Short meromixis in a shallow lake of the semi-arid pampean region

Abstract

Meromictic lakes do not mix entirely due to a strong saline gradient that separates the mixolimnion (superficial, less saline) from the monimolimnion (deep, more saline). The East Lagoon of the Parque Luro Provincial Reserve (La Pampa) is a typical hypersaline (salitral) lagoon that, after an exceptional rainfall, was transformed into a shallow meromictic lake. The present work aimed to survey the changes in the physicochemical characteristics in the water and in the planktonic communities during meromixis, which lasted at least 7 months. Compared to the monimolimnion, the mixolimnion was characterized by lower salinity and nutrients; however, it had higher pH, dissolved oxygen, chlorophyll, and organic solids. The shallow depth of this lake and the action of the winds caused the end of the meromixis. Phytoplankton was dominated almost exclusively by Dunaliella salina and during meromixis, no differences in density or biomass were observed in its vertical distribution. Zooplankton (generally dominated by Artemia persimilis) decreased in richness with increasing salinity. During meromixis, this species coexisted with Boeckella poopoensis, and both had high densities of juveniles in the mixolimnion relative to the monimolimnion. The episode of a short meromixis, as described in this study, allowed the observation of diverse limnological conditions and the development of a greater number of zooplankton species, some endemic to the region. These attributes make this shallow lake a site of particular conservation interest.