Size Distribution of phytoplankton in the central and eastern tropical Pacific

Size separations of phytoplankton biomass and production were performed with Nuclepore filters in the central and eastern tropical Pacific. The results show that there is a relationship between size and distance from land forms, with larger aggregates of phytoplankton dominating close to shore and small (less than 1 μm) forms dominating offshore. Surface nutrient concentrations were high in the offshore regions of equatorial upwelling and there was no apparent relationship between nutrient concentration and size. The phytoplankton biomass and production in the equatorial Pacific from 90°W to 150°W are dominated by small algae which are not retained by 5 μm Nuclepore filters with these organisms accounting for over 80% of the biomass and production. It is hypothesized that the dominance of small forms, as well as the relatively low plant biomass found, is the result of the absence of neritic bloom‐forming centric diatoms in offshore regions. The fact that small phytoplankton dominate in the equatorial Pacific suggests a dynamic food web. A dynamic food web coupled with the vigorous horizontal circulation of the equatorial Pacific supports the notion that some of the new nitrogen upwelled in this region is first exported laterally and then vertically.