Abstract

Aquatic systems, such as the St. Lawrence Estuary and Gulf (SLEG), are represented by a combination of recalcitrant Organic Matter (OM) derived from land-based carbon-based compounds, and marine labile OM, mostly composed from phytoplankton and zooplankton biomass. The consumption of marine labile OM components by aerobic heterotrophic bacteria results in sustained pressure of dissolved O2 concentrations and accumulation of more recalcitrant OM fractions corresponding to slower degradation rates. The SLEG is a cold temperate estuarine system, characterized by large seasonal variations in riverine discharge rates resulting in sudden influxes of fresh and labile OM deposited into the sediment. The accumulation and remineralization of recalcitrant fractions (formed through in situ primary production) could be modulated by the priming effect (PE) where the change in the degradation rate of recalcitrant sediment is modulated by the addition of labile OM. The SLEG is an ideal site to study PE, where climate change has reduced dissolved O2 levels, particularly in the depths of bottom waters and affected the marine life and may have contributed to the onset and worsening of hypoxia. To examine PE, through a batch incubation experiment we attempt to measure changes in the rate of degradation in recalcitrant sediment from SLEG by adding fresh labile OM, specifically 13C-depleted phytoplankton. A time course study was developed to examine the pathway of fresh OM, particularly carbon, remains in the artificial system. Phytoplankton fixes inorganic carbon, specifically CO2 (aq), and transforms it into organic matter via photosynthesis. Since this process preferentially selects carbon-12, we expect to observe an increase of carbon-12 in the system (a lowering of the 13C/12C ratio) over time. Most of the organic matter in sediment originates from primary production in the water column, therefore the sediment’s initial delta13C value can be altered by the amount of CO2 (aq), plankton, nutrients, and light. We performed Dissolved Organic Carbon (DOC) and stable isotope analysis on the seawater samples as well as stable isotope analysis of the samples that underwent incubation. Quenching of the incubations followed by the extraction, quantification and isotopic characterization of the bacterial fatty acids and hydrocarbons allows us to examine whether there is any effect of labile OM on the mineralization of recalcitrant sedimentary OM. Finally, the experimental evaluation of the reactivity of recalcitrant OM indicated that the sedimental bulk was not reactive on the time scale of 20 and 32 days respectively, such that only additions of labile OM were quickly consumed.