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 (pCO₂) in seawater on rates of primary and bacterial production at Station ALOHA (2245’ 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 ¹⁴C-bicarbonate assimilation; ¹⁴C-PP) under elevated pCO₂ (~1100 μatm) compared to ambient pCO₂ (~387 μatm). In 2 of 10 experiments, rates of ¹⁴C-PP decreased significantly (~43%) under elevated pCO₂ 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 ³H-leucine (³H-Leu), while in 1 experiment, rates of ³H-Leu incorporation measured in the dark (³H-Leu_Dark) increased more than 2-fold under high pCO₂ conditions. We also examined photoperiod-length, depth-dependent (0–125 m) responses in rates of ¹⁴C-PP and ³H-Leu incorporation to abrupt pCO₂ increases (to ~750 μatm). In the majority of these depth-resolved experiments (4 of 5 total), rates of ¹⁴C-PP demonstrated no consistent response to elevated pCO₂. In 2 of 5 depth-resolved experiments, rates of ³H-Leu_Dark incorporation were lower (10% to 15%) under elevated pCO₂ compared to controls. Our results revealed that rates of ¹⁴C-PP and bacterial production in this persistently oligotrophic habitat generally demonstrated no or weak responses to abrupt changes in pCO₂. We postulate that any effects caused by changes in pCO₂ may be masked or outweighed by the role that nutrient availability and temperature play in controlling metabolism in this ecosystem.