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Ecosystem dynamics and nutrient inputs to the coastal water bodies are directly coupled with
seasonal fresh water plume formation at marine environment and affect wide range o f biogeochemical and physical processes o f coastal waters. Trincomalee Bay and adjacent coastal
waters are such a distinctive ecosystem which is frequently subjected to alter its biogeochemical processes and cycles by Mahaweli River water discharges with plenty of
agrochemicals and organic particulate matters. Present study was conducted in June 2010 to
December 2011 in order to monitor both phytoplankton and zooplankton variations with
relation to monsoonal driven physico-chemical parameters covering full monsoon cycle,
(North-East, South-West, First and Second inter monsoons). Sixteen (16) sampling sites
representing three distinctive regions (bay, mouth, and offshore) o f Trincomalee coastal waters
were subjected for sub surface, (lm depth) sampling (temperature, salinity, nutrients, dissolved
oxygen, total suspended solids and plankton), with total area o f450 km2.
According to the results, mean phytoplankton abundance varied between 2989-148670
individuals l_i during four monsoons while maximum and minimum numbers were recorded
during North East monsoon at Mahaweli River mouth and second inter monsoon at offshore.
The highest abundance (7.73* 105 individuals l'1) was observed in bay area while the lowest
abundance (492 individuals l*1) was recorded in offeshore during North East monsoon.
Phytoplankton population was dominated by diatoms (100%) in bay area during North East
monsoon while offshore waters invaded by dinoflagellates (40%) during same season.
Phytoplankton abundance was positively correlated with nutrients [Nitrate-N (r=0.51) NitriteN (r=0.34) and phosphate-P (r=0.40) and Silicate-Si (i=0.54)] and negatively correlated with
salinity (r=-0.86). There was no significant varaition between phytoplankton abundance and
other oceanographic parameters such as temperature, dissolved oxygen and total suspended
solids (P>0.05). Maximum zooplankton (34683 individuals m'3) was observed near Mahaweli
River discharge points during North East monsoon while minimum (327 individuals m 3)
recorded at offshore canyon area during 2nd inter monsoon. Here, Calanoids dominated the
zooplankton population while abundance was positively correlated with silicate (r=0.43),
phosphate (r=0.47) and phytoplankton abundance (r=0.59). Salinity impacted negatively (r=-
0.52) on zooplankton population while the highest number o f groups were found in Koddiyar
Bay which is subjected to change in water quality with Mahaweli River discharge. Thus,
phytoplankton and zooplankton were maximum at bay during North East monsoon and closely
correlated with maximum nutrient concentration carried by Mahaweli run off enriched with
eroded soil particles during North East monsoon.
