Journal
MARINE ECOLOGY PROGRESS SERIES
Volume 426, Issue -, Pages 57-70Publisher
INTER-RESEARCH
DOI: 10.3354/meps09001
Keywords
Sargasso Sea; Picoalgae; Bacteria; Prochlorococcus; Synechococcus; Convergence zone
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Funding
- Knud Hojgaards Fond
- Danish Natural Sciences Research Council
- Nordea Fond
- Villum Kann Rasmussen Fond
- Elisabeth and Knud Petersens Fond
- Carlsberg Foundation
- Dansk Expeditions Fond
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Elevated levels of biomass and productivity are often associated with ocean frontal systems. The Subtropical Convergence Zone (STCZ) in the southern Sargasso Sea shows pronounced and stable thermal fronts, but little is known about the ecological consequences of these hydrographic features. With the aim of improving the understanding of physical and lower trophic level processes in the area, we carried out field studies of physical, chemical and biological characteristics along 3 transects crossing thermal fronts associated with the STCZ in March to April 2007. Thermal and chemical stratification were evident at all stations within the STCZ, and a depletion of nitrate was measured in the upper similar to 150 m. The phytoplankton biomass was dominated by Prochlorococcus spp. with highest abundance, up to > 400 000 cells ml(-1), at the chlorophyll a (chl a) maximum at 100 to 150 m depth. Synechococcus spp. were generally located higher in the water column. Picoalgae were less abundant, up to 8000 cells ml(-1), but explained most of the variation in chl a. Even though diatoms and dinoflagellates were few, the biomass of larger phytoplankton equalled or exceeded that of picoplankton at a few stations. Bacterial biomass was roughly equal to half of the phytoplankton biomass. We did not find elevated levels of primary production or biomass of specific phytoplankton groups associated with the STCZ, probably due to a pronounced variability between stations along transects. Nevertheless, distinct increases in chl a were associated with the zone and thermal fronts bordering the STCZ.
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