Biogeochemical effects of iron availability on primary producers in a shallow marine carbonate environment

Abstract

We completed a synoptic survey of iron, phosphorus, and sulfur concentrations in shallow marine carbonate sediments from south Florida. Total extracted iron concentrations typically were Ͻ50 mol g Ϫ1 dry weight (DW) and tended to decrease away from the Florida mainland, whereas total extracted phosphorus concentrations mostly were Ͻ10 mol g Ϫ1 DW and tended to decrease from west to east across Florida Bay. Concentrations of reduced sulfur compounds, up to 40 mol g Ϫ1 DW, tended to covary with sediment iron concentrations, suggesting that sulfide mineral formation was iron-limited. An index of iron availability derived from sediment data was negatively correlated with chlorophyll a concentrations in surface waters, demonstrating the close coupling of sediment-water column processes. Eight months after applying a surface layer of iron oxide granules to experimental plots, sediment iron, phosphorus, and sulfur were elevated to a depth of 10 cm relative to control plots. Biomass of the seagrass Thalassia testudinum was not different between control and iron addition plots, but individual shoot growth rates were significantly higher in experimental plots after 8 months. Although the iron content of leaf tissues was significantly higher from iron addition plots, no difference in phosphorus content of T. testudinum leaves was observed. Iron addition altered plant exposure to free sulfide, documented by a significantly higher ␦ 34

Sulfide intrusion in seagrasses assessed by stable sulfur isotopes—a synthesis of current results

“…Addition of iron to the sediments decreased the dissolved sulfide pools and at the same time δ 34 S increased in the leaves of T. testudinum and Halodule wrightii. This suggests that δ 34 S can be used as an indicator of stress removal in seagrasses (Chambers et al, 2001), and both species grew better after the reduction in sulfide levels in the sediments. Similar addition of iron to organic enriched sediments inhabited by P. oceanica showed reduced sulfide intrusion where the leaf δ 34 S increased from +18.2 in control plots to +20.5 in iron added plots (Marbà et al, 2007).…”

“…Both shoot survival and recruitment rates increased due to the relief of sulfide pressure. The mechanisms behind improved growth due to iron additions remain to be resolved, but the plant enzymatic activities increased, probably due to removal of inhibition by sulfide as well as higher iron availability, resulting in higher growth (Chambers et al, 2001;Holmer et al, 2005a). …”

Eutrophication in shallow coastal bays and lagoons: the role of plants in the coastal filter

“…However, we know less about plant-mediated feedbacks on P cycling as eutrophication proceeds in these environments than we do about N cycling in temperate systems. In oligotrophic habitats, iron oxides that form in the seagrass rhizosphere, effectively bind P in the solid phase and decrease the release of P to overlying waters (Chambers et al 2001, Rozan et al 2002. While it was previously thought that carbonate sediments were a permanent sink for P and that mineral P was not available to plants, recent studies suggest that seagrass metabolism facilitates the dissolution of carbonate minerals in the rhizosphere, releasing bound P (Jensen et al 1998, Burdige & Zimmerman 2002.…”

“…McGlathery 2001, seagrass metabolism will decline. In sediments of highly eutrophic systems, sulfate reduction will likely increase and sulfides will compete with phosphate for oxidized iron (Heijs et al 2000, Chambers et al 2001; this would likely result in the seagrass meadow switching from being a net sink to a source of P (Perez et al 2001). The increase in P release to the overlying water would be a positive feedback further stimulating algal production and prolonging the negative effects of shading associated with eutrophication (Heijs et al 2000, Perez et al 2001.…”

Iron Deficiency in Seagrasses and Macroalgae in the Red Sea Is Unrelated to Latitude and Physiological Performance

“…Iron can also limit primary production in shallow marine systems, especially in carbonated sediments characterized by very low iron pools (Pestana et al, 2003). Observations and field experiments provide evidence of iron deficiency in seagrasses growing on carbonate sediments (Marbà et al, 2008), where iron, addition to sediments stimulates seagrass growth in the Yucatan Peninsula , Southern Florida (Chambers et al, 2001), and the Balearic Islands (Marbà et al, 2007). Field observations on coral reefs, which are also carbonate-rich habitats, also suggest potential iron limitation in the growth of macroalgae, as indicated by higher growth rates of these macrophytes near ship wrecks, which leak iron in micronutrient-poor islands in the Central Pacific (Kelly et al, 2012).…”