The influence of iron on phytoplankton communities in the deep chlorophyll maximum of stratified oceanic regimes

Deep chlorophyll maxima (DCMs) are commonly formed at the interface of the light limiting and nutrient limiting depths of a stratified oceanic water column. The influence of light and macronutrient availability in regulating DCM characteristics has long been recognized, but the potential influence of micronutrients like iron has not been well studied in DCM communities. Because iron requirements for phytoplankton are higher at low light levels it has been hypothesized that iron could limit growth of phytoplankton in the DCM, even when macronutrients are exhausted in surface waters. We conducted field incubation experiments to investigate the interactive effects of iron and light availability on DCM communities in both the subtropical California Current and the eastern tropical North Pacific. In both regions phytoplankton communities were co-limited by iron and light, with diatoms responding most strongly to iron additions. Responses were observed in a variety of parameters measured including chlorophyll a, macronutrients, taxonomic pigments, and Fv/Fm. Iron- light co-limitation significantly altered the size structure of the phytoplankton community by preferentially limiting the growth rate of large diatoms. In turn, the size structure of phytoplankton affects the rate of carbon export and the amount of energy transferred to higher trophic levels. These results show that iron availability, in conjunction with light and macronutrients, is an important, but previously undocumented, factor structuring DCMs.