Effects of sugar cane monocultures on origin and characteristics of dissolved organic matter in the Manguaba lagoon in northeast Brazil

Brazil has extensive sugar cane monocultures, which significantly alter hydrogeochemical material fluxes. We studied dissolved organic matter (OM) fluxes in the Manguaba lagoon-estuary system, which drains a sugar cane monoculture-dominated hinterland and discharges into the Atlantic coastal ocean. The OM fluxes into the lagoon originate from baseflow, field runoff and sugar cane factory effluents. In the study, dissolved organic carbon (DOG) concentration, delta C-13 DOG and UV absorbance were analysed along a freshwater-seawater salinity gradient that encompasses river (DOC 9-11 mg l(-) (1), delta C-13 -22.2 parts per thousand to -25.5 parts per thousand); lagoon (4-11 mg l(-1), -20.5 parts per thousand to -24.8 parts per thousand); estuary (3-9 mgl(-1), -22.6 parts per thousand to -25.3 parts per thousand) and coastal waters (1.64 mg l(-1), -21 parts per thousand) with different intra-seasonal runoff conditions. We used the carbon isotope data to quantify the sugar cane derived DOG. Where river water meets brackish lagoon water, substantial loss of DOG occurs during rainy conditions, when suspended sediment from eroded fields in the river is very high. During dry weather, at much lower suspension levels, DOG increases, however, presumably from addition of photolysed resuspended sedimentary OM. In the estuary, mixing of DOG is strictly conservative. Ca. 1/3 of riverine DOM discharged into the lagoon has a sugar cane source. Within the lagoon on avg. 20% of the bulk DOM is comprised of sugar cane DOM, whereas during heavy rainfall the amount increases to 31%, due to intensified drainage flow and soil erosion. In the estuary, 14-26% is of sugar cane origin. The sugar cane-derived component follows the mixing patterns of bulk DOM. (C) 2010 Elsevier Ltd. All rights reserved.