Modern pollen and vegetation relationships in a mangrove tidal creek, South Andaman, Andaman & Nicobar Islands, India and their palaeoecological implications

Abstract

This paper presents the first modern pollen, non-pollen palynomorphs and grain size data from a pristine mangrove tidal creek on the South Andaman Island, Andaman and Nicobar Islands, India. The surface sediment samples were collected from different mangrove environments of the Wright Myo Creek, South Andaman to study the relationship between the pollen-spore data and present vegetation cover. Results show that pollen from local vegetation more abundant, whereas regional vegetation surrounding the depositional site is poorly represented. Heritiera littoralis, Ceriops tagal, Excoecaria agallocha and Bruguiera gymnorrhiza are markedly abundant in the landward zone. The transitional and seaward pollen assemblages are characterized by Rhizophora mucronata, Avicennia marina and Bruguiera gymnorrhiza pollen types. Pollen from mangroves are locally restricted and reflect the distributions of the source plants and mangrove types. However, the greatest concentrations of allochthonous taxa of evergreen, deciduous and montane components are derived from the surrounding lands. This area receives high rainfall during monsoon favouring long distribution of palynomorphs i.e. allochthonous species from hinterland to the depositional site through connecting drainage system. Abundance of non-pollen palynomorphs suggest not only humid as well as wet conditions, but also the particular salinity and pH characteristics of mangrove types in the area. Such relationships were evident in the Redundancy Analysis (RDA), which revealed that the first two axes of the RDA with self-values of 0.63 and 0.06 respectively explained 68.4% of data variation and 95.2% of the correlation between pollen and the environment. Additionally, the RDA analysis between Non-Pollen Palynomorph (NPP) data and the environmental variables showed that the first and second axes with self-values of 0.60 and 0.04 respectively, explained 68.4% and 96.1% of NPP data variance, respectively. The granulometric analysis clearly indicates that sand fraction is maximum towards sea, whereas suspended sediments (silt and clay) are significantly higher towards transitional and landward zones. This is well corroborated with pollen preservation in this area. At the mouth of the creek, strong hydrodynamic flow and low pollen percentages is observed, while abundant pollen are noticed in the suspended sediments probably due to weak hydrodynamic forces towards land. Therefore, the effects on pollen assemblages from sediment dynamic conditions must be taken into consideration during the reconstruction of palaeovegetation and palaeoclimate. The presented dataset provides new information on modern pollen assemblages and their relationship to the vegetation types and environmental conditions, which can also be effectively applied to other mangrove ecosystems of South and South East Asian regions to reconstruct palaeovegetation and palaeoclimatic changes.