Articles | Volume 13, issue 15
Biogeosciences, 13, 4343–4357, 2016
Biogeosciences, 13, 4343–4357, 2016

Research article 04 Aug 2016

Research article | 04 Aug 2016

Patterns of carbon processing at the seafloor: the role of faunal and microbial communities in moderating carbon flows

Clare Woulds1, Steven Bouillon2, Gregory L. Cowie3, Emily Drake1, Jack J. Middelburg4,5, and Ursula Witte6 Clare Woulds et al.
  • 1School of Geography, University of Leeds, Leeds, LS2 9JT, UK
  • 2Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium
  • 3School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, UK
  • 4Royal Netherlands Institute of Sea Research (NIOZ-Yerseke), P.O. Box 140, 4400 AC Yerseke, the Netherlands
  • 5Department of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, the Netherlands
  • 6Institute of Biological and Environmental Sciences, Oceanlab, University of Aberdeen, Aberdeen, AB41 6AA, UK

Abstract. Marine sediments, particularly those located in estuarine and coastal zones, are key locations for the burial of organic carbon (C). However, organic C delivered to the sediment is subjected to a range of biological C-cycling processes, the rates and relative importance of which vary markedly between sites, and which are thus difficult to predict.

In this study, stable isotope tracer experiments were used to quantify the processing of C by microbial and faunal communities in two contrasting Scottish estuarine sites: a subtidal, organic C rich site in Loch Etive with cohesive fine-grained sediment, and an intertidal, organic C poor site on an Ythan estuary sand flat with coarse-grained permeable sediments.

In both experiments, sediment cores were recovered and amended with 13C labelled phytodetritus to quantify whole community respiration of the added C and to trace the isotope label into faunal and bacterial biomass. Similar respiration rates were found in Loch Etive and on the Ythan sand flat (0.64 ± 0.04 and 0.63 ± 0.12 mg C m−2h−1, respectively), which we attribute to the experiments being conducted at the same temperature. Faunal uptake of added C over the whole experiment was markedly greater in Loch Etive (204 ± 72 mg C m−2) than on the Ythan sand flat (0.96 ± 0.3 mg C m−2), and this difference was driven by a difference in both faunal biomass and activity. Conversely, bacterial C uptake over the whole experiment in Loch Etive was much lower than that on the Ythan sand flat (1.80 ± 1.66 and 127 ± 89 mg C m−2, respectively). This was not driven by differences in biomass, indicating that the bacterial community in the permeable Ythan sediments was particularly active, being responsible for 48 ± 18 % of total biologically processed C. This type of biological C processing appears to be favoured in permeable sediments. The total amount of biologically processed C was greatest in Loch Etive, largely due to greater faunal C uptake, which was in turn a result of higher faunal biomass. When comparing results from this study with a wide range of previously published isotope tracing experiments, we found a strong correlation between total benthic biomass (fauna plus bacteria) and total biological C processing rates. Therefore, we suggest that the total C-cycling capacity of benthic environments is primarily determined by total biomass.

Short summary
Estuarine sediments are important locations for carbon cycling and burial. We used tracer experiments to investigate how site conditions affect the way in which seafloor biological communities cycle carbon. We showed that while total respiration rates are primarily determined by temperature, total carbon processing by the biological community is strongly related to its biomass. Further, we saw a distinct pattern of carbon cycling in sandy sediment, in which uptake by bacteria dominates.
Final-revised paper