Phytoplankton species assemblages and their relationship to hydrographic factors - A study at the old port in Mangalore, coastal Arabian Sea

Several environmental variables were monitored and water samples were examined from over a period of 17 days in December 2005 and January 2006 to understand the taxonomic structure of coastal phytoplankton communities at a coastal site near Mangalore. The phytoplankton community was characterized by identifying and quantifying microalgal taxa; in conjunction, also determined Chl a, inorganic nutrients, oxygen, salinity, temperature, total bacteria, and tidal range. Canonical correspondence analysis (CCA) was used to trace temporal phytoplankton community changes, and to examine the relationships between species composition and environmental factors. These variables were subjected to analysis, pair-wise, to identify correlations. The species diversity of the phytoplankton community changed rapidly, and high species diversity coincided with significantly higher relative diatom abundance, and lower species diversity coincided with significantly higher proportions of phytoplankton taxa other than diatoms and dinoflagellates. In addition, the Chl a concentration was positively correlated to the relative proportion of dinoflagellates. The relative abundance of some species was dependent on specific environmental variables, such as Chaetoceros tenuissimus that was found inversely correlated to salinity. It is also found that the stage of the tides may have implication on the sampling, because it has an influence on species diversity and the proportion of specific microalgal groups. At declined water level, the relative proportion of diatoms and dinoflagellates was low, and generally the Chl a concentrations were also lower during low tide.