Ecosystem metabolism in a stratified lake

Abstract

Seasonal changes in rates of gross primary production (GPP), net ecosystem production (NEP), and respiration (R) were determined from frequent automated profiles of dissolved oxygen (DO) and temperature in a clear‐water polymictic lake. Metabolic rate calculations were made using a method that integrates rates across the entire depth profile and includes DO exchange between depth layers driven by mixed‐layer deepening and eddy diffusivity. During full mixing, NEP was close to zero throughout the water column, and GPP and R were reduced 2–10 times compared to stratified periods. When present, the metalimnion contributed 21% and 27% to whole‐lake areal rates of GPP and R, respectively. Net autotrophy prevailed in the epilimnion (NEP = 11 ± 14 mmol O2 m−3 d−1; mean ± SD) compared to balanced production in the metalimnion (NEP = 2 ± 19 mmol O2 m−3 d−1) and net heterotrophic conditions in hypolimnic waters (NEP = −15 ± 24 mmol O2 m−3 d−1). Positive NEP occurred in the metalimnion during periods when the photic depth extended below the mixed‐layer depth. Although the single‐sonde method estimated higher areal GPP (19%) and R (14%) compared to the two depth‐integrated approaches, differences were not significant. During stratification, daily variability in epilimnetic DO was dominated by metabolism (46%) and air‐water gas exchange (44%). Fluxes related to mixed‐layer deepening dominated in meta‐ and hypolimnic waters (49% and 64%), while eddy diffusion (1% and 14%) was less important. Although air‐water gas exchange rates differed among the three formulations of gas‐transfer velocity, this had no significant effect on metabolic rates.