Water Quality in the Integrated Mariculture Pond Systems (IMPS) at Makoba Bay, Zanzibar, Tanzania

Abstract

The Makoba integrated mariculture pond system project (IMPS) has been on-going since 1998. Except for a period from mid 1999 to 2001, various water quality parameters, namely temperature, salinity, pH, dissolved inorganic nutrients and dissolved oxygen concentration and saturation, have been monitored continuously. In 2002, measurement of sediment oxygen demand (SOD) and consumption of oxygen by sediments was initiated. Dissolved oxygen concentration varied from the highest monthly average of 7.16 mg/l in October 1998, to 2.2 mg/l in March 2000 (r = –0.69). Nutrient concentrations remained moreor- less constant, except in the finfish ponds, where slight variations were recorded. The lowest concentration of ammonium-nitrogen (3.02 mg-at N/l) was recorded in the Kiwani creek in December 2001, and the highest (18.02 mg-at N/l), in the finfish ponds in December 1998. In 2002 the concentrations became much lower and the overall regression from 1998 to 2002 was negative. Dissolved inorganic phosphate concentration in the finfish ponds increased from 4.36 to 7.97 mg-at P/l (r = 0.75) between August 1998 and April 1999. In 2002, however, the concentration was low, making the overall trend also negative. pH ranged from 7.07 in Kiwani creek to 8.54 in the outlet. There was no clear pattern in the pH trends at the individual stations. In 1998 and 1999 there was a general decrease in pH with a corresponding increase in nutrient concentrations and decrease in dissolved oxygen. The SOD values in the ponds and the outlet were higher than in the creek and the reservoir, and the half life of dissolved oxygen was lowest in the ponds compared to the rest of the sites. The nutrient concentrations at Makoba ponds are generally acceptable for the environment and fish farming. INTRODUCTION Aquaculture of animals in most of Africa is still small-scale and freshwater. Recently, however, the Institute of Marine Sciences (IMS) of the University of Dar es Salaam has received an increasing number of requests for advice on subsistence and commercial mariculture (Mmochi & Jiddawi, 1996). In 1986 some attempts at prawn farming were made in Bagamoyo in Tanzania but were stopped due to land controversies (Bryceson, 2002). What would have become the biggest prawn farm in the world (Boyd, 1996; Bryceson, 2002) was about to be started in Rufiji River delta, but joint opposition from interested parties caused the project to be halted in 1999/2000 (Bryceson, 2002). Environmental problems have resulted from the conversion of wetland habitats to aquaculture ponds. These include nutrient, sediment and organic waste accumulation leading to deterioration of water quality, one of the important factors that determine the viability of fish farming. The water quality problem is associated with both physical and chemical problems such as too high or too low dissolved oxygen (DO) concentrations,