Dissolved iron supply limits early growth of estuarine mangroves

Abstract

Three mesocosm experiments were performed in an outdoor facility to quantify the responses of five mangrove species grown from seedling to sapling stage to increasing rates of dissolved iron supply. Stem extension and biomass of mangroves were measured in the first two experiments, and in the third experiment, rates of microbial iron reduction were measured in relation to stem extension of two mangrove species. In all experiments, mangrove growth was enhanced by increasing iron supply, although some species showed iron toxicity at the higher supply rates. In the first two experiments, stem extension rates of Rhizophora apiculata, Bruguiera gymnorrhiza, and Xylocarpus moluccensis best fit Gaussian curves with maximal growth at supply rates of 50-60 mmol Fe x m(-2) x d(-1), whereas growth of Avicennia marina and Ceriops tagal increased to the highest rate (100 mmol Fe x m(-2) x d(-1)) of iron supply. Changes in leaf chlorophyll concentrations and iron content of roots mirrored the growth responses. In the third experiment, rates of microbial iron reduction were greater with R. apiculata and A. marina than in controls without plants; for both species, there was a positive relationship between stem extension and iron reduction. The rates of iron reduction and rates of iron supplied to the plants were well within the range of interstitial iron concentrations and rates of iron reduction found in the natural mangrove soils from which the seedlings were obtained. The responses of these species show that mangroves growing from seedling to sapling stage have a strong nutritional requirement for iron, and that there is a close relationship between plant roots and the activities of iron-reducing bacteria. These results suggest that mangrove growth may be limited in some natural forests by the rate at which iron is solubilized by iron-reducing bacteria. Such biogeochemical conditions have significant implications for successful recruitment, establishment, and early growth of mangroves.

Leaf Nutrient Resorption and Export Fluxes of Avicennia marina in the Central Red Sea Area

“…Our results reveal a low iron resorption efficiency (≤42%) for Avicennia marina in the Central of the Red Sea, about half that for N and P. The comparatively low efficiency in iron resorption, compared to N and P resorption helps explain experimental results reporting that iron additions resulted in significantly taller plants compared with complete fertilizer additions, and consistent with the biogenic nature of the sediments in the Red Sea (Almahasheer et al, 2016b), similar to the iron deficiency of seagrass growing above carbonate sediments reported in the Mexican Caribbean (Duarte et al, 1995). These results also support the findings of a mesocosm experiment (Alongi, 2010) reporting that mangroves growth in some forests may be limited by the rate at which iron is solubilized by iron-reducing bacteria. Particularly when considering the high sediment Fe stock in this study of 1 Kg m −2 compared to around 0.1 Kg m −2 for N and P respectively.…”

“…Additionally, iron has also been found to be limiting (Alongi, 2010;Almahasheer et al, 2016b), and it has been suggested that the low availability of the three elements may be the reason for mangroves absence in several coastlines otherwise having conditions suitable for mangrove growth (Sato et al, 2011). Nutrient resorption may be, therefore, a key strategy for mangroves to grow in oligotrophic environments, such as the Red Sea (Almahasheer et al, 2016b).…”

Nutrient Limitation in Central Red Sea Mangroves

“…The finding of iron-limited growth of Central Red Sea seedlings is consistent with (Alongi, 2010), who reported high iron requirements in the transition stage from seedling to sapling in five mangrove species growing in north Queensland, Australia. Iron limitation in mangrove growth in the Central Red Sea is expected, as iron limitation is characteristic of marine coastal habitats dominated by biogenic carbonate sediments, which are typically iron depleted (Duarte et al, 1995).…”

“…The outcome of these experiments depend on location, with some mangrove stands shown to be limited by phosphorous (Feller, 1995;Koch and Snedaker, 1997;Feller et al, 2003a;Lovelock et al, 2004Lovelock et al, , 2006) and others to be limited by nitrogen (Feller et al, 2003b;Naidoo, 2009). Prior to the present study, iron limitation in mangrove growth had been reported, on the basis of experimental addition, only in mangroves in north Queensland, Australia (Alongi, 2010). The finding that mangroves in the Central Red Sea are nutrient-limited, as evidenced by low nutrient concentrations in their leaves and very low iron levels in the propagules they form, allows us to begin to understand the functioning and drivers of these important components of the Red Sea ecosystem.…”

“…Indeed, low availability of nitrogen, phosphorus, and iron has been suggested as the cause for the absence of mangroves along shorelines that are otherwise suitable for mangrove growth (Sato et al, 2011). Mangrove fertilization experiments have demonstrated site-specific nutrient limitation of nitrogen and phosphorous (Feller, 1995;Koch and Snedaker, 1997;Feller et al, 2003a,b;Lovelock et al, 2004Lovelock et al, , 2006Naidoo, 2009;Alongi, 2011), whereas an experimental study conducted on mangroves in north Queensland, Australia, demonstrated iron deficiency (Alongi, 2010). Previous work has not considered these three nutrients together, which is the focus of this work.…”