Holocene trophic state changes in relation to sea level variation in Lake Blanca, SE Uruguay

Paleolimnological data are presented relating trophic development to sea level variation in Lake Blanca, a small (0.6 km(2)), coastal fresh waterbody in southern Uruguay. Using a sediment core that extended to 7300 years BP, analyses of grain size, thin sections, organic matter, carbonate, total carbon, nutrients, diatoms and palynomorphs, allowed us to infer changes in trophic state and paleosalinities, which were closely related to Holocene sea level variation. Higher trophic states were observed during regressive events, most probably due to increases in runoff and erosion as regression progressed. Four diatom association zones (DAZ) were identified in the sediment core. The basal core section pre-dated the first Holocene marine transgression, contained no diatoms, chrysophyte cysts or non-siliceous microalgae, and showed C/N ratios values higher than 20. Thus, it is likely that the system exhibited terrestrial characteristics. In the second section (similar to6500 - 2200 years BP, following the first Holocene transgression), there was dominance of marine/ brackish diatom species. The lowest trophic states of the core were observed in this section. The third section ( 2200 - 1100 years BP), represented the system as it became separate from the Atlantic Ocean, and showed a dominance of brackish/ freshwater species and increases in trophic state were observed. In the last section ( after 1100 years BP), the system became fully freshwater as no marine or brackish diatom species were found, but a trend to oligotrophication was observed, probably due to nutrient depletion. However, after similar to 1967 AD, eutrophication intensified because of forestry and soil fertilization in the catchment. Pollen association zones (PAZ) allowed us to identify four sections. Below 250 cm ( 2200 years BP), the core contained no pollen grains as redox potential and pH values were not conducive for pollen preservation. After 2200 years BP ( when the system started to separate from the ocean), xerophilic taxa typical of coastal dunes colonized the catchment. Only after similar to 1100 years BP ( after fully freshwater conditions established) pollen grains of trees were observed.