Appendix 4
DETRITUS ASSOCIATED RESPIRATION DURING MACROPHYTE DECOMPOSITION

ABSTRACT

The detritus-associated respiration levels in the dominant tropical aquatic macrophytes were measured along with their decomposition patterns. More resistant plants like Eichhornia and Salvinia showed lower decay losses and respiration rates as compared to Najas and Hydrilla. The respiration of macrophytes during decomposition depended on their composition and foliage along with biomass, indicating a constant oxygen demand in a composite macrophyte system such as a swamp.

1. INTRODUCTION

The aquatic macrophytes play an important role in the production processes, nutrient status and oxygen budgets of water bodies (Wetzel and Hough, 1973). While a considerable quantum of nutrients are locked up by these plants during their vegetative phase, a determinative role is played in the oxygen regime of the systems during the decaying process. Several studies have been made on the decomposition of macrophytes and detritus formation along with changes in their constituents (Kaushik and Hynes, 1968; 1971; Garith and Lawacz, 1976; Godshalk and Wetzel, 1978; Pohuniol, 1982) but those on the removal of oxygen from the total budget due to the decay are scarce. A few measurements carried out on the respiration of macrophytes pertain to their growing phase and not to the oxygen demand during decay.

Macrophytes form a significant part of the tropical aquatic systems, as observed in case of fish ponds of Orissa, India, with the weed cover comprising mainly Eichhornia, ranging up to 100 percent of the pond surface (Olah, 1983). This report describes the oxygen consumption patterns of some of the important macrophytes associated with different stages of decomposition and attempts to relate them with the total community respiration levels in different waters.

2. METHODS

Five species of macrophytes dominating in the swamp in the FARTC campus, Bhubaneswar, India were selected for decay-associated respiration studies. They were Eichhornia crassipes Kunth, Salvinia cucullata Roxb., Hydrilla verticillata Casp., Najas foveolata A.BV. and Ceratophyllum demersum L. Five different stages of weed decay were identified: (i) green and fresh, (ii) slightly brown and frayed margines, (iii) brown, drooping with visible decay, (iv) dark brown to black, losing form and disintegrating, and (v) pulpy forming a mucilogenous mass with cellulose. Known quantities of each plant in different stages of decay were introduced into airtight glass containers with water in duplicate whose initial oxygen was measured using an oxygen meter with electrode (Beckman Monitor II). They were incubated in the dark at in situ temperature for 2–4 hours, at the end of which the final oxygen concentrations were measured in all the containers. The dry weight of the plant material introduced was determined and the respiration values are presented as mg O₂ g^-1 dry weight h^-1.

To study the in situ decomposition rates and associated respiration magnitudes in the swamp, bag method of incubation was used (Ayyappan et al., 1984). Fresh material of Eichhornia, Salvinia, Hydrilla and Najas were collected and uniform weights enclosed in nylon bags (2 mm mesh size; 5 cm dia). Two sets of four bags for each plant were suspended at the surface and bottom in the weed-zone of the swamp. The introduction was made every alternate day and at the end of ten days the incubated material was taken out. The loss of the plant material due to decomposition was determined as a percentage of initial weights on a dry weight basis. The oxygen consumption levels were measured, as explained earlier and respiration expressed as mg O₂ g^-1 dry weight h^-1.

Concentrating on the common weed, water hyacinth, further studies were carried out on its decomposition and respiration in fish ponds with different levels of organic fertilization. Sets of four bags of five leaves of Eichhornia with uniform weight supported on a bamboo pole were introduced on alternate days in a fish rearing pond, where cowdung is applied at the rate of 10 000 kg ha^-1 year^-1.

The samples were collected from the fourth to the twentythird day and twelfth to thirtyfirst day of incubation weights recorded and oxygen consumption levels measured.

A similar experiment was conducted in a series of highly manured ponds, with the Eichhornia leaves incubated at the surface as well as pond bottom. The ponds were manured with cowdung at 10 000 to 100 000 kg/ha/year. After 14 days and 24 days of incubation, decay losses and respiration levels were measured.

3. RESULTS

3.1 Decay-associated respiration in different macrophytes

The respiration levels at different stages of decomposing macrophytes showed a regular trend with regard to decay (Table I). The first stage of decay showed higher oxygen consumption than the fresh material in all cases, except Hydrilla. The setting-in of colonization and decay of organic material had caused the higher oxygen. consumption.With the reduction in easily degradable organic content and retention of resistant portion, the oxygen uptake steadily decreases.

The variations in the respiration values between the macrophytes were also conspicuous. Hydrilla showed highest oxygen requirement when fresh, followed by Najas, Eichhornia, Ceratophyllum and Salvinia. At the same time, the reduction gradient in oxygen uptake with progressive decay was steep in Hydrilla. Maximum levels of uptake during the decaying stages were observed in Najas. While the uptake level remained at a low constant level in Salvinia, that in Eichhornia was medium.

3.2 Decomposition and respiration of macrophytes in the swamp

The weight loss due to decomposition was lower in the hard and resistant weeds, Eichhornia and Salvinia, while it was considerably higher in the softer Hydrilla and Najas plants (Fig. 1). The decay rates were higher at the bottom in all cases. In Eichhornia, the rate showed an increase after the second day until the fifth, stabilized later at the bottom and remained low at the surface. The average daily decay losses at the surface and bottom over the whole period were 0.88 percent and
3.86 percent respectively. The respiration was low, with a maximum of 0.5 mg O₂ g^-1h^-1 on the tenth day. The effect of leaching was pronounced in in case of Salvinia within two days, when the weight loss reached 30 percent in both the levels and stabilized to about 40 percent. The average daily losses at the surface and bottom were 3.32 percent and 4.64 percent respectively. With low respiration values at the beginning, it was about 1 and 2 mg O₂ g^-1 h^-1 at the two levels on the last day. The decomposition of Hydrilla and Najas at the bottom was nearly complete within 10 days, at around 80% in case of surface-incubated material. The decay rate was high until the fourth day, the average daily loss rates at surface and bottom levels for Hydrilla and Najas were 5.81% and 10.74% and 8.36% and 10.93% respectively. The pattern of respiration in the two plants was, however, different. While the oxygen consumption stabilized to about 7 mg O₂ g^-1 h^-1 in Hydrilla by the eighth day, it showed an increasing trend in case of bottom-incubated Najas. A high value of around 20 mg O₂ g^-1 h^-1 was recorded in the plants incubated for nine days. This is in conformity with the observation made with Najas in different decaying stages. However, the surface-incubated leaves had a stabilized uptake of about 4 mg O₂ g^-1h^-1.

3.3 Eichhornia decomposition in the fish-rearing pond

In the experiment conducted to show the effect of incubation time on the decomposition rate of Eichhornia leaves in a fish-rearing pond, the classical pattern of initial higher loss due to leaching followed by stabilization due to breakdown of organic matter and retention of resistant material was pronounced (Fig. 2). In both the sets observed for 23 days and 31 days, the trend was similar with significant overlapping of values. The general trend of respiration values also agreed with the earlier observation, with increase in oxygen uptake from fresh leaves to the first stage of decomposition. By the end of one month of incubation, the leaves were at the initial stages of decomposition, the delay being due to their compactness in the bags. The average daily decay loss in the pond worked out to 4.07 percent. Slightly higher values of oxygen uptake than that studied in the swamp were observed. A stage of stabilization seemed to be setting in by the end of the period.

3.4 Eichhornia decomposition in highly manured ponds

The decay losses and oxygen uptake by decomposing water hyacinth did not present a clear gradient in the ponds with different manure doses, as the fertilizing effect on the water quality itself was yet to be visible. The average decay losses in the surface and bottom levels of the six ponds were 33.17 percent and 35.33 percent in 14 days, the average daily loss being 2.37 percent and 2.52 percent. The respective respiration levels were 0.159 and 0.123 mg O₂ g^-1 dry weight h^-1.During a period of 24 days, the weight losses were 42.0 percent and 55.5 percent in the surface and bottom levels, with the daily average working out to 1.74 percent and 1.31 percent and the respiration levels being 0.978 and 1.42 mg O₂ g^-1 h^-1. While the daily decomposition losses were lower than in the fish-rearing pond, the respiration levels were in general conformity.

4. DISCUSSION

An initial increase in the oxygen requirement level of macrophytes when they begin to decay may be explained by the loss of photosynthetic pigments, degradation of nonresistant organic matter, and bacterial colonization. This also indicates the magnitude to which the weeds use up oxygen from the total budget, when there is a mass collapse (a regular seasonal feature). The variations in the respiration values between the macrophytes are due to the amount of resistant material like lignin, hemicelluloses, fibre, etc., in the plant, pigment content, nature of leaves, etc.Hydrilla showed higher oxygen requirement when fresh, while Najas that is comparatively hard, and had higher uptake levels with the advance of decomposition. The resistant leaves of Eichhornia and Salvinia had comparatively low requirements as their fibre content is greater and the time span of decomposition is longer.

In case of in situ incubation of macrophytes in the swamp, higher levels of decomposition were observed in the bottom due to higher bacterial activity and mechanical breakdown. As mentioned earlier, Najas and Hydrilla exhibited higher decay losses as well as oxygen uptake rates due to their composition, as compared to Eichhornia and Salvinia. In the latter two, the weight loss is due more to leaching than the actual decay which sets in somewhat later. A similar observation of higher respiration rates and photosynthetic efficiencies too in Najas and Hydrilla have been made (Olah, et al., in prep.). It must be stressed that the composition and type of foliage of the macrophytes are also to be considered, along with the actual biomass, while comparing their respiration levels. Depending on the type of products formed at different levels of decay, oxygen consumption varies. Thus, in a water body such as a swamp, containing all these macrophytes, there is a continuous demand for oxygen by different weeds at varying stages of decomposition.

A clear picture of the weight losses and respiration levels in the decomposing Eichhornia was obtained during this study. The first stage marked by leaching followed by colonization and stabilization during detritus formation as proposed by Olah (1972) were observed. The respiration values showed an increasing trend with the onset of decay and tended to stabilize towards the end. It may be mentioned that these represent the transgression between fresh leaves and the first stage of decay, and not complete decomposition as would have happened in case of Najas or Hydrilla. A few isolated higher levels of respiration observed might be due to higher bacterial colonization, which is to be confirmed. Considerably higher value of decomposition as compared to the swamp and ponds with higher manuring may be explained with respect to level of organic enrichment that was lower in this case, and hence a greater potential for decay. Slightly higher values of respiration observed in bag experiments as compared to nature, might be due to the fact that the latter is a long-term process with no oxygen limitation. In the former, the leaves incubated in compact bags were used to measure oxygen uptake, that would show a higher potential during the measurements.

With higher levels of organic fertilization, it was significant to note that the degree of macrophyte respiration in total community respiration had reduced. Also, weight loss in these systems had been due to grazing by macro-invertebrates such as chironomids to a considerable extent. Given the above results, it is obvious that macrophytes, while contributing to the oxygen budget during their vegetation phase, also consume a significant amount on death and decay. In the composite macrophyte system of a swamp, a constant oxygen demand is maintained by the macrophytic decomposition. While aerated systems provide for a faster detritus formation and nutrient recycling, a lack of oxygen leads to anaerobic conditions and further organic accumulations.

5. SUMMARY

1. The detritus-associated respiration levels in the dominant tropical aquatic macrophytes were measured along with their decay rates in the different systems of swamp and manured fish ponds.

2. Five macrophytes, viz., Eichhornia crassipes Kunth, Salvinia cucullata Roxb., Hydrilla verticillata Casp., Najas indica and Ceratophyllum demersum L. were used for studies. Five stages of decomposition ranging from fresh leaves to detritus formation were identified for the purpose.

3. The oxygen requirement was observed to increase in the first stage of decay with a gradual stabilization further. While it was in the low ranges of 0.26 to 0.44 mg O₂ g^-1 h^-1 in the five stages of decomposition in Salvinia, high values of 0.12 to 2.31 mg O₂ g^-1 h^-1 were noticed in Najas.

4. The decay losses were greater in bottom-incubated macrophytes than at the surface. More resistant plants like Eichhornia and Salvinia had lower decay losses and respiration levels compared with Najas and Hydrilla.

5. The classical pattern of decomposition including leaching, colonization and stabilization was observed in case of Eichhornia, whose daily decomposition and respiration levels were monitored.

6. The respiration levels of macrophytes during detritus formation largely depended on their composition, foliage and decay patterns along with the biomass, indicating the constant oxygen demand in the composite macrophyte system of a swamp.

REFERENCES

Ayyappan, S., Olah, J., Raghavan, S.L. and Purushothaman, C.S., 198.Macrophyte decomposition in two tropical lakes.Arch. Hydrobiol., (accepted).

Garith, A. and Lawacz. W., 1976. Breakdown of leaf litter in the litteral zone of a eutrophic lake.Ekol. Pol., 24: 421–430.

Godshalk, G.L. and Wetzel, R.G., 1978. Decomposition in the literal zone of lakes.In: R.E. Good, D.F. Whingharn and R.L. Simpson (Eds.): Freshwater Wetlands. Ecological Processes and Management Potential, 131–143 Academic Press, New York.

Kaushik, N.K. and Hynes, H.B.N., 1968. Experimental study on the role of autumn shed leaves in aquatic environments. J. Ecol., 56: 229–243.

Kaushik, N.K. and Hynes, H.B.N., 1971. The fate of dead leaves that fall into the streams.Arch. Hydrobiol., 68: 465–515.

Olah, H., 1972.Leaching, colonization and stabilization during detritus formation. Mem. st. Ital. Idrobiol., 29: 105–127.

Olah, J., 1983. A programme of investigations on the hydrobiology of fish ponds. FAO Field Document 6: FI: DP/IND/75/031, 43p.

Wetzel, R.G. and Hough, R.A., 1973. Productivity and role of aquatic macrophytes in lakes: an assessment. Pol. Arch. Hydrobiol., 20: 9–19.

Table I: Respiration in different stages of decomposing macrophytes, mg O₂ g^-1 dry weight h^-1

Fig. 1 Decomposition losses and respiration rates of macrophytes in the swamp

Fig. 2 Pattern of Eiohhornia decomposition and respiration in fish pond