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°N, 17-22°W) was studied are the Latitude II cruise 1995 with the aim of examining planktonic food web controls on these fluxes. The transect crossed three distinct water masses, a productive area in the NW African upwelling between 18° and 23°N, a warm water mass between 6° and 13°N, and unproductive waters at the northern range of the transect. The contrasting phytoplankton biomass and production across the eastern tropical N Atlantic appear to be driven by major changes in the supply of nutrients across the thermocline, which averaged 0.99 ± 0.6mmol N m^-2d^-1 and 0.13 ± 0.07 mmol P m^-2d^-1, and was particularly high at the upwelling area off NW Africa. This supply was relatively low in nitrogen relative to that in phosphorus, driving the system towards nitrogen limitation. This was further enhanced by an important sinking flux of nitrogen in dissolved organic matter, which averaged 14.8 ± 6.8 mmol N m^-2d^-1 and exceeded the average internal input of nitrate by seven fold. There was, therefore, a severe nitrogen deficit that must have been offset by lateral and/or atmospheric inputs. Evaluation of the likely size of these sources indicates that lateral inputs must be important in the southern range of the area investigated, while atmospheric inputs seem to deliver the required N to balance losses in the northern area of the eastern subtropical Atlantic. Planktonic biomass was dominated by heterotrophs, and community respiration rates and bacterial carbon requirements exceeded the net primary production in the most unproductive areas. There must be significant allocthonous inputs of organic carbon and nitrogen in this region of the ocean. The distribution of primary production was uncoupled to that of the calculated 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, whereas the less productive areas at the northern and southern edges of the transect 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 largely by a downward dissolved organic carbon flux, partially supported by DOC excretion from phytoplankton cell lysis (30-70% of the net primary production in the area), twice as high as the average upward DIC flux. Provided that heterotrophic carbon requirements exceed the measured net primary production, the net downward flux into the thermocline's interior must be driven by the supply of allocthonous organic carbon, from atmospheric deposition and/or lateral supply.