Abstract
Lagoons along the Red Sea are socio-economically important but most vulnerable ecosystems. Strong evaporation, strengthened due to global warming, leads to high salinity and high temperature in surface water, which exacerbate physiological stress in mangroves. Rabigh Lagoon is characterized by unusually large spatial differences in nutrient dynamics and physicochemical parameters. We conducted a multi-approach study to demonstrate how environmental factors cause growth retardation and stress in mangroves. There were significant variations (P < 0.05) in water salinities and temperatures, nitrogen and phosphorus abundance in sediments, and antioxidants in mangroves among 8 sites across the lagoon. Salinity and water temperature rapidly increased from the northern where Red Sea seawater enters to the southern end, but dissolved oxygen declined. High salinity (44.9), high surface water temperature (28.8°C) and relatively high nutrients at the southern end of the lagoon (S8) corresponded to high concentrations of antioxidants. High δ13C (−12.4‰) and δ15N (4.9‰) in the sediments at S8 also reveal relatively high nutrient level due to stagnant water, camel grazing, and runoff bringing in fertilizer from agriculture activities in the catchments during seasonal flooding events. Principal component analyses showed that N and P limitation at the lagoon entrance is the leading cause for mangrove stress, while high salinity, temperature, and low dissolved oxygen are the predominant factors for high antioxidants concentrations at the southern end of the lagoon.
Highlights
Mangroves play an important role in tropical and subtropical coastal ecosystems by providing food sources, breeding grounds, carbon storage, and protection from coastal erosion (Polidoro et al, 2010; Almahasheer et al, 2016)
Our goal is to demonstrate how nutrients in the sediments, high salinity and temperature interacted to cause stress in mangroves, and shift of the leading factors from the lagoon entrance to the southern end
The texture of sediment was predominantly sandy at the entrance of the lagoon, and loamy at S8, which was due to clay materials brought by runoff and low rate of flow
Summary
Mangroves play an important role in tropical and subtropical coastal ecosystems by providing food sources, breeding grounds, carbon storage, and protection from coastal erosion (Polidoro et al, 2010; Almahasheer et al, 2016). Mangroves are the most productive plants in the sea in terms of average rates of gross and net primary production. Edaphic factors such as salinity, waterlogging, tidal frequency, surface hydrology, soil redox potentials, and nutrient limitation have major impacts on tree height and productivity of mangroves (Feller, 1995; Lovelock et al, 2004, 2006; Naidoo, 2009). Dwarf mangroves are mature forests that are limited in growth by soil conditions that may include low nutrients, high salinity, or excessive sulfide concentrations. High salinities could result in dwarfed plants with small and thick leaves (Patel et al, 2010; Naidoo et al, 2011) as mangroves allocate more biomass resources to roots than shoots (Naidoo and Kift, 2006)
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