Seamount Communities and Pelagic Interfaces: A Tale of Two Seamounts

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Seamount Communities and Pelagic Interfaces A
Tale of Two Seamounts

• Karen Wishner (Univ. of Rhode Island)
• Marcia Gowing (Univ. of Calif., Santa Cruz)
• Lisa Levin (Scripps Inst. of Oceanography)

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Volcano 7 Seamount Eastern Tropical Pacific
Focus on benthopelagic coupling Alvin submersible
(750-3000 m) Benthopelagic (near-bottom)
zooplankton In situ feeding experiments,
trophic webs Benthic photography,
samples MOCNESS tows, CTDs (0-1300 m)
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Benthos and the Oxygen Minimum Zone
Depth (km)
0
1
2
3
Oxygen (mL/L)
gt 1.0
0.5 - 1.0
0.1 - 0.5
lt 0.1
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Vertical Zonation V7 Seamount
Depth Where Taxa First Occurred
Wishner et al. 1990, 1995 Levin et al. 1991
Levin 2002
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Zonation and Physical Oceanography
CTD Profiles
T
Ox
Low Ox Water
Wishner et al. 1995
Mullineaux analysis
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But OMZ pelagic interfaces are biologically and
chemically complex Its not just a physics and
physiology problem, but an array of physical-
biogeochemical interactions. Seamounts are ideal
laboratories for exploring these interactions.
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Major Oxygen Minimum Zones (OMZs) of the
World(and Marine Oxic / Anoxic Interfaces)
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4
lt 0.5
OMZ Midwater depths with Oxygen lt 0.5 mL/L
Annual Mean Oxygen at 500 m (mL/L)
www.nodc.noaa.gov
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Oceanic regions permanently low in oxygen
Oxic / anoxic interface Cariaco Basin
Oxygen minimum zone Arabian Sea
Graph from G. Taylor and M. Scranton

• Organisms specifically adapted to these
  conditions
• Some unusual and significant ecological processes

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Interesting Chemistry in the Redoxcline
nitrification?
Cariaco Basin Taylor et al. 2001
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Enrichments at the Redoxcline
prokaryotes
redoxcline
light scattering layers
Redfield
Modified from Taylor
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Microbial Community at Oxic/Anoxic Interface
Prokaryotes (bacteria)
Free Virions (VLP L-1)
Redoxcline
Depth (m)
Flagellates (bacterivores)
Ciliates (predators)
Figures modified from G. Taylor
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What is fueling the microbial food web?
Enhanced heterotrophic bacterial production
Chemoautotrophic production
Figures modified from G. Taylor
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Plankton and the Oxygen Minimum Zone
Epipelagic
OMZ
Interface
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Zooplankton biomass is reduced within the OMZ
center but increases in association with the
steep oxygen gradients at the lower OMZ interface
Biomass (mM C/m3)
For biomass and distributional effects on oceanic
OMZ zooplankton, the critical range for oxygen is
lt 0.1 mL/L.
0.001
10
0.1
0
Depth (m)
500
1000
0
0.4
Double MOCNESS for vertically-stratified sampling
Oxygen (mL/L)
Wishner et al. 1998
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Diel Vertical Migration into the OMZ
Day
Night

• DVM only into upper OMZ
• Animals in lower OMZ are present day and night

ADCP record of acoustic backscatter (Arabian Sea)
Data of C. Flagg in Morrison et al. 1999
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Feeding rates of copepods living at the lower OMZ
interface can be relatively high compared to
adjacent depths
Abundance (/m3)
In situ Ingestion (µgC/cop/d)
Plankton Net on Alvin
30
0
0
0.4
700
Depth (m)
800
Cod End Incubation Chamber
900
Wishner et al. 1995
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OMZ zooplankton are omnivorous, with even a
single species feeding at multiple trophic levels
Gut Contents include

• Surface Flux
• diatoms, picoautotrophs
• Deep-Sea Detritus
• olive green material, amorphous material
• Zooplankton Remains
• cuticle, nematocysts
• Deep-Sea Aggregates
• bacterial clusters

Bacterial Aggregates
Virus
Cyanobact.
TEMs of gut contents M. Gowing
Algal cell
Gowing and Wishner 1992, 1998
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Microbivore pathway Bacteria and bacteria-like
bodies in zooplankton guts were probably ingested
opportunistically as aggregates because
individual organisms are too small to be eaten
separately
Bacterial aggregates observed in water samples
from OMZ interface Often associated with other
detritus in gut contents Present in many (but
not all) individuals from a variety of species
and taxa Not embedded in gut wall
Bacteria-like bodies in gut contents
Gowing and Wishner 1998, Wishner et al. 2000
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Possible Sources of Microbial Aggregates

• Heterotrophic processes
• Decomposition of suspended and sinking material
  
• Deep OMZ interface may be enriched with
  sinking material because of low zooplankton
  biomass in the suboxic water above
• Chemosynthetic processes (dark C fixation)
• In situ new production
• Observed in some pelagic redoxclines

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Zooplankton (and seamount benthos) may directly
ingest the products of in situ chemoautotrophic
production This could be in the form of
microbial aggregates or other organisms of the
microbial loop that themselves ingest the
chemosynthetic microbes. Zooplankton and
benthos could then repackage and transport
(actively and passively) this material to depth
as fecal pellets, or further into pelagic or
benthic food webs.
If dark C fixation is present in OMZs, then
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Zonation and Physical Oceanography
CTD Profiles
T
Ox
Low Ox Water

• Oscillation of an entire pelagic biogeochemical
  regime and ecosystem
• Complex interactions with benthic ecosystem

Wishner et al. 1995
Mullineaux analysis
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Could these phenomena be more widespread?

• Present day strong OMZs and permanent suboxia
• 2 of oceans benthic continental margin
• Additional extent in midwater regions with
  many seamounts
• Basin-scale oxic/anoxic interfaces
• Paleoceanographic extent of OMZs
• Expansion and contraction on scales of 10s -
  10,000 yr
• Global scale anoxic events in geological record
• Future predictions
• OMZs may increase with global warming
• Some regions with episodic or seasonal suboxia
  may become permanently anoxic

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Kickem Jenny Expedition A NOAA Ocean
Exploration Program (2003)

• Submarine arc volcano in Caribbean near Grenada
• Main purpose to investigate volcanic geology
  and potential hazards
• Opportunistic biological studies

Wishner et al. 2005
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Hydrothermal vents were discovered in the central
crater at 250 m depth. Many vents emitted gas
bubbles.
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Organisms Microbial Mats, Shrimp, Worms
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Abundant seamount community away from vents
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Shrimp

• 3 species of normal mesopelagic shrimp (J.
  Martin)
• Some swimming, others immobile (dead or
  comatose?)
• Hypothesis Shrimp were trapped within the
  crater during diel vertical migration
• Did they succumb to hostile vent environment?
• Or are they potential opportunists?
• But stable isotopes indicate normal pelagic food
• Why were no other vertically migrating taxa
  observed on the seafloor?
• Why were no shrimp found in other non-vent
  secondary craters?

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Questions

• Why are there so few vent-associated species on
  KEJ?
• Are the vent-associated species endemic or
  widespread? Are the seamount species affected
  by the proximity of vents?
• How did they get there? Are midwater animals
  trapped by hydrothermal venting in the crater?
• Do animals utilize chemosynthetic vent
  production?
• How does bubbling gas or vent effluent affect the
  animals and nearby circulation?
• How typical is this of other submarine arc
  volcanoes?
• For the purposes of this meeting, how should we
  mesh studies on vents and seamounts?

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Other interesting examples of vent and midwater
community interactions
Eel City (Staudigel et al.)
Crabs (Jeng et al. 2004)
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Conclusions

• Need to broaden the scope of seamount studies to
  include the full range of physical and
  biogeochemical bentho-pelagic interactions
• OMZ edge redoxclines that intersect seamounts,
  and volcanic arc hydrothermal vent systems at
  midwater depths, provide interesting test sites
  for exploring these interactions
• Many interesting combinations of biology,
  physics, chemistry, geology

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Deep oxygen interfaces are chemoclines, not
pycnoclines
Cariaco Basin
Arabian Sea
Modified from Scranton et al. 2001
Wishner et al. 2000
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Living in SuboxiaImplications for Pelagic
Processes

• Midwater community structure and function
• Unique organisms and assemblages layered in
  midwater redoxclines
• Pelagic diversity and evolution
• Under-appreciated (and under-modeled) complex
  trophic pathways
• Global biogeochemical cycles
• Narrow pelagic zones acting as high impact
  filters for sinking material
• Effects on carbon and particulate fluxes
• Midwater trophic pathways based on new
  production via chemosynthesis?

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Benthopelagic Implications

• Impacts on benthic community
• Effects on organic matter preservation and food
  availability for benthos
• Implications for benthic diversity and evolution
  
• Paleoceanographic reconstruction of past
  environments
• Under-appreciated influence of midwater
  processes on anoxic basin sediments used for
  climate reconstruction