Phytoplankton in an Oligotrophic Ocean: Species Structure and Interannual Variability

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Abstract
I have examined the interannual variability of the rank order of abundance of phytoplankton species in the North Pacific central environment. Discrete data were integrated within the shallow association (0 to °100 m) and the deep association (°100 to 200 m) of each of 10 stations occupied during six summers between 1973 and 1985 (12.2 yr). Two hundred and forty—five and 231 species were observed in the shallow and deep zones, respectively. In both zones, 21 species accounted for 90% of the individuals. Each zone contained a large number of rare species that, in spite of their low abundances, were frequently present. To reduce analytical bias, the evaluation of interannual variability was based on a reduced list of 194 species. The period between 1973 and 1985 represented @>1000 generation times. During this time there was a slow change in the structure of both phytoplankton assemblages. Change was greater among the species of the deeper association and particularly among the diatoms of the layer. The correlations between the rank orders of abundance never approached zero, even for samples separated by 12.2 yr. Samples from within in the same zone were more similar to each other than were deep and shallow samples collected at the same instant of time, even though the vertical separation of these latter samples was 20 m or less. This study supports earlier conclusions that the epiplanktonic populations of the North Pacific central environment are more stable than those of most other planktonic ecosystems. Most of the variance in abundance is concentrated in the smaller spatio—temporal scales. It is possible that whatever mechanisms moderate species coexistence are operating on these scales.