Synthesis of the Iron Enrichment Experiments: SEEDS and SERIES Iron fertilization experiment in the western subarctic Pacific (SEEDS)

An iron enrichment experiment (Subarctic Ecosystem Response to Iron Enrichment Study), SERIES was carried out at Ocean Station Papa (50°N, 145°W) in the subarctic Northeast Pacific Ocean in July 2002, in order to observe the physiological, ecological and biogeochemical responses to a release from iron stress in these high-nitrate, low-chlorophyll (HNLC) waters. The patch was created by injecting FeSO4⋅7H2O (iron source) and SF6 (water mass tracer) into the ship’s wake while a Lagrangian grid of 8 km × 8 km was navigated. During the 27 days of occupation after iron release, the patch grew from < 100 km to approximately 1000 km, and at Days 15–16 of the experiment, the patch shifted from a northward to an eastward drift, towards the centre of an eroded eddy. Shortly after, salinity and nutrient concentrations below ~10 m depth began to increase and, simultaneously (and possibly coincidentally), the iron-induced diatom bloom crashed. Mass balances of nitrogen and silicon were performed for two periods, separated at Day 20, when mixed-layer nutrient concentrations began to increase. Prior to Day 20, drawdown of silicic acid and nitrate was 13 μM and 5 μM, respectively, in the upper 30 m. Si[OH]4 drawdown was balanced by biogenic silica (BSi) accumulation in the upper 50 m and sediment-trap flux at 50 m (export flux), while NO3 drawdown was balanced by the accumulation of particulate organic nitrogen (PON) and NH4, and PON export flux. Because of the injection of nutrients below ~10 m after Day 20, nutrient drawdown was not observed for the latter part of the experiment, but there was a large decrease in BSi and PON concentrations in the upper 50 m. The decrease in BSi concentration was balanced by sediment-trap fluxes of BSi, but the decrease in PON concentration exceeded the sum of NH4 accumulation and PON export flux. The imbalance in the nitrogen budget might be attributable to the poor constraint of changes in the dissolved organic nitrogen (DON) pool, or by the fact that dilution during patch growth has not been considered. These latter factors are the focus of on-going work.