Volume No. 6 Issue No.: 1 Page No.: 12-25 July-Sept 2011

 

SEDIMENT CHARACTERISTICS AND MEIOFAUNA ABUNDANCE IN THREE MANGROVE STANDS DOMINATED BY Rhizophora Mucronata IN MAURITIUS

 

Reshma Sunkur* and Chandani Appadoo

Department of Biosciences, Faculty of Science, University of Mauritius, Reduit (Mauritius)

 

Received on : April 15, 2011

 

ABSTRACT

 

Mangrove sediment has been found to be a key factor in the determination of the mangrove tree distribution and growth throughout the world. The present study is a first concerning meiofauna organisms in Mauritius and the results can form baseline for future researches. The study which was carried out during the months of October 2009 to March 2010, aimed at assessing the differences in the sediment types and the link to the mangrove forest structure as well as the diversity and abundance of the meiofauna in relation to the sediment grain size. Three study sites were chosen: Case Noyale (found in the Western part of Mauritius) which is subdivided into two forests, the first one with average tree height of 300-325 cm, the second one with average tree height of 315-615 cm and Maconde (found in the South Western part of Mauritius) with average tree height of 440-925 cm. Sediment samples were taken randomly in triplicates using a plastic corer 30cm long and 5cm wide; for meiofauna collection, a plastic corer of 15cm long and 3.6cm wide was used. All mangrove trees were surveyed for density per 25m2, height and Diameter Breast Height (DBH) in cm. The sediment samples were allowed to air dry and oven dry to constant mass prior to granulometric analysis; meiofauna sediment was immediately fixed in 5% formalin after coring. Meiofauna was collected on a 42μm sieve, centrifuged with magnesium sulphate thrice and counted under a dissecting microscope after staining with Rose Bengal. Maconde showed the highest tree density at a mean of 39 ± 4.3 trees per 25m2 followed by 37 ± 6.9 trees per 25m2 and 26 ± 3.9 trees per 25m2 in the taller and shorter forest of Case Noyale respectively. The general trend of sediment deposition to a depth of 15cm was coarse grains (8.00mm-1.4mm) in the top 5cm, middle size particles (1mm-125μm) in the middle 10cm and silt and clay material (> 53μm) in the last 15cm. Each of the three sites had a high percentage of one particular sediment type: the shorter forest at Case Noyale was dominated by gravel, the taller forest was characterised by fine sand particles while silt and clay was the main component of the sediment load at Maconde. However, one way ANOVA test revealed no link between the particle grain size and the growth of mangrove trees at the three locations. It was seen that copepods and nematodes were the dominant meiofauna organisms at the three sites though their relative abundance differed: the highest amount was in the seaward region of Maconde at 202 ± 32.3 organisms per corer/cm2, followed by 84 ± 22.9 organisms per corer/cm2 in the shorter forest of Case Noyale and the least in the taller forest of Case Noyale at 75 ± 7.2 organisms corer/cm2. The low abundance of the meiofauna as compared to other studies done on the meiofauna in other tropical areas such as Kenya and Brazil has been attributed to the heterogeneity of sampling points caused by the patchiness of the meiofauna in the diverse habitats of the mangrove environment. Harpacticoid copepods were found to be the dominant organisms at the three sites which also had the highest load of coarse grains in the upper 5cm of the mangrove sediments. Based on previous studies on the meiofauna, food availability and oxygen levels have been regarded as the main drivers of the existing differences in the meiofauna distribution and abundance.

 

Keywords : Mangroves, Rhizophora mucronata, Sediment, Granulometric analysis, Meiofauna

 

 

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