Abstract:
Puttalam Lagoon, the second largest (32,700 ha.) lagoon in Sri Lanka is located in the Puttalam
District of the Northwestern Province. The lagoon is fed by two rivers namely, Kala Oya and Mee
Oya. Phytoplankton is sensitive to the changes in water quality thus is effective indicators of
environmental conditions. The objective of this study was to determine the diversity and
composition of the phytoplankton in Puttalam lagoon. Plankton sampling was done once in July
2017 from fifteen sampling locations. Samples (N = 36 samples/lagoon) were drawn by filtering
50 L of water collected at a depth of one meter from the surface using a 20 µm nylon plankton net.
The filtrate was concentrated to 100 mL for each station. Three drops of Lugol’s solution were
added to each sample to preserve the phytoplankton and allowed the organisms to settle. Then
quantitative analysis was carried out through sub sampling technique using a Sedgwick rafter cell
under a compound microscope. Species were identified to the nearest taxonomic level following
the standards guides. Shannon diversity index of phytoplankton was calculated. A total of 39
phytoplankton taxa belonging to two phylum (Ochrophyta and Dinoflagellata), and three classes
(Bacillariophyceae, Coscinodiscophyceae and Dinophyceae) were identified. Bacillariophyceae
(diatom) is the most abundant class which included 22 species. This was followed by
Coscinodiscophyceae which included three species. Also, thirteen species were included in class
of Dinophyceae. The dominant toxic phytoplankton species were identified as Prorocentrum sp,
Photoperidinium sp, Ceratium furca, and Noctiluca scintillians. The highest abundance (27250 total
individuals/ml) of phytoplankton was observed at sampling location, KL 12 (Northern part of the
lagoon) and would be due to the less human disturbance. The lowest abundance (2467 total
individuals/ml) of phytoplankton was recorded at sampling location, KL 1 (Southern part of the
lagoon near the Puttalam town). This might be due to the high loads of pollutants discharge into
the lagoon from Puttalam town area. The Shannon diversity index of 1.52 indicates lower species
diversity in the lagoon.