Interactions of physical, chemical and biological processes affecting the seasonality of mineral composition and nutrient cycling in the water column of a deep subalpine lake (Lake Garda, Northern Italy)
Salmaso, Nico; Decet, Fabio
Archiv für Hydrobiologie Volume 142 Number 4 (1998), p. 385 - 414
published: Jul 30, 1998
ArtNo. ESP141014204005, Price: 29.00 €
Causes for temporal variations and depth distribution of major ions and algal nutrients in the water column of the deep (Zmax = 350 m) and large (49 km3) oligo-mesotrophic subalpine Lake Garda (Northern Italy) were investigated through monthly samplings from December 1994 to December 1996. The time course of vertical chemical gradients is mainly determined by the balance between algal activity and mineralisation and by the extent of vertical mixing in spring. The increase in calcite saturation (Ω-values up to 9) due to algal CO2 depletion and higher temperatures beginning in late spring is followed by a shift of about two months in triggering of CaCO3 precipitation. Calcite saturation in the hypolimnion is near CaCO3 solubility equilibrium (0.6 < ω <1.3), and such as to favour an incomplete redissolution of the falling calcite crystals, resulting in weak [ca2+] and alkalinity gradients along the water column. The incomplete mixing of the lake since 1992 appears mainly determined by climatic conditions. Consumption of O2 in the hypolimnion is paralleled by an increase of nutrients and dissolved inorganic carbon. However, this correspondence is clearly evident only for SRP, Si and CT whereas major [NO3-] gradients appear mainly confined to the superficial layers. In the hypolimnion, stoichiometric relationships among CT, SRP and O2 are characterised by slopes of the functional lines (inferred from reduced major axis regressions) approximating the Redfield ratios.