Depth-integrated estimates of ecosystem metabolism in a high-elevation lake (Emerald Lake, Sierra Nevada, California)

Steven Sadro, John M. Melack and Sally MacIntyre

Limnol. Oceanogr., 56(5), 2011, 1764-1780 | DOI: 10.4319/lo.2011.56.5.1764

ABSTRACT: A series of vertical profiles of dissolved oxygen (DO) collected periodically over two consecutive ice-free seasons in an oligotrophic high-elevation lake (Emerald Lake, California) were used to investigate volumetric and areal rates of gross primary production (GPP), community respiration (CR), and net ecosystem production (NEP). Diel patterns in DO did not weaken with depth in this lake, where the entire 10-m water column was within the euphotic zone and where a deep chlorophyll a (Chl a) maximum was common during periods of thermal stratification. During stratification, both GPP and CR increased with depth, and heterotrophy (NEP < 0) tended to occur below the thermocline in association with higher Chl a and particulate matter concentrations. With the onset of autumn mixing each year, vertical gradients in metabolism weakened or disappeared and the entire water column was autotrophic. Net autotrophy over the growing season was confirmed using three methods of estimating whole-lake metabolism. During periods of stratification, flux across the thermocline, where eddy diffusivities were near molecular, was small (4% of total epilimnetic fluxes), while within the hypolimnion, where stratification was weaker and eddy diffusivities larger, fluxes between strata were more substantial (12% of total fluxes). For this lake and other small lakes with low wind speeds and Lake numbers near 10, mixing due to turbulence should be included in computations of metabolism within the hypolimnion. However, single-station measurements from within the epilimnion provide a reasonable estimate of seasonal metabolism, especially in the autumn when the lake is mixing on a diel basis.

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