Fatty acids as indicators of sediment organic matter dynamics in a monsoon-influenced tropical estuary

Sedimentary organic matter (OM) composition serves as a biomarker and helps in hindcast analysis of the events in the habitat. The present study was carried out on a monthly basis to evaluate the contribution of different biological communities to the surface sediment in a monsoon-influenced tropical Zuari estuary, west coast of India, using source-specific fatty acid (FA) biomarkers and total organic carbon to nitrogen ratio (TOC/TN). Our observations revealed spatio-temporal variation in the OM composition in this estuary, which is influenced by in situ production, bacterial re-working, riverine inputs, and hydrodynamics. The content of TOC, TN, and total FAs in the sediment ranged from 0.13 to 4.27%, 0.01 to 0.31%, and 29 to 122 mu g g-1 sediment dry weight, respectively. High content of TOC, TN, total FAs, terrestrial plants-specific FAs, and detrital derived FAs at the lower mid-estuarine (inner) stations when compared to the mouth of the estuary, points out lower mid-estuarine region as the major depositional site for the OM. Low amount of OM at the mouth of the estuary is attributed to high bacterial contribution, which seems to be maintained by repetitive cycles of resuspension-deposition of organic materials. Thus, it seems that the estuarine morphology influences the deposition of OM and plays an important role in the functioning of the estuarine ecosystem. The contribution of phytoplankton-specific FAs was high at the banks of the estuary, which form a suitable habitat for benthic organisms including artisanal fisheries. Diatom, dinoflagellate, and zooplankton-specific FAs were high during the post-monsoon season and decreased from the pre-monsoon to monsoon season. Terrestrial plants and bacteria-derived FAs were high during the monsoon suggesting the role of bacteria in the assimilation of terrestrial material. Elucidating the degradation pattern of in situ produced and terrestrial-derived organic matter through the laboratory designed microcosm experiments is a way forward.

Automated summary

The study on sedimentary organic matter composition in a monsoon-influenced tropical Zuari estuary on the west coast of India revealed spatial and temporal variations influenced by in situ production, bacterial re-working, riverine inputs, and hydrodynamics. Lower mid-estuarine region was identified as the major depositional site for organic matter, while the lower content of organic matter at the mouth of the estuary was attributed to high bacterial contribution. Estuarine morphology plays a significant role in organic matter deposition.