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Energy value of biomass within benthic algae of milkfish ponds
Aquaculture, 64 (1987) 31-38 Elsevier Science Publishers B.V., Amsterdam
31 -
Printed
in The Netherlands
Energy Value of Biomass within Benthic Algae of Milkfish Ponds SUN-CH10
FONG and HAAI-PERNG
Institute of Marine Biology, National of China) (Accepted
8 December
JU Sun Yat-Sen
University,
Kaohsiung,
Taiwan (Republic
1986)
ABSTRACT Fong, S.-C. and Ju, H.-P., 1987. Energy value of biomass within benthic Aquaculture, 64: 31-38.
algae of milkfish ponds.
The energy content of benthic algae in milkfish ponds was measured. Algae in fertilized ponds contained 3.8 times more energy than algae in unfertilized ponds. Differences in algal flora pattern and variation of light extinction coefficients were considered as the major factors affecting the energy content of the benthic algae. Energy content during April and August appeared to be higher than at other months.
INTRODUCTION
The milkfish ( Chanos chaos) is a euryhaline species widely distributed in the tropical and subtropical waters of Indo-Pacific areas. Southern Taiwan is located near the northern limits of its distribution so temperature can cause overwintering loss for local fish farms. During the winter the fish are moved to a deep water trench and the ponds are drained and fertilized. The exposure of bottom mud to the sun between October and March guarantees the growth of benthic algae which serve as a major source of food for the milkfish. Such techniques of milkfish culture have been in vogue in Taiwan for more than two centuries, resulting in a production rate of approximately 2000 kg/ha. However, the production of milkfish in shallow water ponds has not increased substantially during the last decade and the so-called deep water method was developed in an effort to increase the yield (Lin, 1981). Although the deep water method has certain advantages over the traditional one, it requires a supply of fresh water and the use of artificial feeds. Using benthic algae as feed for the milkfish may be economical but more basic research on this source of food and its value to fish is needed. Several papers have reported basic studies of milkfish culture. Tang and
0044-8486/87/$03.50
0 1987 Elsevier Science Publishers
B.V.
32
Hwang (1966) studied the preference and digestibility of different algal groups to milkfish. They reported that filamentous blue-greens and benthic diatoms were most desirable and that they can crowd out undesirable algae in ponds. Tang and Hwang (1965) reported that blue-green algae in milkfish ponds stopped growing if the salinity reached 70 ppt but that diatoms may tolerate a salinity level of up to 90 ppt. Chen (1971a) studied the pH value and content of organic matter of the bottom mud. The taxonomy of benthic algae in milkfish ponds was reported by Chang (1969). Lin (1969) reported the difference in feeding habits of milkfish and the fluctuations of dissolved oxygen content within milkfish ponds. Experiments by Chen (1971b) demonstrated that fertilization of ponds with water-soluble silicate significantly affected the growth of benthic algae. Chang et al. (1977) estimated the production of benthic algae, expressed in terms of ash-free dry weight, as between 28 and 47 g/kg. Su and Ting (1980) reported variation in growth rates and feeding rates of milkfish fry within deep water ponds. Culturing techniques for milkfish in deep water ponds were reviewed and discussed by Lin (1981) . We studied the changes in algal production in milkfish ponds using energy value (calorie content/m’ of area) as the indicator of biomass. Variation due to difference in algal group and the relationship between energy content and the degree of light extinction in the water were the primary areas of focus. MATERIAL
AND METHODS
Field experiments were done in shallow water ponds at milkfish farms in the Chee-kwoo area of Tainan county. Six ponds of approximately 4 ha each were employed. They have been used for milkfish culture for several years and adapted to the traditional shallow water culturing methods. Three of them, designated as U6, U8 and UlO, were not fertilized. Three other ponds, desig-, nated as Fl, F2 and F3, were treated with organic fertilizer using standard commercial methods. During the winter, the ponds were filled with sea water to a depth of 10 cm, and allowed to dry under the sun to encourage the development of an algal bed. The fertilization procedure was repeated three or four times between September 1983 and March 1984 until the ponds were ready for stocking. All ponds were filled with approximately 30 cm of water and stocked with fish before 1 April 1984. Each pond was sampled once every 2 weeks through October 1984 when the ponds were drained. Because our operation would unavoidably cause damage to the algal bed near the sampling area, activity was restricted to within 5 m from the shore, and sampling sites were randomly selected around that area. Sites with a very soft bottom and thus not suitable for growth of benthic algae or areas with accumulated floating benthic algae were considered unsatisfactory and excluded. A piece of bottom mud was dug up with a small shovel. If the mud surface appeared to be intact, a specific area
33
(approximately 50 cm2) of the surface layer (about 1 mm thick above the surface, as identified by the grayish color due to oxidation) was carefully scratched off as samples for studies. Samples for analysis of energy content were preserved in a portable ice chest, transported back to the laboratory and immediately oven-dried for 24 h at 70” C until the weight of the sample remained constant. The caloric value of the dried sample was estimated using an adiabatic bomb calorimeter (Shung-Nan company, Taiwan) which was calibrated with benzoic acid standards (Slobodkin and Richman, 1960 1. Three replicates were combusted for each sample. Energy value, expressed in terms of calories/m’ of sampling area, was estimated by dividing the caloric content of each gram of dried sample by the size of the sampling area times the total weight of dried sample. Samples used for algal classification were bottled in filtered pond water in the field and preserved with formalin solution ( less than 10% ) . Observations were conducted within 2 weeks. Each sample was thoroughly mixed, one to two drops were placed on a slide and observed through a microscope. Individual plants were first classified roughly into one of the four major groups: blue-green algae, diatoms, green algae, miscellaneous. Plants in each group were further subdivided according to shape pattern into several size classes, as measured by a micrometer. A size index for each of the size classes was calculated by multiplying length by width. Portions of large individual plants, such as Lyngbya spp., that went off the range of the ocular view were not included. For the tiny cells which appeared as a colony, as in the case of Microcystis spp., the whole patch of cells was taken as one single organism. Thirty observations were taken for each sample and the number of algae of each size class was recorded during each observation. Since the density of each sample collected was different, comparisons of frequencies among samples were considered relative rather than absolute. Light intensity at different depths of water and turbidity was measured at the same time the samples were collected using a small portable luxmeter (ANA Model 500). A water-proof case made of acrylic material was designed for the instrument to operate under the water. When the intensity of light went off the acceptable range of the instrument, a small piece of prestandardized filter was used and the reading calibrated accordingly. RESULTS
Classification of frequently appearing algae was attempted according to Yamaji (1982) and Mizuno (1983). Ten blue-green, eight diatom, two green algae and one dinoflagellate were identified as to genus and are listed in Table 1.
All individual algae, whether identified or not, were counted as belonging to one of the 11 size classes. The results were combined to represent the frequency
34 TABLE
1
Major groups of benthic algae with averaged size index estimated Algae group Fl Dinoflag. Gl Green Dl Diatom
Genus identified
Length (X10 “mm)
Platymonas
sp.
Chlamydomonas
Width (X10
Chlorella sp. Diploneis sp.
“mm)
x
width of individual
algae
LXW
95% Confidence
(X10-“mm)
interval
54 408
44.9-
62.9
24
381.4-
438.4
22
154
133.2-
177.7
71
284
254.9-
313.3
61
976
889.1-
1068.3
252
4032
3544.9-
4342.7
9
sp.
as length
Naoicula
D2 Diatom D3 Diatom D4 Diatom
Bl Blue-green B2 Blue-green
sp.” Nitzschia sp. Synedra sp.” Amphora sp.” Navicula sp. Amphiprora sp. Gyrosigma sp. Pleurosigma sp.” Synechocystis sp. Lyngbya sp.” Phormidium sp.” Spirulina
B3 Blue-green
84.2
10 160
7 2
70 320
53.8144.8-
492.7
29
27
783
683.1-
922.2
252
6
1512
1147.8-
1879.0
448
26
11648
sp.”
Chroococcus
sp.
Merismopedia sp. Microcystis sp.”
B4 Blue-green
Anabaena
B5 Blue-green
Oscillatoria sp.” Lyngbya sp.” Microcoleus
,‘Genus appearing
sp.
8417.3-14892.5
sp.”
most frequently.
distribution of size classes in that sample. Biomass of each class was calculated by multiplying frequency by the value of size index of the respective class shown in Table 1. They were then transformed into relative values by dividing them by the total biomass. Blue-green algae dominated other groups in the fertilized ponds (Table 2 ) . The average relative biomass of blue-green algae and diatoms was 0.92 and 0.05 in fertilized ponds and 0.41 and 0.57 in unfertilized ponds, respectively. The relative biomass of other groups of algae was negligible. It appeared that seasonal variations of the relative size index of blue-green algae and diatoms were not significant. The difference in algal flora between the two types of pond was considered the result of fertilization. The energy value of biomass ( in terms of calorie/m2 ) of fertilized ponds was significantly higher than that of unfertilized ponds. The overall means and 95% confidence intervals were 952 + 142 for fertilized ponds and 253 % 54 for unfertilized ponds. The changes in energy values with time are plotted in Fig. 1. Peak energy values were found in April and May and between August and September. Although the energy content within fertilized ponds was higher than that in unfertilized ponds, both types of ponds showed identical seasonal fluctuations
35 TABLE 2 Relative biomass (in terms of size index) of blue-green algae (BG) and diatoms (DT) in triplicate fertilized (F) and unfertilized (U) milkfish ponds, as a percent of total biomass Fl
Date
8 May 21 May 2 June 23 June 9 July 27 July 13 Aug. 23 Aug. 11 Sept. 24 Sept. 8 Oct. 29 Oct.
F2
U6
U8
BG
DT
BG
DT
BG
DT
BG
DT
BG
0.89
0.11 0.06 0.07
0.05 0.08
0.90 0.87 0.93 0.97 0.92 0.95 0.94 0.97
0.10
0.01
0.14 0.15 0.20 0.34 0.65 0.63 0.37 0.45 0.32 0.12 0.17 0.18
0.86 0.83 0.80 0.65 0.34 0.36 0.63 0.54 0.68 0.87 0.81 0.80
0.93
0.05
0.32
0.67
0.97 0.95
0.02 0.02
0.95 0.92 0.86 0.96 0.95 0.88 0.89 0.96 -
0.92
0.05
0.92
0.94 0.93 0.87 0.97
0.08 0.02
0.97 0.82
Mean
F3
0.02 0.04
0.12 0.03 0.02 0.04 0.05 0.02
0.05
0.13 0.07 0.01 0.04 0.02 0.02
UlO DT
BG
DT
0.66 0.54 0.70 0.61 0.31 0.51
0.29 0.45 0.30 0.38 0.65 0.41
0.04 0.28 0.25 0.25 0.44 0.58 0.36 0.53 -
0.96 0.66 0.74 0.74 0.56 0.40 0.64 0.46
0.56
0.41
0.34
0.64
indicating that season variation was not caused by fertilization but is a natural phenomenon. Light intensities (in terms of natural logarithm of lux) at different levels of water for each survey were regressed against depth as shown in Fig. 2. Table 3 shows the regression coefficients (all tested as significant at a 0.05 level) esti14r
A
M
J
J
A
s
0
N
MONTH
Fig. 1. Changes in energy values of biomass within fertilized Vertical lines stand for the 95% confidence intervals.
(F) and unfertilized
(U)
ponds.
36
20
30 DEPTH
Fig. 2. Relationship
between light intensity
(cm)
and water depth.
mated for each survey. Analysis of variance and multiple comparison of means showed that the extinction coefficients for unfertilized ponds and fertilized ponds were significantly different while fish ponds of the same group were not different from one another. The average extinction coefficients, and the respective 95% confidence intervals, were - 0.0551+ 0.0140 and - 0.0288 + 0.0044 for unfertilized and fertilized ponds, respectively. The figures indicate that light penetrating water was absorbed much faster by the water of unfertilized ponds than of fertilized ponds. A simple linear regression relationship between extinction coefficient (K) and turbidity ( T) was: K= 0.0084 + 0.0020
T
and was significant at P < 0.05. This formula can be used to estimate of K by measuring the turbidity of the water.
the value
TABLE 3 Values of light extinction coefficients for triplicate fertihzed (F) and unfertilized (U) milkfish ponds during each survey estimated as regression of In lux on depth (all coefficients were significantly different from zero) Date
27 July 13 Aug. 23 Aug. 11 Sept. 24 Sept. 8 Oct. 29 Oct. 13 Nov.
Milkfish ponds Fl
F2
F3
U6
F8
UlO
-0.023 -0.031 -0.050 -0.052 -0.035 -0.021 -0.021 -0.014
-0.023 -0.026 -0.047 -0.029 -0.025 -0.025 -0.023
-0.023 -0.024 -0.026 -0.029 -0.039 -0.025 -0.036 -0.015
-0.035 -0.063 -0.027 -0.042 -0.083 -0.043 -0.025
-0.022 -0.039 -0.075 -0.052 -0.112 -0.042 -0.105
-0.075 -0.051 -0.046
-
37 DISCUSSION
It was shown that blue-green algae dominate almost completely if the ponds are fertilized. However, without fertilization, diatoms take over as the major group. Chen (1971a) reported that low temperature during winter favors the growth of diatoms. Tang and Hwang (1965) showed with aquarium experiments that blue-green algae stop growing if the salinity is higher than 70 ppt while diatoms continued to grow at salinities as high as 90 ppt. These facts implied that diatoms were better adapted to rough environments than the bluegreen algae. The respective energy values of blue-green algae and diatoms can be estimated by multiplying the size index with the total energy:
Blue-green
Diatoms
Total
Fertilized ponds
876
48
952
Unfertilized ponds
104
144
253
The blue-green algae in the fertilized ponds contributed eight times the energy they did in the unfertilized ponds while diatoms produced one third as much energy in the fertilized ponds as in the unfertilized ponds. The latter situation is probably the result of unsuccessful competition by the diatoms. The difference in light extinction coefficients between fertilized and unfertilized ponds suggests that well fertilized ponds have clearer water and thus allow a greater amount of light to penetrate onto the surface of the algal bed. It would be interesting to know whether it is the high production of algae that clears the water or whether it is the other way round. The ratio of light intensity between the water surface and at 30 cm deep was calculated from the average extinction coefficient: Fertilized Unfertilized
ponds: exp ( - 0.0288 x 30 ) = 0.4215 ponds: exp ( - 0.0551 x 30) = 0.1915
Assuming that production is linearly related to light intensity in the milkfish pond, the ratio of light intensity for fertilized and unfertilized ponds of 0.42/0.19= 2.2 is not sufficient to account for the ratio of production (952/253 = 3.76) of the two types of ponds in our study. It is concluded that the algae production and light penetration reacted with each other in such a way that neither would succeed completely without the involvement of the other, and clearer water would increase the benthic production to some extent. The existence of the first peak of energy production in April/May could be
38
either the result of the blooming of algae at an early stage of the pond or due to the interference of organic matter within fertilizer or soil during the combustion. The second peak between August and September was possibly the result of the basic performance of benthic algae ‘due to an increase in water temperature and light intensity during the summer. July is also the time for some of the poorly managed commercial ponds to be drained and exposed to sunlight for short periods of time (an operation known traditionally as saibeng-ping). Because a rise in energy content within algae existed after the fertilizers were nearly exhausted, additional fertilizer applied at that time would be an effective way of encouraging further accumulation of energy. ACKNOWLEDGEMENTS
The authors are grateful to the National Science Council, The Republic of China (ROC) for financial support of this project (grant no. NSC73-0204BllO-04). Thanks are also due to Mr. Y.Y. Ting, Director of the Tainan Station, Taiwan Fisheries Research Institute, for his support and advice on this project, Mr. Huang who allowed us to collect samples from his farm ponds, and Ms. P.R. Lin who identified all the algae.
REFERENCES Chang, M.H., Chen, S.S., Lin, K.Y. and Chen, SC., 1977. Experiments on application of tobacco waste as fertilizer and pesticide in milkfish ponds. Bull. Taiwan Fish. Res. Inst., 29: 39-46. Chang, T.P., 1969. Algae of Tainan milkfish ponds. Chin.-Am. Jt. Comm. Rural Reconstr. Fish. Ser., 7: 91-135. Chen, H.C., 1971a. Increase of benthic algae in milkfish (Chanos charms Forsskal) ponds by application of silicate. Chin.-Am. Jt. Comm. Rural Reconstr. Fish. Ser., 11: 71-83. Chen, H.C., 1971b. Studies on milkfish ponds: I. effects of pH value and salinity on the growth of benthic algae. Tung-Kang Marine Laboratory, Aquiculture, l(2): 1-11. Lin, H.S., 1969. Some aspects of milkfish ecology. Chin.-Am. Jt. Comm. Rural Reconstr. Fish. Ser., 6: 68-90. Lin, M.N., 1981. Theory and practice in deep-water systems of milkfish culture, part I. Fisheries Magazine, 4 (May) : 35-37 (in Chinese). Mizuno, T., 1983. Illustrations of the Freshwater Plankton of Japan. Hoikusha Publishing Co., Ltd., Osaka, 353 pp. Slobodkin, L.B. and Richman, S., 1960. The availability of a miniature bomb calorimeter for ecology. Ecology, 41(4) : 784. Su, B.T. and Ting, Y.Y., 1980. The study of improvement of the milkfish culture in deep-water ponds. Bull. Taiwan Fish. Res. Inst., 32: 579-585. Tang, Y.A. and Hwang, T.L., 1965. Studies of milkfish pond fertilization. Bull. Taiwan Fish. Res. Inst., 12: 42-47. Tang, Y.A. and Hwang, T.L., 1966. Evaluation of the relative suitability of various groups of algae as food of milkfish in brackish-water ponds. FA0 Fish. Rep., 3 (44): 365-372. vamaji, T., 1982.Illustrations of the Marine Plankton of Japan. Hoikusha Publishing Co., Ltd., Osaka, 537 pp.
31 -
Printed
in The Netherlands
Energy Value of Biomass within Benthic Algae of Milkfish Ponds SUN-CH10
FONG and HAAI-PERNG
Institute of Marine Biology, National of China) (Accepted
8 December
JU Sun Yat-Sen
University,
Kaohsiung,
Taiwan (Republic
1986)
ABSTRACT Fong, S.-C. and Ju, H.-P., 1987. Energy value of biomass within benthic Aquaculture, 64: 31-38.
algae of milkfish ponds.
The energy content of benthic algae in milkfish ponds was measured. Algae in fertilized ponds contained 3.8 times more energy than algae in unfertilized ponds. Differences in algal flora pattern and variation of light extinction coefficients were considered as the major factors affecting the energy content of the benthic algae. Energy content during April and August appeared to be higher than at other months.
INTRODUCTION
The milkfish ( Chanos chaos) is a euryhaline species widely distributed in the tropical and subtropical waters of Indo-Pacific areas. Southern Taiwan is located near the northern limits of its distribution so temperature can cause overwintering loss for local fish farms. During the winter the fish are moved to a deep water trench and the ponds are drained and fertilized. The exposure of bottom mud to the sun between October and March guarantees the growth of benthic algae which serve as a major source of food for the milkfish. Such techniques of milkfish culture have been in vogue in Taiwan for more than two centuries, resulting in a production rate of approximately 2000 kg/ha. However, the production of milkfish in shallow water ponds has not increased substantially during the last decade and the so-called deep water method was developed in an effort to increase the yield (Lin, 1981). Although the deep water method has certain advantages over the traditional one, it requires a supply of fresh water and the use of artificial feeds. Using benthic algae as feed for the milkfish may be economical but more basic research on this source of food and its value to fish is needed. Several papers have reported basic studies of milkfish culture. Tang and
0044-8486/87/$03.50
0 1987 Elsevier Science Publishers
B.V.
32
Hwang (1966) studied the preference and digestibility of different algal groups to milkfish. They reported that filamentous blue-greens and benthic diatoms were most desirable and that they can crowd out undesirable algae in ponds. Tang and Hwang (1965) reported that blue-green algae in milkfish ponds stopped growing if the salinity reached 70 ppt but that diatoms may tolerate a salinity level of up to 90 ppt. Chen (1971a) studied the pH value and content of organic matter of the bottom mud. The taxonomy of benthic algae in milkfish ponds was reported by Chang (1969). Lin (1969) reported the difference in feeding habits of milkfish and the fluctuations of dissolved oxygen content within milkfish ponds. Experiments by Chen (1971b) demonstrated that fertilization of ponds with water-soluble silicate significantly affected the growth of benthic algae. Chang et al. (1977) estimated the production of benthic algae, expressed in terms of ash-free dry weight, as between 28 and 47 g/kg. Su and Ting (1980) reported variation in growth rates and feeding rates of milkfish fry within deep water ponds. Culturing techniques for milkfish in deep water ponds were reviewed and discussed by Lin (1981) . We studied the changes in algal production in milkfish ponds using energy value (calorie content/m’ of area) as the indicator of biomass. Variation due to difference in algal group and the relationship between energy content and the degree of light extinction in the water were the primary areas of focus. MATERIAL
AND METHODS
Field experiments were done in shallow water ponds at milkfish farms in the Chee-kwoo area of Tainan county. Six ponds of approximately 4 ha each were employed. They have been used for milkfish culture for several years and adapted to the traditional shallow water culturing methods. Three of them, designated as U6, U8 and UlO, were not fertilized. Three other ponds, desig-, nated as Fl, F2 and F3, were treated with organic fertilizer using standard commercial methods. During the winter, the ponds were filled with sea water to a depth of 10 cm, and allowed to dry under the sun to encourage the development of an algal bed. The fertilization procedure was repeated three or four times between September 1983 and March 1984 until the ponds were ready for stocking. All ponds were filled with approximately 30 cm of water and stocked with fish before 1 April 1984. Each pond was sampled once every 2 weeks through October 1984 when the ponds were drained. Because our operation would unavoidably cause damage to the algal bed near the sampling area, activity was restricted to within 5 m from the shore, and sampling sites were randomly selected around that area. Sites with a very soft bottom and thus not suitable for growth of benthic algae or areas with accumulated floating benthic algae were considered unsatisfactory and excluded. A piece of bottom mud was dug up with a small shovel. If the mud surface appeared to be intact, a specific area
33
(approximately 50 cm2) of the surface layer (about 1 mm thick above the surface, as identified by the grayish color due to oxidation) was carefully scratched off as samples for studies. Samples for analysis of energy content were preserved in a portable ice chest, transported back to the laboratory and immediately oven-dried for 24 h at 70” C until the weight of the sample remained constant. The caloric value of the dried sample was estimated using an adiabatic bomb calorimeter (Shung-Nan company, Taiwan) which was calibrated with benzoic acid standards (Slobodkin and Richman, 1960 1. Three replicates were combusted for each sample. Energy value, expressed in terms of calories/m’ of sampling area, was estimated by dividing the caloric content of each gram of dried sample by the size of the sampling area times the total weight of dried sample. Samples used for algal classification were bottled in filtered pond water in the field and preserved with formalin solution ( less than 10% ) . Observations were conducted within 2 weeks. Each sample was thoroughly mixed, one to two drops were placed on a slide and observed through a microscope. Individual plants were first classified roughly into one of the four major groups: blue-green algae, diatoms, green algae, miscellaneous. Plants in each group were further subdivided according to shape pattern into several size classes, as measured by a micrometer. A size index for each of the size classes was calculated by multiplying length by width. Portions of large individual plants, such as Lyngbya spp., that went off the range of the ocular view were not included. For the tiny cells which appeared as a colony, as in the case of Microcystis spp., the whole patch of cells was taken as one single organism. Thirty observations were taken for each sample and the number of algae of each size class was recorded during each observation. Since the density of each sample collected was different, comparisons of frequencies among samples were considered relative rather than absolute. Light intensity at different depths of water and turbidity was measured at the same time the samples were collected using a small portable luxmeter (ANA Model 500). A water-proof case made of acrylic material was designed for the instrument to operate under the water. When the intensity of light went off the acceptable range of the instrument, a small piece of prestandardized filter was used and the reading calibrated accordingly. RESULTS
Classification of frequently appearing algae was attempted according to Yamaji (1982) and Mizuno (1983). Ten blue-green, eight diatom, two green algae and one dinoflagellate were identified as to genus and are listed in Table 1.
All individual algae, whether identified or not, were counted as belonging to one of the 11 size classes. The results were combined to represent the frequency
34 TABLE
1
Major groups of benthic algae with averaged size index estimated Algae group Fl Dinoflag. Gl Green Dl Diatom
Genus identified
Length (X10 “mm)
Platymonas
sp.
Chlamydomonas
Width (X10
Chlorella sp. Diploneis sp.
“mm)
x
width of individual
algae
LXW
95% Confidence
(X10-“mm)
interval
54 408
44.9-
62.9
24
381.4-
438.4
22
154
133.2-
177.7
71
284
254.9-
313.3
61
976
889.1-
1068.3
252
4032
3544.9-
4342.7
9
sp.
as length
Naoicula
D2 Diatom D3 Diatom D4 Diatom
Bl Blue-green B2 Blue-green
sp.” Nitzschia sp. Synedra sp.” Amphora sp.” Navicula sp. Amphiprora sp. Gyrosigma sp. Pleurosigma sp.” Synechocystis sp. Lyngbya sp.” Phormidium sp.” Spirulina
B3 Blue-green
84.2
10 160
7 2
70 320
53.8144.8-
492.7
29
27
783
683.1-
922.2
252
6
1512
1147.8-
1879.0
448
26
11648
sp.”
Chroococcus
sp.
Merismopedia sp. Microcystis sp.”
B4 Blue-green
Anabaena
B5 Blue-green
Oscillatoria sp.” Lyngbya sp.” Microcoleus
,‘Genus appearing
sp.
8417.3-14892.5
sp.”
most frequently.
distribution of size classes in that sample. Biomass of each class was calculated by multiplying frequency by the value of size index of the respective class shown in Table 1. They were then transformed into relative values by dividing them by the total biomass. Blue-green algae dominated other groups in the fertilized ponds (Table 2 ) . The average relative biomass of blue-green algae and diatoms was 0.92 and 0.05 in fertilized ponds and 0.41 and 0.57 in unfertilized ponds, respectively. The relative biomass of other groups of algae was negligible. It appeared that seasonal variations of the relative size index of blue-green algae and diatoms were not significant. The difference in algal flora between the two types of pond was considered the result of fertilization. The energy value of biomass ( in terms of calorie/m2 ) of fertilized ponds was significantly higher than that of unfertilized ponds. The overall means and 95% confidence intervals were 952 + 142 for fertilized ponds and 253 % 54 for unfertilized ponds. The changes in energy values with time are plotted in Fig. 1. Peak energy values were found in April and May and between August and September. Although the energy content within fertilized ponds was higher than that in unfertilized ponds, both types of ponds showed identical seasonal fluctuations
35 TABLE 2 Relative biomass (in terms of size index) of blue-green algae (BG) and diatoms (DT) in triplicate fertilized (F) and unfertilized (U) milkfish ponds, as a percent of total biomass Fl
Date
8 May 21 May 2 June 23 June 9 July 27 July 13 Aug. 23 Aug. 11 Sept. 24 Sept. 8 Oct. 29 Oct.
F2
U6
U8
BG
DT
BG
DT
BG
DT
BG
DT
BG
0.89
0.11 0.06 0.07
0.05 0.08
0.90 0.87 0.93 0.97 0.92 0.95 0.94 0.97
0.10
0.01
0.14 0.15 0.20 0.34 0.65 0.63 0.37 0.45 0.32 0.12 0.17 0.18
0.86 0.83 0.80 0.65 0.34 0.36 0.63 0.54 0.68 0.87 0.81 0.80
0.93
0.05
0.32
0.67
0.97 0.95
0.02 0.02
0.95 0.92 0.86 0.96 0.95 0.88 0.89 0.96 -
0.92
0.05
0.92
0.94 0.93 0.87 0.97
0.08 0.02
0.97 0.82
Mean
F3
0.02 0.04
0.12 0.03 0.02 0.04 0.05 0.02
0.05
0.13 0.07 0.01 0.04 0.02 0.02
UlO DT
BG
DT
0.66 0.54 0.70 0.61 0.31 0.51
0.29 0.45 0.30 0.38 0.65 0.41
0.04 0.28 0.25 0.25 0.44 0.58 0.36 0.53 -
0.96 0.66 0.74 0.74 0.56 0.40 0.64 0.46
0.56
0.41
0.34
0.64
indicating that season variation was not caused by fertilization but is a natural phenomenon. Light intensities (in terms of natural logarithm of lux) at different levels of water for each survey were regressed against depth as shown in Fig. 2. Table 3 shows the regression coefficients (all tested as significant at a 0.05 level) esti14r
A
M
J
J
A
s
0
N
MONTH
Fig. 1. Changes in energy values of biomass within fertilized Vertical lines stand for the 95% confidence intervals.
(F) and unfertilized
(U)
ponds.
36
20
30 DEPTH
Fig. 2. Relationship
between light intensity
(cm)
and water depth.
mated for each survey. Analysis of variance and multiple comparison of means showed that the extinction coefficients for unfertilized ponds and fertilized ponds were significantly different while fish ponds of the same group were not different from one another. The average extinction coefficients, and the respective 95% confidence intervals, were - 0.0551+ 0.0140 and - 0.0288 + 0.0044 for unfertilized and fertilized ponds, respectively. The figures indicate that light penetrating water was absorbed much faster by the water of unfertilized ponds than of fertilized ponds. A simple linear regression relationship between extinction coefficient (K) and turbidity ( T) was: K= 0.0084 + 0.0020
T
and was significant at P < 0.05. This formula can be used to estimate of K by measuring the turbidity of the water.
the value
TABLE 3 Values of light extinction coefficients for triplicate fertihzed (F) and unfertilized (U) milkfish ponds during each survey estimated as regression of In lux on depth (all coefficients were significantly different from zero) Date
27 July 13 Aug. 23 Aug. 11 Sept. 24 Sept. 8 Oct. 29 Oct. 13 Nov.
Milkfish ponds Fl
F2
F3
U6
F8
UlO
-0.023 -0.031 -0.050 -0.052 -0.035 -0.021 -0.021 -0.014
-0.023 -0.026 -0.047 -0.029 -0.025 -0.025 -0.023
-0.023 -0.024 -0.026 -0.029 -0.039 -0.025 -0.036 -0.015
-0.035 -0.063 -0.027 -0.042 -0.083 -0.043 -0.025
-0.022 -0.039 -0.075 -0.052 -0.112 -0.042 -0.105
-0.075 -0.051 -0.046
-
37 DISCUSSION
It was shown that blue-green algae dominate almost completely if the ponds are fertilized. However, without fertilization, diatoms take over as the major group. Chen (1971a) reported that low temperature during winter favors the growth of diatoms. Tang and Hwang (1965) showed with aquarium experiments that blue-green algae stop growing if the salinity is higher than 70 ppt while diatoms continued to grow at salinities as high as 90 ppt. These facts implied that diatoms were better adapted to rough environments than the bluegreen algae. The respective energy values of blue-green algae and diatoms can be estimated by multiplying the size index with the total energy:
Blue-green
Diatoms
Total
Fertilized ponds
876
48
952
Unfertilized ponds
104
144
253
The blue-green algae in the fertilized ponds contributed eight times the energy they did in the unfertilized ponds while diatoms produced one third as much energy in the fertilized ponds as in the unfertilized ponds. The latter situation is probably the result of unsuccessful competition by the diatoms. The difference in light extinction coefficients between fertilized and unfertilized ponds suggests that well fertilized ponds have clearer water and thus allow a greater amount of light to penetrate onto the surface of the algal bed. It would be interesting to know whether it is the high production of algae that clears the water or whether it is the other way round. The ratio of light intensity between the water surface and at 30 cm deep was calculated from the average extinction coefficient: Fertilized Unfertilized
ponds: exp ( - 0.0288 x 30 ) = 0.4215 ponds: exp ( - 0.0551 x 30) = 0.1915
Assuming that production is linearly related to light intensity in the milkfish pond, the ratio of light intensity for fertilized and unfertilized ponds of 0.42/0.19= 2.2 is not sufficient to account for the ratio of production (952/253 = 3.76) of the two types of ponds in our study. It is concluded that the algae production and light penetration reacted with each other in such a way that neither would succeed completely without the involvement of the other, and clearer water would increase the benthic production to some extent. The existence of the first peak of energy production in April/May could be
38
either the result of the blooming of algae at an early stage of the pond or due to the interference of organic matter within fertilizer or soil during the combustion. The second peak between August and September was possibly the result of the basic performance of benthic algae ‘due to an increase in water temperature and light intensity during the summer. July is also the time for some of the poorly managed commercial ponds to be drained and exposed to sunlight for short periods of time (an operation known traditionally as saibeng-ping). Because a rise in energy content within algae existed after the fertilizers were nearly exhausted, additional fertilizer applied at that time would be an effective way of encouraging further accumulation of energy. ACKNOWLEDGEMENTS
The authors are grateful to the National Science Council, The Republic of China (ROC) for financial support of this project (grant no. NSC73-0204BllO-04). Thanks are also due to Mr. Y.Y. Ting, Director of the Tainan Station, Taiwan Fisheries Research Institute, for his support and advice on this project, Mr. Huang who allowed us to collect samples from his farm ponds, and Ms. P.R. Lin who identified all the algae.
REFERENCES Chang, M.H., Chen, S.S., Lin, K.Y. and Chen, SC., 1977. Experiments on application of tobacco waste as fertilizer and pesticide in milkfish ponds. Bull. Taiwan Fish. Res. Inst., 29: 39-46. Chang, T.P., 1969. Algae of Tainan milkfish ponds. Chin.-Am. Jt. Comm. Rural Reconstr. Fish. Ser., 7: 91-135. Chen, H.C., 1971a. Increase of benthic algae in milkfish (Chanos charms Forsskal) ponds by application of silicate. Chin.-Am. Jt. Comm. Rural Reconstr. Fish. Ser., 11: 71-83. Chen, H.C., 1971b. Studies on milkfish ponds: I. effects of pH value and salinity on the growth of benthic algae. Tung-Kang Marine Laboratory, Aquiculture, l(2): 1-11. Lin, H.S., 1969. Some aspects of milkfish ecology. Chin.-Am. Jt. Comm. Rural Reconstr. Fish. Ser., 6: 68-90. Lin, M.N., 1981. Theory and practice in deep-water systems of milkfish culture, part I. Fisheries Magazine, 4 (May) : 35-37 (in Chinese). Mizuno, T., 1983. Illustrations of the Freshwater Plankton of Japan. Hoikusha Publishing Co., Ltd., Osaka, 353 pp. Slobodkin, L.B. and Richman, S., 1960. The availability of a miniature bomb calorimeter for ecology. Ecology, 41(4) : 784. Su, B.T. and Ting, Y.Y., 1980. The study of improvement of the milkfish culture in deep-water ponds. Bull. Taiwan Fish. Res. Inst., 32: 579-585. Tang, Y.A. and Hwang, T.L., 1965. Studies of milkfish pond fertilization. Bull. Taiwan Fish. Res. Inst., 12: 42-47. Tang, Y.A. and Hwang, T.L., 1966. Evaluation of the relative suitability of various groups of algae as food of milkfish in brackish-water ponds. FA0 Fish. Rep., 3 (44): 365-372. vamaji, T., 1982.Illustrations of the Marine Plankton of Japan. Hoikusha Publishing Co., Ltd., Osaka, 537 pp.