Hydrobiologia 341: 21-26, 1996. 21 © 1996 Kluwer Academic Publishers. Printed in Belgium. Factors influencing biomass and nutrient content of the submersed macrophyte Egeria densa Planch. in a pampasic stream Claudia S. Feijo6 1 ,2, Fernando R. Momol, Carlos A. Bonetto 3 & Nuncia M. Tur 3 1Programa de Ecologfa Acudtica, Departamento de Ciencias Bdsicas, Universidad Nacional de Lujdn, C. C. 221, 6700 Lujdn, Buenos Aires, Argentina 2 Comisidn de Investigaciones Cient(ficas de la Provincia de Buenos Aires, Argentina 3 CONICET, Buenos Aires, Argentina Received 17 January 1996; in revised form 10 September 1996; accepted 19 September 1996 Key words: submersed macrophytes, nutrients, biomass, sediment Abstract We identified factors influencing biomass and nutrient content in E. densa in an enriched pampean stream of Argentina. Physical (current velocity, temperature), chemical (pH, conductivity, dissolved oxygen, nutrient content in water and sediments), and biological variables (biomass and nutrient content of E. densa, biomass of periphyton and other macrophytes) were estimated at each sampling occasion, and mean monthly values estimated. Biomass and nutrient content in E. densa were correlated with these physical-chemical and biological variables. Biomass was positively correlated with ammonium in stream water (P<0.05) and sediment total nitrogen (P<0.01). Nitrogen showed a positive relationship with ammonium (P<0.01), and a negative one with nitrate and periphyton biomass (P<0.05). Phosphorus was positively correlated with soluble reactive phosphorus (P<0.01). The growth of other macrophyte species in the stream seemed to influence E. densa biomass, probably through competition for light. Current velocity was low and not significantly related with E. densa biomass; however, a flood at the beginning of the study washed the macrophyte stand downstream. Introduction The 'Pampa' is a vast grassy plain that covers central Argentina. It is composed of loess, deposited during the Quaternary Period, and formed the rich and deep soils (typical argiudolls) that support the cattle and agricultural production of the country. Owing to the gentle slope of the plain (0.5%), the small rivers and streams of the NE of Buenos Aires Province run east- wards towards the rivers Parana and De la Plata (Sala & Auge, 1970). These rivers are mainly fed by local subsurface run-off. Rainfall only contributes c. 12% of the mean water discharge of these rivers, but this con- tribution can increase to 60% during floods (>200 mm precipitation in three days) (Sala et al., 1983). Egeria densa Planch. is a submersed macrophyte native to South America. It has been introduced to other regions where it became a nuisance, especially in North America and New Zealand (Ashton & Mitchell, 1989). Due to its invasive capacity, it has replaced the native aquatic vegetation in some water bodies (Coffey & Clayton, 1987; Haramoto & Ikusima, 1988; Nakanishi et al., 1989; Tanner et al., 1990a, 1990b). In spite of its wide distribution, there are relative- ly few studies on its ecology (Cook & Urmi-K6nig, 1984). The influence of light and temperature on growth and metabolism have been studied (Barko & Smart, 1981), as well as its mobilization of sedimen- tary phosphorus (Barko & Smart, 1980). Haramoto & Ikusima (1988) analyzed its ecophysiological life cycle in a lotic irrigation ditch. However, the relationships between growth of E. densa and the different physical, chemical and biological variables remain unreported in the original environment of the plant. This study thus aimed at identifying those factors that influence biomass and nutrient content in E. densa in a stream of the Pampean Plain.