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Abyssal Plains . Tim Lamothe, Julie van der Hoop & Sara Wanono. Lecture Outline. Physical and chemical characteristics of the abyssal plains Characteristics of abyssal fauna and an overview of deep-sea food supply Research & sampling methods Response of the benthos Whale Fall Ecology

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Abyssal plains l.jpg

Abyssal Plains

Tim Lamothe, Julie van der Hoop & Sara Wanono


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Lecture Outline

  • Physical and chemical characteristics of the abyssal plains

  • Characteristics of abyssal fauna and an overview of deep-sea food supply

  • Research & sampling methods

  • Response of the benthos

  • Whale Fall Ecology

  • Limitations of deep sea science


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Characteristics of the Abyss

  • The abyssal zone (2000-6000m deep) is the single largest habitat on Earth, covering 300,000,000 km2

  • The abyssal plains, located in the aphotic zone at depths of 4000-6000m, are the flattest of all the Earth’s topographical regions.

  • 40% of total seafloor and ¼ of earth’s surface

  • Average slope of less than 1 meter per horizontal kilometer


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Abyssal Sediment

  • Broad, relatively featureless expanses of mainly land-derived sediment, usually carried by turbidity and riverine currents.

  • Underlying topography is blanketed by massive amounts of sediment

  • Range of thickness: 100 meters – more than 1 kilometer

  • The principal sediment constituents on abyssal plains are brown clays and the siliceous remains of radiolarian zooplankton and such phytoplankton as diatoms.


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Properties of Abyssal Plains

  • Water temperature in the abyssal zone ranges from 0 to 4 degrees Celsius.

  • Abyssal salinities range narrowly around 35 parts per thousand.

  • The abyssal zone is characterized by immense pressure, generally ranging between 200 and 600 atmospheres.


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Properties of Abyssal Plains

  • Deep sea waters of the abyssal plains are aerated by the advection of cold, dense, oxygen rich polar water.

  • The nutrient salt concentration is higher in abyssal waters than in overlying waters because the abyssal zone acts as a reservoir for the salts from decomposed biological materials


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Light in the Deep Sea

Complete lack of sunlight precludes any photo-synthetically derived primary productivity

This, then, begs the question, from which so much of thescientific study of the deep sea is born – how can organismsfeed, or even live, in the deep sea?


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Benthic dwellers

  • Epifauna

  • Infauna

  • Nektobenthos

  • Community structure less stable

  • Limiting factor


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Adaptations to obtain prey

  • Photosynthetic production cannot occur

  • Sensory devices

  • Long antenna

  • Detect motion

  • Sharp teeth

  • Hinged jaws

  • Expandable bodies

  • Bioluminesce


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Food sources

  • Season phytoplankton bloom

  • Fecal pellets

  • Crustacean molts

  • Fish dumping


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Food sources

  • Dead fish and mammals

  • Floating algae

  • Detritus

  • Biogenious sediments

  • 1-3% of surface organic primary production reaches the abyssal seabed


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Ecological Trends

  • Whole animal food falls occur on a smaller scale

  • Coastal macroalgae and seagrass have often been encountered in sediment traps

  • Deep sea epifaunal deposit feeders ingest macroalgae and seagrass


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Ecological Trends

  • Food falls provide energy and its presence influences the structure of benthic communities

  • Organic primary production is converted to bacterial tissue


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Ecological Trends

  • Evidence of strong correlation between phytodetrital material found in the deep-sea and surface water productivity.

  • Nutritive values are reduced because of the long residence times in the water column.


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Ecological Trends

  • Phytodetritus deposits are likely a major influencing factor affecting large blooms of phytoplankton in surface waters

  • Variation in the timing and amount of this deposition from year to year.

  • Seasonal drops of phytodetritus are considered a major source of energy for the deep-sea community


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Methods

  • Photography

    • Visual evidence

    • Transects


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Methods

  • Cores: Small, but quantitative measurements

    • Box Cores: Effects of bow wave

    • Tube Cores: Preserve conditions at sediment-water interface

Gage and Bett 2005


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Methods

  • The MEGACORER

    • 12 10cm diameter cores

    • Penetrate 20-40cm into sediment

    • Sample size: 942.5cm2

Gage and Bett 2005


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Methods

  • Phytopigments:

    • Determination of phytodetrital makeup, source, age, depth penetration.

    • Chlorophyll a: intact phytoplankton cells, indicates undegraded material.

    • Phaeopigments: degradation product of chlorophyll, indicates breakdown.

    • Chlorophyll a:Phaeophorbide ratio (R): small for relatively undegraded material.

      Thiel et al. 1988: R=1.64 and 2.04 compared to value of 42.1 in a Holothurian stomach.

Thiel et al. 1988


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Methods

  • Phytopigments

    • Chlorophyll b: terrestrial input

    • Fucoxanthin and other carotenoids: diatoms and dinoflagellates.

  • Inorganic Composition

    • Rarely reported

    • Percentage of CaCO3 can infer relative abundance of coccolithophorids

      • 2% at Sta M (NE Pacific)

      • 62% at PAP (NE Atlantic)


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Methods

  • Sediment Community Oxygen Consumption (SCOC)

    • Measure of the rate of organic matter mineralization by sediment community. Does not differentiate between taxonomic groups.

    • Increase in SCOC following organic matter sedimentation indicates increased respiration

      • Indicates a benthic response


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Benthic Response: SCOC

  • Drazen et al. 1998 (NE Pacific, Sta. M): maxima coincide with periods of peak POC flux. Significant increase in SCOC from Feb to June. No significant difference between years.

  • Smith et al. 2001 (NE Pacific, Sta. M): seasonal fluctuation in relative synchrony with POC flux. Over 8 years, remarkably consistent.

Drazen et al. 1998


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Benthic Response

  • Bacteria colonize & transform detritus

  • Benthic meiofauna quickly colonize:

    response is < 3 h.

  • Affects species composition, distribution, abundances on short term: rapid aggregation and dispersal of specialists.


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Thurston et al. 1994

  • Three N. Atlantic sites separated by 40°N, at similar depths (4850-5440m).

  • Latitude marks separate physical mixing characters; distinct fish communities, benthic groups.

    • PAP: North of 40°N, dominated by “vacuum cleaning” holothurians. Detritivores high.

    • GME and MAP: South of 40°N, dominated by asteroids and decapods. Carnivores high.

  • PAP site receives larger total POC flux, in aggregated forms, on seasonal cycles, than southern sites.

  • Shows that megafaunal organism type and size can be different at the same depth: food abundance and delivery is of great importance to faunal community.


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Depth Profile of Response

  • Drazen et al. 1988

    • Chlorophyll a levels decrease with sediment depth.

    • ATP (measure of respiration of sediment community) also decreases with depth

  • Surface organisms gain a greater benefit from inputs of phytodetritus than deep-sediment dwelling organisms.


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Whale Falls

An oasis in the abyssal desert

  • The periodic falls of large whale carcasses provide massive pulses of labile organic matter to the deep sea

  • Species richness at whale falls rivals that at hydrothermal vents

  • Characterized by four distinct, successional stages:1) mobile scavenger stage2) enrichment opportunist stage3) sulphophilic stage4) reef stage

  • Evidence suggests whale falls act as deep sea stepping stones for various taxa as they make their way across the seafloor to hydrothermal vents and cold seeps.


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Limitations

  • Relatively inaccessible

  • scientists must often rely on “snapshots” (short sampling periods)

  • Greater difficulty of replicated sampling within a relatively small area of seabed when using a surface vessel in deep water.

  • Technology is expensive

  • Bringing deep sea sediment to the surface can result in decompression of sediment and disruption of initial composition.

  • Transporting fauna to surface can interfere with integrity of samples

  • Delicate process: sampling methods can disrupt original state of biogenic structures, sediments, etc.

  • Extensive study of certain areas, but none of others – are findings truly representative of global deep sea trends?



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