Optical absorption spectra of waters from the Orinoco River outflow: Terrestrial input of colored organic matter to the Caribbean

Abstract

An extensive series of optical absorption spectra were recorded for waters in the eastern Caribbean Sea, the Gulf of Paria, and the Orinoco River Estuary during the high‐flow period of the Orinoco River in the Fall of 1988. Evidence for high levels of dissolved, colored organic matter (COM) was found throughout the eastern Caribbean, with these levels increasing substantially at stations nearer the Gulf of Paria and the Orinoco River. The dependence of the absorption at 300 nm ( a 300 ) on salinity exhibited conservative mixing behavior for transects through the Gulf of Paria. In contrast, transects through the Orinoco Estuary revealed that a 300 remained approximately constant or slightly increased with increasing salinity up to 27‰, after which it decreased linearly with increasing salinity. This nonconservative behavior was not produced by COM release from suspended particulate matter at higher salinity, nor did it appear to originate from the sediment. Below 30‰, the slopes of the log‐linearized absorption spectra were independent of salinity (0.0140 nm −1 ). At higher salinities, the slopes apparently increased, suggesting that the COM is modified. Based on specific absorption coefficients measured for COM isolated from this area, COM input by the Orinoco River is estimated to be ∼ 2.5 × 10 12 g C/yr, which represents about 1% of the total global transport of dissolved organic carbon to the ocean. This input is likely to influence substantially the carbon cycle, photochemical properties, and optical characteristics of the waters in this region.