taken with Van Don samplers (3L and 1.5L) and the P. bipes cells were counted under a light microscope. In 1994, samples were also taken with a 20 Ilm mesh plankton net (dia. 30 cm) hauled from a bottom to a surface. AJditionally, in 1994, the growing rate of P. bipes was estimated by monitoring a time-course change of P. bipes cells in the enclosures, which is l-L glass flask enclosed in the surface water and floating on the surface. The vertical temperature profile was measured with a thermometer (Handy Thermo DP-21, EIWA Electoric Instruments Co.). In 1994, flow velocities were also measured with an electromagnetic velocimeter (VM-20L and VNT-2-50-08PS, KENEK) whose sensor was hung and fixed by ropes from Kochi bridge.
NL..f-
Figure \. Observation Points in 1991's and 1992's survey.
Kochi Bridge 45m 15m
~
!
85m
•
a!i
15m
.,
~ -ll~:~~ ~1:~~·~:J-·-·I~~··:·-· b: .• . L···6.... ; 16.5rr
D
15m
0 ...
c
i
14m
34n,
·······0·
A
• Van Don sampler & plankton net • F10w meter o plll1kton net only .. EncIOlu....
Figure 2. Observation Points in 1994's survey.
Accumulation of freshwater red tide
213
RESULTS AND DISCUSSION Status of freshwater red tide at an upstream end Qf the ShQrenii Dam reservQir
Daily variation of P. bipes. Figure 3 shows the daily variatiQns of the mean biQmass within the area of stations (Fig. I) from [-25m] tQ [+275m] and depth from 0 tQ 3 min 1991 and 1992. During this period, the inflow rate was relatively stable and ranged from 3.5 tQ 5.9 m 3/s in 1991, and 1.5 to 4.6 m 3/s in 1992. The mean biQmass of P. bipes increased day by day, and the 11-like specific rates of them were estimated at 0.13 /day in 1991 and O.06/day in 1992.
Figure 3. Daily variation of P. bipes biomass in the study area in 1991 and 1992.
Flow condition in the upstream end. Figure 4 shows vertical prQfiles of temperature and flow velocity in 1994. In this figure, positive and negative values of velocity indicate down· and upstream flow, respectively. Temperature prQfiles show that the water column at the upstream end of the reservoir was thermally stratified. Since the downstream flow almost appeared under depth of I m, it would originate from a plunged inflow. On the other hand, a contour flow occurred in the upper layer. Comparing profiles of temperature and flow, the direction of flow reversed at the layer with temperature between 17 and 18°C.
Temp. rC) 15 1617 '81m
o.
Velocity (cm/s)
~~
Temp ('C)
~5
::::
1617 18 197
Velocity
Velocity
(cm/s) Temp. rC) (cm/s)
~~
:::: ;:'5 161718 '97
~~
:::: ;:
OS
jl. -5
1.5
d'2.0 2.5 3..0 0.0
~~~~+~H=i;M=i=+:ii;M=;;4t:rf1M;:;:;:44
0.5 61.0
:g 1.5 c!j2.0 '" 2.S
u."".£...l"""'J....--.......~""'''''''I~'-Io....l..ol
3.0~L...L..L..u .........~..
Figure 4. Vertical profiles of temperalure and flow velocily in 1994.
Diurnal variation of P. bipes. Figure S shows CQntQur diagrams Qf cell numbers of P. bipes and temperature in the vertical section of the line M (Fig. 2) in 1994. The diagrams for temperature show a weak thermocline which declined from the upstream side. In comparison with Figure 4, the up- and downstream flQW would
214
M. YAMADA et al.
occur above and below this thermocline, respectively. In the diagrams of cell number at 14:00. 6:00. and 12:00 (daytime), there was an area of dense accumulation of P. bipes (greater than 103.4 cellslmL) at the surface of the upstream side and contour lines followed the thermocline under the surface. On the other hand, in the diagrams at 18:00 and 0:00 (night), the area greater than 10 3 cellslmL extended along the thermocline to the lower layer. and the surface discoloration disappeared. Furthermore, the area greater than 103.4 cellslmL appeared in the downstream side from 18:00 to 0:00 on October 27. Point
Point
lfT-lT_--TB--.:;ADr-,:C_-r:-:>
4
Figure S. Contour diagrams of cell number of P. bipes and temperature in study &rea in 1994.
Figure 6 shows the diurnal variation of the mean biomass within the whole study area in 1994. The biomass of P. bipes was changed in approximately a 24-hour cycle. The amplitude of variation was about 109 cellslm 2 and indicates that about 213 of the daily maximum biomass fluctuated and 1/3 of it remained in the study area.
5x1d'"T"""---.--......----...-o--r-...
l
-~--
4x1oB
~.;"
11';;;' 3x1d'
_. ~
:;
0_
--.0;....
"" 1\
--
,,/ I
I
~ 2X1oB-I-'-".:~·o1----.lI--+
~
I
I
1x1oB-l---+-----!I--O+---+-l 18
0
6 12 Time (hour)
18
Figure 6. Diurnal fluctuation of P. bipes biomass in 1994.
Accumulation mechanism Contour diagrams (Figure 5) show the spatial movement and/or transportation of P. bipes in the upstream end. Summarizing information in the literature (Kagawa et al., 1984; Hata, 1991; Nakamoto, 1991) and our observation, we present our concept for the accumulation mechanism of Peridinium near the upstream end in Figure 7. In this concept, the reach close to an upstream end can be longitudinally divided into the 'accumulation area', where red tide can be observed in the daytime. and the 'dispersion area' which is defined as the area of dispersing Peridinuim at night The upstream reach can also be vertically divided into the
Accumulation of freshwater red tide
21.5
'upper layer' which flows upstream, and the 'under layer' which flows downstream. Within this reach, processes related to the vertical movement of red tide's biomass would be the 'vertical migration' of Peridinium and the 'entrainment' of water, while processes related to the horizontal movement of such biomass would be ~e 'import' and the 'export' by flows. Furthermore, the 'inflow' and 'outflow' are an input and output, respectively, of the defined reach of an upstream end. The biomass of Peridinium could be varied by certain biological processes such as growth, death, grazing, en-/excystment, which have not been clearly understood. Thus, these are summarized as the process of 'growth' in the present study.
Upper Layer
Under
~umulation / Area - , , - - - -
;
Layer
Outflow
Figul'C 7. Scheme of accumulation mechanism and its processes.
Biomass chan~e in the accumulation area A dense discoloration of the red tide (grater than 103.4 cell/mL of P. bipes) was usually observed in the study area (Figures I and 2). We consider this area as the 'accumulation area' in Figure 7. Figure 6 shows specific growing rate of 0.13 and 0.06 /day of Peridinium biomass under the relatively stable flow, while O.IS/day was also estimated with enclosures used in 1994's survey. These values are nearly to the specific growth rate of P. bipes in the culture at 1O-25°C, 0.10-0.15 /day (Nishibori et al., 1991). However, the diurnal fluctuation of Peridinium biomass (Figure 6) is much greater than the growth of P. bipes. Since P. bipes was seldom contained in the river water, the 'inflow' is likely to have little effect on the diurnal fluctuation. Thus, Peridinium biomass should be fluctuated by the 'import' and the 'export'. To confirm this reasoning, we estimate the transport by counter flows in the cross section of line C. The transport rate (cells/s) is defined as a net biomass transport in/out through a cross section of the accumulation area per second and is calculated by vertically adding up the transport fluxes (cells/m 2/s), which are multiplying mean cell numbers, flow velocities and depths at each 0.5 m layer of the section. Positive and negative values of them indicate the 'export' and the 'import', respectively. Table I compares diurnal change of the observed and predicted Peridinium biomass. The prediction of biomass was performed by subtracting the transport rate from the observed biomass at each time. The predicted variation of biomass is almost the same as the observed one (r2::0.72) and exhibits a diurnal cycle. This result shows that the Peridinium biomass mainly fluctuates by the horizontal transport of biomass and supports our reasoning. Since the diurnal variation of flow velocity was negligible (Figure 4), the amount of transport will almost depend upon the vertical distribution of P. bipes.
216
M. YAMADA d al.
Table I. Comparison between observed and predicted biomass
Time
Observed Biomass
Transport Rate
Ptedicted Biomass
average·· obs. 10· cells·s" 10· cells·s"
IOu cells
18:00
IOu cells. 3.92
20:00
1.81
6.11
22:00
156
2.41
0:00
2.09
-0.16
2:00
1.41
5.41
4:00
1.43
13.6
6:00
2.65
3.69
8:00
2.21
-lSJ
10:00
2..55
2.08
12:00
3.10
-8.01
14:00
3.15
·6.30
16:00
2.84
8.13
18:00
2..52
3.45
6.11·" 432 0.86 2.33 9.51 8.65 1.06 0.26 3.00 .1.19
0.92 5.79 2.21
0.91 20:00 1.59 i • Vol1l%l\e of estimateduea is 7915 m •• A.verage ofobs. 'lUlIeS in the upper end lower liM. ••• Use 20:00 data only
1.43 1.25 2.03 1.31 0.15 2.03 2.14 2.53 3.32 4.27 2.17 2.11 1.43
An attempt to Control the freshwater red tjde As discussed previously the transport processes by flows have a greater effect upon the variation of red tide's biomass than growth. Therefore, the control of flow, especially the upstream flow, is more likely to be effective for the control of freshwater red tide in the upstream end. Ideally, this flow control should be performed to prevent the import of organisms to the accumulation area, to enhance their export from that area, and ultimately to wash organisms out to the downstream reach where they will never return to the 'accumulation area'. Methods having potential to realize these objectives are, for example, the discharge control of a reservoir, the mixing of lake water by aeration or circulation, and the setting the barrier devices in the upstream end. These methods, however, have been applied to treat the lake water quality problem, such as algal blooms or low DO in eutrophic lakes. We made one attempt to re-evaluate the further use of these methods for the red tide control. Figure 8 shows our concept of fences for the control of red tide accumulation. Just after the 1992 survey, we experimentally installed three fences (made by vinyl and hung to I m of depth) at intervals of ISO m in the upstream end and observed the biomass variation of P. bipes in the same manner as described earlier. Figure 9 shows the daily change of Peridinium biomass before and after the installing of fences. This result suggests that the increase of biomass slacked after the installing of fences, but its overall effect was inconclusive at that time.
Accumulation of freshwater red tide
217
I Segmentation of acelumulaled organisms I Fence
Figure g. Concept of fences for freshwater red tide control.
1 5x10'0 , - - - - . . . - - - - - - - - - - - , ~ Installing lances
N'
~
~ 1.0x10'0
~
IS.OX108
co
---+-.--I----l--• o : _.
. ._-----.. . .
----;----.1 ~:~+'--'.<---_
0
O.Ox10 0
7
14
21
28
35 42 49 56 63
Day Figure 9. Biomass change installed before and after fences.
CONCLUSIONS We performed field observation of the freshwater red tide in the upstream end of Shorenji Dam Reservoir in 199), 1992 and 1994. The results exhibit the distribution patterns of P. bipes which vertically migrated by phototaxis, and vertically entrained at the plunging point, and horizontally moved upstream and downstream each day. The biomass of P. bipes increased at a similar rate as growth day by day, but it diurnally fluctuated with the amplitude of'lJ3 of the daily maximum biomass. Moreover, this diurnal fluctuation is much greater than the growth of P. bipes. We defined the areas where red tide is accumulated and dispersed, and consider the processes related to these variations as 'vertical migration' and 'entrainment'; 'imporl' and 'export'; 'outflow'; and 'growth', The transport rates estimated by the vertical profiles of P. bipes and flow velocity successfully explain that the diurnal fluctuation of P. bipes biomass mainly depended on the horizontal transport of biomass. This evidence suggests that the control of flow is more likely to be effective for the control of freshwater red tide in the upstream end. To control the freshwater red tide, we need further study of the re-evaluation of the current methods for water quality improvement in the aspects of flow control. Additionally, the clarification of the ecological status of red tide, such as en-/excystment. preylpredator relationship, etc.• is also necessary for further understanding of the phenomenon. ACKNOWLEDGMENT This material is supported by Special Coordination Funds for Promoting Science and Technology of the Science and Technology Agency, Japan. We thank all those in The Management Office of Kizu River and Shorenji Dam Reservoir of Water Resources Development Public Co., and in Water Resources Environment Technology Center for their assistance with our observation.
218
M. YAMAnA ~I aI.
REFERENCES Kagawa, H.. !seri. Y. and Ito. T. (1984). Environmental Conditions at the head of a teservoir where freshwater ted tide of Peridimum occurs-In the case of the lshitegawa Dam Reservoir-. SuishitsllbdDJa, K~roJ:y ... 7. 37S-383. (in Japanese) Hala, S (J 991). The Fresh-water Red Tide in Nagase Reservoir (Kochi Ptef.). Suishirsuodaku K~roJ:yu, 14. 293-297. (in Japanese) Nakamoto. N. (1991). On the freshwater Red Tide in Reservoir Kanna·lto. SuishitsllOdaJcM K~roJ:yu. 14. 281-285. (in Japanese) Nishibori. N.• Nishijima, T.• Onoda and Y.• HalA, Y. (1991). Effect of Lightlnltnsity. Temperature. pH. and Nitrogenous Nutrient on the Growth of Peridinium bipes fo. llCCultalum. Nippon SwJsan Gakkaishi. 57. 1729- 1735. (in Japanese)