Nutrient and phytoplankton dynamics in two mangrove tidal creeks of the Indus River delta, Pakistan

We report the results of a 3 yr study on 2 tidal creeks in the northwest corner of the lndus River delta (Pakistan) mangrove ecosystem. We measured light (extinction coefficients), nutrients (nitrate, nitrite, phosphate, silicate and ammonium), chlorophyll a (chl a), suspended load, primary productiv~ty (I4C uptake] and phytoplankton species In Isaro 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 l to 8 PM, NH, from 2 to 15 pM, Si04 from 3 to 20 pM, and PO, from 0.2 to 2 pM (except when it was undetectable during a bloom in lsaro Creek in June). Hence, nitrogen and silicate do not limit primary productivity, but during a large bloom phosphate may become limiting When phosphate samples were reanalyzed without removing the sed~ment (no filtration), phosphate concentrations were about 25 D/o higher. This suggests that phosphate is adsorbed to sediment particles and could potentially be biologically available. There was a high suspended load (100 to 180 mg 1-') because the sediments were fine (silt and clay) and they were kept suspended by high tidal currents (0.5 to 1.5 m S-') caused by tidal ranges of up to 4 m during spring tldes in these shallow (3 to 9 m deep) creeks. Hence, extinction coefficients were very high (1 to 2 m-') and the 1% light depth ranged from < l m in August (the rainy season) to about 4 m in January and February. Consequently, primary productivity appeared to be light-limited throughout the year. Chl a ranged from 1 to 4 pg 1-' with occasional blooms up to 40 pg I' . Primary productivity ranged from 0.2 to over 1 g C mw2 d-' in lsaro Creek. There was no apparent seasonal cycle in chl a or primary productivity. Phytoplankton species were predominantly large centric diatoms which were presumably kept in suspension by tidal currents. Since nutrients are rarely limiting, there is an export of nutrients from the creeks to the coastal area which may stimulate phytoplankton productivity.