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MEPS 426:57-70 (2011)  -  DOI:

Distribution and production of plankton communities in the subtropical convergence zone of the Sargasso Sea. I. Phytoplankton and bacterioplankton

Lasse Riemann1,7,*, Torkel Gissel Nielsen2,6, Theis Kragh3, Katherine Richardson4, Hjalte Parner5, Hans Henrik Jakobsen6,2, Peter Munk6

1Department of Natural Sciences, Linnaeus University, 39182 Kalmar, Sweden
2National Environmental Research Institute, Aarhus University, Department of Marine Ecology, 4000 Roskilde, Denmark
3Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, 3400 Hillerød, Denmark
4University of Copenhagen, Center for Macroecology, Evolution and Climate, 2200 Copenhagen N, Denmark
5International Council for the Exploration of the Sea (ICES), 1553 Copenhagen V, Denmark
6National Institute of Aquatic Resources, DTU Aqua Section for Oceanecology and Climate, Technical University of Denmark, DTU Kavalergården 6, 2920 Charlottenlund, Denmark
7Marine Biological Section, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark
(Second superscript numbers indicate present addresses)

ABSTRACT: 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 ~150 m. The phytoplankton biomass was dominated by Prochlorococcus spp. with highest abundance, up to >400000 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.

KEY WORDS: Sargasso Sea · Picoalgae · Bacteria · Prochlorococcus · Synechococcus · Convergence zone

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Cite this article as: Riemann L, Nielsen TG, Kragh T, Richardson K, Parner H, Jakobsen HH, Munk P (2011) Distribution and production of plankton communities in the subtropical convergence zone of the Sargasso Sea. I. Phytoplankton and bacterioplankton. Mar Ecol Prog Ser 426:57-70.

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