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Role of nitrogen in submerged plant development in Mediterranean climatic zone - a mesocosm experiment

Özkan, Korhan
The effects of increasing nitrogen and phosphorus loading on submerged macrophyte development was tested in a mesocosm experiment for three months. Experiment consisted of three NO3-N loadings with factorial of two PO4-P loadings in a fourfold replicated design. Twenty four enclosures placed at one meter depth were isolated from the lake but kept open to sediment and atmosphere. Each enclosure stocked with ten Myriophyllum spicatum shoots with underyearling fish to reduce zooplankton grazers. Biweekly sampling and weekly nutrient additions were performed for three months. Mean total nitrogen (TN) concentrations sustained in nitrogen treatments through out the experiment were 0.52, 1.99, 8.07 mg/l. Both phosphorus treatments converged to a mean concentration below the targeted level, ranging between 0.05-0.1 mg/l TP. In comparison to mesocosm studies in temperate lakes, higher assimilation rates for nutrients were observed in Lake Pedina. Due to extraordinarily high evapotranspiration and drought in 2007, the water level decreased 0.6 m in enclosures. Total macrophyte biomass remained indifferent to nutrient treatments with continuous growth and failed to validate any direct or indirect negative effect of increasing nutrient concentrations. Phytoplankton biomass differed significantly among factorial treatments but remained low, while periphyton biomass differed among nitrogen treatments. In comparison with other studies the phytoplankton biomass remained low and the periphyton biomass became high for reference TP concentrations, indicating a competitive advantage of periphyton over phytoplankton on nutrient utilization in the enclosures. Zooplankton:phytoplankton biomass ratio was low throughout the experiment and zooplankton community mainly consists of smaller species, reflecting high predation pressure.