Organic matter decomposition in simulated aquaculture ponds

Summary

Different kinds of organic and inorganic compounds (e.g. formulated food, manures, fertilizers) are added to aquaculture ponds to increase fish production. However, a large part of these inputs are not utilized by the fish and are decomposed inside the pond. The microbiological decomposition of the organic matter is a critical factor for water quality control and nutrient recycle. Usually, management practices are developed to control the survival and health of the cultured animals and to maintain good water quality. The microbiological processes are affected by these practices but usually unintentionally. A better control of culture conditions and sustainability of aquaculture ponds is possible with an improvement of the microbiological processes. The present thesis is divided in two parts, the first is a literature review of the microbial ecology of aquaculture ponds and the second is the description of a series of experiments in lab-sc ale aquaculture ponds.

In the first part, the role of the microorganisms in aquaculture ponds is reviewed, focusing on the decomposition of organic matter and its influence on pond dynamics. It was theoretically estimated that the addition of I kg of formulated feed would yield approximately 125 g bacterial biomass. This bacterial biomass is potentially a valuable nutrients source for higher trophic levels. Sedimentation and resuspension processes are important factors affecting the decomposition pathways. Both processes are directly related with the feeding rate and the stocking density applied. The rate of organic matter loading, environmental factors and pond management practices influence the functioning of algae-bacteria interactions, which are extremely important in pond processes. Included is a literature that describes commercial probiotic products that claim to solve: nutritious, water quality and pathogens problems in pond aquaculture, were analyzed. Alternative management practices to steer the decomposition process were also presented (Chapter 2).

The second part describes all the experiments that were conducted in lab scale microcosm systems, simulating the conditions of an intensive fish-less aquaculture pond with daily feeding rates. In Chapter 3 the influence of aerobic and anaerobic conditions and the organic C/N ratios on the decomposition process is described. Under aerobic decomposition less organic carbon remained in the systems. The results from this experiment suggest that a C/N ratio ranging between the tested values (6.3 and 12.8) does not have a significant influence on the carbon mineralization in the short term " 50 days). However, a C/N ratio decrease was observed in all the treatments during the experimental period; this reduction was especially fast and steep under aerobic conditions. This decrease in C/N ratio of the organic matter might explain why in all treatments the rate of decomposition slowed down at the end of the experiment. The C/N ratio also determined the concentration of inorganic nitrogen compounds in the water. Higher concentrations were found for the richest protein diet treatments. No nitrification was measured even though oxygen and ammonia were present.

Bacterial biomass production was quantified testing two formulated fish feed with different protein content under aerobic and anaerobic conditions (Chapter 4). The oxic status significantly influences the bacterial abundance, bacterial biomass, bacterial respiration and bacterial efficiency. More bacterial biomass was produced under aerobic conditions. The two diets did not influence significantly the bacterial growth. The bacterial abundance at the end of the experiment was 3.4 x 109 cells ml-1 in aerobic treatments and 1.9 x 109 cells m1-1 in anaerobic treatments. The remaining amount of carbon, fixed in bacterial biomass and expressed on a per area basis, was 19 g m-2 day-t for aerobic system and 8 g m-2 day-1 for anaerobic systems.

In Chapter 5 the effect of the oxic-anoxic range on fish feed decomposition was investigated. Different ranges, from completely aerobic to completely anaerobic, were tested. To establish intermediate oxic levels the following treatments were used: 1) alternated flows of 02 or N2 at different periods and 2) maintaining the coexistence of aerobic and anaerobic layers while applying short resuspension events. Similar amounts of carbon were converted to CO2 under completely aerobic conditions and under the different ranges of aerobic-anaerobic conditions. Under anaerobic conditions much less carbon was converted into CO2. This means that actually only limited periods of oxic conditions (or resuspension) are needed to stimulate complete organic matter decomposition. From our results it appears that only 6h per day of aerobic conditions or only once mixing of aerobic and anaerobic layers (i.e. resuspension) per four days are needed to reach the same carbon mineralization as in continuous aerobic conditions. Very limited nitrification was observed in the completely aerobic treatment. Nitrification and denitrification were registered for all the systems when aerobic and anaerobic conditions coexisted in time or space. The highest nitrogen removal (around 70%) was found in the resuspension treatments (and 12 h O2 flow treatment).

The use of controlled lab scale microcosm simulating intensive aquaculture ponds allowed us to follow the fate of carbon and nitrogen during particular decomposition processes. The results found in the different chapters are discussed in Chapter 6. Both the quality and the quantity of the organic matter influenced the decomposition process and its products. The use of high protein diets increased the concentration of nitrogen species affecting the water quality. The aerobic and anaerobic conditions determined the nutrients pathway (mineralized, assimilated or partially decomposed). More bacterial biomass was produced under aerobic conditions than under anaerobic. The coexistence of aerobic and anaerobic conditions stimulated organic matter decomposition; it avoided the accumulation of ammonia while maintaining good water quality conditions.

A better understanding and control of the organic matter decomposition in aquaculture ponds is crucial. The anaerobic decomposition only becomes a problem when it predominates in the sediment, causing the aerobic-anaerobic interface to move up into the water column, and thus remains disconnected from the aerobic decomposition. Management practices that link aerobic and anaerobic processes can stimulate fish production by recycling carbon and nitrogen compounds. The recycling of surplus organic matter through bacterial processes, however, has a limit. Increasing fish pond productivity should come along with practices to stimulate the autotrophic and heterotrophic food webs, without exceeding the capacity of this aquatic system.