Biogeochemical behavior of organic carbon in a small tropical river and estuary, hainan, china

The biogeochemistry of a small tropical river and estuary (the Wanquan River (WQR)) in Hainan, China, was studied to obtain information on the sources, transformation and fate of riverine organic matter. Water and total suspended matter were sampled along a salinity gradient during four field campaigns in December 2006, August 2007, July-August 2008 and April 2009. We were able to observe the effect of heavy precipitation associated with the tropical cyclone Kammuri (2008) and a strong first-rain event (2009) on the export of organic carbon. Both dissolved organic carbon (DOC) and particulate organic carbon (POC) generally decreased with increasing salinity in the estuary, with minimal seasonal or annual variations. The POC concentrations were in the range of 18-178μM for the WQR and were significantly correlated with chlorophyll a concentrations. The highest average values of DOC (~190μM) were observed in 2009. These values could be the result of the flushing effect of the first rain. In the high-salinity zone, in situ productivity could be an important source for DOC. The δ¹³C_org values of particles (-29.5‰ to -23.2‰), combined with the ratio of organic carbon to total nitrogen (OC/TN), reflect a mixture of terrestrial organic matter and in situ production. The amplitudes of the POC and DOC variations on the occasion of the first-rain event of 2009 were larger than those observed during Kammuri. It is estimated that 29.8tday^-1 of DOC and 10.4tday^-1 of POC, representing 6-10% of the annual loads, were delivered to the coast during the 10-day period of the first flush event in 2009. Our results demonstrate the important role of short-term aperiodic events on small rivers and estuaries along tropical coasts.