3.1. Data accessibility
After searching, 902 peer-reviewed articles focusing on the distribution and accumulation pattern of metals and metalloids; i.e. Arsenic (As), Cadmium (Cd), Chromium (Cr), Cobalt (Co), Copper (Cu), Mercury (Hg), Lead (Pb), Nickel (Ni), Selenium (Se) and Zinc (Zn) in water, sediment and fish muscle from 2005–2022, 15 articles (Fig. 2) were used for the analysis of the result. After screening all the reports in Ethiopian aquatic environment (Fig. 3), a majority (73.68%) of reports were from the Rift Valley Lakes (RVLs). For instance, from the RVLs, more (47.37%) was done in Lake Hawassa followed by Lake Ziway (26.32%).
3.2. Detection method of metals
Observing the collected data, different methods (instrumental analysis) were used for the detection of metals and metalloids in water, sediment and fish. The most used detection method (Fig. 4) for metals and metalloids analysis was Atomic Absorption Spectrophotometer (AAS = 66.7%).
2.1. Metals and metalloids in water, sediment and fish muscle
The concentration of 10 metals and metalloids (i.e. As, Cd, Cr, Co, Cu, Hg, Pb, Ni, Se and Zn) were collected from previously done literatures. An overview of the concentration of metals and metalloids in water (µg/L), sediment (mg/Kg dw) and fish muscle (mg/Kg dw) of Ethiopian surface waters are presented in Table A1, A2 and A3 (see in Annex I) respectively. Zn has the highest concentration (575.90 µg/L) of metals and metalloids in water which has shown a similar pattern as compared to previous studies (Nigussie et al., 2011; Gebrekidan et al., 2013; Amare et al., 2014; Kassaye et al., 2016; Abebe et al., 2022). The average concentration of Cu, Cd, Pb, Ni, Cr, Co, Hg, As and Se in water were 176.43 µg/L, 66.60 µg/L, 59.07 µg/L, 191.84 µg/L, 35.32 µg/L, 109.04 µg/L, 0.32 µg/L, 2.10 µg/L and 0.82 µg/L respectively. The concentration of metals and metalloids in dry season was higher than found in rainy season which may be affected by the adsorption of metals and metalloids on suspended matters in rainy season (Abebe et al., 2022). In terms of specific location, more data were collected from Rift Valley Lakes, Lake Hawassa, with highest concentration of Zn 1059.76 µg/L and was similar with other studies (Nigussie et al., 2011; Amare et al., 2014; Kassaye et al., 2016; Abebe et al., 2022). In sediment, the concentration of Cr (4529.21 mg/Kg dw) and Ni (4422.56 mg/Kg dw) was higher when compared with other metals and metalloids which is similar compared to Kassaye et al. (2016) in Lake Ziway and Hawassa. The concentration of Cu, Cd, Pb, Zn, Co, Hg, As and Se were 32.93 mg/Kg dw, 24.49 mg/Kg dw,16.10 mg/Kg dw,, 259.30 mg/Kg dw, 23.17 mg/Kg dw 8.15mg/Kg dw, 2.08 mg/Kg dw and 0.18 mg/Kg dw respectively. The highest concentration of Cr in sediment may be ingested by fish and later accumulated. Overall, the concentration of metals and metalloids in sediment was higher than in water.
Analysis of the results also revealed metals and metalloids levels in fish muscle (Table A3). For instance, in all fish species the concentration of Zn was highest which was 18.48mg/Kg dw, 15.40mg/Kg dw and 31.00 mg/Kg dw in O. niloticus, C. gariepinus and B. intermedius respectively. This shows a similar pattern with previous studies (Desta et al., 2006; Dsikowitzky et al., 2013; Abebe et al., 2022), which may due to the essentiality of Zn for physiological role and uptake from the aquatic environment by fish preferentially. In O. niloticus, Cd (0.72mg/Kg dw), Pb (0.50mg/Kg dw), Cr (0.71mg/Kg dw), Co (1.16mg/Kg dw), Ni (0.51mg/Kg dw), Cu(3.18mg/Kg dw), As (0.79mg/Kg dw), Se(14.47mg/Kg dw), and Hg (0.49mg/Kg dw) respectively. In C. gariepinus Cd (0.73mg/Kg dw), Pb (0.32mg/Kg dw), Cr (2.64mg/Kg dw), Co (1.10mg/Kg dw), Ni (0.73mg/Kg dw), Cu (2.31mg/Kg dw), As (0.22mg/Kg dw), Se (1.29mg/Kg dw), and Hg (0.29mg/Kg dw) respectively. In B. intermedius Cd (0.03mg/Kg dw), Pb (1.39mg/Kg dw), Cr (1.01mg/Kg dw), Co (0.31mg/Kg dw), Ni (0.08mg/Kg dw), Cu (0.86mg/Kg dw), As (0.48mg/Kg dw), Se (1.51mg/Kg dw), and Hg (0.28mg/Kg dw) respectively. Similar to previous studies (Desta et al., 2006; Dsikowitzky et al., 2013; Abebe et al., 2022), the finding of this study also reveal that the concentration of most metals and metalloids were relatively higher in carnivorous fish than in herbivorous fish.
3.3. Bioaccumulation-and Biota Sediment accumulation Factors
The average concentration of detected metals and metalloids in water (µg/L), sediment (mg/kg dw), fish muscle (mg/kg dw, and their BAF and BSAF are summarized in Table 1. For instance, the BAF of Hg in O. niloticus and B. intermedius and As in O. niloticus was greater than 1. The BSAF of Se in all fish species and As for O. niloticus and Co for B. intermedius was also greater than 1. Moreover, BSAF of As and Se for O. niloticus was greater than 2. In contrast, previous study have shown a higher BAF for Cd (Gebrekidan et al., 2013) and Cr and Hg (Abebe et al., 2022) and BSAF of all metals or metalloids was also less than 1 (Abebe et al., 2022). Overall, BAF and BSAF value greater than 1 indicates that metal or metalloid may be accumulated in fish muscle, while a value less than 1 indicates that a metal or metalloid is not accumulated in fish tissue directly from water and associated with sediment respectively (Salam et al., 2020). In addition, an organism tissue with BSAF > 2, 1 < BSAF < 2 and BSAF < 1 can be considered as a macro-concentrator, micro-concentrator and de-concentrator respectively (Dallinger, 1993). Therefore, the results of this study illustrated that O. niloticus is a bio-concentrator of As and Se. While, authors like Arnot and Gobas (2006) suggest that BAF values < 1000, 1000 < BAF < 5000 and > 5000 are also considered as having no probability of accumulation, bio-accumulative and extremely accumulative respectively.
Table 1
The average concentration of detected metals and metalloids in water (µg/L), sediment (mg/kg dw), fish tissue (mg/kg dw, (where; ON = O. niloticus, CG = C. gariepinus and BI = B. intermedius)), and respective BAF and BSAF values.
Variable
|
Detected metals and metalloids
|
Cu
|
Cd
|
Pb
|
Ni
|
Zn
|
Cr
|
Co
|
Hg
|
As
|
Se
|
Water
|
176.43
|
66.60
|
59.07
|
191.84
|
575.90
|
35.32
|
109.04
|
0.32
|
2.10
|
0.82
|
ON
|
0.72
|
0.50
|
0.71
|
1.16
|
0.51
|
3.18
|
18.48
|
0.79
|
14.47
|
0.49
|
BAF
|
0.004
|
0.008
|
0.012
|
0.006
|
0.001
|
0.090
|
0.169
|
2.450
|
6.901
|
0.60
|
CG
|
0.73
|
0.32
|
2.64
|
1.10
|
0.73
|
2.31
|
15.40
|
0.22
|
1.29
|
0.29
|
BAF
|
0.004
|
0.005
|
0.045
|
0.006
|
0.001
|
0.07
|
0.14
|
0.68
|
0.62
|
0.35
|
BI
|
0.03
|
1.39
|
1.01
|
0.31
|
0.08
|
0.86
|
31.00
|
0.48
|
1.51
|
0.28
|
BAF
|
0.0002
|
0.021
|
0.017
|
0.002
|
0.0001
|
0.02
|
0.28
|
1.49
|
0.72
|
0.34
|
Sediment
|
32.93
|
24.49
|
16.10
|
4422.56
|
259.30
|
4529.21
|
23.17
|
8.15
|
2.08
|
0.18
|
ON
|
0.72
|
0.50
|
0.71
|
1.16
|
0.51
|
3.18
|
18.48
|
0.79
|
14.47
|
0.49
|
BSAF
|
0.02
|
0.02
|
0.04
|
0.0003
|
0.002
|
0.001
|
0.80
|
0.10
|
6.97
|
2.82
|
CG
|
0.73
|
0.32
|
2.64
|
1.10
|
0.73
|
2.31
|
15.40
|
0.22
|
1.29
|
0.29
|
BSAF
|
0.02
|
0.01
|
0.16
|
0.0002
|
0.003
|
0.001
|
0.66
|
0.03
|
0.62
|
1.66
|
BI
|
0.03
|
1.39
|
1.01
|
0.31
|
0.08
|
0.86
|
31.00
|
0.48
|
1.51
|
0.28
|
BSAF
|
0.001
|
0.06
|
0.06
|
0.0001
|
0.0003
|
0.0002
|
1.34
|
0.06
|
0.73
|
1.60
|
3.4. Ecological risk assessment
Based on data analysis, in surface waters of Ethiopia, the dissolved concentration of Cu, Cd, Pb, and Ni were higher compared to the water quality standards of the Ethiopian Environmental protection Authority (EEPA, 2003) and the European Union water quality standard guidelines for inland surface freshwater bodies (OECD, 2007) (Table A1). Likewise, in sediment of surface waters (Table A2), Cu, Pb, Ni, Zn and Cr were above TEC values and Cd, Ni and Cr were above PEC levels when compared to the United States Consensus-Based Sediment Quality Guidelines (SQGs) for freshwater which represent Threshold/Tolerable Effect Concentration (TEC) and Probable Effect Concentration (PEC) values (MacDonald et al., 2000), and this was similar to a previous study (Abebe et al., 2022) in Lake Hawassa. This indicates that these detected metals in water and sediment may pose risk to aquatic organisms or benthic fauna.
3.5. Human health risk assessment
The concentration of all detected metals and metalloids in water, except Cd, Pb and Ni, were lower than the recommended values for water quality, for drinking water, by WHO (1987) which implies water, in terms of the detected metals, is not fully safe for drinking. Fish muscle is consumed by people in most countries (Ataro et al., 2005; Abebe et al., 2022). Analysis of human health risk (hazard quotient of metals) associated with metal or metalloid-contaminated fish consumption (Table A4), HQ for Cd and As for all edible fish species and Cr for C. gariepinus was > 1. This was unexpected because previous studies found, except Hg, metals and metalloids were not hazardous to human consumers (Asefa & Beranu, 2016; Dsikowitzky et al., 2013; Abebe et al., 2022). A HQ > 1 is considered to be hazardous and a HQ < 1 is non-hazardous (Lemly, 1996; Onsanit et al., 2010; Pinzón-Bedoya et al., 2020). In case of the worst case scenario which is calculated using a maximum concentration of metals and metalloids in fish muscle, MEA of most metals and metalloids in all edible fish species was higher than the average daily national fish consumption of Ethiopians. This indicates that consumption of fish contaminated with metals or metalloids, except Cd, As, and Cr, may not pose adverse health effects for consumers. However, local consumers who live close to each freshwater ecosystem may be more exposed to health hazards.