The influence of abrupt increases in seawater <i>p</i>CO₂ on plankton productivity in the subtropical North Pacific Ocean

<div><p>We conducted a series of experiments to examine short-term (2–5 days) effects of abrupt increases in the partial pressure of carbon dioxide (<i>p</i>CO₂) 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 ¹⁴C-bicarbonate assimilation; ¹⁴C-PP) under elevated <i>p</i>CO₂ (~1100 μatm) compared to ambient <i>p</i>CO₂ (~387 μatm). In 2 of 10 experiments, rates of ¹⁴C-PP decreased significantly (~43%) under elevated <i>p</i>CO₂ 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 <i>p</i>CO₂ conditions. We also examined photoperiod-length, depth-dependent (0–125 m) responses in rates of ¹⁴C-PP and ³H-Leu incorporation to abrupt <i>p</i>CO₂ 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 <i>p</i>CO₂. In 2 of 5 depth-resolved experiments, rates of ³H-Leu_Dark incorporation were lower (10% to 15%) under elevated <i>p</i>CO₂ 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 <i>p</i>CO₂. We postulate that any effects caused by changes in <i>p</i>CO₂ may be masked or outweighed by the role that nutrient availability and temperature play in controlling metabolism in this ecosystem.</p>