The influence of abrupt increases in seawater pCO(2) 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 (pCO(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 pCO(2) (~1100 μatm) compared to ambient pCO(2) (~387 μatm). In 2 of 10 experiments, rates of (14)C-PP decreased significantly (~43%) under elevated pCO(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 pCO(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 pCO(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 pCO(2). In 2 of 5 depth-resolved experiments, rates of (3)H-Leu(Dark) incorporation were lower (10% to 15%) under elevated pCO(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 pCO(2). We postulate that any effects caused by changes in pCO(2) may be masked or outweighed by the role that nutrient availability and temperature play in controlling metabolism in this ecosystem.