The winter overturn plays an important role in the lake metabolism. Full
homothermy generally occurs between December and March, when water
temperatures are between 14 and 16°C. Thermal and chemical seasonal stratification
extends from April-May to mid- or the end of December. During the
stratified period, the epilimnion (temperature 15-30°C) has high levels of
dissolved oxygen (7.5-8.5 mg I"1
), but is depleted in nitrogen (N) and phosphorus
(P). The hypolimnion rapidly becomes anaerobic, with high concentrations
of sulfide (5-6 mg 1~') and ammonium (NH4
+
; >600 ug N I"1
). Throughout the year,
ambient levels of soluble reactive phosphorus (SRP) in the trophogenic layer are
low (1-5 jig P I"1
) with a maximum of -10 ug P I"1
during the period of overturn
(Berman et aL, 1997).