Root fouling organisms of Rhizophora mucronata in Rekawa Lagoon, Sri Lanka

Abstract

Mangrove underwater roots provide habitats for a variety of root fouling organisms. Since there is a dearth of information locally on the fauna and flora growing on such habitats, a study was carried out to study the biodiversity of mangrove root fouling organisms at the Rekawa lagoon (05° 58’ N and 80° 50’ E). In this study, a mangrove prop root that is hanging freely into the water from each one of the 20 randomly selected Rhizophora mucronata trees was cut to collect the fouling organisms from four different zones, namely, the top most, upper middle, lower middle and near the bottom along the root. The number of solitary countable organisms were recorded per unit surface area (m-2) of each one of the four root zones. The abundance of the non-countable colony-forming organisms were determined using the % volume values following the points (volumetric) method. Abundance data were analyzed non-parametrically using the Kruskal Wallis test. Further, the diversity of organisms in each zone of the root were also calculated. Twenty seven different taxa of fouling organisms including countable taxa such as Isopods, Amphipods, Copepods, Polychaets, Bivalves, Heliozoans, Diatoms, Oscillatoria sp., Nematodes, Cryptophyta, Bacillariophyta, Phytoplankton, Foraminfera, Chlorellacea and colony forming non-countable taxa such as Rhodophyta, porifera, Hydra sp., Ascidiacea, Ulothrix sp., Ciliophora and Chlorophyta were recorded. The median abundance of countable organisms was significantly higher in the upper middle zone (2461 m-2) than that in the bottom zone (1057 m-2). Further, the median diversity of these fouling organisms in the upper middle zone was significantly higher (196 m-2) than that in the bottom layer, which showed the minimum diversity (70.83 m-2). However, the median % volume of non-countable organisms did not differ significantly between different root zones. Variation of the abundance and diversity of countable and non-countable fouling organisms in the Rekawa lagoon could be attributed to the prevailing environmental conditions.