Effect of deep water nutrient enrichment on plankton metabolism in the N and S Atlantic gyres

Observations indicate that the metabolic balance in the epipelagic N and S Atlantic gyres differ as a result of regional differences in both photosynthesis (P) and respiration (R) rates. However, the trophic and biogeochemical processes causing these patterns are mostly unknown. With the aim to explore if regional differences exist in the response of epipelagic communities to nutrient enrichment, we carried out two microcosm (20L) experiments in the N (19º 51 N; 29º 08 W) and S (18º 33 S; 25º 04 W) Atlantic gyres. 1, 2, 3 and 4 L of deep (ca. 220 m depth) seawater were added to surface seawater samples (2 m depth) up to final volumes of 20L, and were incubated during 8 (N Gyre) or 5 (S gyre) days, together with untreated controls, in a deck incubator refrigerated with surface seawater. In both the N and S gyres, P and chlorophyll a concentration increased rapidly after the addition of deep seawater, especially in the pico- and nanophytoplankton. Increases in R rates were slower and smaller than for P. This changed the community metabolism from marginally heterotrophic at the beginning of the experiments (-0.32 ± 0.21, -0.19 ± 0.14 mmol O₂ m^-3 d^-1 in the N and S gyre, respectively) to net autotrophic (up to ca. 2 and 4 mmol O₂ m^-3 d^-1 on days 5 and 8 in the +3L and +4L treatments of the N and S gyre experiments, respectively). Here we analyze the response of community P and R rates, together with the dynamics of nutrient concentration, size-fractionated chlorophyll a concentration, and abundance of Prochlorococcus, Synechococcus, total bacteria and viruses.