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

Abstract

Red Sea mangroves occur in an oligotrophic sea without permanent freshwater inputs. Understanding the mechanisms to cope with nutrient limitation is, therefore, important to understand their distribution and nutrient dynamics in coastal ecosystems. We measured total number of meristems to estimate their leaves production and nutrients (N, P, and Fe) as a function of age in Avicennia marina leaves. Then estimated resorption rates; the recovery of nutrients from senescing leaves before they are shed in a total of 91 leaf from four different mangroves stands in the Central Red Sea. We found that the concentration of N and P but not Fe declined with age. Nutrient content also declined in the older leaves with high resorption capacity of 69 and 72% in N and P vs. low resorption of 42% in Fe. The role of Fe resorption is poorly studied in plants, nevertheless, this study could provide an insight into our knowledge of iron resorption in the mangroves, which has never been assessed before. The leaf nutrient export flux from senescing leaves in monospecific stand of Avicennia marina was 9, 0.4 and 1 g m −2 year −1 for N, P, and Fe respectively, suggesting mangrove litter-fall to be an important source of bioavailable iron in particular, due to its low resorption, to the adjacent oligotrophic ecosystem.

Rates and drivers of Red Sea plankton community metabolism

“…The Red Sea is a semi-enclosed highly oligotrophic basin (Acker et al, 2008;Raitsos et al, 2013). It is known as one of the warmest tropical seas, with maximum sea surface temperatures ranging from 33.0 to 33.9 • C during summer (Chaidez et al, 2017;Osman et al, 2018), and up to 34-35 • C in certain parts of the basin (Rasul et al, 2015;Garcias-Bonet and Duarte, 2017;Almahasheer et al, 2018). Due to the prevailing arid conditions, the Red Sea experiences large evaporation rates (nearly 2 cm yr −1 of freshwater from the surface layers) while the lack of river runoff and low precipitation rates make this system one of the saltiest seas on the planet (Sofianos, 2002;Sofianos and Johns, 2015;Zarokanellos et al, 2017).…”

“…In tropical and subtropical oligotrophic regions, the high temperatures may amplify the metabolic imbalances in plankton communities as CR tends to increase faster than GPP (Harris et al, 2006;Regaudie-de-Gioux and Duarte, 2012) if the allochthonous sources of organic carbon are enough to subsidise their carbon demand. These allochthonous inputs may be delivered from land through riverine discharge, from the atmosphere through atmospheric deposition of dust and volatile organic carbon (Jurado et al, 2008), or are exported from productive coastal habitats (Duarte et al, 2013; Barrón and Duarte, 2015).…”

Remobilization of Heavy Metals by Mangrove Leaves

“…Although this will lead to a reduction of heavy metals in soils, shedding of e.g., mangrove leaves could transfer metals from deep soil layers into the soil surface and elsewhere in the ecosystem, so that rather than sequestering metals mangroves could be remobilizing pollutants within the ecosystem. Despite pollution in mangroves have been widely studied Zhang et al, 2014;Alzahrani et al, 2018;Kulkarni et al, 2018), and previous studies showed that mangroves have the capacity to reabsorb nutrients before shedding the leaves (Alongi et al, 2005;Zhou et al, 2010;Almahasheer et al, 2018), the fate of metals remains largely unknown (Saenger and McConchie, 2004). Therefore, the elemental fluxes between bio-and geospheres of mangrove ecosystems and in particular, the dynamics of heavy metals in mangrove leaves remain, to the best of our knowledge, unknown.…”

“…Recently, we examined the capacity of Avicennia marina in the Red Sea to reabsorb N, P, and Fe prior to leave senescence, and despite evidence of Fe deficiency, mangroves had low resorption of Fe (42%) compared to N and P (69 and 72%, respectively; (Almahasheer et al, 2018). Whether these applies to other heavy metals or not is unknown.…”

Reply to reviewer 2

“…In tropical and subtropical oligotrophic regions, the high temperatures may amplify the metabolic imbalances in plankton communities as CR tends to increase faster than GPP Regaudie-de-Gioux and Duarte, 2012) if the allochthonous sources of organic carbon are enough to subsidise their carbon demand. These allochthonous inputs may be delivered from land through riverine discharge, from the atmosphere through atmospheric deposition of dust and volatile organic carbon (Jurado et al, 2008), or are exported from productive coastal habitats Barrón and Duarte, 2015). The Red Sea is a semi-enclosed highly oligotrophic basin (Acker et al, 2008;Raitsos et al, 2013).…”