Distribution of diatoms along the equatorial transect in the western and central Pacific during the 1999 La Niٌa conditions

Diatoms in the surface waters were sampled at 22 stations (7°N, 134°E–0°, 170°W) in the western and central equatorial Pacific during December 1998–January 1999 in order to determine the distribution, abundance and composition of taxa. In addition, two types of oceanographic condition and nutrient regimes were investigated. The observed diatom distribution is significantly constrained by the water masses: the Western Pacific Warm Pool (WPWP) located in the west and the EUR (Equatorial Upwelling Region) located in the east. Along a transect from west to east, total standing stocks slightly increased. While the standing stock of pennate diatoms was almost uniform throughout the sampled stations, that of centric diatoms increased in the EUR. In addition, a more diverse species assemblage was found in the EUR than in the WPWP. Nitzschia bicapitata, a pennate diatom, was the dominant species and occurred ubiquitously both in the WPWP and the EUR. Thalassionema nitzschioides, another pennate species, was also present, with high relative abundance throughout the sampled stations. The majority of centric taxa, such as the Thalassiosira complex, Chaetoceros atlanticus var. neapolitana and Rhizosolenia bergonii, occurred only in the EUR. Such a clear-cut distribution of the key taxa is attributed to changes in environmental conditions from west to east along the Equator: increase in salinity, nutrients, and decrease in temperature. Thus, the diatom assemblages in the western and central equatorial Pacific reflect the hydrographic and nutritional conditions of the surface waters.