High nutrient retention in chronically nutrient-rich Pampean streams

VICTORIA GARCÍA; CECILIA HEGOBURU; LORETA GIMENEZ; PATRICIA GANTES; FRANCESC SABATER; CLAUDIA FEIJOÓ

Santander

Congreso; XVII Congress of the Iberian Association of Limnology; 2014

Iberian Association of Limnology

Nutrient retention was generally analyzed in streams with low nutrient concentrations or in impaired streams with high nutrient levels, but information about nutrient retention in chronically enriched streams is scarce. We measured nutrient (PO4 and NH4) retention in two enriched Pampean streams without nutrient point sources along the year. We also measured several variables that could influence nutrient uptake, including physico-chemical (pH, conductivity, dissolved oxygen and nutrient concentrations), hydrological (flow, size of the transient storage zone or As/A), and biological (biomass of the different basal compartments) variables. Metabolism was also estimated using the chamber method for the basal compartments (macrophytes, macroalgae, epiphyton, epipelon) and the open-channel one-station technique for the whole ecosystem metabolism. We observed a high percentage of nutrient retention in both streams (63 and 49 % for PO4, and 87 and 84 % for NH4) despite their high dissolved phosphorus concentrations (0.11 and 0.15 mg P-PO4/l). Phosphorus retention was more variable along the year than ammonium retention, and showed no saturation within the range of the observed basal dissolved phosphorus concentrations. Nutrient retention was not associated to As/A and the biomass of aquatic macrophytes. Phosphorus retention was positively related to water temperature, the degree of heterotrophy of epiphyton and epipelon (measured by the autotrophic index), and the ecosystem respiration, while ammonium retention was related to water temperature, the degree of heterotrophy of epiphyton and seston, epiphyton biomass and the ecosystem P/R. In addition, nutrient uptake rates (U) of PO4 and NH4 were positively associated to each other, indicating an stoichiometric regulation of nutrient uptake. High nutrient retention in these streams may be influence by the activity of several functional compartments, but the heterotrophic processing may play a preponderant role in this process.