The effects of hydrology on plankton biomass in shallow lakes of the Pampa Plain

Climatic and hydrological variability is usually high in the Pampa Plain (Argentina). However it has not studied yet how this variability may affect the phytoplankton and zooplankton biomass and community structure in aquatic systems of this region. The main purpose of this study was to assess flushing effects on nutrient and plankton dynamics in two interconnected very shallow lakes of the Pampa Plain. In order to study the impact of hydrology on the plankton biomass and community structure, we compared the summer plankton community among three consecutive years with contrasting hydrological characteristics. Water residence time varied an order of magnitude among years and this variability was correlated to strong changes in physicochemical and biological lake characteristics. Depending on the water discharge level, the hydrological regime within the lakes ranged from lentic to more lotic conditions. Nutrient and phytoplankton biomass were positively related to water discharges. During high flushing periods, nutrients import from intensive agriculture lands leads to a dramatic increase in trophic conditions. On the other hand, macrozooplankton biomass was positively related to water residence time and showed a dramatic decrease during high flushing years. Rotifers biomass was not affected by interannual water discharge variability during the study period. Our results support that in case of lakes with high flushing rates, zooplankton development is dependent on water residence time and that hydrology may have stronger effects on macrozooplankton biomass than top-down control by planktivores.