The metabolic balance of the planktonic community in the North Atlantic Subtropical Gyre: The role of mesoscale instabilities

Abstract

We have studied the net community production (NCP) balance in the Eastern region of the Subtropical North Atlantic during two cruises, in August 1998 and April 1999. In August, heterotrophic bacteria were more abundant than picophytoplankton, which resulted in net heterotrophy (NCP = –129 mmol O2 m−2 d−1 ± 18; mean ± SE), whereas these differences in plankton components were not apparent in April 1999, when the community was in metabolic balance (NCP = –13 mmol O2 m−2 d−1 ± 19). In April, the metabolic balance of microplankton communities was net heterotrophic outside (NCP = –57 mmol O2 m−2 d−1 ± 26) and lower than inside (NCP = 19 mmol O2 m−2 d−1 ± 19) the mesoscale structures present in the area (subtropical front, cyclonic eddy Leticia, and the Great Meteor Tablemount). Positive NCP inside the cyclonic eddy (NCP = 20 mmol O2 m−2 d−1 ± 17) was due to lower respiration rather than to higher gross primary production rates. Gross extrapolation suggests that the regional organic carbon deficit within the Eastern Subtropical Gyre (0.98–1.88 Gt C yr−1) should increase by ~14% and 52% in the absence of mesoscale structures.

Rates of dissolved organic carbon production and bacterial activity in the eastern North Atlantic Subtropical Gyre during summer

“…It was very similar to that reported by Zubkov et al (1998) ). However, our measurements were, on average, about 2-fold lower than the values reported by González et al (2001) for the subtropical North Atlantic in August 1998 and by for the same region in August 1996. Although the bacterial abundances reported by González et al (2001) and ourselves were obtained on the same cruise, caution must be taken in comparing the two sets of data, since different methods for bacterial quantification were used -flow cytometry by Gonzá-lez et al (2001) and microscopic observation in the present study (see Sieracki et al 1995).…”

“…The average BP rate obtained in this study (Table 1) was 3-fold higher than the rate reported by Zubkov et al (2000) during September 1995 for the subtropical North Atlantic, and very similar to the mean value given by for the Sargasso Sea and by González et al (2001) for the same region during April 1999. The mean BP:NPP ratio was in the lower extreme of the global average (0.1 to 0.2) calculated for the oceans (Ducklow 2000).…”

“…Carbon fluxes through the microplankton community in the euphotic zone of the North Atlantic Subtropical Gyre during August 1998 were computed using all the rates quantified and estimated in this study, and the averaged O 2 community respiration rate measured by González et al (2001) in the same region during the same cruise (Fig. 6).…”

“…6). The average community respiration rate given by González et al (2001) was transformed to carbon units using a respiratory quotient equal to 0.8 (Robinson et al 2002). BCD was calculated using an estimated BGE, as explained described in 'Materials and methods'.…”

Protist control of phytoplankton growth in the subtropical north-east Atlantic

“…data), while the euphotic layer during Cruise Az2 was less stratified and ambient temperature was lower (González et al 2001). Chl a concentration was always below 0.1 mg m -3 in the surface water, while initial chl a in experiments conducted with DCM water ranged from 0.07 to 0.39 mg m -3 (P 0 : Table 1).…”