Samples were collected from 11 different locations along Zanzibar Island ( Figure 1 ). Sampling sites with a range of different characteristics were chosen to guarantee representativeness. The various socio-economic activities represented at the chosen sampling stations included ports, recreation, fishing, agriculture, tourism, shipyards, cargo shipping, and ferries. A summary of the activities for each sampling station is shown in Table 1 .

Total of 33 water samples were collected from 11 sampling sites of Zanzibar Island ( Figure 1 ). In the process Niskin water sampler was used to collect 1 L of water at least 0.5 m down the subsurface layer in all stations to avoid taking the contaminated micro layer and poured in sampling bottles pre-cleaned with acetone. The samples were then kept under 4^oC using ice box and brought to laboratory for diuron analysis within ten days from the sampling date.

Water samples were filtrated by vacuum filter with a glass fiber filter grade GF with nominal pore size of 0.7 µm and diameter of 25 mm. Diuron in water was extracted following the solid phase extraction (SPE) and high performance liquid chromatography (HPLC) method of Diuron in crude oil as recommended by the Ministry of Plantation Industries and commodities, Malaysia ( Muhamad et al., 2010 ).

Water samples were pre-concentrated in the solid phase extraction cartridges column (SUPELCLEAN^TM, ENVI^TM-18 Sigma Aldrich, Germany). Prior to the extraction of diuron the columns were conditioned with 10 mL of acetonitrile, followed by methanol and milli Q water respectively. 10 mL of 0.2 M EDTA was added to 1 L of water and then pH of water was adjusted to 3.5. Then 1 m L of 1 mg/L diuron D-6 (C₉H₄Cl₂D₆N₂O) was spiked as surrogate standard in order to monitor the recovery of Diuron. Water samples were automatically concentrated in the solid phase extraction column by using solid phase extraction controller with flow rate of 20 m L/min. Supelclean^TM ENVI^TM-18 cartridges were then dried under nitrogen gas for 5 min. Diuron was eluted from the column using 5 mL of acetonitrile. Finally, acetonitrile was evaporated to 0.2 mL with a stream of pure nitrogen gas.

Diuron was analyzed using High Performance Liquid Chromatography (Agilent 1260 HPLC, G1315D Diode Array Detector). The Supelcosil^TM LC-18 HPLC column (25 cm × 4.6 mm 5 µm) (Agilent, USA) was used. Separation was obtained using gradient elution at flow rate of 1mL/min with solvent A (80% milli-Q water) and solvent B (20% Acetonitrile). The determination wavelength was set at 254 nm and the column temperature was kept at 40˚C, the retention time was 3.12 min and injection volume was 1.0 µL. The detection limit was 0.86 ng/L. The calibration data ( Table 1 ) and the curve ( Figure 2 ) are given below. Recovered concentrations from injected standards were collected from the following formula;

Concentration (μg/L) = (Peak Area + y-intercept)/Slope

where y-intercept = 0.231 and slope = 67.4095 as indicated in calibration curve. Table 1. Calibration curve data for Diuron (ppm) analysis using HPLC.

For the purpose of quality control of the analytical procedure, 1 L of Milli-Q water was treated the same as the real water samples for each batch of analysis as the water sample procedural blank. The recovery tests were also conducted for the purpose of monitoring the efficiency of the methodology. 1 L of Milli-Q water was spiked with known concentrations of diuron D-6 to determine recovery. The sample was processed following same method as real water samples. The recoveries of diuron D-6 were 92.34% ± 4.94% in the spiked samples. The recoveries of Diuron D-6 are shown in Table 2 .

One-way repeated measures analysis of variance (ANOVA’s F test at 5% level of significance) and Tukey’s honest significance test (Tukey’s HSD) was used to test whether the mean Diuron concentration differs by the study sites and to compare spatial variation of the data.

Diuron was detected in seven (64.6%) sites among 11 sampled sites in Unguja Island with Bwawani1 (BWN1) recorded a maximum value of 1366 ng/L ( Table 3 ). The station is closer to the major dockyard in Zanzibar. It is also close to an overcrowded harbor in the archipelago. Furthermore, the station BWN1 is positioned in a bay area with little tides movement and water exchange, which may contribute to the elevation of Diuron. The lowest value of diuron among the detected sites was 260 ng/L at Bawe1 (BWE1). However, Diuron was below detection limit (BDL) in 4 (36.4%) sites, which are Kwale (KW stations), Chumbe (CHU stations), Mtoni (MTN stations), and Murongo (MRG stations) as seen in ( Table 3 ).

Chumbe (CHU) is a coral park and one among the Marine Protected Areas (MPAs) in Zanzibar. Although the concentration of Diuron in Chumbe (CHU) was Below Detection Limit (BDL), ecological monitoring needs to be consolidated over a longer period and widely in the region. Monitoring and assessment methodology needs to be standardized and harmonized in order to make the results comparable. Much as the regional collaboration is desirable, it is important to recognize that institutional collaboration has to start at the national level for successful management of the MPAs in the region ( UNEP-WCMC, 2023 ).

The detected values of diuron from each site are summarized in Figure 3 . Results from one-way repeated measures analysis of variance indicated no significance difference in Diuron concentration among measured concentrations at each site ( Figure 3 ). It is evident the distributions reflect anthropogenic activities as a possible source of diuron in the sampling areas ( Ali et al., 2021 ; Sheikh et al., 2016 ; Lam et al., 2005 ; Hall Jr. et al., 1999 ).

Note: BDL = Below Detection Limit; MPA = Marine Protected Areas.

The maximum permissible concentration of 430 ng/l for Diuron has been set by the appropriate Dutch authorities ( Lamoree et al., 2002 ). Diuron is no longer approved for use in the UK as an active ingredient in antifouling paints, on any size of vessel. In this study the highest Diuron concentration in average is 1321.67 ± 52.30 ng/L recorded at Bwawani (BWN) while the lowest is 263.33 ± 4.00 recorded at Bawe, BWE ( Table 4 ).

Spatial variations in diuron mean concentrations were noted at Harbour (HBR), Bwawani (BWN) and Kizimkazi (KZ) with little variation observed in Pange (PNG) ( Figure 4 ). However, all sites except Bawe (BWE) regardless of those below detected limits, have shown Diuron concentration above maximum permissible concentration (MPC) of 430 ng/L, as proposed by Dutch National Institute of Public Health and Environment ( Giacomazzi & Cochet, 2004 ) ( Figure 5 ). The average Diuron concentrations detected around Unguja Island were considerably different (p = 0.05; at 5% significance level) from the allowed Diuron value of 430 ng/L ( Figure 5 ).

Over the years, increasing pressure has been placed on the marine environment including Marine Protected Areas. The development in technology has brought about an increase in the range of use of marine resources and access to marine environments. For example, the development of large scale fishing industries, tourism, aquaculture and the development of new forms of drugs from marine biodiversity has led to exploitation of marine resources and destruction of the marine Environment ( National Research Council, 1994 ). It should be noted that Mnemba Island (MN site), which is Marine Conservation Area in Zanzibar, has recorded the diuron concentration of up to 625 ng/L ( Figure 5 ), which is above MPC. The main reason may be due to tourism activities taking place in this area. Although diuron concentrations were not record in several study sites, the results of this study indicating that diuron in Zanzibar is a little higher since the majority of the sites (54.6%) records the value above the maximum permissible concentration (MPC) of 430 ng/L. The ubiquitous presence of diuron is unsurprising since it appears one of the most commonly used antifouling paint booster biocides whereas also having wide spread non-antifouling uses. Predominantly, the use of diuron has been associated with weed control in non-agricultural applications. Several studies have investigated the impact of non-antifouling use of diuron release into the aquatic environment ( Albanis et al., 2002 ; De Almeida Azevedo et al., 2000 ; Field et al., 2003 ). It is suggested that the source of diuron in estuaries and coastal areas is supplemented by non-antifouling paint inputs since the estuaries are subject to relatively low volumes of shipping and yachting when compared to marinas and ports water. There is the possibility that some freshwater and seawater from ports and marinas near estuaries might receive considerable agricultural drainage, containing diuron [9].

Diuron residues showed significant differences between average concentrations of the study sites ( Table 5 ) with least of 0.000 for p ˂ 0.05 (ANOVA). It is evident that, distributions reflect anthropogenic activities as a possible source of diuron in the sampling areas ( Ali et al., 2021 ; Sheikh et al., 2016 ; Lam et al., 2005 ; Hall Jr. et al., 1999 ). Moreover, statistical analysis of the results between the clusters; Reef (CPN and MN); Islands (BWE and PNG); Harbor (HBR and BWN); and Coast (KZ) showed a significant difference (p ˂ 0.05) of the concentration of Diuron between the clusters ( Table 6 ). The findings showed that the distribution of Diuron was based on the characteristics of the relative site. The highest average concentration of 1164.17 ± 184.78 ng/L was found at the area of dockyards and harbor because the economic activities conducted like cargo shipping, passenger speed boat, fishing, tourism activities such as snorkeling and local cargo increase the level of diuron contamination ( Figure 6 ). Liu et al. (1999) reported that fishing boats require heavy applications of antifouling paint to prevent the growth of fouling organisms, due to the faster wear off of the antifouling chemicals through their heavy use. The lowest average concentration (449.33 ± 91.19 ng/L) was detected at the coastal area. This is probably due to the fact that the most economic activities in this area are agriculture, which have low possibility of producing high amount of contamination of diuron ( Figure 6 ).

The average concentration in coral reefs areas was 925.20 ± 276.29 ng/L ( Figure 6 ). The coral reefs were positioned at the intermediate contamination of Diuron. The elevated value of Diuron in Reef areas (CPN and MN) is probably due to high level of tourism activities taking place daily compared to other clusters such as coast and Island.

Note: *The mean difference is significant at the 0.05 level.

Note: *The mean difference is significant at the 0.05 level.

In addition to that, the statistical information between each category varied, where their mean variation was significant in some stations. For example, the average concentration of Diuron between Reef category and Small Island showed statistically there is no significant variation (p ˃ 0.05). On the other hand, the average concentration between Reef and Harbour, Reef and Coast, and between Coast and Island showed there were not significant. However, the average concentration between Island and Harbour and between Harbour and Coast statistically showed significant variation ( Table 6 ). Variation was probably due to difference of the activities and boat density ( Ali et al., 2021 ; Sheikh et al., 2016 ). The main sources of the biocides could come from increasing in activities in harbor/dockyard such as passengers, cargo and fishing as well as tourism boating activities during snorkeling and dolphin watching. It is important to continue monitoring of the biocides.

The maximum concentrations measured from different regions are much higher compared to the maximum concentration found around coastal areas of Unguja Island. For example concentration up to 3,050 ng/L was detected in Seto Inland Sea, Japan ( Okamura et al., 2003 ); 42,000 ng/L in Lagoon water Italy ( Gennaro et al., 1995 ); 6742 ng/L estuaries UK, ( Thomas et al., 2002 ); 2,000 ng/L in Mediterranean coast, Spain, ( Martinez et al., 2000 ). In contrast to the said examples, however, the findings of the current study demonstrate that the level of Diuron around coastal Unguja Island is relatively low. Different researches have verified numerous effects of diuron on corals, such as the loss of symbiotic algae in M. digitata and S. hystrix ( Jones, 2004 ), reduction the calcification rate of G. fascicularis ( Sheikh et al., 2009 ), tissue retraction of P. damicomis ( Negri et al., 2004 ), reduction of respiration of P. cylindrical ( Råberg et al., 2003 ), the detachment of soft tissue of juvenile of Acropora tenuis ( Watanabe et al., 2006 ) and the reduction of ¹⁴C incorporation in Madracis mirabilis ( Owen et al., 2003 ). Definitely, detection of diuron in marine environments causes a number of ecological effects.