Macro algae as biofilters of effluents from integrated mariculture fishpond systems in Zanzibar, Tanzania

Growth rates and biofiltration capacity of Ulva reticulata, Eucheuma denticulatum and Gracilaria crassa were investigated in serially arranged finfish - shellfish and macroalgae systems under laboratory conditions and field conditions. In the laboratory, U. reticulata showed the highest biofiltration capacity. The daily growth rates of the three macroalgae grown in a media containing 68.81 ± 0.20 µg-at NL-1 ammonia-N and 39.72 ± 0.39 µg-at.PL-1 phosphate-P at 30.5°C were 5.78% for U. reticulata stocked at lkg m-2, 4.64% for E. denticulatum and 3.82% for G. crassa both stocked at 4 kg m -2. The biofiltration capacities were 56% ammonia-N and 41 % phosphate-P for U. reticulata; 51% ammonia-N and 34% phosphate-P for E. denticulatum; 49% ammonia-N and 30% phosphate-P for G. crassa. In the field, the macroalgae showed negative growth rates of 2.62% for U. reticulata, 3.21 % for E. denticulaum and 2.16 for G. crassa. These were possibly due to low levels of dissolved inorganic nutrients supply from the finfish ponds of 14.56 ± 0.59 ug-at.NL- 1, 5.46 ± 0.24 ug-at.P L-1 and 0.40 ± 0.01 pg-at.N03-NL-1; poor vertical visibility (< 0.45 m), high sedimentation rates (5.67 mg.m2. day 1), an insufficient water movement, epiphytes and ice-ice diseases mostly in E. denticulatum. Such conditions favored the growth of microphytobenthos particularly the genera Spirullina and Oscillatoria, which influenced nutrient movement towards the sediment at a flux of 2.19 pg-at.N.m -2h-1 ammonia-N and 0.49ug-at.P.m -2h-1 phosphate-P during light incubations. Thus contributing to nutrient decrease as you move from the finfish to the macroalgae ponds through the shellfish ponds. The findings will contribute to some of the basic management and development operations needed for mariculture activities to be environmentally acceptable through liquid waste management using biological methods