eighteen Nutrient cycles and fertilization

Static water aquaculture is the practice of applied limnology. The limnological principles of production and aquatic ecology are used to artificially augment natural production in the aquatic system of the fish pond through aquaculture management. The traditional management strategy involves the addition of organic or inorganic material, or a combination of the two, to increase productivity, which ultimately results in increased fish production. As long as photosynthesis is not carbon limited, as indicated by low levels of alkalinity, the addition of fertilizer nutrients will increase fish production until it is limited by low dissolved oxygen levels caused by excessive plankton respiration.

Aquatic food chains transform energy into fish biomass by two basic mechanisms. Autotrophic production is the direct proliferation of photosynthate by algae, Cyanobacteria, and photosynthetic protozoans. Heterotrophic production is the release of nutrients from organic matter through the microbial loop, accomplished primarily by bacteria. Both processes are integral to the function of aquatic systems. Autotrophic production is stimulated by the addition of inorganic fertilizers, which are manufactured products with high levels of soluble nitrogen (N) and phosphorus (P). Heterotrophic production is stimulated by the addition of organic matter, such as plant material or manure, produced directly or indirectly by photosynthesis that occurs outside of the system to which it is added.

Solar energy is transformed directly to photosynthate biomass, which may be consumed directly by fish or made into natural foods for fish by secondary production. The autotrophic energy pathway involves the production of photosynthetic microbes which are consumed by zooplankton and protozoa, which are in turn consumed by fish or invertebrates. The energy originally captured in photosynthesis is ultimately transferred to carnivores in the aquatic system.

The energy contained in organic fertilizers is released through microbial decomposition. The resulting microbial biomass and the particulate organic matter that they produce are consumed by protozoa, which are in turn consumed by zooplankton and ultimately turned into fish biomass.

As organic material is decomposed, small amounts of N and P are released that result in an autotrophic component to the production of organically fertilized fish ponds. Autotrophic and heterotrophic production both result in unconsumed biomass at all trophic levels, which is decomposed through the heterotrophic pathway, adding a heterotrophic component to the production of organically fertilized fish ponds.

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