Abstract
Responses of phytoplankton community in subtropical surface waters by the addition of deep ocean water (DOW) were examined under natural sunlight. Two culture experiments were carried out in September and October, 1999; DOW collected from ca 800 m was mixed into the surface water at high concentrations (13 and 27% (v/v)) in 9 l polycarbonate bottles and incubated in an outdoor incubator (EXP 1), and DOW from ca 400 m was daily added to ca 800 l of surface water at low concentrations (0.6 and 1.2%) in polyethylene bags moored in a harbor (EXP 2). Both experiments showed that the DOW enrichment, equivalent to more than 0.15 μmol nitrate l-1 day-1, increased concentrations of chlorophyll a (Chl a) after a lag period of 5 to 7 days and reached maximum concentrations at 10 to 12 days after the start of the experiments. The maximum levels of Chl a were proportional to the total addition of nitrate; 1 mol nitrate = around 1.4 g Chl a. The specific rates of increase of Chl a concentration were also in proportion to the amount of added DOW. Low Chl a share percentage of>2 μm fraction, such as 15% of total Chl a, in the initial surface water was changed to more than 70%. These results indicated that the DOW enrichment effectively enhanced the growth of nano- and microphytoplankton. Within those phytoplankton, some diatom species, being extremely less abundant in the subtropical surface water, increased rapidly. Results indicate primary production even in subtropical ocean can be raised effectively to enhance effective phytoplankton community for fish production by the DOW enrichment. Since the enhancement of the phytoplankton growth by the DOW enrichment was observed after 5 to 7 days, DOW should be added to keep at least 0.1 μmol nitrate l-1 day-1 during the lag period for initialing the fertilization effects in the subtropical waters.
