Collection of sediment and plant samples
In November 2012, the study site was uniformly divided into twenty sampling
points along the Island with a distance of about 200 m between two consecutive
sampling points (Fig. 1). At each sampling point, three random samples of surface
sediment were collected by a rectangular sampler (10 cm long10 cm
wide15 cm deep). To analyze the pollution history of heavy metals in the
mangrove, three core sediment samples were collected by a PVC core (10 cm in
diameter66 cm deep) in the unvegetated area at sampling points N5, N10 and
N15 to minimize the effect of root uptake (Fig. 1). According to the previous results
of 210Pb dating, the sedimentation rate in Nansha mangrove is approximately
2 cm yr1 (Chen and Luo, 1991). Thus, the sediment sample in each core was cut
into 33 layers from the top at 2 cm depth interval by a PVC knife to represent the
pollution history from 1979 to 2011.
Roots, stems and leaves of the dominant plants, namely S. apetala and
C. malaccensis, were also collected from sampling points N5, N10 and N15. Stems
and leaves of S. apetala were collected by cutting. Roots of S. apetala were collected
together with sediment using the PVC core, followed by washing away the sediment.
C. malaccensis was collected by carefully removing the whole individual from
sediment and each collected individual was then separated into roots and leaves.
At each sampling point, three individuals were collected for each plant species.