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

Abstract

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 ($6500-2200 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 $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 $1100 years BP (after fully freshwater conditions established) pollen grains of trees were observed.

South American lake paleo-records across the Pampean Region

“…2). Paleolimnological results indicate that, over long timescales, similar trophic states to those observed at present existed during the Holocene, due to climate and sea level variations (García-Rodríguez et al, 2004;.…”

Phytoplankton community structure in five subtropical shallow lakes with different trophic status (Uruguay): a morphology-based approach

“…Except for Cisne Reservoir, all other lakes (Blanca, Escondida, Clotilde, and García) were originated after the Holocene marine transgression (García-Rodríguez et al, 2004) and nowadays are completely isolated from the sea. The lakes encompass a trophic gradient from oligo-mesotrophy to hypereutrophy (Kruk et al, 2006(Kruk et al, , 2009b, according to Salas & Martino (1991) criteria (Table 1).…”

Climate change impacts on lakes: an integrated ecological perspective based on a multi-faceted approach, with special focus on shallow lakes

“…It is, however, possible to track ecosystem change with negligible anthropogenic impact if the records cover several centuries (Gregory-Eaves and Beisner, 2011) or are obtained from the currently more pristine ecosystems of the globe, such as lakes and ponds in the Arctic (Smol et al, 2005). In the sub-tropics (with a short history of anthropogenic impacts), analysis of sediment cores has suggested that climate amelioration (i.e., warmer and more humid conditions) has co-occurred with events of natural eutrophication (García-Rodríguez et al, 2004;Inda et al, 2006). However, in more populous regions with a long history of catchment activity, it has generally proved very difficult to separate the effects of recent climate change from other anthropogenic pressures, in particular point and diffuse source nutrient addition , acidification (Simpson and Anderson, 2009) and atmospheric nitrogen deposition (Hobbs et al, 2010).…”