Phytoplankton composition in a mangrove ecosystem at sandspit,Karachi,Pakistan

The phytoplankton occurring in the mangrove habitat of sandspit,karachi was assessed qualitatively and quantitatively during a period from june 2013 to may 2014. species of diatoms appeared to be more diverse and dominant forms. in contrast dinoflagellates exhibited very low species diversity and were observed in samples collected from june,august and september. phytoplankton cell density varied seasonally and their abundance ranged from 0.25 x106 -7.044x106 and 0.042x106 -5.172x106 cells l‾1 during high and low tide,respectively. pennate diatoms were highly diversified when compare to centric diatoms (3 centric and 23 pennate). centric diatoms were dominated by cyclotella cf. meneghiniana,whereas cyilndrotheca closterium was the most abundant species in the pennate group followed,by navicula sp.,nitzschia sp.,pleurosigma sp. and gyrosigma sp. respectively. the dinoflagellates were represented by very few species such as alexandrium sp.,prorocentrum micans and gyrodinium spirale. seasonal variations in hydrographical parameters were also recorded and the data ranges for salinity (33 -42 psu),temperature (16 -32˚c),ph (7.0 -7.7),dissolved oxygen (0.08 -6.18mg/l),total suspended solids (0.97 -13.8 mg/l) and chlorophyll a (0.0006-0.431 µg/l). during the period of this investigation sandspit backwaters received a load of pollution from the layari river and therefore the resistant diatom species dominated and dinoflagellates occurred in very low abundance and diversity. implication of pollution and changing hydrographical parameters in the coastal waters of karachi with phytoplankton structure need regular monitoring and research. © 2017,pakistan botanical society. all rights reserved.