Food-web structure and elemental (C, N and P) fluxes in the eastern tropical North Atlantic

The flow of biogenic elements (C, N and P) in the upper tropical Atlantic ( 29-5 degreesN, 17-22 degreesW) was studied are the Latitude N cruise 1995 w ith the aim of examining planktonic food web controls on these fluxes. The transect crossed three distinct water masses, a productive area in the NW A frican upwelling between 18 degrees and 23 degreesN, a warm water mass betw een 6 degrees and 13 degreesN, and unproductive waters at the northern rang e of the transect. The contrasting phytoplankton biomass and production acr oss the eastern tropical N Atlantic appear to be driven by major changes in the supply of nutrients across the thermocline, which averaged 0.99 +/- 0. 6 mmol Nm(-2) d(-1) and 0.13 +/- 0.07 mmol P m(-2) d(-1), and was particula rly high at the upwelling area off NW Africa. This supply was relatively lo w in nitrogen relative to that in phosphorus, driving the system towards ni trogen limitation. This was further enhanced by an important sinking flux o f nitrogen in dissolved organic matter, which averaged 14.8 +/- 6.8 mmol Nm (-2) d(-1) and exceeded the average internal input of nitrate by seven fold . There was, therefore, a severe nitrogen deficit that must have been offse t by lateral and/or atmospheric inputs. Evaluation of the likely size of th ese sources indicates that lateral inputs must be important in the southern range of the area investigated, while atmospheric inputs seem to deliver t he required N to balance losses in the northern area of the eastern subtrop ical Atlantic. Planktonic biomass was dominated by heterotrophs, and commun ity respiration rates and bacterial carbon requirements exceeded the net pr imary production in the most unproductive areas. There must be significant allocthonous inputs of organic carbon and nitrogen in this region of the oc ean. The distribution of primary production was uncoupled to that of the ca lculated CO2 flux across the ocean surface. The productive area off the NW African upwelling acted as a source of CO2 to the atmosphere (0.5 mmol C m( -2) d(-1)), driven by high DIC inputs from the thermocline's interior, wher eas the less productive areas at the northern and southern edges of the tra nsect acted as net sinks for atmospheric CO2 (up to 2.5 mmol C m(-2) d(-1)) . The carbon sink in these unproductive regions seems to be supported large ly by a downward dissolved organic carbon flux, partially supported by DOC excretion from phytoplankton cell lysis (30-70% of the net primary producti on in the area), twice as high as the average upward DIC flux. Provided tha t heterotrophic carbon requirements exceed the measured net primary product ion, the net downward flux into the thermocline's interior must be driven b y the supply of allocthonous organic carbon, from atmospheric deposition an d/or lateral supply. (C) 2001 Published by Elsevier Science Ltd.