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

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 > 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.

Localised mixing and heterogeneity in the plankton food web in a frontal region of the Sargasso Sea: implications for eel early life history?

“…The non-corrected estimates are reported here. Note that there are minor differences in the magnitude of the primary production estimates reported here and in Riemann et al (2011) owing to differences in the method used to fit the curves to the P vs. E parameters between the 2 studies.…”

“…The detection limits were 0.06, 0.1, 0.04, 0.3 and 0.2 μmol kg −1 for PO 4 , NO 3, NO 2 , NH 4 and silicate, respectively. A detailed description of the distribution of inorganic nutrients is given in Riemann et al (2011). Therefore, only nutrient information relevant for the current analyses is presented here.…”

“…One hypothesis is that this frontal zone might comprise more rewarding feeding grounds for the larvae than surrounding waters. However, two studies (Andersen et al 2011, Riemann et al 2011) carried out concomitantly with the Munk et al (2010) study failed to identify significant differences in either primary or secondary production between frontal and non-frontal waters -although the Andersen et al (2011) study did suggest a greater abundance of some zooplankton forms in the frontal region than elsewhere.…”

“…The present study is based on data collected on the same cruise as the Munk et al (2010), Andersen et al (2011) and Riemann et al (2011) studies. We hypothesized that the frontal dynamics described above might potentially create heterogeneity in the plankton production characteristics within the frontal re gion.…”

Biophysical characteristics of a morphologically-complex macrotidal tropical coastal system during a dry season

“…6b and d). It is well-accepted that frontal regions are often characterised by high phytoplankton biomass, due to either in situ production in the front or convergent transport toward the front (Franks, 1992a,b;Fu et al, 2009;Riemann et al, 2011). Phytoplankton biomass in the frontal region was most likely stimulated by the supply of nutrients (particularly nitrogen) from the vertically well-mixed water (Fig.…”