Instability of shallow lakes: A matter of the complexity of factors involved in sediment and water interaction?

Instability and extreme fluctuations in water quality characterizing shallow lakes are to a large extent due to rapid changes in the internal supply rates of nutrients. In fact, is in these systems that sediment and water interaction plays a major role in nutrients ’ dynamics. For this study, a double-approach perspective with both field measurements and laboratory experiments has been used in order to determine the contribution of physical, biological, and chemical mechanisms to nutrients ’ dynamics in two shallow adjacent lakes in Andalusia (Spain). Despite their geographic proximity, strong differences between the study systems have been recognised. In the Lake Honda, the high nutrient concentrations, which ultimately support a large algal bio-mass, are the result of: i. Resuspension of the surface sediment favoured by its morphometry, hydrologic regime and sediment granulometry. ii. Intense organic matter mineralization due to the labile nature of the organic settled matter (planktonic). In Lake Nueva, by contrast, physical constrains (i.e. wind-induced resuspension) have a limited effect due to the coarse surface sediment and to the development of macrophytes (Najas marina, Potamogeton pectinatus). In addition, the structurally more complex organic matter of its sediment regulates the low nutrients turnover. In this lake, nutrient exchange rates across the sediment-water interface are also controlled by chemical processes, such as P adsorption onto CaCO3, a mechanism that is favoured by the high Ca+2 concentration in the interstitial water. In this way, the joint effect of physical, chemical, and biologi-cal mechanisms determine the fast nutrients ’ benthic regeneration in Lake Honda, while in contrast, a large fraction of the nutrients present in the sediment of the Lake Nueva is in particle form