Inter-annual abundance variation in some genera of diatom and zooplankton in a mangrove ecosystem

Abstract

This study presents data for the abundance of phytoplankton and zooplankton and related habitat parameters for Lothian Island mangrove ecosystem located at the coastal boundary of the Ganges River delta. Over the 3 study years, total abundance of phytoplankton and zooplankton was lowest during the monsoon period. In 2008, the most common diatom genera (by % relative abundance) were Skeletonema cf. costatum (14.2), Thalassiothrix (9.36), Nitzschia sigma (8.16), Coscinodiscus radiatus (7.65), Chaetoceros (6.64), Pleurosigma (3.05), Thalassionema (1.77), Ditylum brightwellii (1.71), and Cyclotella (1.0), whereas in 2010 they were Skeletonema cf. costatum (41.7), Chaetoceros (11.1), Thalassiothrix (6.04), Nitzschia sigma (4.49), Coscinodiscus radiatus (3.96), Cyclotella (2.61), Thalassionema (2.11), Pleurosigma (1.22) and, Ditylum brightwellii (1.01). Relative abundance of the zooplankton size classes typically followed the order meso > micro > macro and Copepods were the most abundant taxa (54.6 % of total zooplankton abundance). Total abundance of both phyto and zooplankton almost doubled in 2010 relative to 2008 although the ratio of the two remained constant (Zooplankton: phytoplankton = 0.002). The N:P ratio of water did not change much over the study and did not seem to be responsible for phytoplankton abundance changes; rather interannual variations in phytoplankton abundance could be explained by a simple predator–prey-relationship with zooplankton.