Decoupling between bacterial production and primary production over multiple time scales in the North Pacific Subtropical Gyre

We measured rates of ³H-leucine (³H-Leu) incorporation, as a proxy for bacterial production, at Station ALOHA (22°45'N, 158°W) in the oligotrophic North Pacific Subtropical Gyre (NPSG). We report measurements conducted between January 2011 and April 2013, examining variability in ³H-Leu incorporation over diel, daily, and monthly time scales. Rates of ³H-Leu were evaluated in the context of contemporaneous ¹⁴C-based primary productivity (¹⁴C-PP) to identify potential temporal coupling between these measures of productivity. Throughout the upper ocean (0–125 m), rates of ³H-Leu incorporation measured in the light (³H-Leu_Light) were stimulated (1.5-fold, on average) relative to measurements in the dark (³H-Leu_Dark). At monthly scales, rates of ³H-Leu_Light and ³H-Leu_Dark varied 4.9-fold and 3.8-fold, respectively, while rates of ¹⁴C-PP varied 1.7-fold. Rates of ¹⁴C-PP were often elevated during summer months (May through August) when incident light flux was greatest, while rates of both ³H-Leu_Light and ³H-Leu_Dark often peaked in early fall (August through October) when seawater temperatures were maximal. Near-daily measurements of ³H-Leu incorporation and ¹⁴C-PP conducted over a 62-day period in the summer of 2012 revealed that rates of ³H-Leu_Light and ³H-Leu_Dark varied ~2.5 and 2.0-fold, respectively, similar to ~1.8-fold daily variability observed in rates of ¹⁴C-PP. Over diel time scales, rates of ³H-Leu_Light and ³H-Leu_Dark demonstrated different patterns, with rates of ³H-Leu_Light elevated at mid-day and rates of ³H-Leu_Dark greatest in the early evening. Together, these results suggest that in this oligotrophic ecosystem, photosynthetic production of organic matter and bacterial production can be temporally uncoupled across daily to seasonal scales.