A phosphorus budget for the eco-tourist resort of Chumbe Island Coral Park, Zanzibar

Abstract: In this case study, a nutrient budget was made to estimate import and export flows of phosphorus (P) on the ecotourism resort of Chumbe Island, Zanzibar. The P content in imported foods and goods was calculated using supply lists containing information on quantities of items bought and standard food composition data. Foods and goods not included in the standard data were subsampled for P analysis. Total P import to Chumbe Island was 41.0 kg/year. More P was imported with foods to the guest kitchen than to the staff kitchen (13.2 and 9.1 kg P/year, respectively) although the guest kitchen served fewer meals (9333 and 14113 meals, respectively). This is explained by the more P-rich foods served in the guest kitchen. When charcoal and firewood were included, the staff and guest kitchens imported equal amounts of P since the staff kitchen used both firewood and charcoal while the guest kitchen only used charcoal as a complement to gas. Ash and biodegradable waste such as fruit peel were composted on-site and used to facilitate the degradation process in composting toilets. Biodegradable waste from both kitchens contributed 4.8 kg P/year to the compost heap, while the ash from kitchen stoves contributed an estimated max. 17.4 kg P/year. Despite uncertainties regarding P amounts in the kitchen waste, estimations showed that P was lost from the compost heap during all stages of decomposition. Animals and birds scattering the compost during feeding and leaching during the rainy season are possible reasons for these losses. In the guest kitchen greywater system, sludge and grease were removed before the greywater was discharged into a small mangrove swamp, while staff kitchen greywater was poured untreated into a mud-hole in rocky ground. The guest kitchen contributed slightly more to P discharge (0.3 kg P/year) than the staff kitchen (0.2 kg P/year). The sludge (discharged at unspecified locations on Chumbe) contained another 0.1 kg P/year. The grease (added to compost) contained no P. Food scraps from both kitchens containing 2.4 kg P/year and composted toilet waste containing 21.6 kg P/year were re-exported to Zanzibar, i.e. approx. half the P imported was exported. The other half of imported P was lost from compost, greywater outlets, as sludge or dissipated through fly ash dispersal. To avoid P accumulation on Chumbe Island, I recommend that all toilet waste and ash be exported in future, since they contain nutrients useful on agricultural land on Zanzibar but harmful to the reef. Furthermore, Chumbe Island management should act to counteract current sources of P losses.