Adding nitrate and phosphate separately or together in the Central Indian Ocean: a nutrient enrichment experiment

Abstract

Nutrient enrichment experiments were carried out in the Central Indian Ocean during the Chinese First Around-the world Research Cruise, adding nitrate, phosphate, or a mixture of both of them to surface seawater. The concentration of nitrate, nitrite, ammonia, and phosphate were analyzed spectrophotometrically, the chlorophyll-a con-5 centration with fluorescence analysis, and the temperature variation during the experiment recorded. Addition of nitrate resulted in rapid growth of phytoplankton concomi-tant with depletion of nitrate in the water samples. No apparent variation occurred in chlorophyll-a concentration when phosphate was added. Combining nitrate and phosphate proved to be best to promote phytoplankton bloom, and nitrate was depleted 10 prior to phosphate. After nitrate was consumed, a substantial amount of phytoplank-ton survived on the supplied phosphate. No correlation was found between the nitrate to phosphate ratio and chlorophyll-a or phytoplankton growth rate. We also found no correlation between water temperature and chlorophyll-a or phytoplankton growth rate. We conclude that neither nitrate to phosphate ratio nor water temperature control the 15 growth of phytoplankton.