Physical and Chemical Controls Methane Bubbling from Two Tropical Reservoirs

University of Utah Department of Geology and Geophysics

Abstract

Methane production in lake bottom sediments is an important geochemical process in tropical reservoirs, as well as being a critical component of the global methane budget. Tropical reservoirs are especially productive because warm temperatures and large organic inputs promote vigorous biogeochemical cycling. Quantifying methane flux to the atmosphere from lakes via ebullition (bubbling) is difficult, as bubbling is episodic and is dependent upon several physical and chemical factors. In an effort to quantify these factors in tropica l climates, Lake Loíza in east-central Puerto Rico and Lake Gatun in Panama have been studied in detail over the past two years.

Methane emission by bubbling was measured at each of the two sites using floating traps over 35 hour to 73 hour study periods. Bottom current velocities, wind velocities, dissolved oxygen content, pH, and temperature were recorded at both lakes throughout the experiment period. Some correlation was observed between high current velocities and increased gas flux, indicating bottom shear stress as a trigger for gas release, but such relationships were not apparent at lower current velocities. Current acceleratio n often demonstrated a better correlation with ebullition events than current velocity alone. A general inverse relationship between gas flux and water depth was observed in both lakes. Additionally, a 3 day long period of very low flux was observed in the ordinarily productive Lake Loíza following a rapid increase in water depth, indicating that fluctuating water levels in man-made lakes play an important role in methane flux to the atmosphere. A laboratory experiment testing the effect of sudden changes in water depth on methane bubbling confirmed the hypothesis that rapid increases in water level have the potential to temporarily inhibit bubbling. Relationships between the dissolved oxygen content of the water column and methane flux were not evident at shallow sites (less than or equal to 6 m), indicating that the oxygen content of shallow tropical lakes may not be useful in predicting methane flux to the atmosphere.

• WEBB Project Bibliography
• WEBB Project Chief Home Page
• Caribbean Water Science Center home page