Abstract
Iron can limit primary production in shallow marine systems, especially in tropical waters characterized by carbonated sediments, where iron is largely trapped in a non-available form. The Red Sea, an oligotrophic ecosystem characterized by a strong N-S latitudinal nutrient gradient, is a suitable setting to explore patterns in situ of iron limitation in macrophytes and their physiological performance under different iron regimes. We assessed the interactions between environmental gradients and physiological parameters of poorly-studied Red Sea macrophytes. Iron concentration, chlorophyll a concentration, blade thickness, and productivity of 17 species of macrophytes, including 7 species of seagrasses and 10 species of macroalgae, were measured at 21 locations, spanning 10 latitude degrees, along the Saudi Arabian coast. Almost 90% of macrophyte species had iron concentrations below the levels indicative of iron sufficiency and more than 40% had critically low iron concentrations, suggesting that iron is a limiting factor of primary production throughout the Red Sea. We did not identify relationships between tissue iron concentration, chlorophyll a concentration and physiological performance of the 17 species of seagrass and macroalgae. There was also no latitudinal pattern in any of the parameters studied, indicating that the South to North oligotrophication of the Red Sea is not reflected in iron concentration, chlorophyll a concentration or productivity of Red Sea macrophytes.
Highlights
Iron (Fe) is an essential nutrient that can act as a limiting factor controlling primary production in freshwater and marine systems (Coale et al, 1996; Sterner et al, 2004)
We found that seven species of macrophytes in the Red Sea, two seagrasses (T. hemprichii and T. ciliatum) and five macroalgae
Benthic macrophytes in the oligotrophic Red Sea were characterized by low iron concentration, with 86% of our seven species of seagrasses and 90% of our ten species of macroalgae having iron concentration below the levels indicative of iron sufficiency (600 mg Fe kg DW−1, Duarte et al, 1995) in their blades (Figure 2, Table 4)
Summary
Iron (Fe) is an essential nutrient that can act as a limiting factor controlling primary production in freshwater and marine systems (Coale et al, 1996; Sterner et al, 2004). Iron can limit primary production in shallow marine systems, especially in. 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 (Duarte et al, 1995), Southern Florida (Chambers et al, 2001), and the Balearic Islands (Marbà et al, 2007). Experimental iron additions in plots of seagrasses growing on coralline carbonate sediments in the Caribbean increased the chlorophyll a (Chl a) concentration in seagrass leaves and enhanced seagrass growth (Duarte et al, 1995). There is a paucity of studies in situ across a broad range of iron inputs to assess effects on primary production in marine benthic ecosystems (Marbà et al, 2008)
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