Abundance and distribution of nanoplankton in the epipelagic subtropical/tropical open Atlantic Ocean

ISSN： 1464-3774

Source： Journal of Plankton Research, Vol.25, Iss.12, 2003-12, pp. : 1535-1549

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Abstract

The objective of this research was to quantify the abundance and distribution of nanoplanktonic particles available to frequently occurring juvenile and adult planktonic copepods in the epipelagial of the open subtropical and tropical ocean. We chose locations off Puerto Rico and off the Bahamas to achieve our purpose. The hydrography for all stations revealed a thin upper mixed layer, a thermocline with varying temperature decreases, salinities approaching and surpassing 37‰, and chlorophyll concentrations increasing with depth from 0.05 to 0.25 g l⁻¹. Nanoplankton was grouped into five size ranges from 2–4 to 10–20 m equivalent spherical diameter (ESD), and four cell types (photosynthetic nanoflagellate, Pnano; heterotrophic nanoflagellate, Hnano; photosynthetic dinoflagellate, Pdino; heterotrophic dinoflagellate, Hdino). The 10–20 m range had the highest total cell volume, and the 8–10 m range the lowest values; for cell type, volumes of Pnano and Hdino were highest, and Hnano lowest. Total particle concentrations covering all depths and stations ranged from 3.4 to 17.5 g C 1⁻¹. At a large percentage of sampled depths total cell volumes of heterotrophs came close to those of autotrophs. Total autotroph volume was higher in the smaller size ranges than in the larger ones, while the opposite was found for heterotrophs. Calculations for ingestion rates of females of the oceanic calanoid Paracalanus aculeatus at environmental food levels and size distributions revealed that the 10–20 m ESD particle size fraction contributed between 46 and 82% of the total ingested nanoplanktonic cell C. At nearly all the observed abundances in our study this copepod would obtain sufficient amounts for basal metabolic needs, and at the majority of stations and depths have additional energy assimilated to cover enhanced metabolic demands and limited reproduction.