Free-floating macrophyte dynamics effects on phytoplankton ecology

O´FARRELL, INÉS; DE TEZANOS PINTO, P.; RODRÍGUEZ, PATRICIA; CHAPARRO, G.; PIZARRO, HAYDÉE

Punta del Este, Uruguay

Congreso; Structure and function of world shallow lakes; 2008

Free-floating macrophyte dynamics effects on phytoplankton ecology O’Farrell, Inés(1)(2), de Tezanos Pinto, Paula(1)(2), Rodriguez, Patricia L(1)(2)., Chaparro, Griselda(1)   and Pizarro, Haydée N. (1)(2)  (1)Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina.  (2)CONICET (Consejo Nacional de Investigación Científica y Tecnológica), Argentina This field experiment highlights several key processes triggered by free floating plants (FFP) dynamics on natural phytoplankton composition, diversity and production and on the physico-chemical conditions. We simulated three commonly encountered scenarios in warm temperate floodplain shallow lakes: FFP persistent dominance, 5-day FFP fluctuation and FFP absence. The experiment was performed in late summer 2006, from March 8 to April 6, using field enclosures settled in the sediments. FFP effect was mimicked with dark meshes. Physico-chemical variables and phytoplankton were analysed every 5 days. Primary production determinations were performed on three occasions. The effects of persistent floating mats on light interception triggered a complex ecosystem response where photosynthesis did not overcome anoxia, marked phytoplankton species and functional group loss and low biomass occurred, yet maintaining a high diversity and productivity. The resulting anoxic conditions promoted phosphate and ammonia release from sediments, which added up to a low biological phytoplankton uptake due to light limitation, determined enhanced dissolved nutrient concentrations. High richness and biomass, but with low functional diversity, characterized the system lacking FFP where light was sufficient but nitrogen limiting. Fluctuating FFP cover determined periodic shifts between these two limiting resources, which most probably explain the maintenance of richness, diversity and production in these shallow lakes. Nitrogen and phosphorus were replenished during each dark period and decreased in the free water phases on behalf of phytoplankton uptake. The imposed cyclic interruption of light produced concomitant abundance oscillations of several functional groups; their persistence under oligophotic conditions provided an inoculum for their recovery as soon as conditions turned favorable. Our results confirm the maintenance of high functional diversity by externally imposed disturbances and its drop in the non-disturbed uncovered situation. However, in the permanently covered scenario diversity remained high in spite of the lack of disturbance due to high evenness. FFP patchiness in shallow lakes allows wind-driven displacements of mats to generate disturbances affecting the community attributes. This study contributes to sharpen the recently proposed framework of FFP alternative stable states, providing experimental field evidence of the mechanisms involved in the shifts between floating plants and phytoplankton dominance.