Primary productivity (PP) in the North Pacific Subtropical Gyre: Understanding drivers of variability via 14C-tracer incubations and PP diagnosed via the diurnal cycle of particulate carbon.

The rate of primary production (PP) in the ocean is a fundamental step in the ocean’s food web and biological carbon pump. For more than 50 years oceanographers have relied primarily on estimates of PP based on in vitro measurements of ¹⁴CO₂ uptake rates. Yet, it is difficult to reconcile PP rates measured in vitro with in situ rates. Here we present diurnal cycles of optically-derived particulate organic carbon (POC) and particle size distributions measured over a series of cruises in the North Pacific relative to traditional ¹⁴C-based PP measurements. We have calculated net PP from the daytime increase in optically-derived particulate organic carbon (POC) and the sum of respiration, grazing and sinking from the nighttime POM decrease. Comparison of optically derived NPP to parallel 12-hr ¹⁴C incubations are highly significant. The variability in productivity measurements over daily to seasonal to annual time-scales are discussed relative to predominant chemical, physical and climactic forcing.