Western Indian Ocean J O U R N A L O F Marine Science Volume 15 | Issue 2 | Jul – Dec 2016 | ISSN: 0856-860X Chief Editor José Paula Western Indian Ocean J O U R N A L O F Marine Science Chief Editor José Paula | Faculty of Sciences of University of Lisbon, Portugal Copy Editor Timothy Andrew Editorial Board Louis CELLIERS Blandina LUGENDO South Africa Tanzania Lena GIPPERTH Aviti MMOCHI Serge ANDREFOUËT Sweden Tanzania France Johan GROENEVELD Nyawira MUTHIGA Ranjeet BHAGOOLI South Africa Kenya Mauritius Issufo HALO Brent NEWMAN Salomão BANDEIRA South Africa/Mozambique South Africa Christina HICKS Jan ROBINSON Australia/UK Seycheles Johnson KITHEKA Sérgio ROSENDO Kenya Portugal Jared BOSIRE Kassim KULINDWA Melita SAMOILYS Kenya Tanzania Kenya Atanásio BRITO Thierry LAVITRA Max TROELL Mozambique Madagascar Sweden Mozambique Betsy Anne BEYMER-FARRIS USA/Norway Published biannually Aims and scope: The Western Indian Ocean Journal of Marine Science provides an avenue for the wide dissemination of high quality research generated in the Western Indian Ocean (WIO) region, in particular on the sustainable use of coastal and marine resources. 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ISSN 0856-860X Cover image: A seagrass meadow (Thalassia hemprichii) at Inhaca Island, Mozambique (© José Paula, 2017) Original Article WIO Journal of Marine Science 15 (2 ) 2016 9-17 9 Nutrient and Bacteria Concentrations in the Coastal Waters off Zanzibar Town Frank Nyanda 1, Regina P. Mtei 2, Salim M. Mohammed 1, Aviti J. Mmochi 1* 1 Institute of Marine Sciences, University of Dar es Salaam, P.O. Box 668, Mizingani Road, Zanzibar, Tanzania 2 Chemistry Department, University of Dar es Salaam, P.O. Box 35061, Dar es Salaam, Tanzania * Corresponding author: avitimmochi@gmail.com Abstract This study assessed the concentrations and distribution of nutrients (ammonium, nitrate, nitrite, soluble reactive phosphorous) and bacteria (total and faecal coliforms) in the waters off Zanzibar Town. The study covered both the SE and NE monsoon and the two transition periods for a total of one year. Nutrient concentrations near a sewage outlet in the Bwawani area and Port sites showed values that exceeded concentrations considered acceptable for healthy reef ecosystems. The mean concentrations of total coliforms were within the ranges reported previously in the area, but faecal coliform concentrations were comparatively higher. Stations close to a sewage outlet showed the highest mean concentrations of contaminants compared to those further from the pollution source. Stations located in the shallow waters off Bawe and Changuu Islets showed slightly higher mean values compared to those located in the deeper waters between Zanzibar Town, and Bawe and Changuu Islets, which showed lowest mean values. The study has shown that the surface coastal waters off Zanzibar Town are contaminated with nutrients and coliforms due to raw sewage effluents from the Zanzibar Municipality and the distribution of these contaminants are mainly determined by seasonal monsoon winds. Therefore, there is an urgent need to treat the waste before discharging it into the coastal waters. Keywords: Water quality, nutrients, coliforms, sewage, contaminants. Introduction in these waters to assist decision-makers in matters Zanzibar Town, in the Zanzibar Urban District, is the pertaining to coastal resource management and pol- most densely populated district in Tanzania, with a lution control. population of 223,033 and a density of 13,940 people/ km2 (URT, 2013). Unfortunately, the domestic sewage This study used nutrients and bacteria as indicators of infrastructure is poor, with only 25% of residences pollution in waters off Zanzibar Town, and included connected to the sewerage network. The remaining an assessment of the distribution and seasonal varia- residences depend on soak pits, pit latrines and septic tion in concentrations of these indicators. tanks, which discharge untreated sewage, storm water, solid waste and debris into marine waters off Zanzi- Methods bar Town (Baur, 1993; UNEP 1998). This has a direct Study Location and Climate impact on recreational activities, natural resources This study was carried out in waters off Zanzibar and fisheries in waters off Zanzibar Town. Few pol- Town, Unguja Island, Zanzibar, Tanzania, which lies lution studies have been carried out in these waters. off the east coast of Africa in the West Indian Ocean Bjork et al. (1995), Johnstone and Suleiman (1997), between 39º 05’ E and 39º 55’ E, and 4º 45’ S and 6º 30’ Mohammed (1997), and Moynihan et al. (2012) have S (Fig. 1). The climate is strongly influenced by mon- indicated that sewage pollution is a major problem soons. There are two peak rainfall seasons, the long in Zanzibar Town. Unfortunately, the concentrations rainfall season between March and May, and the short and distribution of contaminants in these waters is rainfall season between October and November. The not well understood. There was thus a need to study sampling stations were located between the Zanzibar the concentrations and distribution of contaminants Town coastline and Bawe and Changuu Islets. Stations 10 WIO Journal of Marine Science 15 (2 ) 2016 9-17 | F. Nyanda et al. Figure 1. Map showing the study area and sampling stations. 1 and 2 were located close to a sewage outlet (Fig. 2) Because Stations 1, 5 and 6 were located in very shal- and the Port respectively. The stations were selected low waters (< 1 m depth), water samples were collected with an aim of assessing concentrations of contami- only at 0.5 m depth. At each depth three replicate water nants near the discharge point. Stations 4, 5, 6 and 7 samples were collected. The samples were transported were located further offshore near Bawe and Changuu to the laboratory at the Institute of Marine Sciences, Islets, while Stations 3 and 8 were located between the where the analysis was done. islets and the main island. The stations thus form two transects, one from the coastline (source of contam- Ammonium, nitrate/nitrite and soluble reactive phos- ination) to Bawe Islet and the other from the coast- phorus (PO4-P) concentrations were analysed accord- line to Changuu Islet. Sampling was carried out for ing to methods described by Parsons et al. (1984). one year between 2012 and 2013 and was designed to cover all seasons, including the Southeast (SE) and Northeast (NE) monsoons and transition periods. Sampling and Analysis for Total and Faecal Coliforms Three replicate water samples were collected at a depth Sampling and Analysis for Nutrients of 0.5 m at all stations using 250 mL autoclaved sam- Water samples were collected using a 1.5 L Nansen pling bottles. After collection the samples were stored water sampler at depths of 0.5 m, 15 m and 20 m. in a sterilized cooler box filled with ice and transported 11 F. Nyanda et al. | WIO Journal of Marine Science 15 (2 ) 2016 9-17 Figure 2. Sewage outlet in Bwawani area, from which waste discharges into coastal waters. to the laboratory for analysis. Coliforms were analyzed different seasons were significantly different (ANOVA, within six hours of sample collection using the mem- p < 0.05). Ammonium and PO4-P concentrations were brane filtration technique (USEPA, 1986). higher during the SE monsoon (Fig. 3 and 5 respectively), while the nitrate/nitrite concentration was Statistical Analysis highest in the NE monsoon (Fig. 4). Data for nutrients were analyzed using one-way analysis of variance (ANOVA), while those for total and faecal coliforms were analyzed using Kruskal-Wallis Concentrations of Total and Faecal Coliforms in Surface Waters ANOVA. In all cases significant differences were deter- The mean concentrations of total (TC) and faecal mined at the 95% confidence level. coliforms (FC) in surface waters were significantly different between stations (Kruskal-Wallis ANOVA, p Results < 0.05) (Tables 1 and 2 respectively). The mean con- Concentration of Nutrients in the Water Column centrations of TC bacteria at Stations 1 and 2 near the The concentrations of nutrients in the water column sewage outlet in the Bwawani area were higher than are shown in Figures 3 - 5. The results are shown in at other Stations in the shallow waters off Bawe (Sta- spatial and temporal variations, including seasons and tions 4 and 5) and Changuu Islets (Stations 6 and 7), depth, with the exception of stations 2, 5 and 6 which and the intermediate waters between Zanzibar Town were located in shallow waters where only samples at and the Islets (Stations 3 and 8). In spite of being fur- 0.5 m depth could be collected. The highest concen- thest from the main sources of these bacteria, Stations trations were usually recorded at 0.5 m depth in all 4, 5, 6 and 7 showed higher mean concentrations than four seasons. Only ammonium showed significant dif- Stations 3 and 8 located between the islets and main ferences in mean concentrations of the vertical pro- island. The mean concentration of TC bacteria in sur- files (ANOVA, p < 0.05) (Fig. 3), while nitrate/nitrite face waters at Bawe Islet was slightly higher compared and SRP were not significant (ANOVA, p > 0.05) (Fig. 4 to Changuu Islet (Table 1). and 5 respectively). FC bacteria were detected in the Bwawani area, and Differences in mean concentrations of nutrients intermediate waters toward Bawe Islet (Table 2). The between stations were significantly different (ANOVA, highest mean concentration of FC bacteria in surface p < 0.05) (Figs. 3, 4 and 5). The highest mean concentra- waters was in the Bwawani area. At most stations, how- tions were usually for Stations 1 and 2 in the Bwawani ever, no FC bacteria were detected. area and the Port, followed by Stations 4 – 7 near the islets and then Stations 3 and 8 between the shore- Seasonally, both TC and FC bacteria showed higher line and islets. Mean concentrations of nutrients in mean concentrations during the peak SE monsoon 12 WIO Journal of Marine Science 15 (2 ) 2016 9-17 | F. Nyanda et al. SEASON: SE to NE Mean Mean±SE 8 6 4 2 0 SEASON: NE ( M) 8 Conc. NH4 + 6 SEASON: NE to SE 8 4 2 0 6 4 2 0 6 4 ST1 ST2 ST3 ST4 ST5 ST6 ST7 ST8 0 ST1 ST2 ST3 ST4 ST5 ST6 ST7 ST8 2 ST1 ST2 ST3 ST4 ST5 ST6 ST7 ST8 SEASON: SE 8 DEPTH(M): 0.5 DEPTH(M): 15 DEPTH(M): 20 Figure 3. Mean concentration of ammonium by depth and season. SEASON: SE to NE Mean Mean±SE 2.0 1.6 1.2 0.8 0.4 0.0 SEASON: NE 1.6 1.2 0.8 0.4 SEASON: NE to SE 0.0 2.0 1.6 1.2 0.8 0.4 0.0 2.0 1.6 1.2 0.8 DEPTH (M): 0.5 M DEPTH (M): 15 M Figure 4. Mean concentration of nitrate/nitrite by depth and season. DEPTH (M): 20 M ST8 ST7 ST6 ST5 ST4 ST3 ST2 ST1 ST8 ST7 ST6 ST5 ST4 ST3 ST2 ST1 ST8 ST7 ST6 ST5 ST4 ST3 0.0 ST2 0.4 ST1 SEASON: SE Conc. Nitrate/ Nitrite ( M) 2.0 13 F. Nyanda et al. | WIO Journal of Marine Science 15 (2 ) 2016 9-17 SEASON: SE to NE Mean Mean±SE 0.8 0.6 0.4 0.2 0.0 SEASON: NE Conc. PO4-P ( M) 0.6 SEASON: NE to SE 0.8 0.8 0.4 0.2 0.0 0.6 0.4 0.2 0.0 0.6 0.4 ST1 ST2 ST3 ST4 ST5 ST6 ST7 ST8 0.0 ST1 ST2 ST3 ST4 ST5 ST6 ST7 ST8 0.2 ST1 ST2 ST3 ST4 ST5 ST6 ST7 ST8 SEASON: SE 0.8 DEPTH(M): 0.5 DEPTH(M): 15 DEPTH(M): 20 Figure 5. Mean concentration of SRP (PO4-P) by depth and season. months and inter-monsoonal period between the NE probably a result of nutrient enrichment by sewage and SE monsoon seasons. At the peak of the SE mon- from Zanzibar Town, as sewage tends to disperse in soon, FC bacteria were detected only in the Bwawani surface waters after it has been discharged into the area (Station 1). During the peak of the NE monsoon receiving seawater body (Alloway and Ayres, 1997; and inter-monsoonal period between the SE and NE Chow et al., 2004). The lower concentrations at the monsoon seasons, FC bacteria were detected at the mid depth (15 m) suggest that there is poor vertical three stations closest to the coastline of the main island, water mixing, retaining the nutrients at the surface. although the concentrations were lower compared to The concentrations of nutrients in the water column the concentrations recorded at the peak of SE monsoon correspond with the results reported by Sierra et al. and inter-monsoonal period between the NE and SE (2007), who observed highest nutrient concentrations monsoon seasons (Table 2). The mean concentrations near the surface followed by bottom waters, and the of TC bacteria in surface waters were significantly dif- lowest concentration at mid depths. However, only ferent (Kruskal-Wallis, p < 0.05) in the different seasons. ammonium showed significant differences in mean concentrations in the vertical profiles (Fig. 3). This is Discussion in agreement with Mohammed’s (2000) results, which Concentration of Nutrients in the Water Column showed significant differences in ammonium concen- Nutrient concentrations in the waters off Zanzibar tration in the water column while those of nitrate/ Town in this study were higher than concentrations nitrite and PO4-P were not significant. reported previously (Mohammed, 2000). This could be attributed to an increase in the population of Zan- Nutrient concentrations at stations close to the sewage zibar Town, as population increase is linked directly outlet in the Bwawani area and the Port had predict- to an increase in waste water generation. As reported ably higher mean concentrations of nutrients which previously, the study area is mainly affected by sewage often exceeded the concentration of 1.0 µM that is pollution (Bjork et al., 1995; Mohammed and John- considered acceptable for healthy reef ecosystems stone, 1995; Johnstone and Suleiman, 1997; Moham- (Bell, 1992), compared to stations situated further med, 1997; Moynihan et al., 2012). The high nutri- from the sewage outlet. On the other hand, stations ent concentrations in surface waters in all seasons is located in shallow waters off Bawe and Changuu Islets 14 WIO Journal of Marine Science 15 (2 ) 2016 9-17 | F. Nyanda et al. Stations Table 1. Mean concentration (MPN/100 mL + SE) of TC in surface waters. Seasons SE to NE NE NE to SE SE ST1 4000 + 11 2800 + 12 10000 + 14 11270 + 16 ST2 1240 + 13 2400 + 13 4760 + 9 8230 + 11 ST3 500 + 6 640 + 10 302 + 7 3530 + 13 ST4 120 + 4 520 + 9 310 + 8 480 + 5 ST5 240 + 9 282 + 6 540 + 5 192 + 7 ST6 290 + 7 320 + 9 740 + 10 250 + 5 ST7 102 + 5 170 + 7 260 + 6 100 + 11 ST8 104 + 8 350 + 9 120 + 9 97 + 4 showed slightly higher concentrations than stations affects surface flows in the Zanzibar Channel and are between the these islets and the main island, suggest- controlled by prevailing monsoon winds throughout ing a source of nutrients from the islets. This might the year, while mean current at the bottom remains reflect inputs from tourist hotels on the islets. The low relatively constant. Therefore, the increased mean mean concentrations of nutrients at stations between concentrations of nutrients (particularly ammonium the main island and the islets probably reflect dilution and PO4-P) in the water column at the stations located of the nutrients from these source areas. further from nutrient sources during the peak of the NE monsoon suggests the contaminants were trapped There were significant seasonal differences in nutrient during the SE monsoon, but dispersed throughout the concentrations. This implies that seasonal monsoon study area during the NE monsoon. rainfall has a significant influence on the concentration of nutrients in the study area. The highest mean Concentrations of TC in Surface Waters concentrations of ammonium and PO4-P in the water Concentrations of TC measured in surface waters for column for all stations were observed during the SE this study are within the ranges reported by Lyimo to NE transition, followed by the SE monsoons, possi- (2007) at Ocean Road near Dar es Salaam City, and bly due to increased rainfall at these times. This could Mohammed (1997) in seawaters off Zanzibar Town. facilitate the entry of nutrients and bacteria into the The concentrations were, however, higher than those sea, while the south easterly winds trap the contam- reported by Abbu and Lyimo (2007) and Namkinga inants at the source (McClanahan, 1988). The highest et al. (2013) for Mtoni Kijichi and Mzinga Creek (Dar mean concentrations of nitrate/nitrite in the water es Salaam), Pangani estuary (Tanga), and the Ruvu column observed during the peak of the NE mon- estuary (Pwani), all located on the Tanzanian Main- soon are possibly associated with nitrogen fixation land coast. This implies that waters off Zanzibar Town processes. Several studies (Bryceson, 1977; McCla- receive higher bacteria loads from anthropogenic nahan, 1988; Lugomela et al., 2002) indicate higher sources than Mtoni Kijichi, Pangani estuary, Ruvu nitrate concentrations in the water column during estuary and Mzinga Creek areas. Stations located close the NE monsoon period linked to nitrogen fixation to the sewage outlet showed higher mean concentra- by the planktonic cyanobacteria Trichodesmium, which tions compared to stations further from the sewage appears in high abundance during this period in outlet (Table 1). The results correspond with the work the study area. During the peak SE monsoon, mean of Shiaris et al. (1987), who reported that sewage-borne concentrations of nutrients at stations near the main bacteria (and nutrients) decline significantly with dis- island (particularly ammonium and PO4-P) increase tance from sewage outfalls. The shallow waters off while during the NE monsoon they are flushed away. It Bawe and Changuu Islets showed slightly higher mean is evident that changes in monsoon winds in the study concentrations of TC compared to stations located area affects dispersion of nutrients in the waters off between the main island and islets. This is possibly Zanzibar Town. This is in agreement with the findings due to discharges from hotels on the islets. It has been of Nyandwi (2013), who stated that current direction shown that microbiological contamination of coastal 15 F. Nyanda et al. | WIO Journal of Marine Science 15 (2 ) 2016 9-17 Stations Table 2. Mean concentration (MPN/100 mL + SE) of FC in surface. Seasons SE to NE NE NE to SE SE ST1 1540 + 12 2350 + 10 5240 + 17 8050 + 21 ST2 320 + 9 1050 + 8 1020 + 13 0 ST3 80 + 3 150 + 5 0 0 ST4 0 0 0 0 ST5 0 0 0 0 ST6 0 0 0 0 ST7 0 0 0 0 ST8 0 0 0 0 mental factors and human activities (Hennani et al., Distribution of Nutrients and Bacteria in the Study Area 2012). Bawe Islet showed slightly higher mean values The distribution of nutrients and bacteria in waters off both of TC in surface waters compared to Changuu Zanzibar Town were mainly determined by monsoon Islet. This may be due to its closer proximity to the rainfall and wind variations. However, biogeochem- Maruhubi sludge dumping site, but could also be due ical processes and station location were also shown to the higher population of tourists on Changuu Islet. to influence distribution. The nutrient and bacteria The mean concentrations of TC at the Bwawani area concentrations were generally highest during the SE and the Port were higher than those recommended and NE monsoon periods. During peak SE monsoon, for marine recreation, shellfish harvesting and aqua- nutrients and bacteria accumulated near the Bwawani culture in other countries (WHO, 1999), but were area. FC was only detected at Bwawani area during this below the guidelines at other stations. period and the mean concentration was much higher waters is a result of the combined effects of environ- (8050 + 21 MPN/100 ml) compared to other seasons. Concentrations of FC in Surface Waters This implies that winds trap FC in the Bwawani and Concentrations of FC measured in surface waters Port areas. However, the situation was reversed during for this study were higher than mean concentrations peak NE monsoon when contaminants were flushed reported by Mohammed (1997) in the same area. out toward offshore waters. For example, during It is reasonable to assume the increase in FC con- this period station 3 and 8 located in deeper waters centrations is related to an increase of the residen- between Zanzibar Town and the Islets showed relative tial population on the study area. Unlike TC, FC was higher mean concentration values of contaminants only detected at three stations; probably because of compared to other seasons. This indicates that mon- dilution effects (Davenport et al., 1976; Ji, 2008). The soon winds under this period facilitate transport and decrease and absence of FC further from the origin is distribution of contaminants from point of discharge due to transformation processes such as dilution, dis- toward offshore waters. persion and decay that affect spatial distribution (Shiaris et al., 1987; Alloway and Ayres, 1997; Sierra et al., Conclusion and recommendations 2007). The FC results showed that water quality in the Coastal water quality off Zanzibar Town is affected by Bwawani area and the Port was poor, and unsuitable nutrients and coliforms derived from sewage generated for recreation and aquaculture (WHO, 1999). Unlike in Zanzibar Town. The distribution of contaminants in TC, the presence of FC bacteria in the surface waters waters off Zanzibar Town is determined by seasonal indicates faecal contamination and the presence monsoon variation, location of a particular area relative of enteric pathogens from warm-blooded animals, to anthropogenic sources of nutrients and bacteria, and including humans (Byamukama et al., 2000; Karaf- biogeochemical processes. There is an urgent need to istan and Arik-Colakoglu, 2005). 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