Ultraplankton distribution in surface waters of the Mozambique Channel – flow cytometry and satellite imagery

Abstract

The composition of ultraplankton (UP) in near-surface samples collected underway every 1 to 6 h from a ship sailing from Durban to the Seychelles was determined by flow cytometry, using both autofluorescence pigments and fluorescence DNA staining. Prochlorococcus (Pro) (17 to 160 x 103 cells ml-1) numerically dominated the ultraphytoplankton (UPP), followed by Synechococcus (Syn) (4.5 to 57 x 103 cells ml-1) and eukaryotic algae (EA) (0.6 to 4.2 x 103 cells ml-1). The abundance of heterotrophic bacterioplankton (HB) was 0.4 to 1.3 x 106 cells ml-1. A strong correlation (r = 0.8 to 0.97) was observed between sea-viewing wide field of view sensor (SeaWiFS) satellite estimates of total chlorophyll a (chl a) concentration and chl a concentration, abundance and biomass of EA as well as abundance and biomass of HB. This shows the potential for deducing spatial distributions of these 2 groups for ecosystem modelling using satellite data. Although the correlation between satellite chl a estimates and Syn chl a concentration was strong (r = 0.83 to 0.88), the correlation with its abundance and biomass was poor (r < 0.6) due to high variability (factor of 12) in cellular chl a content and to a lesser extent to diurnal cycles. The relationships were similar when either only daytime or all UP measurements were compared with the satellite data. No relationship was found between satellite data and Pro chl a concentration, abundance or biomass, even after correction for a pronounced diel cycle, suggesting that the SeaWiFS instrument might not detect Pro chl a.

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