Appendix 2
COMMUNITY METABOLISM IN TROPICAL UNDRAINABLE RURAL FISH PONDS

ABSTRACT

The process of community metabolism including primary production and community respiration in eighteen undrainable rural fish ponds in India was quantified using the diurnal oxygen curve method. A considerable portion of the gross primary production that was in the moderate ranges, was consumed for the community respiration. Negative, and low positive values of net primary production indicated the significance of the allochthonous supply in these organic enriched systems. The low values of sediment respiration signified the reduced condition of stagnating sediment acting as an energy trap.

1. INTRODUCTION

During the last decade, there has been an emphasis on the extension of fish culture activity to monsoon-fed undrainable rural ponds in Orissa, India, initiated by the Freshwater Aquaculture Research and Training Centre, Dhauli. The culture practice in these fish ponds corresponds to the management level of organic fertilization technology. The fish production in these village ponds is based mainly on the utilization of the natural fish food resources sustained and enhanced by the introduction of organic carbon. A study of the community metabolism of these ponds was made in which the basic production processes were measured using the diurnal oxygen curve analysis.

2. METHODS

The few studies published on the primary production of the Indian freshwaters have been carried out by the classical light and dark bottle method, measuring the changes of oxygen concentration in the bottles after long incubation periods (Sreenivasan, 1963, 1976; Vijayaraghavan, 1971). In these hypertrophic waters, the use of this method often results in a significant underestimate of the planktonic primary production as well as community respiration (Kalbe, 1972; Olah et al., 1978). Also, the primary production and respiration of the macrophyte and benthic communities are not included in this measurement. Hence, a method for the analysis of the diurnal oxygen curve was employed, measuring the in situ levels by a Beckman Monitor II system oxygen meter. As significant variations were noticed in the size, area, water depth and organic loads of these undrainable ponds, a simplified version of the diel curve analysis was used. Diurnal variations of dissolved oxygen were monitored with three measuring points and the McConnell (1962) equation was used to calculate the primary production and community respiration. The only drawback in the analysis of diurnal oxygen curve is that the diffusion rate at the air-water interface is not incorporated in the calculations. However, this error on estimations is minimal, as there is very little turbulence in these ponds due to high embankments and thick vegetation around them.

To determine the magnitude of the sediment oxygen consumption in the whole community respiration, intact sediment cores were collected manually in glass tubes (20 cm length; 3.5 cm dia), with at least 10 cm of the overlying bottom water remaining undisturbed. After measuring the initial oxygen concentration in the water above the sediment, the tubes were incubated under in situ conditions and in dark for 2 hours. The final oxygen levels were measured. The production and respiration values were calculated as gC m^-2d^-1 using a conversion factor of 0.375 for the measured oxygen values.

3. RESULTS

The community metabolism in 18 and sediment respiration in 10 undrainable rural ponds were measured in February of 1985. Some of the rural ponds monitored are old, up to 100 years, with deep, organic-rich soft sediment (Table I). Most of them, however, are new or reconstructed within 2 to 8 years. Majority of the ponds have a water surface area below 1 ha, with the average being less than 1 000 m². The largest pond, No. 4, however, has an area of 2.13 ha. The water depth varied from as low as 48 cm to 225 cm, the average being around 100 cm. The stocking structure and density varied from 2 800 to 7 000 ha^-1, with the three Indian major carps, catla, rohu and mrigal and the common carp. In some ponds, it was supplemented with silver carp. These small ponds serve many purposes in the life of the villages and the organic enrichment is also realized through these activities. Thus the magnitude of organic fertilization relates to the size of human population and animals associated with the particular ponds. The human population ranged between 10 and 1 250 ha^-1. They are the point-source of organic enrichment during dry seasons and diffuse source during the monsoons.

The gross primary production in the ponds ranged from 1.76 to 4.79 gC m^-2d^-1, except in pond 1, where it was extremely high, being 12.3 gC m^-2d^-1 (Table II). The average gross primary production was 3.73 gC m^-2d^-1. No significant correlation between the primary production levels and the main characteristics of these ponds could be discerned. However, production levels were high in old ponds with moderate organic load as in pond 1, or moderately old ponds with high organic load as in pond 10. The production was generally low in young, shallow ponds with moderate organic load (pond 3) or moderately old ponds with low organic load (pond 9). Production levels were observed to reach higher levels even in young ponds receiving scant organic load through human population and animal livestock, as in the case of pond 12, largely because of the additional organic manuring and supplementary feeding.

The magnitude of total community respiration was close to the gross production values, being in the range of 1.66 to 4.69 gC m^-2d^-1, except a high value of 11.34 gC m^-2d^-1 in pond 1 (Table II). The average community respiration value was 3.60 gC m^-2d^-1. The benthic community respiration ranged widely, amounting to 5.34 to 29.95 percent of the total community respiration. The quantum of respiration at the sediment-water interface as a percentage of the primary production was lower in the more productive ponds, as compared to the low productive ones.

The net primary production as an end-result of the daily production and consumption processes was negative in seven ponds and positive in the other eleven. The negative net production ranged between 28.28 and 5.16 percent of the daily gross production, with a high value of 61.72 percent in pond 2.In ponds with positive net production, the values ranged between 0.70 and 35.62 percent of the gross primary production.

4. DISCUSSION

The perennial undrainable ponds in tropical monsoon lands with longer duration of sunshine are ideal fish culture systems. Multipurpose use of these ecosystems by the villagers facilitates continuous organic enrichment and hence the fish culture practice in such systems depend largely on the management level of organic fertilization where the bacterial-detrital food chain dominates, besides the algal food chain in the production process.

The community metabolism in this type of fish rearing ecosystem is usually represented with negative net primary production values. In the present study, however, only seven ponds were characterized by negative net primary production out of the eighteen ponds.Although the picture on an annual basis might change, i.e., more ponds may show negative values, the present results represent the average conditions of the production processes in these village ponds. In case of positive net primary production, the surplus between the organic carbon input through daily primary production together with average daily allochthonous organic enrichment and the output quantified as total community respiration, appears in the carbon accumulation sedimented at the bottom. The difference between the larger input due to organic enrichment and the smaller output with sediment organic carbon form the actual values of the daily net primary production.

The main feature of the undrainable ponds is the thick accumulation of organic matter in these ponds. For instance, pond 1 which is about 100 years old, has a sediment depth of 144 cm with 3.11 percent organic carbon content, 12 dm^-3m^-2 of gas accumulated as well as a significant amount of reduced substances (Olah et al., in prep.). In these shallow ponds, organic accumulation creates anaerobic conditions at the sediment-water interface due to low winddriven turbulence and convection currents which is further aggravated by low molecular and eddy diffusion. In the surveyed 10 ponds, the magnitude of the sediment respiration or benthic community respiration was generally low. In most cases it was smaller than it would correspond to the Hargrave's relation describing the percentage of the sediment respiration to the primary production, as a function of water depth (Hargrave, 1973). This may also be due to the limited transport mechanism at the sediment-water interface.

The primary production was moderate in all the surveyed fish ponds, except pond 1. Studies on the seasonal pattern of the primary production in Indian waters show that the month of February is a period with average values (Vijayaraghavan, 1971; Ayyappan, 1977) and hence, the measured values may represent the annual average situation in these village ponds. Similar primary production levels have been measured in other Indian freshwaters (Sreenivasan, 1972; Ayyappan, 1977), but with the light and dark bottle method, whose underestimation has been established (Kalbe, 1972; Olah et al., 1978). Even with organic enrichment in these ponds which though not precisely quantified, but seems considerable from the human and animal populations surrounding them, the primary production is in the moderate ranges (Table III). Evidently, the organic-rich sediment layers in these ponds are acting as energy traps due to their anaerobic and reduced conditions, as also proved by high values of their chemical respiration.

5. SUMMARY

1. The quantification of community metabolism including primary production and community respiration of eighteen undrainable rural fish ponds using the diurnal oxygen curve method has been attempted for the first time. Earlier attempts have used the light and dark bottle method, that has been proved to underestimate the production levels.

2. The gross primary production values in terms of gC m^-2d^-1 varied from 1.76 to 4.79, except in pond 1 with a high value of 12.30. Considering the method employed, they provided the first real estimates of the primary production in these ponds that was in the moderate ranges.

3. The total community respiration values were close to the production values, being 1.66 to 4.69 gC m^-2d^-1, with an exception of 11.34 gC m^-2d^-1 in pond 1.The sediment or the benthic community respiration values measured in ten ponds ranged from 0.15 to 1.27 gC m^-2d^-1.

4. While low values of positive net primary production of 0.03–1.35 gC m^-2d^-1 were observed in 11 ponds, negative values of -1.29 to-0.13 gC m^-2d^-1 were recorded in the remaining seven. This indicates the significance of allochthonous organic matter and a host of other factors operating in these organic-enriched systems.

5. Though similar values of primary production have been reported in other Indian freshwaters as in the present case, the former have generally underestimated the production by the light and dark bottle method. The present values appear to be the real estimates, being in the moderate ranges of productivity.

REFERENCES

Ayyappan, S., 1977. Observations on plankton with reference to hydrography of Ramasamudra tank, Dakshina Kannada, M.F.Sc. dissertation, submitted to the University of Agricultural Sciences, Bangalore, 198p.

Baumgardner, R.K., 1966. Oxygen balance in a stream receiving domestic and oil refinery effluents.Ph.D. Thesis, Oklahoma State University, 70p

Cole, J. and Fisher, S.G., 1978. Annual metabolism of a temporary pond ecosystem. American Midland Naturalist, 100 (1): 15–22.

Duffer, W.R. and Dorris, T.C., 1966. Oxygen balance in a southern great plains stream in Southeastern Oklahoma. Limnol. Oceanogr., 11 (2): 141–151.

Fontaine III, T.D. and Ewel, K.C., 1981. Metabolism of a Florida lake ecosystem.Limnol. Oceanogr., 26 (4): 754-63.

Hannan, H.H., 1967. Macrophyte standing crop and metabolism in a constant temperate river.Ph.D. Thesis.Oklahoma State University, 47p.

Hargrave, B.T., 1973. Coupling carbon flow through some pelagic and benthic communities.J. Fish. Res. Bd Can., 30: 1317–1326.

Kalbe, L., 1972. Sauerstoff und Primarproduktion in hypertrophen Flachseen des Havelgebietes.Int. Rev. Ges. Hydrobiol., 57 (6) : 825–862.

Kelly, M.H., Fitz Patrick, L.C. and Pearson, W.D., 1978. Phytoplankton dynamics, primary productivity and community metabolism in a north-central Texas pond. Hydrobiologia, 58 (3) : 245–260.

McConnell, W.J., 1962.Productivity relations in carboy microcosm.Limnol. Oceanogr., 7: 335–343.

Mathis, B.J. and Myers, S.A., 1970.Community metabolism in Lower Peoria lake. Trans. Illinois State Acad. Sci., 63 (2): 207–213.

Olah, J., Zsigri, A. and Kintzly, A.V., 1978.Primary production estimators in fish ponds by the mathematical evaluation of daily oxygen curves. Aquacultura Hungarica, 1: 3–14.

Sreenivasan, A., 1963.Primary production in three upland lakes of Madras State, India.Curr. Sci., 32: 130–131.

Sreenivasan, A., 1972. Energy transformations through primary productivity and fish production in some tropical freshwater impoundments and ponds. Pages 505–514 in Z. Kajak and A. Hillbricht-Ilkowska (editors) : Productivity problems in freshwaters. Polish Scientific Publishers, Warsaw, Poland.

Sreenivasan, A., 1976. Limnological studies of and primary production in temple pond ecosystems. Hydrobiologia, 48 (2): 117–123.

Vijayaraghavan, S., 1971. Seasonal variations in primary productivity in three tropical ponds. Hydrobiologia, 38 (3–4) : 395–408.

Table I. Main characteristics of undrainable rural fish ponds in Orissa, India

Table II. Community metabolism in undrainable rural fish ponds, g C m^-2 d^-1

Table III. Ranges of community metabolism in freshwater ecosystems, g C m^-2 d^-1