It has been known for over a decade that the limiting micronutrient iron, supplied from shelf sediments, can fuel the productivity of the nutrient-rich coastal upwelling systems of central California. The significance of iron supply as a factor in community production in the more nutrient-poor waters of southern California, however, has been unstudied. Conventional wisdom would suggest that, given the relative scarcity of the macronutrients nitrate, phosphate and silicic acid, availability of these resources would be the only significant “bottom up” control on phytoplankton communities in southern California waters. New studies by CCE scientists, however, indicate that the supply of iron does indeed influence the overall productivity and biogeochemistry of the southern California Current System (sCCS).
In the sCCS, both coastal boundary upwelling and offshore wind stress curl upwelling supply macronutrients to surface waters. Wind stress curl is the dominant process, accounting for about twice as much vertical transport as coastal upwelling in this region. But, in contrast to coastal upwelling, wind stress curl upwelling occurs far from a continental shelf source of iron, leading to iron limitation effects on the offshore phytoplankton community. CCE researchers used a combination of time series measurements of dissolved iron concentration in surface waters and shipboard incubation experiments to document the impact of iron supply patterns on the planktonic ecosystem in the sCCS. Iron availability has now been shown to limit phytoplankton growth rates and influence phytoplankton community composition not only near the sea surface, but also in sub-surface waters.
In stratified regions like the offshore sCCS, phytoplankton communities can extend well below the surface mixed layer to the base of the sunlit layer (the euphotic zone). In many ocean regions, the lower euphotic zone has a zone of elevated phytoplankton biomass known as the subsurface chlorophyll maximum (SCM). In low-light regimes such as the SCM, dissolved iron requirements increase due to the need of phytoplankton to synthesize more Fe-containing photosynthetic proteins to harvest and process light. CCE researchers have obtained some of the first field evidence for the effects of iron limitation on these deeper communities using a matrix of microcosm grow-out experiments conducted at low light levels reproducing SCM conditions.
The effects of iron limitation at the base of the marine food web readily propagate to higher trophic levels, affecting the abundance of zooplankton grazers and plankton-feeding fish. Iron supply is an important variable that can interact with long-term climate changes within the California Current System, such as increased surface temperatures, ocean stratification, and hypoxia.