Allocation of particulate organic carbon from different sources in two contrasting estuaries of southeastern USA

Particulate organic carbon (POC) is an important component in the carbon cy cle of estuarine systems. Many studies have utilized carbon isotopic compos ition (delta C-13-POC) to estimate the origin of POC. Most of the early stu dies were restricted to areas of minimum terrestrial inputs of POC, whereas later studies used multiple stable isotope analysis. In our study, we asse ss the proportion of POC from C-3 and C-4 plants and microalgae in two terr estrially affected, contrasting estuaries of the southeastern U.S.A.: the A ltamaha (Piedmont) and the Satilla (blackwater) Rivers. Our analysis was ba sed on a mass balance equation, taking into consideration the estimated amo unts of microalgae and C-3 and C-4 plant material. Analysis of delta C-13 O f extracted chlorophyll a (Chl a) was used to estimate the isotopic signatu re of microalgae to constrain the three endmember mass balance. The maximum concentration of POC fluctuated between 9 and 11 mg C L-1, coinciding with the period of maximum riverine flow. The POC represented 50-70% of the tot al organic carbon. However, large fluctuations were observed, suggesting im portant tidal- and wind-driven resuspension. The delta C-13-POC ranged from -21 to -28 parts per thousand, being more depleted at lower salinities. Th is indicates rapid dilution of terrestrially derived material with estuarin e- or marine-derived material. The Chi a concentration was maximal during s ummer in upstream stations of the Altamaha estuary, while the Satilla gener ally showed a midestuary maximum. The delta C-13 of Chi a ranged from -20.7 to -31.43 parts per thousand, indicating isotopically depleted inputs of m icroalgal-derived material (MaDM). Our results suggest that MaDM is the dom inant component of POC in the Altamaha River during summer, whereas the C-3 component is dominant during periods of lower productivity and high flushi ng. Although MaDM is significant in the Satilla estuary, most of the POC co nsists of material derived from C-3 and C-4 plants.