Early decomposition of Ruppia cirrhosa (Petagna) Grande and Potamogeton pectinatus L. leaves

Resum

The decomposition of Ruppia cirrhosa and Potamogeton pectinatus was studied in laboratory in order to determine the release of nitrogen and phosphorus to water. We compare differences in short-term decomposition rates between R. cirrhosa and P pectinatus, and we discuss the possible effect of the decomposition on the physical and chemical characteristics of the water. Senescent leaves of the two macrophyte species were put into plastic vessels with filtered water from La Tancada lagoon (Ebro River Delta, NE Spain). We analyzed biomass, C, N and ash content in the macrophytes samples and pH, conductivity, temperature and alkalinity and oxygen, N and P concentration in the water. We observed a decrease in oxygen concentration and pH, and an increase in alkalinity during the first 12 hours of incubation. Phosphorus and nitrogen as ammonium increases were higher in water with macrophytes than in water without macrophytes (control). Oxydized nitrogen decrease and SRP and ammonium increase were observed coinciding with dissolved oxygen depletion. Pattern of variation in N content was different in both macrophytes related to the chemical nature of the detritus and to inorganic nutrient concentration in the water. A quick nitrogen release in inorganic form during the four days of incubation was observed. Increase of SRP concentration may be related to leaching and to P solubility increase under anaerobic conditions.