The bacterial decomposition and nutrient recycling in ponds are greatly influenced by the anaerobic nature of the sediment and inadequate oxygen. The total oxygen production in pond 1 on 18 February 1983 was about 15.4 g m-2 and the total respiration was only about 9.4 g m-2. On this particular day there was more organic matter produced than mineralized in the whole community. Similarly, on 26 February 1983 the oxygen production was 23.8 g m-2 whereas the oxygen consumption was 21 g m-2. However, during the active decomposition of the Microcystis on 15 March 1983 the total community respiration (6.6 g m-2) was higher than the total oxygen production of 5.7 g m-2. The formalin-treated sediment core was investigated separately for biological and chemical oxygen demands. In the ponds with thick anaerobic sediment, the chemical oxygen demand ranged from 70 to 90 percent of the total oxygen consumption. This high chemical oxygen demand is the result of the bacterial fermentative processes in the anaerobic sediment. In the absence of oxygen, the bacteria decompose the long organic molecules into smaller ones still having some reducing power and able to react directly with the oxygen or indirectly through chemical side-reactions. A good example of the reduced end-products of the anaerobic bacterial decomposition is the large quantity of gases accumulated in the pond sediment.
Using the special gas collector manufactured at FARTC, the amount of gas measured and the values were 3–12 dm 3 m-2. The slow release of gases and other reduced compounds and their contact with oxygen result in such chemical oxygen consumption. During low atmospheric pressure, however, larger amounts of the sediment gases enter the water column and consume much more oxygen which results in fish mortality.
The decomposition rate in the different layers of the water column and of the sediment was tested by a series of in situ experiments in pond 1. Leaves of water hyacinth and pieces of cellulose filter paper were separately packed with a 2 m long plastic net bag in such a way as to expose them to the different depths of water column and sediment, and attached to a bamboo rod. The separate bamboo rods were pushed into the sediment so deep that 8 pieces of model substrates (80 cm of bamboo rod) were positioned in the sediment. After 9 days, the bamboo rods with model substrates were removed and the dry weight loss of water hyacinth leaves and the cover of the yellow pigmented cellulose decomposers on the filter paper were measured. The decomposition rate in the water column increased downwards from the 110 cm water depth to the bottom water layer (Table 8). The dry weight loss in the 110 cm layer was about 49 percent, while in the bottom water the decomposition of the 4 water hyacinth leaves was almost complete reaching the weight loss value of 99 percent. At the sediment water interface the decomposition rate was still high, about 74 percent, but after that in the sediment layers there was a pronounced decrease in the decomposition. In the deepest sediment layer, about 80 cm, the dry weight loss of the 4 water hyacinth leaves was only 36 percent. The decomposition of pure cellulose in the form of filter paper has shown a similar vertical pattern but the absolute value for the decomposition of the long and resistant mollecules of cellulose was much less. In the deeper sediment layer there was no sign of decomposition. The cellulose filter paper remained practically intact after the 9-day incubation period. Presently, there are not sufficient data available to explain this particular vertical distribution of the decomposition rate in these ponds. The very high increase downward in the bottom water layer and the low decomposition rate in the sediment, however, have a direct relation to the very slow oxygen and nutrient transport mechanism within the water column and at the sediment water interface.
The different bacterial populations taking part in the carbon nitrogen phosphorus and sulphur cycles have been counted in pond 1. To count the total number of heterotrophic bacteria a nutrient agar medium is widely used. However, this medium contains a large amount of organic matter and inhibits the growth of most of the water bacteria living in the water with much less organic matter.
This inhibitory effect is more pronounced in the water of these small ponds characterized by high nutrient deficiency. To overcome this, a number of new media have been evaluated for counting the heterotrophic bacterial populations in the water and sediment of undrainable fish ponds. Comparison of these media with the nutrient medium used shows the advantages of the new media containing small amounts of organic nutrient (Table 9). Much higher numbers of bacteria were developed on these media than on nutrient agar. The largest number of bacteria developed on sodium caseinate agar; the bacterial colonies were small, easily distinguishable and countable compared to colonies grown on nutrient agar. This medium was later selected for counting the heterotrophic bacteria to replace the nutrient agar media. Even more bacteria were grown on pond water agar containing only nutrients present in pond water than on nutrient agar.
Among the bacterial populations decomposing the organic carbon the numbers of cellulose decomposers and methane producers were determined (Table 10). The number of cellulose decomposers was high at the sediment water interface and much lower in the water column. In the water column the yellow pigmented bacteria of the Cytophaga genus predominated while in the upper sediment layers the Actinomycetes were present in high numbers to the value of about 4 000/lg of wet sediment. The microaerophilic nature of Actinonomycetes explains their relative abundance in the uppermost sediment layer and their complete lack in the water column. No methane producers were found in the water column while their numbers reached the value of 33 000/lg of wet sediment. Their high value resulted in the large amount of gases (mostly methane) in the sediment of this pond.
Seven populations of bacteria taking part in the nitrogen cycle were counted (Table 11). The number of aerobic nitrogen fixers were low both in the water column and in the sediment. The anaerobic nitrogen fixers of the Clostridium genus were present in very high numbers in the upper sediment layer, surpassing the value of 300 000/lg of wet sediment. Their high number suggests the importance of the sediment nitrogen fixation in those ponds with an overall nitrogen deficiency. The number of ammonifyers was relatively high in the sediment which explains the large amount of ammonia absorbed by inorganic sediment particles or dissolved in the sediment water, and locked-in because of inadequate transport mechanisms at the sediment water interface. The number of urea decomposers having direct relevance to the urea fertilization was also high both in the water column and in the sediment. The numbers of both groups of nitrifyer bacteria were very low; this is also connected with the ammonia deficiency in the water column and the inadequate oxygen supply at the water sediment interface. The number of denitrifyers was small even in the sediment due to the slow nitrification process supplying them with the nitrate substrate.
The population of bacteria dissolving the precipitated inorganic phosphate in the sediment was large, and those producing sulphide from sulphur containing protein was small (Table 12).
The vertical distribution in the sediment layers of the protein decomposers producing sulphide and the methane producing bacterial populations indicates the activity of the sediment down to the measured 25 cm depth (Table 13). The number of protein decomposers producing sulphide decreased only in the deepest sediment layer, while the number of methane producers remained high even in the deepest layer. This distribution patter corresponds well to the vertical distribution of the gas caverns and bubbles in the sediment layers.
The distribution and quantity of all the bacterial populations counted are closely connected with and governed by the general nutrient deficiency in the water column and nutrient accumulation and oxygen deficiency in the sediment.
