Iron chemistry in seawater and its relationship to phytoplankton: a workshop report

Abstract

. In coastal and shelf waters, substantial external inputs of iron come from riverine sources and bottom sediments, leading to markedly higher dissolved and particulate iron concentrations. Much of the particulate iron in nearshore waters is inorganic and processes that solubilize this reservoir, making it accessible to phytoplankton, are especially relevant (see below). The greater iron inputs to coastal and shelf regions compared to the open ocean are accompanied by high iron requirements of neritic phytoplankton species (Brand et al., 1983; Sunda et al., 1991). These systems therefore might be strongly influenced as the comparatively rich iron resource is diminished by phytoplankton blooms. The single largest reservoir of iron in the surface waters of HNLC regions may be the biota itself. New evidence indicates that this biological pool of iron is recycled on the time scale of days, much like N and P (Hutchins et al., 1993). This “input” of regenerated iron to surface waters is estimated to be more than an order of magnitude greater than the external supply rate of iron (Bruland, this meeting; Morel, this meeting) and may largely satisfy the iron-demand of phytoplankton in these systems. This view is supported by recent results of Price et al. (1994) showing that iron uptake rates of plankton in the equatorial Pacific are sufficient to entirely turn over the dissolved iron pool within half a day or less. Presently, there is no indication of the chemical forms of this regenerated iron or whether these forms are directly reassimilated by phytoplankton. B. What are the sinks of iron? The removal of iron from surface waters is fairly well constrained within a geochemical (i.e. mean residence time) perspective, however, the mechanisms and dynamics of this removal is not well understood. Mechanisms for removing iron from surface waters include: * sorption and precipitation, 0 biological assimilation, aggregation of inorganic or organic colloids, and . sinking of mineral and biogenic particles. While much of the particulate iron introduced via rivers, sediment resuspension, or as mineral aerosols will be removed by settling, ascertaining the underlying basis for the removal of “dissolved” iron forms is much more difficult. In regions with a high sinking flux of inorganic mineral particles (e.g. in some coastal and well mixed shelf waters), dissolved iron may be removed abiotically by sorption to surfaces of these particles. Similarly, sinking organic particles also can scavenge soluble iron from surface waters (Morel and Hudson, 1985). Dissolved iron also is “removed” via direct assimilation by phytoplankton. The subsequent sinking of live cells or fecal matter will transport a portion of this biogenic iron from surface waters. In addition to direct assimilation and sorption onto sinking (mineral and biogenic) particles, iron may sorb to colloidal organic matter which is abundant in surface waters (Wells and Goldberg, 1992, 1994). The stability of this colloidal phase is a topic of much dispute (Honeyman and Santschi, 1989; Bauer et al., 1992; Moran and Buesseler, 1992; Wells and Goldberg, 1993), but the extremely large colloidal surface area combined with the particle reactive nature of iron suggests that aggregation of organic colloids could be important for removing iron. Significant unresolved issues regarding iron removal include (1) identifying the specific mechanisms of iron sorption to abiotic and biotic sinking particles, which will shed light on how changes in iron speciation may affect this removal pathway, (2) changes in the iron “export efficiency” of the assimilation pathway with shifts in primary productivity or species assemblage, and (3) the abundance and reactivity of iron in the colloidal reservoir. The relative importance of abiotic, bioM.L. Wells et al.iMarine Chemistry 48 (199.5) 157-182 163 tic and colloid aggregation removal processes will vary from regime to regime and with season. (2) What are the chemical speciation and forms of iron among the soluble, colloidal and particulate fractions, including the rates and mechanisms of transformations among these forms? There are some large gaps in our knowledge of iron chemistry in seawater. The development of trace metal clean techniques for seawater collection and analysis over the past decade (Bruland et al., 1979; Gordon et al., 1982; Landing and Bruland, 1987) has given us reliable profiles for iron in the traditional categories of particulate (> 0.4 pm) and “dissolved” (< 0.4 pm) fractions; however, the chemical forms of iron within these fractions has been largely speculative. For example, there now is evidence that iron exists, at least partially, in the small colloidal phase (Wells and Goldberg, 1991; Wu and Luther, 1994; Powell and Landing, abstract) which is included in operationally defined “dissolved” fractions. Determining how iron is partitioned among these phases, and among various chemical forms within each phase, is central to understanding iron speciation in seawater.