Seasonality and vertical distribution of epiphytic diatoms on the pneumatophore surface of mangrove Avicennia officinalis L.

Abstract

Mangrove pneumatophores (respiratory roots) form a unique habitat about which very little ecological information is available. In this study, epiphytic diatoms on the pneumatophore surface of Avicennia officinalis (L.) were investigated with a focus on their seasonal and vertical distribution. The results indicated that among the seasons, highest diatom abundance was observed in monsoon. Vertically, maximum abundance was observed on 0-5 cm of the pneumatophore during post-monsoon and pre-monsoon and then gradually decreased upwards, but during monsoon, 5-10 cm section hosted maximum abundance. There was a seasonal and vertical variation on the diatom abundance and their interactive effect. This is because diatoms probably avoid the basal section (0-5 cm) of the pneumatophore during monsoon due to excessive sediment burial caused by intense riverine flux and settlement of suspended material. Overall it was found that vertically, diatoms (>70%) are mainly concentrated on the lower 0-10 cm of the pneumatophore, probably because this section remains moist during emersion period (low tide). Among the diatoms, Melosira nummuloides and Bacillaria paradoxa exhibited dominance during the monsoon, whereas Thalassiosira spp., Navicula spp., and Nitzschia spp.were prevalent during post- and pre-monsoon. To the best of our knowledge, this is the first study describing the composition of epiphytic diatoms from the pneumatophore surface of A. officinalis. Keeping in mind that these aerial roots together greatly enhance the surface area for attachment of epiphytic diatoms, their cumulative primary production would probably surpass that of mangroves on a quantitative basis and needs future investigation