L10 Continental shelves and Ocean floor

Question 1

How is the ocean floor mapped?

Answer 1

Using sonar technology such as multibeam sounders.

Historically lowered a chain overboard to record depth - Effective for general topography but not final detail.

Question 2

7 points about continental shelves

Answer 2

• underwater during glacial periods
• shelves end at shelf break (av depth 150m), after that is the abyssal plain.
• Benthos is subtidal/sublittoral and water column above is the neritic pelagic zone
• 8% of global sea surface
• v. productive regions
• Important for fisheries, shipping, aquaculture.
• proximity to coast and shallow depth makes them good for other human activities: shipping, oil/gas extraction, renewable energy, recreation.

Question 3

Why are continental shelves so productive?

Answer 3

run off from land means nutrients are barely limited, so high productivity in water column and benthos

Question 4

Describe the soft bottomed benthic substrate

Answer 4

Variation of sediment type, wave action, salinity, light and temperature
Vegetation free, sediment rich substrates
Sediment type depends on wave action, water movement, upwelling and geological history of the region.
Coarser sediments found close to shore

Question 5

what is the difference in benthos if more or less turbulent?

Answer 5

More turbulent - Sand, more oxygenated, less detritus, many suspension feeders
Less turbulent - Mud, less oxygenated, more detritus which gets stuck between spaces in sediment, many deposit feeders.

Question 6

What are microbiota, meiofauna, macrofauna, epifauna?

Answer 6

Microbiota - Dinoflagellates, diatoms, bacteria
Meiofauna - 0.063 -0.5mm
Macrofauna - >0.5mm
Epifauna - Emerge from seabed

Question 7

What are the dominant macrofauna

Answer 7

Polychaetes, echinoderms, crustaceans, molluscs.

Abundance changes due to sediment. Soft sediment = soft bodied biota - eg anemones and sea pens

Question 8

Describe the hard bottomed sub littoral

Answer 8

bedrock, boulders, compacted shell gravel.
Difficult to burrow into so communities are dominated by epifauna.
sessile animals: sponges, hydroids, anemones, bryozoans, tube polychaetes, barnacles, seasquirts.
Motile species: Chitons, urchins, limpets, abalones

Question 9

Where are kelp forests found and what are they formed of?

Answer 9

cool, temperate waters
Brown macroalgae, order laminariales
Fronds sometimes 30m
Rapid growth, form canopys, v dense
extremely productive

Question 10

Describe the community in kelpforests

Answer 10

diverse community living on fronds or holdfasts
Sea urchins are important grazers
Strong trophic interactions - since prevention of otter hunting, killer whales started huntung otters, caausing urchin increase rapidly. Caused loss of kelp forests due to grazing. (top down interaction)

Question 11

describe the deep sea benthos

Answer 11

Rich organic ‘ooze’.
Small amount of prmary production reaches depths below 200m (only 1-3% reaches sea floor).
Few suspension feeders and mainly deposit feeders.

Question 12

What fauna are in the deep sea benthos?

Answer 12

Infaunal meiofauna - <0.5mm, nematodes, foraminifera, copepods
Infaunal macrofauna - >0.5mm, polychaetes, amphipods, bivalves
Epifauna - mobile scavengers Plesiopenaeus (shrimp), Sessile filter feeders Chondrocladia lyra - carnivorous harp sponge.

Question 13

Describe changes of fauna with depth

Answer 13

Deep water fauna shifts with depth. at 100-200m sponges are huge part of biomass. Deeper, holothurians and starfish dominate. Scavengers become more common and sessile species less common.
Biomass decreases with depth exponentially
Often decrease in size with depth, apart from scavengers increase. eg giant isopods and amphipods.

Question 14

What are seamounts?

Answer 14

Undersea mountains rising >1km above abyssal floor.

Knolls rise from 0.2-1km. Typically 3-5km below surface.

Question 15

Where have seamounts been found?

Answer 15

Yessen et al identified 33,400 seamounts and 138000 knolls along fault lines, often extinct volcanoes.

Question 16

Why are seamounts important ecologically?

Answer 16

Interfere with ocean currents, driving eddies over seamount, causing upwelling and concentrate nutrients.
Provide a hard subsrtate which is uncommon in the deep sea.
Habitat for deep water corals and fish
Study by Morato et al of long line catch records showed these areas contained highest diversity of large pelagics close to seamount.

Question 17

Examples of 2 species at seamounts

Answer 17

Deep water roughy fish - 150 years
Lophilia pertusa (coral) can live >200 years.

Question 18

How have seamounts been exploited?

Answer 18

Developing fishing technology and methods.
Not sustainable fisheries and soon will be non profitable
Trawling caused substantial damage especially to corals
Some are now protected.

Question 19

Where were hydrothermal vents discovered and where do they form?

Answer 19

in 1977 at 2700m deep, 200 miles NW along fault lines off galapagos. Where magma is close to seabed, permeates seabed floor and superheated water is ejected back carrying minerals.

Question 20

What is first in the food chain in hydrothermal vents

Answer 20

Chemoautotrophic bacteria are foundations of food chain eg Beggiata - hyperthermophile. 85-115 degrees.
2H2S +O2 = 2H2O +2S

Question 21

What dominates vents in the Atlantic?

Answer 21

Bathymodiolus thermophylus [Vent mussels]

symbiotic bacteria in gill filaments, and can filter feed too. Must live close to sulfides to feed bacteria.

Question 22

Give an example of 2 polychete worms in hydrothermal vents

Answer 22

1. Alvinella - [heterotrophic polychaete, pompeii worm]
   Grow up to 13cm long and aggregate in paper thin tubes. Hairs are cultures of bacteria - perhaps insulation? Feed on filamentous autotrophic bacteria. Most thermally tolerant EK known. Usually > 65 degrees
2. Vestimentiferans [polychaete tube worm]
   no digestive tract, symbiotic bactria in throphosome tissue instead. Specialized Hb binds to H2S and transports it to bacteria.

Question 23

What are 2 other chemosynthetic based systems?

Answer 23

1. Whale falls - Lipids in bones leave highly anoxic sulphide based conditions. Colonisation by specialised polychaetes and mussels, and bone eating tube worms - Osedax.
2. Cold seeps - Where hydrocarbons seep from sediment. Methane hydrate ‘ice’ forms when gas escapes in low temperature and supports some microbes. H2S also released, and over time mussels colonise. Often occurs with ‘Brine pools’.