Linking diel patterns in community respiration to bacterioplankton in an oligotrophic high-elevation lake

We examined the role of heterotrophic bacterioplankton in structuring diel patterns in community respiration (CR) in an oligotrophic alpine lake. We used continuous measurements of dissolved oxygen from both free-water and in situ incubation chambers to characterize a consistent diel pattern in oxygen drawdown with two distinct phases. CR rates averaged 15.4 times higher during the late afternoon to early night phase, which lasted for up to 5 h past dusk before rapidly transitioning to the lower rate phase, which lasted through dawn. Bacterioplankton always accounted for the majority of CR (73% seasonally averaged). We examined patterns in dissolved organic carbon (DOC) and bacterial growth to determine the degree to which bacterioplankton were affecting diel variability in respiration rates. There was a diel pattern in DOC accumulation, with the magnitude of overnight drawdown of DOC (2.0 ± 0.8 μmol L−1) matching our oxygen-based estimates of the amount of DOC required to sustain higher CR rates (2.4 ± 1.3 μmol L−1). Bacterioplankton communities cultured on dusk water had growth-rate constants two times higher than those cultured on dawn water and removed the excess DOC in dusk water. Our results indicate that higher daytime CR of bacterioplankton is directly coupled to daily production of DOC at a timescale of hours, likely through phytoplankton exudation. Ignoring diel variability in CR may result in substantial underestimation of daily CR and gross primary production.