The Availability of Iron in Seawater: A Perspective

Abstract

There is evidence that iron availability may regulate primary production in some oceanic systems, yet we are still unable to determine what proportion of total iron in seawater is available for uptake by phytoplankton. Definitions of available iron that are based on size fractionations (i.e., dissolved vs. particulate) are inadequate; new conceptual approaches are needed for defining what constitutes biologically available iron in seawater. It is argued here that iron availability can be characterized by measurements of chemically labile, or reactive iron. Field data show that the labile proportion of total iron can vary considerably over short time scales, indicating that the chemical nature of iron, and hence its availability to phytoplankton, is a dynamic feature in natural systems. Further, a significant proportion of particulate iron in seawater is chemically labile, suggesting that it may be a ready source of available iron. The vertical and cross-shelf transport of suspended sediments may therefore be an important mechanism for distributing available iron to phytoplankton. In open ocean environments, where atmospheric dust deposition is the principal source of iron, there is evidence that photolysis of colloidal and particulate iron may be an important pathway for converting crystalline, unavailable iron into biologically utilizable forms.

Keywords:

• Iron limitation
• iron
• phytoplankton
• availability
• lability
• photolysis