Phytoplankton processes during a mesoscale iron enrichment in the NE subarctic Pacific: Part III—Primary productivity

As part of the Canadian SOLAS program, a large scale iron (Fe) enrichment experiment (Subarctic Ecosystem Response to Iron Enrichment Study; SERIES) was performed in the NE subarctic Pacific in July of 2002. Dissolved Fe was added to a 77 km2 patch of seawater and the evolution of the subsequent phytoplankton bloom was monitored for 26 days. Particulate organic primary productivity (OPP) inside the patch began to increase in all phytoplankton size-fractions (picophytoplankton, nanophytoplankton and microphytoplankton) relative to outside the patch within 48 h. After day 10, microphytoplankton (>20 μm) were responsible for the vast majority of both OPP and phytoplankton biomass. Maximum OPP of ca. 15 mmol C m−3 d−1 was achieved on day 15, representing a 20-fold increase from average OPP measured outside the patch. Water-column integrated, biomass (chl a)-specific OPP (Pbint) of the total phytoplankton assemblage peaked twice, once following the first Fe infusion on day 4 (2.9 mmol C mg chl a−1 d−1) and then coinciding with maximum OPP on day 15 (2.6 mmol C mg chl a−1 d−1). Maximum Pbint achieved on day 4 represented a 5-fold increase relative to Pbint measured outside the patch. Water-column integrated OPP also peaked on day 15 at ca. 251 mmol C m−2 d−1, and coincided with a rapid decline in silicic acid (Si(OH4)) concentrations. At this time, microphytoplankton accounted for ca. 90% of total OPP. Patch-averaged chlorophyll a (chl a) concentrations were maximal (∼5 mg m−3, >16 times the outside patch) on day 18, during which time microphytoplankton OPP had begun to decline. In addition to OPP, particulate inorganic primary productivity (IPP) also increased due to an elevated coccolithophore abundance, reaching a maximum of 0.25 mmol C m−3 d−1 achieved 9 days after the initial Fe enrichment, which then decreased back to rates similar to those measured outside of the patch. Changes in primary productivity were also assessed using pulse amplitude-modulated (PAM) fluorometry. Relative electron transport rates (ETR) obtained by PAM fluorometry were significantly correlated (p<0.001, r2=0.82) with the 14C-based primary production rates during the Fe enrichment experiment. The increase in all measured photosynthetic parameters with Fe enrichment provides compelling evidence that primary productivity in the NE subarctic Pacific is regulated by Fe availability during the summer.