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Phytoplankton community structure in the eastern Equatorial Pacific (EEP) and the Peru upwelling system was determined using ChemTax analysis of pigment data. Photosynthetic efficiencies (Fv : Fm), concentrations of macronutrients, iron (Fe), and the redox proteins flavodoxin (Flv) and ferredoxin (Fd) were used to assess the physiological status of the algal communities with respect to Fe. Diel periodicity in Fv :Fm ratios was observed in Fe-stressed populations of Prochlorococcus and type 4 haptophytes (Phaeocystis) in the EEP with daytime values (0.05-0.30) displaying a distinct noontime minimum. On the Peruvian continental shelf, upwelling of Fe-rich subsurface waters led to massive diatom blooms (reaching chlorophyll a [Chl a] values >45 µg L-1) with near-maximal photosynthetic efficiencies (Fv: Fm >0.6). Assimilation and luxurious uptake of Fe, scavenging processes, and Ekman transport off the shelf break led to rapid decreases in Fe concentration and dramatic shifts in phytoplankton assemblages. The shelf break region was dominated by cryptophyte populations with relatively high photosynthetic competency (Fv:Fm = 0.4-0.5). In this region, subnanomolar Fe concentrations limited further diatom growth. As Fe concentrations decreased offshore, haptophyte (coccolithophorids and Phaeocystis) populations contributed approximately 40-80% toward the total Chl a concentration. Haptophyte populations were Fe stressed, as determined by high Fly indices (Flv [Flv + Fd]-1) and low photosynthetic efficiencies (Fv:Fm ∼0.3). The Flv index was very high in regions where Fe concentrations were <0.2 nmol L-1, but it was 0 when Fe levels were >0.4 nmol L-1. In the Peru upwelling, Fe concentrations between 0.2 and 0.4 nmol L-1 represented a transitional region that marked the threshold switch in community Fd and Flv expression. [ABSTRACT FROM AUTHOR]

Abstract: Surface water transects and vertical profiles for dissolved iron, macronutrients, chlorophyll a (Chl a), and hydrographic data were obtained in the Peru upwelling regime during August and September 2000. The supply of the micronutrient iron, relative to that of the macronutrients nitrate, phosphate and silicic acid, is shown to play a critical role in allowing extensive diatom blooms to develop in the Peru upwelling system. The extremely high-chlorophyll “brown waters of Peru” (with Chl a concentrations between 20 and 45 μg/l) result from massive diatom blooms with maximal photochemical efficiencies (Fv/Fm >0.6) occurring in the iron-rich upwelling region observed over the broad continental shelf off northern and central Peru. The source of the upwelled water in this region is the nutrient-rich subsurface countercurrent in contact with the organic-rich shelf sediments. This subsurface shelf water is suboxic and has extremely high concentrations of dissolved Fe (>50 nM) in the near-bottom waters. In marked contrast, relatively low-chlorophyll “blue waters” (Chl a <2 μg/l) with low concentrations of dissolved Fe (<0.1 nM) and high unutilized macronutrient concentrations are observed in the coastal upwelled waters along the southern coast of Peru and in the offshore regions of the Peru Current. Southern Peru is a region without a wide shelf to serve as a source of iron and, as a result, dissolved Fe concentrations in the near-bottom suboxic waters of this region are an order-of-magnitude lower than observed off northern and central Peru. In addition, the offshore Peru Current is a broad, Fe-limited, high-nitrate, lower than expected chlorophyll region extending hundreds of kilometers offshore into the northeast region of the South Pacific subtropical gyre and northwestward into the South Equatorial Pacific. [Copyright &y& Elsevier]