AME 15:277-284 (1998)  -  doi:10.3354/ame015277

ABSTRACT: Phytoplankton species composition, succession and water quality were monitored over 7 d in 6 penaeid prawn ponds. They were filled simultaneously with water; 3 were fertilized with urea and 3 with sodium nitrate to give equivalent nitrogen concentrations. Neither chlorophyll a concentration nor phytoplankton cell counts differed between treatments. Phytoplankton species diversity was low with only 2 phytoplankton species present, the diatom Nitzschia sp. and the dinoflagellate Prorocentrum minimum. Adjacent ponds had similar trends in cell numbers irrespective of nitrogen source. Initially Nitzschia sp. grew more rapidly, outcompeting P. minimum in all ponds. There were differences in the growth of Nitzschia sp. that appeared to be related to the position of the pond along the inlet channel. Urea and nitrate uptake was rapid, with concentrations decreasing from 34 μM nitrogen to below 1 μM within 3 d. Nitzschia sp. numbers decreased in 2 ponds after this time but in the other 4 continued to increase until Days 6 and 7. P. minimum numbers remained low for the first 4 to 5 d then increased in 5 of the 6 ponds. There were no significant differences in water temperature, oxygen, phosphate and ammonia concentrations between the 2 treatments. pH was significantly higher in the nitrate treatment than in the urea treatment, probably due to the dissolution products of these compounds in seawater. The results suggest that the differential response of phytoplankton species to nutrients, and interactions between species, may be more important than the form of nitrogen available in determining phytoplankton species composition and succession.

KEY WORDS: Urea · Nitrate · Nutrients · Penaeid · Shrimp · Prorocentrum · Nitzschia