A survey was conducted on 15th November, 1970, in mangrove forests and backwater regions of a section of the Vellar-Coleroon estuarine complex; a total of 19 stations were occupied. Detailed investigations on nutrients, pigments, and plankton were carried out. The following ranges in values were recorded: salinity, 10.40 to 30.50‰; pH, 7.50 to 8.30; temperature, 29.50° to 30.50°C; total phosphorus, 0.72 to 3.34 μg at/l; inorganic phosphate, 0.19 to 1.59 μg at/l; ammonia, 0.34 to 0.36 μg at/l; nitrite, 0.11 to 0.25 μg at/l; nitrate, 2.85 to 6.94 μg at/l; silicate, 18.49 to 134.92 μg at/l. Dissolved oxygen content ranged from a minimum of 3.69 ml to a maximum of 5.44 ml/l. Chlorophyll a ranged from an undetectable amount to 1.01 mg/m3, chlorophyll b from 0.02 to 0.85 mg/m3, chlorophyll c from 0 to 0.41 mg/m3 and carotenoids from 0 to 0.74 MSPU/m3. The plankton displacement volume ranged from a negligible amount to 3.60 cm3/m3; seston varied between 0.29 and 0.91 g/l. Phytoplankton was abundant at 3 stations; at other stations zooplankton was abundant. Coscinodiscus, Asterionella and Ditylum were the dominant forms among the phytoplankton; Oithona, Acrocaanus, Euterpina, Centropages, Corycaeus, Lucifer and Oikocreura were dominant among the zooplankton. Phytoplankton and zooplankton populations, as percentage of the total plankton, varied between 3.70 and 89.00% and between 11 and 96.30%, respectively. Average gross production values in the mangrove and back-water stations were 7.56 and 3.33 g C/m3/day, and the net production values 6.29 and 2.67 g C/m3/day, respectively.