Appendix 3
SEDIMENT OXYGEN CONSUMPTION IN TROPICAL UNDRAINABLE FISH PONDS

ABSTRACT

The magnitudes of sediment oxygen consumption in the rural undrainable fish ponds of Orissa, India, were quantified and partitioned into bacterial, animal and chemical uptakes. It was in the low ranges, comprising a maximum of 30 percent of the total community respiration. Chemical uptake was generally dominating, followed by bacterial respiration and a low macroinvertebrate respiration. The limiting effect of oxygen levels, mechnical disturbances and bioturbation operating on the uptake rates in the ponds were demonstrated. Proving that the sediment layers are acting as energy traps, measures are suggested for improved sediment-water interaction and enhanced nutrient recycling.

1. INTRODUCTION

The chemical and biological processes in the pond sediment which is a reservoir of nutrients for the overlying water, are influenced by various factors. The mutual regulation and interactions between the sediment and water are vital for the productivity and lifespan of a water body (Wetzel, 1975). The oxygen uptake level of the sediment has been considered to be a good indicator of benthic community metabolism as well as intensity of the mineralization process (Edwards and Rolley, 1965; Hargrave, 1969 a; Pamatmat 1971; Grancli, 1978). While many studies have been carried out at quantifying the magnitude of sediment respiration in a variety of freshwater ecosystems in the temperate (Hargrave, 1973; Edberg and Hofsten, 1973; Edberg, 1976; Grancli, 1978) and on the factors affecting the same (Hargrave, 1972: Grancli, 1979 a) no similar attempt has been made in the tropical aquatic systems.

While working in the community metabolism of undrainable rural fish ponds of Orissa, India, the net primary production values seemed to be in the low ranges, in spite of the systems being originally enriched through allochthonous sources. It was observed that the deep sediment layers, instead of actively interacting with the water medium, were acting as nutrient traps and even led to fish kills at times of incidental disturbances. Hence, a study on the determination of sediment oxygen consumption and further partitioning into specific uptakes was planned. Influencing factors such as oxygen levels, mechanical disturbances, bioturbation, etc. , were taken into consideration in a series of experiments. The present communication reports the first quantified estimates of sediment oxygen consumption and the related aspects in these undrainable fish ponds.

2. METHODS

Eleven undrainable perennial fish ponds representative of rural ponds in Orissa, India, were selected for studies from a cluster of 32 ponds surveyed earlier (Olah, 1983). Information on their morphometric features such as age, water surface and depth and sediment depth were collected. The sediment organic carbon content and quantum of benthic fauna were determined in accordance with standard methods. Gross primary production and total community respiration were calculated based on the analysis of diurnal oxygen consumption with three oxygen measurements (McConnell, 1962) and presented elsewhere. No standard method has been evolved for the measurement of sediment oxygen uptake and a few investigations have been made on the reliability of in situ or laboratory measurements (Pamatmat, 1971 a; James, 1974; Edberg, 1976 and Grancli, 1978).

In order to quantify the sediment oxygen consumption and separate it into bacterial and animal respiration and chemical oxygen consumption, three treatments were planned, viz., in situ incubation, treatment with an antibiotic and mercuric chloride, along with other experiments. Various toxic substances like phenol, potassium cyanide and formalin have been used to eliminate biological respiration and in the present studies, mercuric chloride was used, considering the oxygen consumption by the inhibitors themselves. An undisturbed sediment sample was brought to the surface using Hargrave's sampler. The cores were collected in glass tubes (20 cm long; 3.5 cm dia) with the overlying water intact. After marking the tubes for different treatments in duplicate, initial dissolved oxygen concentration was measured using an oxygen meter (Monitor II Beckman model) with electrode. While the first set was undisturbed, the second set of tubes was sealed with an aquatic solution of oxytetracycline to a final concentration of about 300 mg/l. The third set was injected with a saturated solution of mercuric chloride to a concentration of 3 mg/l. The tubes were incubated in dark at in situ temperature for two hours, after which the dissolved oxygen levels were measured. While the last set of tubes gives a measure of chemical oxygen uptake, the second set includes the uptake by animal and bacterial communities. The difference between the oxygen contents of the tubes not treated with antibiotic and the treated ones indicates the bacterial respiration. The volume of water in the tubes and diameter of the tubes were noted for further computation.

An experiment was conducted to know the vertical pattern of the potential oxygen consumption in sediment layers in pond 1. Unit weights of sample from each of the three sediment layers, 0–2 cm, 6–8 cm and 14–16 cm, were introduced into an airtight container with water, whose oxygen contents are measured. They were stirred well and the oxygen content measured to obtain the immediate potential consumption. The containers were incubated in the dark at in situ temperature for two hours, at the end of which the oxygen content was measured, to obtain the cumulative potential uptake.

The oxygen uptake of sediment-water systems is unfluenced by oxygen availability, mixing of water and sediment and biomass and kind of organisms in the sediment (Grancli, 1978). The effect of oxygen concentration on the bacterial and chemical oxygen consumption at the sediment-water interface of pond 1 was measured at in situ and enriched levels. Disturbance of the core gives the oxygen requirement of the entire soft sediment layer and also removes the artificial oxygen gradients in water. The effect of mechanical disturbance on the respiration was measured in the cores of pond 1 as well as that of the swamp in FARTC campus, for a comparison. Disturbance effect in swamp sediment was tested at the in situ and enriched oxygen levels to obtain the potential uptake.

The profound sediment cores from a highly organic-loaded earthern pond were used for measuring the effect of bioturbation by chironomid larvae, usually found in large numbers (1 415–25 889 m^-2 and even up to 65 000 m^-2). Fifth instar Chironomus larvae were collected from the mud with a 0.4 mm mesh sieve and thirty of them were introduced into each core (42 440 m^-2), together with a control to measure the oxygen consumption of the larvae themselves. The enhanced oxygen uptake levels due to movement of the larvae were measured, following the procedure explained earlier, with in situ incubation in the dark, for two hours.

3. RESULTS

3.1 Important features of the ponds studied

The rural undrainable fish ponds studied are small perennial water bodies, with considerable vegetation surrounding them. The study covered ponds newly constructed two years ago as well as those 100 years old. The water surface area varied from 0.01 to 0.75 ha. While the water depth in these ponds ranged from 60 to 160 cm, the sediment depth was 47–144 cm. Multipurpose use of the ponds by villagers provides them the source of organic enrichment, the human population and animal livestock surrounding the ponds being 83–850 ha^-1 and 7–641 ha respectively.

The organic carbon content of the deep sediment layers ranged from 0.74 to 3.11 percent and the gas accumulation was as high as 3–12 m³ m^-2, largely comprising of methane. The reduced condition was visible and the interstitial ammonia content was 2–6 ppm. The benthic community mainly comprising of chironomid larvae and oligochaets ranged from 80 to 440 m^-2 by number, but was conspicuous by its absence in pond 1. This pond, more than 100 years old, has a deep sediment layer (144 cm) with a permanent bloom of Microcystis in the water medium. Hence, all further studies have been conducted in the sediment cores from this pond. The fish stocking densities in these ponds ranged from 3 400 to 7 000 ha^-1, comprising of the three Indian major carps, catla (Catla catla), rohu (Labeo rohita), mrigal (Cirrhinus mrigala) and a lower proportion of the common carp (Cyprinus carpio var. communis).

3.2 Magnitudes of sediment oxygen uptake

The sediment oxygen consumption ranged between 0.39 and 3.39 g O₂ m^-2d^-1 (Table I).A high value of 7.49 g O₂ m^-2d^-1 was observed in one pond, of which the chemical uptake formed about 96.13 percent and remaining was bacterial consumption. The present magnitudes of sediment oxygen uptake comprised 5.14–29.74 percent of the gross primary production and 5.34–29.95 percent of the total community respiration, their actual values being presented elsewhere (Olah, et al., in prep.)

Either the bacterial or chemical oxygen uptake levels formed the major portion of the total sediment uptake. The bacterial consumption varied up to 77.27 percent, the value recorded in pond 2. While pond 9 did not show any bacterial uptake, that in pond 1 was still in the low ranges (9.17%).The animal consumption was significant only in ponds 3 (66.93%), and 10 (43.24%) and remained low in others (5.88–7.20%), while it was nil in six cases. The chemical uptake ranged from 15.53 percent in pond 2 to 100 percent in pond 9. As the animal respiration was negligible, the bacterial and the chemical uptakes generally showed an inverse trend in their magnitudes.

The importance of chemical and bacterial oxygen consumption in these reduced sediments was further shown in the experiment on the vertical distribution of the potential oxygen consumption in pond 1 (Table II). The immediate uptake accounted for 54.17–69.23 percent of the total consumption in the three layers, during two hours of incubation. A large quantum of benthic animals would have maintained this uptake rate at a uniform level. Again, the decrease in the uptake rates with depth was not great, being about 50 percent of that of surface even at 14–16 cm, in cases of both immediate and cumulative consumptions.

3.3 Significance of oxygen concentration in sediment respiration.

The sediment oxygen uptake was influenced by the initial oxygen concentrations and an oxygen-limitation of the uptake was visible in all these ponds (Fig 1). The uptake rates in the different ponds varied directly with the in situ dissolved oxygen contents of the overlying waters.This was further proved in the experiment on the effect of oxygen concentration on the sediment oxygen uptake in pond 1. At an enriched oxygen level of 9 ppm, the chemical and the total consumption increased by more than 50 percent of that at in situ levels and was 75 percent with regard to bacterial uptake.

3.4 Oxygen uptake as influenced by turbulance

In the swamp sediment used for comparison, oxygen consumption showed a conspicuous difference, especially in the disturbed core (Table IV). While the uptake rate increased by 1.15 times that of in situ levels in the undisturbed core at the enriched level of 9.5 ppm, it was 2.38 times in the disturbed core While the former indicates the actual consumption, the latter is an estimate of the requirement at the time of turbulance, mechanical, biological or otherwise.

The disturbances of the sediment-water interface have a significant inference on the sediment respiration. The differences in the undisturbed and disturbed cores show the potential sediment consumption, as well as indicate the reduced nature of the amount of organic matter in the sediment. While the increase was 1.64 times of the undisturbed core in case of pond 1, it was 2.60 times in the swamp sediment, which further rose to 5.42 times at an enriched level. The differences are due to the increased organic content in the swamp sediment, which when disturbed, consumed higher oxygen. This also gives a measure of the suppression of mineralization process due to lack of oxygen transport to sediment layers.

The effect of chironomid bioturbation on the sediment respiration was significant. While the total oxygen consumption of 30 chemical larvae themselves was only 1 515 g O₂ m^-2d^-1, and that of the sediment was 2.72 g O₂ m^-2d^-1, the indirect effect or the larval-induced oxygen uptake due to their movements was as high as 4.20 g O₂ m^-2d^-1, being about 50 percent. This also was more than two-and-a-half times that of the respiration amount, indicating the importance of benthic animal activities.

4. DISCUSSION

The magnitudes of sediment oxygen consumption in the different freshwater bodies collected from various sources are presented in Table V. It was seen that though the value of sediment oxygen uptake was considerable, the quantum as a percentage of gross primary production was low compared with others (Hargrave, 1969a; 1973). Similar was the case in the share of the community respiration also, reflecting the limited role of sediment in the community metabolism of these ponds. Grancli (1978) cautions that oxygen uptake values might underestimate total mineralization of organic material during periods of low oxygen availability since they do not measure anaerobicmetabolism in the sediment. This must be considered while comparing them with the primary production. However, the total uptake including chemical oxidation provides reliable estimates. The latter represents the intensity of anaerobic fermentative processes utilizing their end product. As already mentioned, the chemical uptake in the sediment uptake was considerable ranging even up to 100 percent. The increasing chemical oxidation has been directly related to the amount of reduced substances and low oxidation-reduction potentials (Teal and Kanwisher, 1961; Pamatmat, 1971a; Hargrave, 1972).

The uptake potential corroborate with the chemical data and accumulation patterns of the sediment of these ponds. Pond 11 needs special mention as fish kills were reported in 1983 (Radheyshyam et al., in prep.) and the sediment chemical uptake was as high as 7.20 g O₂ m^-2d^-1 forming 96.13 percent of the total.

Bacterial oxygen consumption was reported to vary between 30 and 45 percent of the sediment uptake in the Marion lake by Hargrave (1969 a), while it was assigned the main role in an American lake (Brewer et al., 1977). In both cases, temperature, rather than oxygen concentrations, was found to play an important role in enhancing the bacterial uptake. The bacterial respiration in the total uptake was considerable in these undrainable fish ponds.

Aquatic sediments are modified to a considerable extent by benthic invertebrates which increase the depth of oxygen penetration into the sediment (Edwards, 1958; Edwards and Rolley, 1965). Considerable values of respiration up to 33 percent by the benthic microfauna representing 96 percent in community biomass has been observed by Hargrave (1969 a) in Lake Marion. Further, chironomids were the single most important animal group accounting for 9 percent of the respiration. In contrast, the benthic faunistic respiration, mainly contributed by the chironomids, was in the low ranges reflecting on their density as well as anaeorbic conditions of the sediment.

The vertical distribution of sediment oxygen uptake preserved a gradual pattern up to 16 cm depth, while significant reductions have been observed even within 1 cm of sediment layer in a eutrophic lake. But the reduction in oxygen uptake in formalin-treated cores was not marked in deeper layers, relating to higher animal activity in the surface sediment layer (Hargrave, 1972). The reduced state of the sediment of undrainable ponds and the anaerobic conditions thereby created, in addition to lack of benthic fauna, are discernible from this observation.

Low levels of oxygen consumption by the sediment at reduced oxygen concentrations and an almost linear dependence of the uptake on oxygen concentration has been reported (Hargrave, 1972; Edberg, 1976; Grancli, 1978; 1979 b). Though temperature has been observed to be a confusing factor in most of the temperate lakes (Hargrave, 1969 b), oxygen level appears to be of utmost importance in these tropical undrainable ponds, being more pronounced in the case of the swamp. This suggests that the sediments of most of the undrainable ponds are in a reduced condition due to lack of a proper oxygen transport mechanism and the differences between actual uptake and potential uptake are often considerable.

The effect of stirring or disturbance on the sediment oxygen uptake has been found to be considerable in earlier studies (Edwards and Rolley, 1965; Carey, 1967; Hargrave, 1969 b). Comparing the sediment accumulation and oxygen uptake, Grancli (1978) also concluded that aerobic sediment mineralization is seriously limited by physical factors such as mixing an oxygen supply, resulting in rapid filling of shallow and eutrophic lakes. The effect of disturbance on the oxygen uptake was observed to be considerable also in the present case, the absence of which has caused large organic accumulation in the pond bottoms.

The effect of bioturbation by the chironomid larvae was conspicuous in the experiment. An increase in the oxygen uptake in proportion to number of added larvae was reputed by Grancli (1979 b). He also observed the increase in oxygen uptake to be three to four times of the respiration and attributed the simultaneous effect to larval burrows. The low benthic fauna and even its complete absence in pond 1 have already been explained with respect to reduced sediment-water exchange processes.

The field studies and laboratory observations on the sediment oxygen uptake of the undrainable rural fish ponds confirm the hypotheses that the deep sediment layers rich in organic matter are playing a limited role in the community metabolism of these ponds. Instead of serving as the nutrient source for the overlying waters, they are acting as energy traps with reduced substances and anaerobic conditions. The situation is aggravated by comparatively low benthic fauna and practically no oxygen transport mechanism by wind-driven turbulence in the systems. The density of fish stocked is also not having a determining influence on bioturbation to increase the sediment water interactions. Measures, such as mechanical raking of the pond beds, introduction of bottom feeders such as common carp, and enriching the benthic community would be useful in accelerating the transport mechanisms and enhance the nutrient recycling processes in these undrainable ponds. Maintenance of an active sediment-water interaction and an aerobic condition at the sediment surface would also prevent any hazards like fish kills at times of sudden occasional disturbances.

5. SUMMARY

1. The quantification of sediment oxygen consumption and further partitioning into chemical, bacterial and macro-invertebrate respiration were carried out in the rural undrainable fish ponds of Orissa, India, along with studies on the influencing factors like oxygen levels, mechanical disturbances, bioturbation, etc.

2. The sediment oxygen uptake values varied between 0.39 and 3.39 g O₂ m^-2d^-1, except for one high value of 7.49 g O₂ m^-2d^-1. They were in the low ranges of 5.14–29.74 percent of gross primary production and 5.34–29.95 percent of total community respiration.

3. The chemical and bacterial oxygen uptake formed the major portion of total sediment consumption, the respective percentages being 15.53–100.00 and 0–77.27 and the animal uptake was generally negligible. The vertical distribution of potential sediment oxygen uptake did not present steep gradients, suggesting the dominance of chemical oxidation.

4. A direct relation between the uptake rates and oxygen concentrations was observed and the latter was a controlling factor in these ponds. The effects of mechanical disturbances and bioturbation by chironomid larvae on the uptake were considerable, the absence of which has caused considerable organic accumulation to the pond bottoms.

5. The studies proved that the deep sediment layers rich in organic matter are playing a limited role in the community metabolism of the undrainable fish ponds. They are acting as energy traps with reduced substances and anaerobic conditions, aggravated by low benthic fauna and wind-driven turbulence in these systems.

6. Certain measures like mechanical raking of pond beds, introduction of bottom-feeding fish and enriching benthic community are suggested for accelerating transport mechanisms. These would enhance the nutrient-recycling processes and also prevent hazards such as fish kills at times of occasional disturbances.

REFERENCES

Brewer, W.S., Abernathy, A.R. and Paynter, M.J., 1977. Oxygen consumption by freshwater sediments. Wat. Res., 11: 471–473.

Carey, A.G. Jr., 1967. Energetics of the benthos of Long Island Sound I. Oxygen utilization of the sediment Bull. Bringham Oceanogr. Collect., 19: 136–144.

Edberg, N., 1976. Oxygen consumption of sediment and water in certain selected lakes. Vatten, 32: 2–12.

Edberg, N. and Hofsten, B.V., 1973. Oxygen uptake of bottom sediments studied in situ and in the laboratory. Wat. Res., 7: 1285–1294.

Edwards, R.W., 1958. The effect of larvae of Chironomus riparius Meigen on the redox potentials of settled activated sludge.Ann. Appl. Biol., 46: 457–464.

Edwards, R.W. and Rolley, H.L.J., 1965. Oxygen consumption of river muds. J. Ecol., 53: 1–19.

Grancli, W., 1978.Sediment oxygen uptake in south Swedish lakes. Oikos, 30: 7–16.

Grancli, W., 1979 a.A comparison of carbondioxide production and oxygen uptake in sediment cores from four south Swedish lakes. Holarctic Ecology, 2: 51–57.

Grancli, W., 1979 b. The influence of Chironomus plumosus larvae on the oxygen uptake of sediment.Arch. Hydrobiol., 87 (4): 385–403.

Hargrave, B.T., 1969 a. Epibenthic algal production and community respiration in the sediments of Marion lake. J. Fish. Res. Bd. Can., 26: 2003–2026.

Hargrave, B.T., 1969 b.Similarity of oxygen uptake by benthic communities. Limnol. Oceanogr., 14 (5): 801–806

Hargrave, B.T., 1972 Oxidation-reduction potentials, oxygen concentration and oxygen uptake of profundal sediments in a eutrophic lake. Oikos, 23: 167–177.

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

James, A., 1974. The measurement of benthal respiration. Wat. Res., 8: 955–959.

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

Olah, J., 1983. A programme of investigations on the hydrobiology of fish ponds.FAO Field document No. 6, FI: DP/IND/75-031, 43p.

Famatmat, M.M., 1971 Oxygen consumption by the seabed. IV.Shipboard and laboratory experiments. Limnol. Oceanogr., 16(3): 536–550.

Teal, J.M. and Kanwisher, J., 1961. Gas exchange in a Georgia salt marsh. Limnol. Oceanogr., 6: 388–399.

Wetzel, R.G., 1975. Limnology.Surrenders, Philadelphia, pp. 589–621.

Table I. Partitioning benthic community respiration in sediments of undrainable rural fishponds g O₂ m^-2 d^-1

Table II.Vertical distribution of the potential oxygen consumption in the sediment of Pond 1, mg O₂ g^-1 dry wt

Table III. Effect of oxygen concentration on the bacterial and chemical oxygen consumption of the sediment-water interface of Pond 1, g O₂ m^-2 d^-1

Table IV. Disturbance effect on the total sediment respiration g O₂ m^-2 d^-1

Table V. Ranges sediment oxygen uptake in freshwater ecosystems

Fig. 1 Relation between sediment oxygen consumption and natural in situ oxygen concentrations