Nutrient Limitation in Central Red Sea Mangroves

Abstract

As coastal plants that can survive in salt water, mangroves play an essential role in large marine ecosystems (LMEs). The Red Sea, where the growth of mangroves is stunted, is one of the least studied LMEs in the world. Mangroves along the Central Red Sea have characteristic heights of ∼2 m, suggesting nutrient limitation. We assessed the nutrient status of mangrove stands in the Central Red Sea and conducted a fertilization experiment (N, P and Fe and various combinations thereof) on 4-week-old seedlings of Avicennia marina to identify limiting nutrients and stoichiometric effects. We measured height, number of leaves, number of nodes and root development at different time periods as well as the leaf content of C, N, P, Fe, and Chl a in the experimental seedlings. Height, number of nodes and number of leaves differed significantly among treatments. Iron treatment resulted in significantly taller plants compared with other nutrients, demonstrating that iron is the primary limiting nutrient in the tested mangrove population and confirming Liebig's law of the minimum: iron addition alone yielded results comparable to those using complete fertilizer. This result is consistent with the biogenic nature of the sediments in the Red Sea, which are dominated by carbonates, and the lack of riverine sources of iron.

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

“…However, to the best of our knowledge, iron resorption had not been previously assessed in mangroves. 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.…”

“…The average leaf nitrogen concentration in mg N g DW −1 tended to be low, about half of the levels of 2% N of DW considered to represent nitrogen-sufficient plants, similarly, the average leaf phosphorous concentration was also low, about 1/3 of the 0.2% P of DW considered to represent P-sufficient plants (cf. Almahasheer et al (2016b). And about half of the concentrations that was found the same species in Kenya of about 20 mg g DW −1 , yet comparable to other species e.g., Xylocarpus granatum, Lumnitzera racemosa, Sonneratia alba in Kenya .…”

“…The Red Sea ranks amongst the most oligotrophic seas due to lack of river runoff and low nutrient supply (Mandura, 1997), and mangrove trees have been shown to be strongly nutrient limited, particularly by iron, in the Central Red Sea (Almahasheer et al, 2016b). Mangroves in the Central Red Sea develop as dwarf trees (<2 m height) along a narrow belt.…”

“…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).…”

Characterization of the CO2 System in a Coral Reef, a Seagrass Meadow, and a Mangrove Forest in the Central Red Sea

“…We do not have information regarding the balance between sulfate reduction‐sulfite oxidation at our mangrove forest and seagrass meadow sites. However, there are arguments in the literature against net sulfate reduction in carbonate sediments containing low iron (Burdige et al, ; Holmer et al, ; Hu & Burdige, ; Krumins et al, ), such as in our seagrass meadow and mangrove forest sites (Almahasheer, Duarte, & Irigoien, ; Anton et al, ; Saderne et al, ).…”