1 Dinoflagellates in a mesotrophic, tropical environment influenced by monsoon

The changes in dinoflagellate community structure in both- the water column and sediment in a mesotrophic, tropical port environment were investigated in this study. Since the South West Monsoon (SWM) is the main source of climatic variation, observations were made during two consecutive post-monsoon periods (2001 and 2002) and an intervening pre-monsoon period (2002). The pre-monsoon period supported a more diverse dinoflagellate community in the water column compared to both post-monsoon periods. Heterotrophic dinoflagellates were abundant in the water column as well as sediment. A seasonal cycling between vegetative and resting cysts of autotrophic and heterotrophic dinoflagellates governed by the environmental characteristics of the study area was observed. Temperature, salinity and suspended particulate matter were the main factors affecting dinoflagellate community structure in both the water column and sediment. The dominant dinoflagellates in the water column differed during both post-monsoon periods that followed two dissimilar monsoon events. Prorocentroids and gonyaulacoids dominated the water column subsequent to the 2001 SWM, whereas dinophysoids and unidentified tiny dinoflagellates dominated during the next post-monsoon period. The 2001 SWM started in May, peaked during June-July and reduced gradually to end in October. The 2002 SWM was erratic; it started late (in June) and ended earlier (in September). These observations highlight the potential of the SWM to influence the community structure of dinoflagellates in tropical waters and points to the importance of long-term studies to discern robust variations in dinoflagellate communities in response to fluctuating monsoon regimes