Spatial Distribution and Accumulation of Heavy Metals and Nutrients in Mangrove Sediments, Leaves, and Macro-invertebrates

Description

This dataset examines the spatial distribution and bioaccumulation of heavy metals and nutrients in mangrove ecosystems along the Dar es Salaam coastline, Tanzania, under the hypothesis that locations nearer to the city exhibit elevated concentrations of heavy metals and nutrients compared to those situated farther away, attributable to urban wastewater, industrial effluents, and stormwater runoff. Data were gathered from three mangrove locations representing varying proximities to the city: Mtoni (adjacent to the city), Kunduchi (moderately distant from the city), and Ras Dege (remote from the city). The data encompass concentrations of six heavy metals (Cu, Zn, Pb, Cd, Cr, Ni) in surface sediments (0–30 cm), mangrove leaves of Avicennia marina, and the fiddler crab Uca inversa. Sediments were air-dried, homogenised, and subjected to acid digestion; leaves and crabs were manually collected, oven-dried, crushed, and digested; heavy metals were assessed using High-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS, Thermo Finnigan Element II), while nitrogen and phosphorus were quantified via DR 4000 spectrophotometer. Bioaccumulation factors (BAFs) were determined as the ratio of metal concentration in organisms to that in sediments. Results indicate that sediments serve as significant reservoirs for Zn, Pb, and Cr (most pronounced at Mtoni); crabs exhibit efficient bioaccumulation of Cu and Cd, but mangrove leaves demonstrate minimal metal transfer (BAFs < 1). Nutrient enrichment (N and P) is most pronounced at Mtoni and has a positive correlation with Zn, Pb, and Cu, suggesting shared anthropogenic causes. This data establishes a foundation for tracking heavy metal and nutrient contamination, endorses the utilisation of U. inversa and A. marina as bioindicators, and guides coastal pollution management and mangrove conservation initiatives.

Steps to reproduce

Field data collection took place from June 2024, immediately after the long rainy season (March–May). This period was chosen because heavy rainfall increases surface runoff, transporting pollutants into the mangrove ecosystems. Sediment samples for heavy metal analysis were collected from 2 m × 2 m plots using a plastic spoon at a depth of 0–30 cm, and were carefully transferred and covered with plastic scrapers. During the leaf sampling process, one mature Avicennia marina tree was selected from each plot. Avicennia marina is recognized for its exceptional resilience to harsh environmental circumstances, encompassing variations in temperature and salinity. This species has shown a significant ability to store heavy metals in its tissues, especially in the roots, as well as in the leaves and bark, without displaying rapid or severe toxicity signs. In each 2m x 2m plot, Crabs (Uca inversa) species were also collected, washed with distilled water to remove any adhering sediment particles, and stored in clean polyethylene bags and stored in an icebox. Crabs (Uca inversa) were selected as the representative macroinvertebrate species for heavy metal analysis. This species was chosen for this study due to the recognition of fiddler crabs in the genus Uca as primary consumers of microalgae and bacteria within mangrove ecosystems. Consequently, they serve as a crucial taxon for elucidating the effects of sewage exposure on mangrove benthic community’s Numerous infauna species, including crabs and polychaetes, are essential for sustaining healthy mangrove ecosystems by facilitating mineralization and nutrient cycling through their burrowing and irrigation behaviours. The vast majority of the nutrient pool of mangrove forests is stored in the soil and not in the trees. In each 2m x 2m plots, 2 cm upper layer of sediment samples was collected in pre-sterile glass jar by using a spoon and immediately placed in an ice box for selected nutrients analysis. Laboratory analysis :Metal analysis was carried out using a high-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS, Thermo Finnigan Element II). Samples of sediment, leaves, and macroinvertebrates were diluted tenfold before ICP-MS analysis. Metal standard solutions were prepared by serial dilution of stock standard solutions: ICM 224 (radion), SM 70 (radion), and XIII (merck). All plant samples were rinsed with pure water to eliminate the residues of seawater and soil attached. Crabs, leaves, and aliquots of the sediment samples were air-dried at room temperature. The leaves were milled with the addition of liquid nitrogen. Aliquots of 1 g were dried in an oven at 105°C according to DIN EN 212 (2003) until a constant mass was reached. The dry mass was used as the basis for the calculations of the concentrations of elements. Six heavy metals Cu, Zn, Pb, Ni, Cr, and Cd were analysed in each leaf, sediment, and macroinvertebrate sample.

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