Community Respiration Rates in Lake Superior

Abstract

Phytoplankton photosynthesis and community respiration are two key components of the carbon cycle that determine the magnitude of net ecosystem production and the balance between oxygen production and oxygen consumption in lakes. As part of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project, rates of community respiration were measured in 1998 and 1999 in nearand offshore waters along the Keweenaw Peninsula in Lake Superior. Because of the difficulties in measuring low rates of respiration, three methods were employed: bottle incubations, measurements of changes in hypolimnetic oxygen inventories, and rates of CO 2 evolution from the lake surface. All three techniques yielded similar rates of CO 2 production. Rates of community respiration (bottle incubations) ranged from 2 to 166 μg C/L/d; rates of hypolimnetic oxygen consumption ranged from 3 to 12 μg C/L/d; and rates of CO 2 evasion from the lake (positive flux is out of lake) ranged from < 0 to 270 mg C/m 2/d corresponding to volumetric rates of < 0 to 11 μg C/L/d. Little change in respiration rate with water depth was noted, but respiration rates near-shore were significantly higher than rates in offshore waters. Higher rates of respiration were measured in the El Niño year of 1998 as compared to 1999, but higher temperatures are not thought to be the direct cause. Rates of respiration were higher than simultaneously measured rates of photosynthesis, and there was a net evolution of CO 2 from the lake; the lake appears to be net heterotrophic.