Pore water gradients below microbial mat surface of Umm Alhool sabkha in Qatar

Abstract

The mineralization of organic matter in marine sediments by microbial activity was studied in Umm Alhool sabkha. In intertidal surface sediments, the development of steep compositional and physico-chemical gradients was a common phenomenon. Rapidly, oxygen is consumed within the upper few mm of marine mats and sediments. In permeable sediments, however, oxygenated bottom waters may have flew through the upper part of the surface sediments leading to enhanced participation of oxygen in element cycling. Whereas in microbial mats, the surface sediments are locally formed, indicating a disturbance in the balance of the biogeochemical processes. Umm Alhool sabkha, situated between Umm Sa'id (Mesaieed) and Al Wakrah, drew our attention to study the biogeochemical cycling because both microbial mats and mangroves ecosystems affect its biogeochemistry. In the present study, the chemistry of pore water below mats surfaces of intertidal sandy sediments was investigated in winter 2011 using a number of different techniques. Pore water was sampled down to 20 cm below surface using pore water lances, diffusion samplers, and centrifugation of sediment core sections. Microsensor measurements of sulfide and pH were also performed on the upper 2 cm. Specifically, we measured salinity, dissolved O2, pH, SO4²¯, H2S, Cl¯, TN, TOC, PO4³¯, NO3¯, NH4+, H4SiO4, and microbial sulfate reduction rates have been analyzed using intact sediment cores. Sulfidic sediments were characterized by high sulfate reduction rates exhibiting maxima between about 5-15 cm, associated with decreased oxygen penetration depths, and proton activities. Anaerobic metabolic activity in pore waters below mat surface lead to significantly enhanced concentrations of sulfide, ammonium, dissolved inorganic carbon, phosphate, silica (steep gradients), and a net consumption of sulfate. They acted as windows for the liberation of reduced substances into the bottom water or the atmosphere. This study represents the first comprehensive investigation of the chemical composition and sulfate reduction rates in Umm Alhool microbial mat ecosystem. It shows how dynamic and self-fueling the system is.