During darkness the metabolism of the phytoplankton changes from photosynthesis to respiration (the process by which organic molecules are oxidized to obtain energy); oxygen is consumed and carbon dioxide is released. The latter adds to the other respiration products (e.g. ammonia) of the crustaceans being farmed (and those of nearly all other non-plant organisms in the pond) with the result that oxygen concentrations and pH fall by night to reach a minimum around dawn. Unfortunately an excess of oxygen at one time of day does not compensate for a deficit at another, and both high and low concentrations can be harmful to crustaceans. In fact, abrupt changes in any water quality factor are likely to be stressful and will adversely affect growth and susceptibility to disease (Lee & Wickins, 1992). When circulation within a pond is poor, often as a result of calm weather, water can become strongly stratified with regard to temperature, pH and oxygen. In the absence of mixing, conditions deteriorate at the bottom of a pond as oxygen is consumed, pH declines and ammonia concentrations rise (Lee & Wickins, 1992).

Pattern of variation of pH, temperature and oxygen within a diurnal cycle in an outdoor pond

Within the natural food web the phytoplankton are the primary producers that provide food for organisms at higher levels in the food chain. Zooplankton, for example, will graze on phytoplankton, and plankton productivity as a whole will support a community of micro-organisms (bacteria, fungi, protozoa) and invertebrates (worms, molluscs, crustaceans) on the pond bed, principally by providing a rain of nutrient organic material (faecal pellets, dead organisms, exuviae). The monitoring and control of algae populations are critical aspects of pond management. If very dense phytoplankton populations develop they can rapidly exhaust supplies of inorganic nutrients and undergo a catastrophic mortality known as a 'crash'. The resulting mass of decomposing organic matter can consume much of the available oxygen and endanger the crop once more. Benthic algal mats and uncontrolled growth of macroalgae are also likely to jeopardise crustacean production (Lee & Wickins, 1992).

Although natural productivity alone, or enhanced with fertilizer, can be adequate for extensive cultures, semi-intensive and intensive farming operations require the addition of supplementary feeds to maintain rapid growth rates. However, in addition to boosting growth, the application of feeds has important implications for water quality and must be controlled if the dangers of anaerobic conditions and ammonia toxicity are to be avoided. Feed is not only consumed by the shrimp crop; uneaten and partially digested fragments are also consumed (decomposed) by bacteria, other micro-organisms and invertebrates on the pond bed. Although this community often provides food for the crop, a major part of its impact relates to its heavy demand for oxygen (Lee & Wickins, 1992).