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
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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). This causes enteric
treat waste before discharging it into the coastal waters,
illness when water is used for recreational purposes or
to eliminate the negative impacts of contaminants in
through the consumption of contaminated seafood.
the waste. It is recommended that further studies on
16
the extent of degradation among the adjacent aquatic
communities (mangroves, sea grass, coral reefs, fish
etc.) due to raw sewage effluents discharged in the study
area be performed, and to isolate the pathogenic species in order to understand the associated risk.
Acknowledgements
F. Nyanda acknowledges Dominion Tanzania Ltd for
financial support and the Institute of Marine Sciences
of the University of Dar es Salaam for laboratory analyses and field assistance.
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