Allochthonous organic matter supplements and sediment transport in a polymictic reservoir determined using elemental and isotopic ratios

Christopher T. Filstrup, J. Thad Scott, Owen T. Lind

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6 Scopus citations

Abstract

Because allochthonous organic matter (OM) loading supplements autochthonous OM in supporting lake and reservoir food webs, C and N elemental and isotopic ratios of sedimenting particulate OM were measured during an annual cycle in a polymictic, eutrophic reservoir. Particulate organic C and N deposition rates were greatest during winter and lowest during spring. C:N ratios decreased through our study indicating that OM largely originated from allochthonous sources in winter and autochthonous sources thereafter. δ13C were influenced by C4 plant signatures and became increasingly light from winter through autumn. δ15N indirectly recorded the OM source shift through nitrate utilization degree with maximum values occurring in May as nitrate concentrations decreased. Unlike relationships from stratified systems, δ13C decreased with increasing algal biomass. This relationship suggests that minimal inorganic C fixation relative to supplies maintained photosynthetic isotopic discrimination during productive periods. Water column mixing likely maintained adequate inorganic C concentrations in the photic zone. Alternatively, OM isotopic composition may have been influenced by changing dissolved inorganic nutrient pools in this rapidly flushed system. δ15N also recorded increased N2 fixation as nitrate concentrations declined through autumn. Secondary sediment transport mechanisms strongly influenced OM delivery. Particulate organic C and N deposition rates were 3× greater near the sediment-water interface. Isotopic ratio mixing models suggested that river plume sedimentation, sediment resuspension, and horizontal advection influenced excess sediment deposition with individual mechanisms being more important seasonally. Our findings suggest that allochthonous OM loading and secondarily-transported OM seasonally supplement phytoplankton production in productive reservoirs.

Original languageEnglish (US)
Pages (from-to)87-100
Number of pages14
JournalBiogeochemistry
Volume96
Issue number1
DOIs
StatePublished - Sep 2009

Bibliographical note

Funding Information:
Acknowledgments We thank Dr. Bradley Christian and Lisa Vajdos for their help with field sampling. We thank Dr. Joseph White and Jeffrey Back for advice during manuscript preparation. We thank Tom Conry and the City of Waco (Texas) for project support. Stable isotope analyses were conducted at the Colorado Plateau Stable Isotope Laboratory. This research was funded by the Environmental Protection Agency through an ENSR, Inc. subcontract (no. 102200) to O. T. Lind, and by a grant from the Texas Water Resources Institute (TAES contract no. 570251) to J. T. Scott. Partial funding was provided by the Folmar and Gardner Graduate Student Research Grants through Baylor University Department of Biology to C. T. Filstrup. We thank an anonymous referee whose critiques greatly improved a previous manuscript version.

Keywords

  • Allochthonous
  • Autochthonous
  • C:N ratios
  • Organic matter
  • Sediment resuspension
  • Stable isotopes

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