Evidence for a heterotrophic subtropical northeast Atlantic

The mean (+/-SE) depth-integrated gross production (P) of 2,600 +/- 271 mg O-2 m(-2) d(-1) derived from a compilation of data from nine cruises conducted between 1991-2000 in the subtropical NE Atlantic was found to be significantly lower (t-test, P = 0.005, N = 33) than the mean (+/-SE) community respiration (R) of 3,821 +/- 276 mg O-2 m(-2) d(-1). Two-thirds of the stations investigated were heterotrophic, and the P/R ratio of the communities tended to increase as P increased, such that communities where P < 3,000 mg O-2 m(-2) d(-1) tended to be heterotrophic. The tendency for R to exceed P (P/R < 1.0) was statistically significant (Wilcoxon ranked sign test, P < 0.05) in the upper and deep layers of the photic zone, with an overall balance between P and R at intermediate depths. These results provide evidence that the subtropical NE Atlantic is a heterotrophic ecosystem, where planktonic communities respire more organic carbon than they produce, thereby acting as net sources of CO2.