Changes in paleoproductivity and lake regimes and its relation with past climate anomalies based on photosynthetic pigments in shallow lakes of the Pampa plain (Argentina) over the last millennium.

Joint research project

Project leaders: Andrea Lami, María De Los Ángeles González Sagrario
Agreement: ARGENTINA - CONICET - Consejo Nacional de Investigaciones Científicas y Técnicas
Call: CNR/CONICET 2015-2016
Department: Earth system science and environmental technologies
Thematic area: Earth system science and environmental technologies
Status of the project: New

Research proposal

Lake sediments contain many different lake productivity proxies, among which photosynthetic pigments, representing a fraction of the organic matter, constitute the most direct and accurate indicator. Therefore, photosynthetic pigments can be used to reconstruct historical trends of lake primary production, changes in lake regimes or the response of the phototrophic community to environmental changes such as climate anomalies, UV radiation, water level fluctuations and also, to track changes in lake functioning as, for example, nutrient dynamics or the onset of the stratification regime (Guilizzoni et al. 2009, Lami et al. 2009, Romero-Viana et al. 2010, Guilizzoni et al. 2011, Cantonati et al. 2014).
Shallow lakes are complex ecosystems and have become the archetypical example of ecosystem with alternative stable states or regimes (Scheffer and van Nes 2007). Temperate shallow lakes may be dominated alternatively by charophytes, submerged angiosperms, green algae, diatoms or cyanobacteria (Scheffer and Nes 2007, Allende et al. 2009). In the first two cases, the lake is under a clear state or regime, whereas if algae or cyanobacteria dominate the lake is in a turbid regime. Clear and turbid regimes occur at low/intermediate and intermediate/high nutrient levels, respectively.
Regime shift can be defined as the sudden drastic transition from one persistent dynamical regime to another and may be due to different mechanisms, such as drastic impact on the system or stepwise change in some important external condition(Scheffer and Jeppesen 2007). Lakes can oscillate between clear and turbid regimes in a cyclic (van Nes et al. 2007) or irregular (Hargeby et al. 2007) way as response to external event like extreme weather conditions, as high precipitation or drought. For example, the Pacific Decadal Oscillation (PDO) has changed the phenology of lake phytoplankton at the community and species level and the onset and duration of the stratification period in Lake Washington (Arhonditsis et al. 2004, Walters et al. 2013). The transition from holomixis to meromixis occurred ca. 1661 AD in Lake La Cruz and has been a process mainly driven by climatic forcing associated with the Maunder minimum (Romero-Viana et al. 2010). In South America, two climatic anomalies, the Little Ice Age (century XV - XVIII, dry and cold phase) and the Medieval Climate Anomaly (century XI - XIV, warm and humid phase), have been documented to affect the north-central area of the Pampa plain during the last millennium (Piovano et al. 2009). These anomalies could have affected lake regimes synchronously in the Pampa plain area during the last 1000 years.
A bimodal transparency pattern in shallow lakes has been described for South America (Kosten et al. 2012), and, in particular, clear, turbid organic (due to phytoplankton) and turbid inorganic lakes has been recognized in the Pampa plain (Quirós et al. 2002, Allende et al. 2009). This area is highly rich in shallow lakes, which are mostly eutrophic, and regime shifts have been related to lake size and hydrological conditions in neo-limnological studies. Specially, regime shifts have been documented to occur linked with precipitation/drought mostly in lakes of small size (Kosten et al. 2012). Furthermore, neo-limnological studies revealed that phytoplankton composition differed between clear and turbid lakes. The first ones, are dominated by mobile nanoplanktonic species like cryptophyceans and crysophyceans, while turbid lakes are dominated by bloom-forming cyanobacteria and large pennate diatoms or cynobacteria and chlorophytes (specially chlorococcaleans) (Allende et al. 2009, Torremorell et al. 2009, Izaguirre et al. 2012). Therefore, the differential composition of photosynthetic primary producers would allow the detection of clear regime in past lake productivity through the analysis of the ratio of pigment concentrations like total cryptophyceans and crysophyceans markers to total chlorophyll.
Lake phytoplankton is affected by several factor besides climate, like nutrient concentration, the light environment, onset and duration of the stratification, concentration of dissolved organic carbon or grazing pressure. Paleo-limnological studies can infer and reconstruct the impact of some of this variables like nutrient level (through transfer functions) or grazing pressure (through biomarkers, invertebrates remains; e. g. Jeppesen et al. 2001, Guilizzoni et al. 2011). The evolution of Lake Lonkoy, Nahuel Rucá and Hinojales (South east of the Pampa plain) have been reconstructed based on pollen, non-pollen palynomorphs and macrofossil remains for the last 5000 years(Stutz 2010, Stutz et al. 2012). Future studies must address changes on the pattern of paleoproductivity and its related causes, the impact of grazing by zooplankton and the degree of dependence on autochthonous versus allochthonous resources in the Pampa plain area. The evolution of shallow lakes in the south east area had been similar and synchronic(Stutz et al. 2012); therefore, further research is necessary to elucidate if these pattern can be detected at a regional scale and could be related to a climatic forcing.

Added value of the cooperation: The interaction of paleo- and neolimnological research lines to solve ecological research questions about shallow lake functioning dates back to 2010. The proposed project will add a new component in the analysis of lake paleoproductivity based on pigment marker and diatom remains interpretation to enlighten the understanding of different feature of lake functioning e.g. regime shifts, primary production changes. We expect that the present proposal will set the basis for future collaboration between our research group and institution (IIMYC-CONICET/UNMDP) and CNR-Instituto per lo Studio degli Ecosistemi.

Research goals

The research project presented here will clarify the occurrence of changes in shallow lake regimes (turbid and clear) and its relation with past lake productivity and climate events during the last millennium in shallow Pampean lakes. This aim will be accomplished through the combination of cladoceran remains, macrophyte pollen and macro-remains, non-pollen palinomorphs (including cianobacteria and chlorophycean remains), analyzed by the Argentinian team and pigment analyses performed at the CNR laboratory. The multi-proxy approach will allow to reconstruct the lake primary productivity changes in lakes located along a precipitation gradient from the coast to the inner part of the continent. With this research, we expect to advance on the understanding of:
1- Diagenetic process in shallow lakes sediments and possible implications for paleoproductivity reconstruction.
2-Timing and characterization of turbid and clear lake regimes based on primary producer taxa and functional groups along the studied period.
3-Patterns of primary producer dominance, as pigment concentration or ratios (e. g. cyanobacteria or cryptophycean marker to total chlorophylls,) and its relation with grazing pressure.
4-Past climate anomalies (Little Ice Age, Medieval Climate Anomaly) impact on shallow lake paleoproductivity and lake regimes.
5-Regional sensibility of phothosynthic organisms to climate changes.
6-Changes in lake metabolism (autotrophy versus heterotrophy) along the studied period.

Last update: 25/04/2024