Physical, chemical and biological characteristics of distinctive regions in paddlewheel aerated shrimp ponds

Abstract

Intensive shrimp ponds are commonly aerated by placing paddlewheel aerators parallel to the pond dikes. This placement creates a relatively rapid water current along the pond banks, while leaving a center region with much slower flow. Implications of this flow mode were studied in three 0.25-ha ponds. Measurements of water flow, sludge depth, dissolved oxygen and pH were taken throughout a pond, and contour graphs were created and verified by several transect measurements on different dates in the same pond and in two other ponds. The distribution of the measured parameters suggests these ponds have two primary regions: a relatively well-mixed and aerated outer region, and a virtually stagnant, vertically stratified inner region. During the early morning, the inner region had significantly lower DO than the outer region in all ponds and on all dates. In contrast, during the afternoon, inner surface water DO ranged from nearly twice to nearly half that of outer surface water. This wide range in the inner region was facilitated by the minimal effect of mechanical aeration in this region and the consequentially larger affect of variable photosynthesis and oxygen demand. The separation between inner and outer regions was very distinct, and characterized by a drop in water speed from about 10 cm/s to less than 2 cm/s within 5 m. Sludge depth and organic matter accumulation were higher in the inner region, facilitating the lowest DO and pH measurements in the bottom water of this region. Shrimp abundance, as indicated by trap catches, was significantly lower in the center region, suggesting conditions may have been less favorable for shrimp growth and health over as much as 30% of the pond area. The results obtained in this work raise the need for renewed thought, development and assessment of alternative aeration strategies in intensive shrimp ponds.