Seasonal variation in sources and processing of particulate organic carbon in the Pearl River estuary, South China

• POC concentration, δ13C and Chl a in four seasons along the Pearl River estuary.
• Freshwater POC was mainly of aquatic origin, except of detritus soil origin in May.
• Estuarine brackish water POC was dominated by in-situ produced OC.
• δ13CPOC was directly related to δ13CDIC, with offset increase in lower or outer PRE.

Particulate organic carbon (POC) in the Pearl River estuary (PRE), South China, along a salinity gradient from freshwater to seawater in four months was studied in order to determine its temporal and spatial changes in source and processing. Analytical parameters included chlorophyll-a (Chl-a), POC, and carbon isotopic composition of POC and the dissolved inorganic carbon (DIC) (δ13CPOC, δ13CDIC). POC varied greatly from freshwater to seawater, exhibiting a significant power law distribution with a rapid decrease (from >2.5 mg l−1 to <0.9 mg l−1) in a narrow salinity range of 0–5 and then a slow decline to ∼0.4 mg l−1 along the large salinity gradient in the estuary. POC was sourced predominantly from in situ phytoplankton, and hence largely reflective of primary production, in February, August, and November as indicated by mostly lower POC/Chl-a values (<200), and significant correlation between POC and Chl-a, as well as between δ13CPOC and δ13CDIC. But in May, soil-derived OC was dominant in freshwater and low salinity estuarine water, as suggested by low POC% in total suspended substance, low Chl-a values and high POC/Chl-a ratios, and higher δ13CPOC values that was not in parallel with δ13CDIC excursion. The offset between δ13CPOC and phytoplankton δ13C (inferred from δ13CDIC) was trivial or positive in salinity <12, but then became negative downstream, which was likely suggestive of biogeochemical change from net respiration in the upper estuary to net production in the lower and outer estuary. Our results demonstrated that in situ phytoplankton was the dominant source to the estuarine POC pool during most seasons of a year, except in May in the first phase of wet season when rainfall and river flux increased abruptly causing intensive flushing effect. We further suggested that POC may be undergone intensive processing within the PRE, which is important for understanding organic carbon delivery in this vigorous land–ocean interface.