Biogeochemical study of the Altamaha estuarine system
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Estuarine systems receive organic matter from multiple sources (e.g. terrestrial vascular plants, salt marsh macrophytes, and marine/river phytoplankton), which is subsequently recycled through physical and biochemical processes. However, many questions remain unanswered. For example, how do organic matter inputs vary in different discharge seasons, how are the organic substrates from different sources utilized by estuarine organisms or degraded by related biochemical processes, and what are important factors controlling the bioreactivities of different organic material? My research aims to address these issues by combining field sample measurements and laboratory experiments. The analytical results indicate that chemical and isotopic compositions of the three typical end members in the Altamaha River estuary (marine diatom, salt marsh plant and land grass) changed differently from material to material and from compound to compound during degradation. There was a linkage (-3‰~-6‰ depletion) between 1313´C of bacteria-specific fatty acids and TOC, implying that ´C of bacteria-specific fatty acids can be used to trace the carbon sources of the microbial communities. The field observations indicate that terrestrial and marine sources dominated the organic matter inputs during the high discharge period while salt marsh plants contributed a large fraction at one site during the low discharge period. In addition, organic matter deposited in the high discharge period was relatively fresh while that in low discharge period was highly degraded. The laboratory simulation experiments further demonstrate an order of biochemical reactivity for different materials: marine diatom > land grass > salt marsh plants. It appears that redox conditions had a small influence on degradation of fresh organic matter in aged sediments, while co-metabolism could have positive and negative effects on organic matter degradation, depending on the type of organic materials coexisting in the system and the diagenetic status of the materials. As primary catalyst in estuarine system, the bacteria communities preferentially utilized organic carbon from phytoplankton. But when salt marsh plants became a dominant input into the sediments at a given site during a low discharge period, bacteria efficiently used this organic matter as well.