Biological variables- Chl a measurement: For Chl a determination, 305 ml of water was withdrawn from each bag at each subsampling time. The water was filtered through 25 mm GF/F filters at ≤ 70 mm Hg vacuum and filters were frozen at –20c until analysis. Acetone (90) was added to extract the pigments 24h prior to analysis Pigments were quantified with a Turner 10AU fluorometer as described in Strickland and Parsons (1972)
Primary production: Total primary production (PP) was measured with the dual labeling technique after Slawyk et al. (1977). Separate incubations were run on aliquots taken from each treatment bag 24 h after the ash or Fe addition (at time equals I d [T1]) and at the end of the 6 d incubation period (T6). For each experiment, 500 mL, of water was withdrawn from the respective treatment bag and transferred to a 500 ml, polycarbonate(PC) bottle. For T1 , a PC bottle was also filled with seawater collected from 10m depth with the Teflon diaphragm pump at the same time as the treatment bag to serve as a blank. At T6, one extra 500 mL, sample was withdrawn from one bag of each treatment to serve as a blank. The PC bottles were inoculated with NaH13CO3 to obtain a final concentration of 200 umol L-1. The blanks were filtered immediately on precombusted 25 mm GF/F filters. The parallel incubation bottles were placed in the incubators for a period of 4-6 h (between 17:00 h and 23:00 h Pacific Daylight Time) and ended with sunset. The experiment was stopped by filtering the water through precombusted Whatman 25 mm GF/F filters. The filters were placed in open cryovials were then capped and kept in Drierite until onshore analysis. The 13C enrichment of the organic material on dried filters was measured in the laboratory using an isotope-ratio mass spectrometer (Delta V advantage, Thermo Electron) coupled to an elemental analyzer (Costech ECS-4010) following the procedures described in Blais et al. (2012).
DMS and DMSP concentrations: DMS and DMSP concentrations were measured following the techniques.