Carbon and oxygen dynamics of shallow aquatic systems: Process vectors and bacterial productivity

Abstract

In shallow aquatic systems subject to heavy allochthonous (terrestrial) organic loading, bacterial processing of organic matter can be a significant component in the ecosystem C‐cycle. Because this bacterial processing of organic matter also produces reduced species (Fe+2, Mn+2, S−2, NH3, etc.), these processes also create an additional chemical oxygen demand. Hence, net ecosystem operation can deviate significantly from the “Redfield Line” defined by the traditional photosynthesis/respiration reaction stoichiometry. Here, a 3D process vector concept is presented in terms of (gas‐exchange‐corrected) CO2 or O2‐TDIC‐time to characterize significant bacterial contributions to the net O2 and TDIC dynamics of the ecosystem. The direction of the process vector provides an important clue to the internal process dynamics and the biogeochemical pathways that govern the net ecosystem function. Using instrumented in situ measures of O2 and TDIC in a small pond (ca. 0.7 m deep), this ecosystem is shown to operate significantly off the “Redfield Line” with multiple periods producing a net increase in both (gas‐exchange‐corrected) O2 and CO2 and exporting of both to the atmosphere. A methodology for the quantification of the minimum bacterially produced TDIC in the ecosystem (MBP) is developed. The rates (38–91 mmoles C m−2 day−1) are consistent with measured carbon fluxes from shallow terrestrial aquatic systems as well as anticipated cell numbers in the sediment that likely contribute to this bacterially produced carbon. The process vector concept can be extended to additional dimensions (CH4, NO3−, NH4+, Fe+2, etc.) and may provide a tool for visualization of observatory data streams.