NUTRIENT AND PHYTOPLANKTON DYNAMICS IN 2 MANGROVE TIDAL CREEKS OF THEINDUS RIVER DELTA, PAKISTAN

We report the results of a 3 yr study on 2 tidal creeks in the northwe st corner of the Indus River delta (Pakistan) mangrove ecosystem. We m easured Light (extinction coefficients), nutrients (nitrate, nitrite, phosphate, silicate and ammonium), chlorophyll a (chi a), suspended lo ad, primary productivity (C-14 uptake) and phytoplankton species in Is aro and Gharo Creeks. There was no clear seasonal cycle in any of the 5 nutrients in either of the 2 tidal creeks. NO3 ranged from 1 to 8 mu M, NH4 from 2 to 15 mu M, SiO4 from 3 to 20 mu M, and PO4 from 0.2 to 2 mu M (except when it was undetectable during a bloom in Isaro Creek in June). Hence, nitrogen and silicate do not Limit primary productiv ity, but during a large bloom phosphate may become limiting. When phos phate samples were reanalyzed without removing the sediment (no filtra tion), phosphate concentrations were about 25 % higher. This suggests that phosphate is adsorbed to sediment particles and could potentially be biologically available. There was a high suspended load (100 to 18 0 mg l(-1)) because the sediments were fine (silt and clay) and they w ere kept suspended by high tidal currents (0.5 to 1.5 m s(-1)) caused by tidal ranges of up to 4 m during spring tides in these shallow (3 t o 9 m deep) creeks. Hence, extinction coefficients were very high (1 t o 2 m(-1)) and the 1% light depth ranged from <1 m in August (the rain y season) to about 4 m in January and February. Consequently, primary productivity appeared to be light-limited throughout the year. Chi a r anged from 1 to 4 mu g l(-1) with occasional blooms up to 40 mu g l(-1 ). Primary productivity ranged from 0.2 to over 1 g C m(-2) d(-1) in I saro Creek. There was no apparent seasonal cycle in chi a or primary p roductivity. Phytoplankton species were predominantly large centric di atoms which were presumably kept in suspension by tidal currents. Sinc e nutrients are rarely limiting, there is an export of nutrients from the creeks to the coastal area which may stimulate phytoplankton produ ctivity.