Abstract

The deep chlorophyll maximum (DCM) is a widespread feature in most stratified, oligotrophic waters. In addition to their well-established importance for many surface phytoplankton communities, more recent evidence suggests that iron, light or co-limitation may also be important drivers for some DCM communities. To test this hypothesis, we describe the results from six grow-out experiments, four from the Equatorial Pacific Ocean (between 150°E and 140°W), one in Western Pacific Warm Pool (9°S, 170°E) and one in the middle of the Tasman Sea (36°S). Photosynthetic efficiency (Fv/Fm) and biomass response, including Chl a and phytoplankton community structure (pennate diatoms, photosynthetic eukaryotes, Synechococcus, Prochlorococcus, and major Prochlorococcus ecotypes), were assessed over five days in control, +Fe, +Light or +Fe +Light treatments. Photosynthetic efficiency did not change dramatically in any of the treatments at any of the locations, except at 0°N 140°W where the control and +Fe bottles had elevated efficiency relative to both +Light treatments. Except for some ecotypes of Prochlorococcus (eMIT9313 and eNATL2A), phytoplankton populations were most strongly limited by light in the DCM. Pennate diatoms and other photosynthetic eukaryotes showed the most enhancement with the addition of iron and light at some stations and may be co-limited, but no phytoplankton populations were enhanced by adding iron alone. Although the duration and magnitude of the responses varied depending on initial macronutrient concentrations, they were generally consistent across the locations sampled. These results suggest that light is the primary limiting resource of the DCM for this vast region, but that iron can play an important additive role in limiting phytoplankton populations in locations where flux to the DCM is reduced.

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