Extreme drought decouples silicon and carbon geochemical linkages in lakes

Abstract

Silicon and carbon geochemical linkages were usually regulated by chemical weathering and organism activity, but had not been investigated under the drought condition, and the magnitude and extent of drought effects remain poorly understood. We collected a comprehensive data set from a total of 13 sampling sites covering the main water body of the largest freshwater lake system in Australia, the Lower Lakes. Changes to water quality during drought (April 2008–September 2010) and post-drought (October 2010–October 2013) were compared to reveal the effects of drought on dissolved silica (DSi) and bicarbonate (HCO3−) and other environmental factors, including sodium (Na+), pH, electrical conductivity (EC), chlorophyll a (Chl-a), total dissolved solids (TDS), dissolved inorganic nitrogen (DIN), total nitrogen (TN), total phosphorus (TP) and water levels. Among the key observations, concentrations of DSi and DIN were markedly lower in drought than in post-drought period while pH, EC and concentrations of HCO3−, Na+, Chl-a, TDS, TN, TP and the ratio TN:TP had inverse trends. Stoichiometric ratios of DSi:HCO3−, DSi:Na+ and HCO3−:Na+ were significantly lower in the drought period. DSi exhibited significantly negative relationships with HCO3−, and DSi:Na+ was strongly correlated with HCO3−:Na+ in both drought and post-drought periods. The backward stepwise regression analysis that could avoid multicollinearity suggested that DSi:HCO3− ratio in drought period had significant relationships with fewer variables when compared to the post-drought, and was better predictable using nutrient variables during post-drought. Our results highlight the drought effects on variations of water constituents and point to the decoupling of silicon and carbon geochemical linkages in the Lower Lakes under drought conditions.