MEPS 544:107-130 (2016)  -  DOI: https://doi.org/10.3354/meps11587

ABSTRACT: Bacteria influence carbon cycling in coastal sediments by decomposing the organic matter produced in situ as well as that delivered from the landscape and neighboring ecosystems. Disturbances, such as eutrophication, and seasonal swings in primary production may influence sediment organic matter composition and, as a result, the sources of carbon available to bacteria. We evaluated whether bacterial carbon sources varied between 2 salt marsh tidal creeks with different water column nutrient levels (fertilized versus ambient) and if they changed over a growing season through field observations and stable isotope labeling experiments. Sediment organic matter composition, determined using lipid biomarkers and stable isotopes, was similar between the tidal creeks and did not change over time. In the field, isotopic analysis (natural abundance δ¹³C) of bacterial lipids and potential organic matter sources revealed that bacteria assimilated carbon from several sources but mainly relied on microalgae. In the experiments, ¹³C was added to sediment cores from both creeks as sodium bicarbonate (June, August, October) or Spartina alterniflora detritus (August) and traced into lipids representing benthic microalgae (BMA) and bacteria. Bacteria assimilated ¹³C from BMA and S. alterniflora at similar rates. Further, ¹³C from S. alterniflora was rapidly transferred to BMA, likely via bacterial respiration. Overall, nutrient enrichment did not affect bacterial carbon sources in the field or incorporation of ¹³C from BMA and S. alterniflora in the experiments. Tight coupling between BMA and bacteria may explain why nitrogen enrichment effects were limited as well as provide a mechanism for retaining newly fixed and detrital carbon in coastal sediments.

KEY WORDS: Carbon cycling · Eutrophication · Benthic microalgae · Spartina alterniflora · Lipid biomarker