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Processes affecting the concentration and isotopic composition of dissolved inorganic carbon (DIC) were investigated in pore waters of two freshwater intertidal areas of the Scheldt Estuary, Belgium. Pore water delta13CDIC values from marshes and mudflats varied from -2.7 to +1.34%, these very large variations reflect the contribution of different carbon sources to the DIC pool. In pore waters of the upper mudflat, river water DIC and dissolution of calcite contribute to a lesser extent (10% and 16% respectively) to the total DIC pool. Results indicate that inorganic carbon added to the porewater of the mudflats has a delta13C value of +2.03% in May 1998. These strongly enriched delta13CDIC values suggest that the major contribution (up to three-quarters) to total DIC is CO2 derived from methanogenesis. In pore watersof the marshes, CO2 derived from organic matter degradation (-2.75%) and river DIC (-1.15 to -1.61%) are the major sources of inorganic carbon contribution to the total DIC pool. In pore waters from a marsh site colonised by willow trees, the contribution from CO2 derived from organic matter degradation is larger than inpore waters from an area with only reed vegetation. In the latter case river water DIC is the major source of pore water DIC. [ABSTRACT FROM AUTHOR]

We investigated the seasonal and geographical variation in the stable carbon isotope ratios of total dissolved inorganic carbon (Delta13CDIC) and suspended matter (Delta13CPOC) in the freshwater part of the River Scheldt. Two majorsources of particulate organic matter (POM) occur in this riverine system: riverine phytoplankton and terrestrial detritus. In winter thelowest Delta13CDIC values are observed due to enhanced input of CO2 from decomposition of 13C-depleted terrestrial plant detritus (average Delta13CDIC = ?/14.3). During summer, when litter input from terrestrial flora is the lowest, water column respiration on POM of terrestrial origin is also the lowest as evidenced by less negative Delta13CDIC values (average Delta13CDIC = ?9.9). In winter the phytoplankton biomass is low, as indicated by low chlorophyll a concentrations (Chl a < 4.5 Mu gl-1), compared to summer when chlorophyll a concentrations can rise to a maximum of 54 Mu gl-1. Furthermore, in winter the very narrow range of Delta13CPOC (from ?26.5 to ?27.6) is associated with relatively high C/N ratios (C/N > 9) suggestingthat in winter a major fraction of POC is derived from allochthonousmatter. In summer Delta 13CPOC exhibits a verywide range of values, with the most negative values coinciding with high Chl a concentrations and low C/N ratios (C/N < 8). This suggestspredominance of phytoplankton carbon in the total particulate carbonpool, utilising a dissolved inorganic carbon reservoir, which is already significantly depleted in 13 C. Using a simple two source mixing approach a reconstruction of the relative importance of phytoplankton to the total POC pool and of 13 C/12C fractionation by phytoplankton is attempted. [ABSTRACT FROM AUTHOR]

In situ decomposition of senescent leaves of two abundant mangrove species (Rhizophora mucronata Lamarck and Ceriops tagal (Perr) C.B. Rob), enrichment of nitrogen and activity of dinitrogen fixing bacteriaduring decomposition were investigated during both rainy and dry seasons in a tropical coastal lagoon (Gazi, Kenya). Rates of leaf decomposition were higher for R. mucronata than for C. tagal and were highest, for both species, during rainy season. Rates of decomposition, expressed as percentage dry mass loss, over a decomposition period of 50 days was: C. tagal (rainy season), 69%; C. tagal (dry season), 27%;R. mucronata (rainy season), 98%; and R. mucronata (dry season), 48%. High rainfall and diurnal tidal inundation appear to enhance the leaf decomposition process. Maximum rates of nitrogen fixation were 380nmol N2 h-1 g-1 dw for C. tagal (rainy season), 78 nmol N2 h-1 g-1 dw for C. tagal (dry season), 390 nmol N2 h-1 g-1 dw for R. mucronata (rainy season) and 189 nmol N2 h-1 g-1 dw for R. mucronata (dry season). Although N2 fixation rates were highest during rainy season,total nitrogen immobilised in the leaves was highest during the dry season. Biological nitrogen fixation can account for between 13 to 21% of the maximum nitrogen immobilised in the decaying mangrove leaves. Nitrogen fixation, as a source of allochthonous nitrogen, sustains a nitrogen input to the mangrove ecosystem, which adds significantly to the nitrogen input through leaf litterfall. [ABSTRACT FROM AUTHOR]