Spatiotemporal changes in biogeochemistry of a temperate seagrass bed

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Spatio-temporal changes in biogeochemistry of a
temperate seagrass bed

• Andrew B. Hebert
• Ph.D. candidate
• Department of Oceanography
• Texas AM University
• hebert_at_ocean.tamu.edu
• http//ocean.tamu.edu/hebert

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Outline

• I. Introduction Seagrass importance and
  biogeochemical processes
• II. Spatio-temporal changes in sedimentary
  geochemistry of a temperate seagrass bed (Zostera
  marina) and adjacent unvegetated sediments
• Microelectrode profiles
• Sulfate reduction rates
• Pore water and solids geochemistry
• III. Spatio-temporal scale lengths for SH2S
• IV. Sediment-seagrass diagenetic modeling efforts
• V. Conclusions

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I. Introduction Why Study Seagrass
Biogeochemistry?

• Determine extent of aquatic stressors on
  estuarine ecosystems
• Examine sediment abiotic interactions with
  biomediated processes
• Scant sedimentary geochemical data

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I. Introduction Why Study Seagrass
Biogeochemistry?

• Atmospheric CO2 sink
• Nursery habitat for juvenile fishes (commercial
  fisheries)
• Current attenuation and wave action inhibitors
  (beachfront property)
• Base of food chain for many waterfowl and
  invertebrates (hunting/fishing/tourism)

Figure courtesy of Jim Kaldy
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Study Site
X
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Slide credit Jim Kaldy
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Yaquina Bay Carbon Sources
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Benthic microalgae
Seagrass and epiphytes
Epiphytes
Macroalgae
Phytoplankton
Adapted from Garber et al., 1992
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Light
Waves
Current
Chlorophyll a
TSS
DIN
DIP
Epiphytes
SOM
TSS
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Typical Reduced Sulfur Pool
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Outline

• I. Introduction Seagrass importance and
  biogeochemical processes
• II. Spatio-temporal changes in sedimentary
  geochemistry of a temperate seagrass bed (Zostera
  marina) and adjacent unvegetated sediments
• Microelectrode profiles
• Sulfate reduction rates
• Pore water and solids geochemistry
• III. Spatio-temporal scale lengths for SH2S
• IV. Sediment-seagrass diagenetic modeling efforts
• V. Conclusions

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II. Spatio-temporal changes in sedimentary
geochemistry of a temperate seagrass bed (Zostera
marina) and adjacent unvegetated sediments

• Questions
• How do the dissolved and solid phase geochemical
  parameters behave in early seagrass/unvegetated
  sediment diagenesis (C,H,N,S,O2,Mn,Fe)?
• How are geochemical parameters between and within
  seagrass and unvegetated sediments different and
  how do they change between light and dark cycles?
• Objectives
• To investigate a much broader range of
  sedimentary geochemical parameters and to better
  understand the affect of light and dark
  conditions on seagrass sediment diagenesis and
  whether or not it is different from adjacent
  unvegetated sediments.

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II. Field Work and Sample Collection
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II. Field Work and Sample Collection
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Microprofile incubation experiments
Profiles every 4 hrs, 24 hr
12 hr light/12 hr dark
40 cm
15 cm total depth
Experiment replicated with fresh cores
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II. Microprofile Cores Dissolved H2S in a
seagrass core
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II. Microprofile Cores
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II. Microprofile Cores

• Biomass and Total Sulfur

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II. SH2S and Belowground Biomass
Critical Biomass Index?
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II. Microelectrode Summary

• Sulfide concentrations become depleted when
  belowground biomass increases?
• The temporal variability of SH2S is lost in the
  spatial variability
• Spatial variability of SH2S decreases below the
  root zone
• A Critical Biomass Index could aid managers in
  determining toxic levels of sulfides and
  maintenance of healthy seagrass ecosystems

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II. Field Work and Sample Collection
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II. Sulfate Reduction Rates
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II. Sulfate Reduction Rates Summary

• SRR increased in seagrass sediments during light
  periods
• SRR in seagrass sediments varied more with depth
  and were higher in magnitude compared to
  unvegetated sediments
• SRR agreed well with daytime SH2S increase for
  SG2 microprofiles

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II. Field Work and Sample Collection
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II. Nutrients
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II. Dissolved Carbon
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II. Reduced Sulfur

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Reactive Metals

• Tests the null hypothesis that mean trace metal
  concentrations between light and dark conditions
  are the same (95 confidence)

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II. Sediment Geochemistry Summary

• DIC and nutrients were higher during the day than
  at night for seagrass sediments which agreed with
  SRR
• TRS was spatially heterogeneous compared to
  unvegetated sediments and TRS exhibited a diurnal
  cycle
• TRS may be oxidized hence liberating trace metals
  enabling bioavailability

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Outline

• I. Introduction Seagrass importance and
  biogeochemical processes
• II. Spatio-temporal changes in sedimentary
  geochemistry of a temperate seagrass bed (Zostera
  marina) and adjacent unvegetated sediments
• Microelectrode profiles
• Sulfate reduction rates
• Pore water and solids geochemistry
• III. Spatio-temporal scale lengths for SH2S
• IV. Sediment-seagrass diagenetic modeling efforts
• V. Conclusions

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III. Spatio-temporal scale lengths for SH2S

• Questions
• What are the spatial scales associated with
  changes in seagrass sediment geochemistry and how
  does the variability compare to adjacent
  unvegetated sediments?
• What is the optimum sampling interval for
  sedimentary geochemical parameters?
• Objectives
• To use the autocovariance function for the
  determination of appropriate sampling intervals
  for SH2S in both vertical and horizontal
  dimensions for seagrass and adjacent unvegetated
  sediments.

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III.Vertical scales
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III. Lateral Scales
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III. Lateral scale lengths
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III. Spatio-temporal scale lengths for SH2S

• What is the significance of scale length?
• Process-defined lengths
• Diversity-controlled (bacterial or
  meio/microfaunal)
• Topographical features of sediment
• Burrowing organisms establishing length of scale
• Random aggregates of SRB

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III. Summary

• Vertical scale lengths did not vary between light
  and dark cycles or between seagrass and
  unvegetated sediments
• Lateral scale lengths approximated our sampling
  interval and were smaller than vertical scale
  lengths
• Vertical scale lengths agreed well with those
  from three years prior
• Results may be used to optimize sampling interval
  without losing the dominant source of variability

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Outline

• I. Introduction Seagrass importance and
  biogeochemical processes
• II. Spatio-temporal changes in sedimentary
  geochemistry of a temperate seagrass bed (Zostera
  marina) and adjacent unvegetated sediments
• Microelectrode profiles
• Sulfate reduction rates
• Pore water and solids geochemistry
• III. Spatio-temporal scale lengths for SH2S
• IV. Sediment-seagrass diagenetic modeling efforts
• V. Conclusions

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IV. Modeling

• Questions
• How precise are our current diagenetic models?
• How valid is current diagenetic theory and can it
  accurately be applied to field observations?
• Objectives
• To calibrate the Eldridge and Morse (2000)
  sediment-seagrass diagenetic model from
  subtropical Thalassia testudinum to temperate
  Zostera marina and run sensitivity analysis for
  raw data and modeled data.

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IV. ModelingCalibration results for SG2 Light
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IV. ModelingCalibration results for SG2 Light
(cont.)
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IV. ModelingCalibration results for SG2 Dark
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IV. ModelingCalibration results for SG2 Dark
(cont.)
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IV. Modeling ResultsSensitivity analysis

• Normalized root-mean-squared differences (N-RMSD)
  revealed that SH2S and TOC were most sensitive to
  changes in physical characteristics (irrigation,
  advection, and biodiffusion)

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Modeling Summary

• Model calibrations were of the same magnitude as
  raw data except for Fe2
• Sensitivity analysis revealed that perhaps a
  tighter coupling exists (mutualistic behavior)
  between bioirrigators and sedimentary sulfide
  concentrations
• A dynamic model may be more appropriate to assess
  spatial heterogeneity

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Outline

• I. Introduction Seagrass importance and
  biogeochemical processes
• II. Spatio-temporal changes in sedimentary
  geochemistry of a temperate seagrass bed (Zostera
  marina) and adjacent unvegetated sediments
• Microelectrode profiles
• Sulfate reduction rates
• Pore water and solids geochemistry
• III. Spatio-temporal scale lengths for SH2S
• IV. Sediment-seagrass diagenetic modeling efforts
• V. Conclusions

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V. So What?...

• Data showed that sedimentary solids (pyrite-Fe),
  and not just pore water, can change on diurnal
  time scales
• Implications on toxic sulfide pool
• Implications on trace metal bioavailability
• Geochemical parameters (both dissolved and solid)
  are largely heterogeneous, especially in seagrass
  sediments

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V. Conclusions (continued)

• Appropriate scale lengths for measuring sulfide
  (in this system) were determined
• Using a broader range of sedimentary geochemical
  parameters allowed better interpretation of
  processes, provided the framework for calibrating
  diagenetic models, and challenged antiquated
  theory
• Sediment geochemical parameters may potentially
  be used as bioindicators of estuarine and
  seagrass health/productivity

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Future Studies

• Determine an appropriate CBI, correlating sulfide
  toxicity experiments with belowground biomass and
  O2 translocation rates
• Get BAMS going and monitor H2S concentrations at
  depth
• Seasonal differences
• Benthic infaunal/seagrass coupling
• with regard to sediment ventilation

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Acknowledgements

• John Morse
• Jay Pinckney
• Pete Eldridge
• Steve Dimarco
• Richard Loeppert
• Jim Kaldy
• Cheryl Brown
• Bruce Boese
• Luis Cifuentes and Brian Jones
• Bob Taylor and Bryan Brattin
• Rolf Arvidson
• Dwight Gledhill
• Megan Singer
• Amy Degeest
• Alyce Lee
• Karen Sell
• Elizabeth Hebert
• TAMU OGC
• USEPA CEB WED

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Questions?
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II. Total Organic Carbon
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Grain Size Distribution
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II. Microprofile Cores Dissolved H2S SG1
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II. Microprofile Cores Dissolved H2S B2
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II. Microprofile Cores Dissolved Fe2 SG1
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II. Microprofile Cores Dissolved H2S SG2
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II. Microprofile Cores Dissolved H2S UV1
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II. Microprofile Cores

• Mean SH2S standard deviation (µM)
• Biomass and Total Sulfur