Abyssal Plains

1. Abyssal Plains Tim Lamothe, Julie van der Hoop & Sara Wanono

2. 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

3. 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

5. 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.

6. 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.

7. 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

8. 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?

9. Benthic dwellers • Epifauna • Infauna • Nektobenthos • Community structure less stable • Limiting factor

10. Adaptations to obtain prey • Photosynthetic production cannot occur • Sensory devices • Long antenna • Detect motion • Sharp teeth • Hinged jaws • Expandable bodies • Bioluminesce

11. Food sources • Season phytoplankton bloom • Fecal pellets • Crustacean molts • Fish dumping

12. Food sources • Dead fish and mammals • Floating algae • Detritus • Biogenious sediments • 1-3% of surface organic primary production reaches the abyssal seabed

13. 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

14. Ecological Trends • Food falls provide energy and its presence influences the structure of benthic communities • Organic primary production is converted to bacterial tissue

15. 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.

16. 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

17. Methods • Photography • Visual evidence • Transects

18. Methods • Cores: Small, but quantitative measurements • Box Cores: Effects of bow wave • Tube Cores: Preserve conditions at sediment-water interface Gage and Bett 2005

19. Methods • The MEGACORER • 12 10cm diameter cores • Penetrate 20-40cm into sediment • Sample size: 942.5cm2 Gage and Bett 2005

20. 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

21. 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)

22. 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

23. 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

24. 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.

25. 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.

26. 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.

27. 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.

29. 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?

30. Questions?