Sediment Raking as a Tool for Enhancement of Phosphate and Productivity of Water in Tropical Pond System

Changes of orthophosphate in water, induced by raking of surface sediment in various ways, were compared. Twelve limnocorrals were installed in a shallow mesotrophic pond using four treatments with triplicate replication:(1) raking of surface sediment (R) by means of a rake for 5min per week (2) fish (F), by introducing six common carp and two silver carp per limnocorral, (3) raking plus fish (R+F) and (4) control (C), without raking and fishless. After day 122, treatments were reversed in limnocorrals byremoving (netting) or introducing fish and stopping or initiating raking. Samples of water and sediment were collected from each limnocorral at weekly interval and monitored for orthophosphate (OP), primary productivity, phosphate solubilizing bacteria (PSB) and sediment available phosphorus. Raking in presence of fish caused 280 and 160% increase in OP concentrations in surface and bottom water, respectively, as compared with raking in absence of fish. Net effects of fish and raking were 13-81 and 26-241%, respectively in surface water and 48-100 and 46-95% in bottom water. This implied that in bottom water, net raking effect was 13-161% greater than net fish effect. Increase in PSB population, due to raking plusfish (R+F) was 6-82% higher than increase by fish alone (F). Responses of available-P in sediment were opposite to that of water, being highest in concentration in control (C) followed by fish (F), raking (R) and raking plus fish (R+F). The primary productivity showed variations parallel with that of orthophosphate of water. This suggests that raking accelerated the transformations of available-P fromsediment to overlying water mediated through oxygen-dependent sediment water exchange mechanisms.