Zooplankton response to different Light regimes caused by floating vegetation in a floodplain wetland from the Lower Paraná River (Argentina).

GRISELDA CHAPARRO; MARÍA SOLEDAD FONTANARROSA; PAULA DE TEZANOS PINTO; PATRICIA RODRÍGUEZ; INÉS O'FARRELL

Punta del Este, Uruguay

Congreso; Structure and function of Shallow Lakes; 2008

Universidad de la República

Dense floating vegetation mats are characteristic of subtropical or warm temperate latitudes floodplain shallow lakes. Notwithstanding, their effect on zooplankton has been scarcely considered. In this study we address the influence of different light regimes associated to floating plants on the aquatic environment and the zooplankton from a floodplain lake. We simulated three scenarios typically encountered in these lakes by using plastic meshes that mimicked floating plant light attenuation: permanent cover, 5-day fluctuating cover and free waters. The experiment was performed for a month on late summer 2006 with in situ enclosures buried in the sediments. Several physico-chemical variables and zooplankton were sampled every 5 days integrating the water column; temporal dynamics of the functional groups was analysed in relation to environmental variables. Zooplankton initial assemblages in the three treatments reflected a lake community of small-bodied taxa characterised by the dominance of rotifers (79%), accompanied by copepods (20%) and cladocerans (1%).  The simulation of a persistent dense macrophyte cover determined poor light conditions with consequent low chlorophyll a concentrations, anoxic water column and high nutrients levels. Zooplankton abundance was very low due to the drastic drop of small herbivores (rotifers and nauplii). Notwithstanding, the nauplii final relative share was 70% of the assemblage. The opposite scenario, enclosures free of cover, was characterised by high chlorophyll a and dissolved oxygen, but low nutrient concentrations. Zooplankton abundance was very high during the initial fifteen days, with enhancement of small herbivores and predators. The final assemblage was heterogeneous and all functional groups co-occurred. Total zooplankton density was positively correlated to phytoplanktonic chlorophyll a. The light fluctuation imposed every 5 days determined a periodic alternation of the resulting environmental variables as typified in the opposite above described scenarios. Nevertheless, zooplankton abundance varied with a 10-day frequency between low and high values, mainly on behalf of the dynamics of small-bodied herbivores. Likewise to the enclosures free of floating plants, no functional groups prevailed by the end of the experiment. The simulation of different light regimes caused significant changes in total zooplankton density (p<0.05), but such alterations were not reflected in a significant change in diversity (Shannon Index).