Hydrobiologia vutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 392: 65-71,1999. H.L. Golterman (ed.], Sediment-Water lnteraction 9. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 65 Pore water N and P concentration in a floodplain marsh of the Lower Paraná Ríver" Carlos Villar 1 , Laura de Cabo/, Panchabi Vaithiyanathan 'VUTSRQPONMLKJIHGFEDCBA & Carlos Bonetto 'BA 1 Instituto de Limnologia Ringuelet, Av. Calchaqui km 23,5 1888 Florencio Varela, Buenos Aires, Argentina (Be- cario CIC) 2 Museo Argentino de Ciencias Naturales Bernardino Rivadavia, CC 220, Suco 5, 1405 Buenos Aires, Argentina 3 Duke University Wetland Center, 16139 Okeechobee Blvd., Loxahatchee, FL 33470, U.S.A. Key words: nitrogen, phosphorus, pore water, macrophyte, floodplain, Paraná River Abstract Inorganic nitrogen and soluble reactive phosphate (o-P) concentrations were measured in the water of a marsh and in its interstitial water at two sites, and in the river water of a floodplain marsh of the Lower Paraná River. These values were compared with the N and P concentration in sediments and macrophyte biomass in order to assess nutrient availability, fate and storage 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 1- 1 , but showed a pronounced minimum (0.5-0,7 mg 1- 1 ), close to (March 95), or relatively soon after (May 94) the end of the macrophyte growing season. Soluble phosphate showed a large variation between P = 0.1-1.1 mg 1- 1 without any discernible seasonal pattern. NH4+ depletion in the pore water concentration and low NIP ratios (3.7 by weight) within the macrophyte biomass at the end of the growing period suggest that available 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, suggesting a permanent flux of nutrients from the sediments. o-P accumulate in the marsh leading to higher concentrations than in the incoming river. NH4+ did not accumulate in the marsh, and no significant differences were observed between the river and the marsh water, while the N03 - contributed by the river water was depleted within the marsh, caused probably by coupled nitrification-denitrification at the sediment-water interface. Although an order of magnitude smaller, the pore water pool can supply enough nutrients to build up the macrophyte biomass pool, but only if a fast turnover is attained. The Paraná floodplain marsh retains a large amount of nutrients being stored mainly in the sediment compartment. Introduction Floodplains often sustain high primary production as- sociated with a high nutrient contribution from the river. Within the deltaic marshes of the Paraná River large volumes of water are exchanged between the river and the floodplain, their magnitude depending on river discharge, tidal amplitude and wind influ- ence. Marsh water is enriched in soluble reactive phosphorus (o-P) but poor in inorganic N with re- spect to the incoming river water (Villar et al., 1996). Marshes are covered by dense stands of emergent mac- * The authors use the abbreviations as suggested by the editor. See page 1. rophytes, Cyperus giganteus and Scirpus californicus being dominant, and both sustained by rhizomes. Low NIP ratios found in macrophyte tissues suggests N limitation for plant growth (Villar et al., 1996). Since the emergent macrophyte biomass is five times larger below ground than above ground (Bonetto, 1996) it may be expected that the sediment interstitial water is the main nutrient source for plant growth. The aim of this paper is to improve our knowledge on the fate of nutrients, their availability and the storage capacity of the Paraná River floodplain marshes. Therefore, we assessed the N and P concentrations in the sediment interstitial water and compared them with the amounts present in the marsh water, in the sediment profile and in the macrophyte biomass.