Carlos Villar

Litter decomposition of emergent macrophytes in a floodplain marsh of the Lower Paraná River

Abstract The role of litter decomposition on organic matter accumulation and nutrient cycling was studied in a floodplain marsh of the Lower Parana River by means of in situ litterbag experiments. The effect of waterborne nutrients on decomposition rates was studied through a laboratory litterbag experiment. Litter decomposition was rather slow, remaining 40–50% of the initial mass after 2 years incubation. Similar decomposition rates were observed in laboratory and field experiments. Water fertilization did not significantly affect decomposition rates. Since organic matter production is faster than decomposition a net accumulation takes place in the upper layers of the marsh soil. N and P litter concentration increased during the decomposition experiment. Floodplain marshes represent effective sinks of nutrients through litter accumulation.

Cd, Cu and Zn concentrations in sediments and the invasive bivalves Limnoperna fortunei and Corbicula fluminea at the Río de la Plata basin, Argentina

Cd, Cu and Zn content in sediments and bivalves (Limnoperna fortunei and Corbicula fluminea) were studied in the lower 500 km stretch of the Paraná River and in the right margin of the Río de la Plata Estuary. Both bivalves species showed higher metal contents at the Paraná River, possibly related to water chemistry. The Paraná water is circumneutral and soft, while the Río de la Plata coastal water is alkaline, hard, with a higher organic content. Bivalves sampled at the thermal effluent of the Atucha Nuclear Power Plant, at the Paraná River, showed the highest metal content, either from increased absorption at the higher water temperatures (12 °C higher than the river water) or from metal corrosion of the power plant cooling system. L. fortunei and C. fluminea appeared to be successful biomonitor organisms for Cd pollution in the basin, while Cu and Zn content in bivalve tissues was apparently not related to metal load.

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

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 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 macrophyte 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 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 NO3- 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.

Zinc toxicity in the fish Cnesterodon decemmaculatus in the Paraná River and Río de La Plata Estuary.

Acute static tests were performed to assess Zn toxicity on adults of Cnesterodon decemmaculatus (not sexed), a neotropical fish of wide distribution in the Río de la Plata basin. Tests were carried out with ZnCl2 dissolved in natural waters from the Lower Paraná River, the Río de la Plata Estuary, reconstituted waters of similar composition, tap and distilled waters. Median lethal concentration at 24 h was positively correlated to conductivity and the concentrations of Ca2+, Mg2+, and HCO3- (r=0.99, 0.96, 0.97, 0.99, p<0.05, respectively), being higher at the Río de la Plata Estuary (61.2 mg l(-1)) than at the Lower Paraná River (35.3 mg l(-1)). Contaminant load in the natural waters tested was similar at both sites, being Zn concentration 40 and 44 microg l(-1), respectively. An assay with reconstituted water of similar alkalinity to the estuary showed a simultaneous decrease in HCO3- and dissolved Zn, while the Zn speciation, predicted with a computer program, suggested that Zn2+ precipitated as ZnCO3. The effect of Ca2+ on Zn toxicity is also discussed. Zn toxicity was higher in the Río de la Plata Estuary water than in synthetic water of similar ionic composition (24-h LC50 = 93.2 mg l(-1)), suggesting the synergistic effect of other toxic pollutants such as metals, present in the former which were absent in the later.

Trace metal concentrations in coastal marshes of the Lower Paraná River and the Río de la Plata Estuary

Cu, Cr, Pb, Zn and Fe in water, sediment and tissue of the emergent macrophyte Schoenoplectus californicus were studied in three coastal environments differing in aquatic chemistry and tide amplitude: a deltaic floodplain marsh, a coastal vegetated strip of the river and a tidal marsh at the south margin of the Río de la Plata Estuary. Metal content in water was not significantly different between the river and the floodplain marsh. Metal content in sediments showed higher values in the marshes than in the river, suggesting a retention in the marsh. Except for Cu, the highest metal content in sediments and water was observed in the tidal marsh. Pb in plants was undetectable. Metal content was higher in rhizomes, except Cr, which was mainly stored in stems. Rhizome metal content was higher in the river, probably related to the higher redox potential of its sediments. Inflorescence metal content was higher in the marshes: the highest Zn, Cr and Fe contents were measured in the tidal marsh; the highest Cu content was in the floodplain marsh, together with the highest organic matter content of the sediments. Zn, Cr and Fe increased in standing drying stems and in inflorescences in late fall. Inflorescences showed the highest metal content after seeds had been released.

Macrophytic primary production and nutrient concentrations in a deltaic floodplain marsh of the Lower Paraná River

Biomass changes across an annual cycle were followed at two sampling sites in the floodplain marsh of the Lower Paraná River: close to the river-shore and 800 m inside the floodplain marsh, both dominated by Scirpus californicus and Cyperus giganteus. Tidal influence determines a daily exchange of water between the river and the floodplain marsh.Estimated net primary production was higher in the river (2820 against 1770 g m−2). Contents of nitrogen and phosphorus in plant tissue decreased from the river to the floodplain (0.62 to 0.45% N and 0.18 to 0.14% P). In spite of the important water exchange between the river and the floodplain, a decrease in nitrate, oxygen and suspended matter, and an increase in soluble reactive phosphorus in the water were observed from the river towards the floodplain marsh.A primary production gradient exists from the river to the inner floodplain marsh, where production is nitrogen-limited, sustained mainly on nutrients supplied by the river. Floodplain marshes are nitrate sinks, probably through denitrification losses and macrophyte uptake.

Metals contents in two fishes of different feeding behaviour in the Lower Paraná River and Río de la Plata Estuary

Cd, Cr, Cu and Pb concentrations were measured in the water and these plus Zn were measured in the diet, muscle and liver of the detritivorous fish Prochilodus lineatus and the omnivorous fish Pterodoras granulosus at two sites (Lower Paraná River and Río de la Plata right margin) located upstream and downstream, respectively, from Buenos Aires, an important source of xenobiotics. Cu and Cr concentrations in water were higher in the Río de la Plata coastal waters, while Cd and Pb were below detection limits. The stomach contents of P. lineatus showed a higher metal content at the estuary, attaining concentrations within the range reported for contaminated sediments. However, metals in fish tissue were not higher at the estuary and, except for Cd in liver, were lower than in the organic fraction of the stomach content. Thus, only Cd was accumulated. The P. granulosus diet was composed mainly of river vegetation debris and clams, Corbicula fluminea, at the estuary. Cd, Cu and Pb in liver were higher at the estuary while only Cu was higher in the diet at this site. The different nature of the diet seems associated with a differential bioavailability of its metal content. P. granulosus accumulates Cu at both sites and Cd at the estuary. Both fishes showed higher metal content in liver than in muscle. Cr in liver was below detection limits. No relation was found between metal content and fish size. Overall, metal content in fish tissues were low, except for Cu in P. granulosus and Cd in P. lineatus, similar to those of uncontaminated sites, suggesting that homeostatic mechanisms prevent metal accumulation.

Acute Cu toxicity, metal bioaccumulation and ion loss in the Neotropical cyprinodontiform Jenynsia multidentata

The acute toxicity of Cu, metal bioaccumulation and ion loss in the cyprinodontiform Jenynsia multidentata , a highly eurihaline Neotropical freshwater fish of wide distribution in the Rio de la Plata basin, was established during a 96 h static exposure. The median lethal concentration at 96 h was 229 µg Cu l -1 . The effect of CuSO 4 5H 2 O was tested in natural freshwater in two-liter Pyrex glass chambers at controlled temperature, with natural light and artificial aeration. During the first hours of exposure to concentrations above 130 µg Cu l -1 , an increase in aquatic surface respiration, air gulping and erratic swimming were observed, showing evidence of the affection of the respiratory system. The concentration of Cu, Na and K in the whole body burden at the end of each experiment showed by correlation analysis Cu bioaccumulation and loss of Na + and K + . The concentration of Cu in water was positively correlated with its bioaccumulation (r=0.79, p=0.06), and negatively correlated with the whole body burden of K + (r=-0.84, p=0.037). K + loss was positively correlated with Na + loss (r=0.88, p=0.02). Cu LC50 for this cyprinodontiform seems rather low when compared to fishes often utilized in toxicity tests

Tidal exchange of water between a coastal marsh and the Río de la Plata estuary: the effect on the main physical and chemical variables

At Punta Blanca, a Schoenoplectus californicus marsh of about 1,190 m long and 175 m wide is separated from the Rio de la Plata estuary by a sand bar interrupted at one extreme by a channel through which tidal exchange of water occurs. Water movement in and out the marsh was estimated by installing a water stage height meter and following stage height variations through complete tidal cycles. Incoming river water was sampled twice and outcoming marsh water three times along six tidal cycles, and main physical and chemical variables were determined. The marked decrease of the suspended matter concentration in the outcoming water points out the large retention of sediments by the marsh. Oxygen concentration and water pH decreased in the outcoming water in four over six samplings. Higher concentrations of soluble reactive phosphorus in the outcoming tidal water suggest P release related with riverine suspended matter transformation upon sedimentation, in response to acidification and reduction within the marsh environment. Nitrate impoverishment in the outcoming water suggests denitrification losses. Since ammonium concentration in the outcoming water was larger than in the incoming water, a net flux from the sediments is suggested. Nevertheless, as a result of nitrate depletion and ammonium release, a net loss of inorganic nitrogen took place.