Sources of organic matter in sediments from the Ord River in tropical northern Australia

The sources of organic matter in the sediments from the Ord River and estuary were assessed using stable carbon isotope values (δ 13C), lipid biomarkers and distributions of chlorophylls and carotenoids as part of a larger multidisciplinary study examining the biogeochemistry of this tropical, macrotidal and large scale estuary. Two field trips were undertaken to obtain samples from both the dry (September, 2002) and wet (February, 2003) seasons. Both biomarker and stable isotope data indicate predominant contributions of organic material from the surrounding vegetation. However, the character of the terrestrial organic matter changes from the freshwater to the marine sections of the river. The sterol profiles in the freshwater sediments are relatively simple and dominated by the C 29 sterols 24-ethylcholest-5-en-3β-ol (sitosterol) and 24-ethylcholesta-5,22E-dien-3β-ol (stigmasterol) with small quantities of 24-methylcholest-5-en-3β-ol (campesterol), all of which are common sterols in higher plants. In the dry season survey, triterpenoid alcohols were generally absent from the freshwater sediments, but in the wet season there was a small contribution from compounds such as α- and β-amyrin and lupeol, indicating additional higher plant input, possibly from further inland in the catchment. Within Cambridge Gulf and the Ord estuary, the waterways are fringed with mangroves which contribute substantial quantities of organic matter as shown by distinctive distributions of triterpenoid alcohols dominated by taraxerol. However, this is not the major triterpenoid in the mangrove trees studied, but seems to be considerably more stable to biodegradation and so dominates the triterpenoids found. Isotope, biomarker (fatty acids, phytol and sterols) and pigment data suggest that only a small part of the sedimentary organic matter is derived from microalgae. Contributions from diatoms (24-methylcholesta-5,22E-dien-3β-ol; fucoxanthin), green algae (Δ 7-sterols), cyanobacteria (C 32 hopanol; zeaxanthin) and dinoflagellates (dinosterol; peridinin) could be recognized. In this environment the high turbidity of the water column severely limits phytoplankton production, although productivity of microphytobenthos on the banks of the river can potentially be high.