Metabolic response of oligotrophic plankton communities to deep water nutrient enrichment

Studies on upper ocean plankton production and respiration in oligotrophic ecosystems are taken to imply that these regions are net heterotrophic (i.e. gross primary production [GPP] < respiration [R]). In this study we examined the hypothesis that the regularly observed state of net heterotrophy in the oligotrophic North Pacific Ocean may be interrupted by periodic bursts of net autotrophy when GPP becomes decoupled from R. To examine this hypothesis, we conducted a series of nutrient addition experiments in the oligotrophic North Pacific Subtropical Gyre (NPSG) in which nutrient-replete deep seawater was mixed with nutrient-depleted mixed-layer water. We monitored time-dependent changes in rates of GPP and R as well as dynamics of chlorophyll and nutrient concentrations. Our results consistently demonstrated that following the addition of nutrients: (1) phytoplankton biomass (chl a) and primary production increase; (2) the photoautotrophic plankton assemblage shifts from small (<2 μm diameter) to large (>10 μm diameter), chl c-containing and Siutilizing cells; and (3) community metabolism shifts from near balance, or slightly net heterotrophic, to demonstrably net autotrophic metabolism. These results suggest that nutrient availability partly regulates plankton community structure and dynamics, and could serve as a mechanism to temporally and spatially decouple GPP and R in oligotrophic open ocean ecosystems.