Temporal and spatial distribution of trace metals in the Rufiji delta mangrove, Tanzania

Andrew Minu,Rikard Kalén,Mårten Dario,J Val Klump,Mario Bilosnic,John F Machiwa,Joyanto Routh

doi:10.1007/s10661-018-6707-2

Abstract

Spatial and temporal distribution of trace metals and their cycling is a key issue for understanding the ongoing biogeochemical processes in coastal environments. Sediment cores were collected from six different sampling locations from the Rufiji delta mangrove forests in southeastern coastal Tanzania that are perceived to be impacted by urban development and agricultural activities in the catchment, and pollution in upstream sections of the Rufiji River. The chronology and sediment accumulation rates at these sampling sites were derived based on the distribution of 210Pbexcess method. The trace metals (As, Cd, Cr, Cu, Ni, Pb, and Zn) were sequentially extracted as per the BCR method and analyzed. The results indicate that the mass accumulation rates range from 0.40 g cm−2 year−1 (cores NR3 and NR4) to 1.75 g cm−2 year−1 (core SR1). Trace metals in the cores are mainly associated with the residual phase and their abundances in sediments are ranked as Cr > Zn > Ni > Cu > Pb > Cd. The results imply that trace metals in the Rufiji delta mangroves are mainly of crustal origin, and they are less sensitive to weathering. Further, these metals are least available for uptake by plants and they pose limited threat to the biota.

Highlights

Mangrove forests located in the inter-tidal zones of tropical and sub-tropical regions are both ecologically as well as economically important coastal ecosystems
The measured 210Pb activity in the Rufiji delta mangrove sediments does not reach the background level at NR2, and the age could not be validated at this site
Sites cores from NR1 and SR1 sites have the lowest sedimentation rate amongst the six sampled stations because these sites are inundated by tides, and they receive considerable amount of runoff water resulting in erosion and loss of sediments

Summary

Introduction

Mangrove forests located in the inter-tidal zones of tropical and sub-tropical regions are both ecologically as well as economically important coastal ecosystems. Mangrove sediments are characterized by their high organic matter (OM) content, poor nutrient quality, high amount of sulfide, and low oxygen levels (Ranjan et al 2013) This makes mangrove sediments a sink (Aderinola et al 2009), or for that matter even a source for various metalbound particles that actively participate in the ongoing biogeochemical reactions (Marchand et al 2012). Metal concentrations in these sediments depend on sediment particle size and stability of sulfide-rich minerals under anoxic conditions. Depending on the pore water conditions (Eh-pH, temperature, and salinity), these metals can transform their oxidation state, and concentrate in plant tissues resulting in long-term

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