Pore water N and P concentration in a floodplain marsh of the Lower ParanaRiver

Inorganic nitrogen and soluble reactive phosphate (o-P) concentrations were measured in the water of a marsh and in its interstitial water at two site s, and in the river water of a floodplain marsh of the Lower Parana River. These values were compared with the N and P concentration in sediments and macrophyte biomass in order to assess nutrient availability, fate and stora ge capacity. High variability was found in the interstitital water using a 1 cm resolution device. Nitrate was never detected in the pore water. Depth averaged NH4+ concentrations in the upper 30 cm layer often ranged from N = 1.5 to 1.8 mg l(-1), but showed a pronounced minimum (0.5-0.7 mg l(-1)), close to (March 95), or relatively soon after (May 94) the end of the macro phyte growing season. Soluble phosphate showed a large variation between P = 0.1-1.1 mg l(-1) without any discernible seasonal pattern. NH4+ depletion in the pore water concentration and low N/P ratios (3.7 by weight) within the macrophyte biomass at the end of the growing period suggest that availa ble N limits plant growth. NH4+ and o-P concentrations were 35 and 7 times higher, respectively, in the pore water than in the overlying marsh, sugges ting a permanent flux of nutrients from the sediments. o-P accumulate in th e marsh leading to higher concentrations than in the incoming river. NH4+ d id not accumulate in the marsh, and no significant differences were observe d between the river and the marsh water, while the NO3- contributed by the river water was depleted within the marsh, caused probably by coupled nitri fication-denitrification at the sediment-water interface. Although an order of magnitude smaller, the pore water pool can supply enough nutrients to b uild up the macrophyte biomass pool, but only if a fast turnover is attaine d. The Parana floodplain marsh retains a large amount of nutrients being st ored mainly in the sediment compartment.