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In a fall 2008 cruise off the coast of California, on-deck incubations with surface seawater were conducted to compare the effect of increasing carbon dioxide (CO2) on nitrogen (N)limited versus N-replete (+ NO3) phytoplankton. Based on previous laboratory studies, we expected that under N-limitation C:N ratios would be more sensitive to CO2 availability. In particular, we wanted to test whether down-regulation of RuBisCO at high CO2 leads to an increased C:N ratio, since RuBisCO is thought to be a large pool of N in many autotrophs. In 4 out of 5 N limited experiments, the C:N ratio of phytoplankton biomass increased slightly from low to high CO2, and this increase was due chiefly to an increase in particulate organic carbon (POC), with little change in particulate organic nitrogen (PON). In contrast, the N-replete experiments did not show a change in C:N ratio, but some experiments exhibited higher rates of carbon fixation and growth at high CO2. In the one N-limited experiment that did not show a change in C:N ratio with CO2, the ambient population had a higher proportion of cyanobacteria compared with the other experiments. The concentration of RuBisCO decreased at high CO2 in several N-limited experiments, but this reduction only accounted for a minor proportion of the overall change in C:N ratio. An increase in C:N ratios at high CO2 in N-limited phytoplankton could provide negative feedback to increasing atmospheric CO2.

Modest increase in the C:N ratio of N-limited phytoplankton in the California Current in response to high CO<sub>2