Limnology of an equatorial high mountain lake — Lago San Pablo, Ecuador: The significance of deep diurnal mixing for lake productivity

Abstract

Equatorial high mountain lakes are a special type of lake occurring mainly in the South American Andes as well as in Central Africa and Asia. They occur at altitudes of a few thousand meters above sea level and are cold-water lakes (<20 °C). Relatively little is known about them. A long-term limnological study was therefore undertaken at Lake Lago San Pablo, Ecuador to analyze the basic limnological processes of this lake, which has a tendency for eutrophication. Lago San Pablo is spread over an area of 668 hectares, has a maximum depth of 35 m, and is located 2660 m above sea level. Its thermal stratification is a monomictic one, with only 1–2 °C difference between the epi- and hypolimnion; overturn is achieved by strong winds during the dry summer period. The stratification phase is characterized by an oxygen deficit in the lower part of the hypolimnion. Besides, strong convective currents occur due to nocturnal cooling, and partial lake mixing was observed during the nocturnal period. This type of lake mixing is called atelomixis, which is characterized by the partial mixing of isolated layers (difference in temperature or ionic content) during stratification. The nutrient level of the lake is quite high: mean P total concentration = 0.22 mg/l, mean N total = 1.05 mg/l, soluble reactive phosphorus (SRP) > 0.01 mg/l, and soluble inorganic nitrogen > 0.2 mg/l. Nitrogen and phosphorus are available in the epilimnion all year round (N sol. inorg·. = 0.3 to 1.7 mg/l N, SRP = 0.04 to 0.63 mg/l P). The N/P ratio is sometimes > 14, sometimes < 10, indicating a variability of the limiting nutrient factor. Considering the nutrient level, the phytoplankton biomass is quite low (about 4,000 cells per ml on average; maximum cell number: 13,000 in 1998 and 10,000 in 1999). The mean epilimnic chlorophyll content (Chl a was 10 mg/l in 1998 and 11 g/l in 1999, and the maximum Chl a content was 16 and 22 g/l in 1998 and 1999, respectively. Phytoplankton production can be limited by nutrients, mainly nitrogen, but convective currents can also cause a significant loss of biomass. The lake's euphotic zone is smaller than its epilimnic zone, indicating that light radiation is limiting in the deeper water body, this is caused by a weak thermocline due to destratification by nocturnal cooling, the atelomixis.