Regulation of the maximum quantum yield of phytoplankton photosynthesis by iron, nitrogen, and light in the eastern equatorial Pacific

The surface waters of the eastern equatorial Pacific are characterized by persistent macronutrients and low phytoplankton biomass. The so-called "high nutrient-low chlorophyll" (HNLC) condition is enigmatic because the stable water column, high irradiance, and high nutrient concentrations would seem to be optimal for phytoplankton growth. Investigations spanning 25 years have shown that low iron concentrations may be partially responsible for the HNLC condition. The exact role of iron in the maintenance of HNLC is unknown; contemporary theory states that low iron availability limits the growth of large diatoms, which could otherwise escape control of the small protistan grazers. Alternatively, iron may limit the photosynthesis and therefore growth of the bulk equatorial flora.; I investigated the hypothesis that low iron availability limits phytoplankton photosynthetic performance in the eastern equatorial Pacific. Phytoplankton photosynthetic performance was characterized with parameters describing photosynthesis versus irradiance (P-I) curves, chlorophyll-specific spectral absorption coefficients, and maximum quantum yields of photosynthesis. These measurements were made on a transect across the equatorial Pacific at 140{dollar}spcirc{dollar}W, on a survey in the vicinity of the Galapagos Islands, and during an open-ocean iron addition experiment performed 200km south of the Galapagos. The maximum quantum yield of photosynthesis was less than 50% of the expected nutrient-replete value in the upper water column at 0{dollar}spcirc{dollar}, 140{dollar}spcirc{dollar}W, in spite of NO{dollar}sb3{dollar} concentrations exceeding 6{dollar}mu{dollar}M. The maximum quantum yield was even lower to the east of the Galapagos, where NO{dollar}sb3{dollar} exceeded 8{dollar}mu{dollar}M. Downstream from the Galapagos and within the iron-enriched patch, {dollar}phisb{lcub}rm max{rcub}{dollar} was enhanced by a factor of 2, and no near-surface depression was observed. These results show that in situ iron availability limits phytoplankton photosynthesis and growth by increasing susceptibility to photoinhibition. Climatic factors which increase low-latitude winds and the extent of deserts can be expected to cause increased primary production in the tropical Pacific.