The Effect of Secondary-Treated Municipal Wastewater Discharge on Nutrient Concentrations in Leaves and Soils of Gray Mangrove in Bahrain

Abstract

The wastewater discharge into the marine environment may affect the integrity of the mangrove ecosystem. In this context, the possible effect of secondary-treated municipal wastewater discharge on the nutrient concentrations in the leaves and soils of Tubli Bay gray mangrove (Avicennia marina (Forsk.) Vierh.) was assessed at three sites. Physical properties and the percent of organic matter content in mangrove soil were measured. The nitrogen concentration was measured using the Kjeldahl method. Extractable phosphorus was quantified using the dry-ash method. Plant and soil concentration of K, Ca, Mg, and Na was determined using an inductively coupled plasma analyzer. Soil organic matter was estimated using the dry-ash method. The results showed no significant differences in the mangrove leaves’ nutrient concentration between sites regarding all the measured variables except N and Na. Wastewater discharge significantly affects mangrove leaves N content at the Tubli site where heavy wastewater loads are discharged. Leaf nutrients’ concentration followed the order: Na> K> N> Mg> Ca> P. Nutrients were concentrated in the topsoil layers in the following order: Ca > Mg > Na > K > N > P. Nutrient level showed a decreasing pattern with soil depth, except for Ca. Significant differences were observed in N and P’s levels in the soil layers between the affected site and the other two sites. Furthermore, the soil analysis indicated significant differences in N and P levels in the Tubli site soil compared to the other two sites due to wastewater discharge. No significant correlations were found between nutrient levels in the leaves of mangrove and its underlying soils. Additionally, the release of wastewater into the Bay significantly increased soil organic matter in the affected site. The study’s findings indicate that the continued release of the secondary effluent into the Tubli Bay may alter the Bay ecosystem’s Physico-chemical properties in general and mangrove survival in particular.