ABSTRACT
In order to describe and evaluate the past and current status, and predict the future status of
mangrove ecosystem in relation to changing human activities in the coastal areas and its
environment in Tanzania, mangrove sediments from the Mtoni estuary were analysed for elemental
(C, N) and isotopic (δ13C, δ15N) signatures to establish the role of anthropogenic influence on the
contribution of organic carbon and nitrogen in the tropical mangrove sediments. Moreover, metals,
PCDD/Fs and dioxin-like PCBs were analysed to describe their current sources and fates in response
to anthropogenic and natural changes. Lastly, the study intended to determine and compare the
variability in the distribution of potential micro-pollutants in the mangrove sediments of Tanzanian
coast.
Sandy particles dominated the mangrove sediments, with less than 6% of organic matter (OM) in
the estuary and less than 9% in the tributaries. Similarities in the levels of total organic carbon (TOC)
and total nitrogen (TN) with depth were observed at all stations, indicating a homogenous, wellmixed top 9-cm sediment layer. A clear gradient of TOC and total nitrogen (TN) levels from the
riverine (Kizinga and Mzinga) stations to the estuarine mouth was observed with highest content in
most upstream stations and decreasing towards the mouth of the estuary. No clear seasonal trend
of C/N ratio was observed except in the confluence region where the ratios were higher in wet than
dry season. Wet season δ15N values were lower while δ13C values were higher compared to dry
season values for all the stations. Mzinga δ15N values were higher than Kizinga values in both
seasons while the opposite was observed for δ13C, but all were lower than the confluence values.
Correlations of the geochemical parameters for both wet and dry seasons suggest that OM in Mtoni
sediments originated mainly from sources that are 15N enriched and 13C depleted and are linked to
OM degradation processes. Combined elemental and isotopic signatures indicated that wet season
values were dominated by sewage material while dry season values were dominated by mangrove
material. Quantitative estimation of the contribution of the identified sources indicated that
sewage material contributed between 60% and 90% of the sedimentary OM in the wet season and
mangrove litter between 44% and 98% in the dry season.
Metal levels in the Mtoni sediment layers (0-3, 3-6, and 6-9 cm) showed neither distinct depth
gradient nor variation between wet and dry seasons. A clear gradient from the upstream of the
Kizinga River to the estuarine mouth was observed for all metals, except As. This was also observed
from upstream of the Mzinga River to the estuarine mouth. Pearson correlation matrix and Principal
component analysis (PCA) indicated that the selected compounds could be separated in 3 groups: (1)
Cu, Ni, Cr, Mn, Zn, Al, Fe, As, Pb; (2) Cd and TOC, which were strongly anti-correlated and (3) Sr.
Enrichment factors (EF) indicated that Sr was most probably derived from natural origin, while other
metals in the Mtoni estuary originated from anthropogenic activities. The levels of anthropogenic
metals are indicative of human induced environmental change and have repercussions on the future
status of the estuary and of the coastal area of Tanzania as a whole.
PCDD/Fs values as analysed by chemically activated luciferase gene expression (CALUX) method and
expressed as bioanalytical equivalency (BEQ) values ranged from 5.7 ± 1.4 to 39.9 ± 5.8 pg BEQ/g
sediment in wet season and from 14.1 ± 2.0 to 32.8 ± 4.7 pg BEQ/g sediment in the dry season. High
levels were observed in Kizinga River and stations close to the mouth of that river. Dioxin-like PCB
(dl-PCB) levels ranged from 0.21 ± 0.03 to 0.53 ± 0.03 pg BEQ/g sediment in wet season and from
0.22 ± 0.03 to 0.59 ± 0.04 pg BEQ/g sediment in the dry season. Higher PCDD/F and dl-PCB levels in
sediments are probably related to open burning of plastic scraps, household burning of wood or
charcoal and traffic related emissions, which all occur in the Dar es Salaam region. The denser
population and the more intense industrial activities in the Kizinga River Basin may explain the
enhanced PCDD/F and dl-PCB levels observed in the sediments of that River compared to those in
the Mzinga River Basin. Enhanced levels in the Kizinga River (up to 400 pg-BEQ/g) observed in the
third sampling campaign showed a clear decreasing concentration gradient in the downstream
direction. The presence of these pollutants in the sediments poses a threat to the biological
community living in the Mtoni estuary.