Comparing the Bioavailability of a Natural and Synthetic Iron Source: Do Past Experiments Accurately Model Diatom Growth in Response to Episodic Iron Addition?

Episodic iron input plays an important role in dictating phytoplankton growth, physiology, and community structure in high-nitrate low-chlorophyll (HNLC) waters of the Gulf of Alaska. Experiments using FeCl₃ as an iron source have resulted in diatom blooms with significant implications for ecosystem productivity and resilience. However, if FeCl₃ and natural sources of iron differ in bioavailability, re-interpretation of these results is warranted. To test the hypothesis that iron is differentially bioavailable across natural and synthetic sources, in summer 2019 we performed a deck-board incubation exposing HNLC phytoplankton from the Northern Gulf of Alaska to dissolved iron from three sources: FeCl₃, the Copper River plume, and HNLC waters. Over a five-day incubation, differences in phytoplankton growth, physiology, and community composition were observed across treatments. Cells > 20 µm exposed to FeCl₃ had higher net growth rates (0.025 µg chl L^-1 day^-1) than cells exposed to iron from the Copper River plume (0.014 µg chl L^-1 day^-1) or HNLC waters (0.008 µg chl L^-1 day^-1). Trends identical to those for cells >20 µm were observed for cells 5- 20 µm and < 5 µm, though net growth rates were slightly lower for these size fractions compared to the > 20 µm size fraction. Photosynthetic efficiency (Fv/Fm) ranged from 0.31- 0.49 for cells <5 µm and from 0.25- 0.61 for cells >20 µm across the three treatments. Fv/Fm was greatest for phytoplankton exposed to FeCl₃ and trends in Fv/Fm highlight community dynamics within incubation bottles. We also present results on diatom community composition and nano-phytoplankton abundance and physiology (e.g. pigment production). We conclude that FeCl₃ is more bioavailable to diatoms than natural sources of iron and highlight the need to compare bioavailability of natural iron sources to more accurately understand phytoplankton productivity in HNLC waters.