The influence of abrupt increases in seawater pCO2 on plankton productivity in the subtropical North Pacific Ocean

We conducted a series of experiments to examine short-term (2–5 days) effects of abrupt increases in the partial pressure of carbon dioxide ( p CO 2 ) in seawater on rates of primary and bacterial production at Station ALOHA (22°45’ N, 158° W) in the North Pacific Subtropical Gyre (NPSG). The majority of experiments (8 of 10 total) displayed no response in rates of primary production (measured by 14 C-bicarbonate assimilation; 14 C-PP) under elevated p CO 2 (~1100 μatm) compared to ambient p CO 2 (~387 μatm). In 2 of 10 experiments, rates of 14 C-PP decreased significantly (~43%) under elevated p CO 2 treatments relative to controls. Similarly, no significant differences between treatments were observed in 6 of 7 experiments where bacterial production was measured via incorporation of 3 H-leucine ( 3 H-Leu), while in 1 experiment, rates of 3 H-Leu incorporation measured in the dark ( 3 H-Leu Dark ) increased more than 2-fold under high p CO 2 conditions. We also examined photoperiod-length, depth-dependent (0–125 m) responses in rates of 14 C-PP and 3 H-Leu incorporation to abrupt p CO 2 increases (to ~750 μatm). In the majority of these depth-resolved experiments (4 of 5 total), rates of 14 C-PP demonstrated no consistent response to elevated p CO 2 . In 2 of 5 depth-resolved experiments, rates of 3 H-Leu Dark incorporation were lower (10% to 15%) under elevated p CO 2 compared to controls. Our results revealed that rates of 14 C-PP and bacterial production in this persistently oligotrophic habitat generally demonstrated no or weak responses to abrupt changes in p CO 2 . We postulate that any effects caused by changes in p CO 2 may be masked or outweighed by the role that nutrient availability and temperature play in controlling metabolism in this ecosystem.