16 from the continental shelf to the deep sea notes for marine biology: function, biodiversity,...

16 From the Continental Shelf to the Deep Sea

Notes for Marine Biology: Function, Biodiversity,

EcologyBy Jeffrey S. Levinton

©Jeffrey S. Levinton 2001

Sediment Type and Life Habit

• Most subtidal bottoms consist of soft sediments

• Muds generally dominated by deposit feeders

• Sands generally dominated by suspension feeders

Sediment Type and Life Habit 2

Suspension feeders do poorly in muds: • (1) near bottom turbidity owing to

resuspension inhibits suspension feeders; • (2) deposit feeders burrow in sediment, making

it unstable and watery - makes suspension feeding difficult;

• (3) deposit feeding bioturbation makes sediment unstable, results in high near-bottom turbidity, which inhibits suspension feeding

Sediment Type and Life Habit 3• Trophic group amensalism - negative

effect of deposit feeders on suspension feeders, owing to the strong disruption of sediment stability and increase of near-bottom turbidity caused by deposit feeding activity

Sediment Type and Life Habit 4• Deposit feeders do more poorly in

subtidal sands, owing to relative lack of sedimenting organic matter

50 60

0

2

4

Depth (cm)

Percent water

Cross-sectional photograph of an intertidal mud dominatedby deposit feeders. Note how water content is high in upper centimeter

Hours

sand

mudC

ondi

tion

inde

xS

uspe

nded

so

lids

(m

g/l)

mud

sand

Effect of mud on a suspension-feeding bivalve, Rangea cuneata.; note that soft tissue in better condition in sands. Bottom showsThat suspended solids are more common over mud than sand bottoms

Seston

Bacterial Decomposition

Resuspension Solution

Bacterial decompositionDeposit feeders

2-3 cm

Permanentdeposit

98-99.5%

Feces

Movement of fine particles over muddy bottoms

Patchiness of Organisms on the Seabed 1

• Subtidal soft bottoms appear superficially to be homogeneous

• In reality, organisms are distributed non-randomly in patches

Patchiness of Organisms on the Seabed 2• The bottom is often heterogeneous owing to

presence of sedimentary structures, such as ripple marks caused by bottom currents

Patchiness of Organisms on the Seabed 3• The bottom is often heterogeneous owing to

presence of sedimentary structures, such as ripple marks caused by bottom currents

• Large bioturbating invertebrates also create a patchy landscape

Patchiness of Organisms on the Seabed 4• The bottom is often heterogeneous owing to

presence of sedimentary structures, such as ripple marks caused by bottom currents

• Large bioturbating invertebrates also create a patchy landscape

• Organic matter is distributed discontinuously, generating local sites of high food value - extreme case, food falls of kelps transported down the continental slope, sinking dead whales

Patchiness of Organisms on the Seabed 5• The bottom is often heterogeneous owing to

presence of sedimentary structures, such as ripple marks caused by bottom currents

• Large bioturbating invertebrates also create a patchy landscape

• Organic matter is distributed discontinuously, generating local sites of high food value - extreme case, food falls of kelps transported down the continental slope, sinking dead whales

• Patchy dispersal and recruitment may produce patchily distributed populations

Sedimentcone

AnusTubes of Euchone

Mouth

€

≥20cm

Sediment microtopography generated by the subtidal burrowingSea cucumber Molpadia oolitica in Cape Cod Bay, Massachusetts

Succession on the Seabed

• Succession in soft sediments involves colonization of disturbed and abiotic substrata by pioneer species, which are surface deposit feeders that burrow only to very shallow depths; at this stage sediment is rich in hydrogen sulfide and poor in pore water oxygen

• Late colonization involves deeper burrowing and feeding animals that turn over and oxygenate the sediment to a deeper depth

Anaerobic sediment

Oxidized sediment

Physical Disturbance Normal

Time

3

2

1

0

Dep

th (

cm)

Successional sequence on the soft-bottom subtidal sea floorfollowing a disturbance

Sampling the Subtidal Benthos

• Sample a large area of bottom• Sample a defined area and uniform depth

below the sediment-water interface• Sample uniformly in differing bottom

substrata• Have a closing device to prevent washout

of specimens as sampler is brought to the surface

A good sampler should:

Sampling the Subtidal Benthos 2• Visual observation is crucial

• Observations and sampling can be done by submersibles, manned and unmanned

Sampling the Subtidal Benthos 3

• Dredges, heavy metal frames with cutting edges that dig into sediment

• Sleds, dredges with ski-like runners that allow only shallow sampling of sediment

• Grabs, samplers that sample only a defined area at a time

• Corers, small tubes that are dropped into sediment (useful for microbiota, sediment samples)

Types of bottom samplers:

Anchor dredge: digs to a specified depth

Peterson Grab

BoxCorer

The Ventana, an unmanned remote operating vehicle, which is equippedwith a grabbing arm, a slurp gun, and video for controlled samplingand observation. Used by scientists at the Monterey Bay Aquarium Research Institute, Pt. Lobos, California

Grabbing arm

Video camera

The manned submersible Johnson Sea-Link, used by scientists working from the Harbor Branch Foundation marine laboratory

The Shelf-Deep Sea Gradient

• We shall now move along a transect from subtidal soft bottoms on the continental shelf to analogous habitats in the deep sea

Surface primary productivity 1• Nearshore on continental shelf primary

production high, as is input of particulate organics to bottom; also a peak of production over shelf-slope break, due to incursion of slope nutrient-rich waters to surface

Surface primary productivity 2• Nearshore on continental shelf primary

production high, as is input of particulate organics to bottom; also a peak of production over shelf-slope break, due to incursion of slope nutrient-rich waters to surface

• Open ocean primary production much reduced, also a much smaller proportion is left ungrazed to sink to the bottom

Input of organic matter 1

• Therefore open deep sea bottoms receive very little nutrient input, owing to small primary production and supply from above, and due to great distance from shore

Input of organic matter 2

• Therefore open deep sea bottoms receive very little nutrient input, owing to small primary production and supply from above, and due to great distance from shore

• Time for material to travel to bottom makes it refractory, owing to bacterial decomposition on the way down

• Input of organic matter from water column declines with depth and distance from shore: continental shelf sediment organic matter = 2-5%, open ocean sediment organic matter = 0.5 - 1.5%, open ocean abyssal bottoms beneath gyre centers < 0.25%

Input of organic matter 3

Microbial Activity on Seabed 1

• Accident highlighted low rate of decomposition: Submersible Alvin was lost in 1968 and recovered a year later. Scientists lunches (including bread, soup prepared from meat extract) were remarkable intact, showing relatively little spoilage.

• Mechanism is not so clear. Could relate to high pressure (depth over 1000m) or perhaps low rates of microbial activity in deep sea.

Microbial Activity on Seabed 2

• Oxygen consumption on deep sea floor is 100-fold less than at shelf depths

Microbial Activity on Seabed 3

• Oxygen consumption on deep sea floor is 100-fold less than at shelf depths

• Bacterial substrates such as agar labeled with radioactive carbon are taken up by bacteria at a rate of 2 percent of uptake rate on shelf bottoms

Microbial Activity on Seabed 4

• Oxygen consumption on deep sea floor is 100-fold less than at shelf depths

• Bacterial substrates such as agar labeled with radioactive carbon are taken up by bacteria at a rate of 2 percent of uptake rate on shelf bottoms

• Animal activity is more complex. Deep sea benthic biomass is very low and some benthic fishes are poor in muscle mass, reflecting low organic matter input, but others are efficient predators and attack bait presented experimentally in bait buckets. Also some special environments with high nutrients (more later)

Environmental stability in the deep sea 1• Physical variables such as salinity and

temperature are much more variable and unpredictable in shelf waters, relative to deep sea bottoms

Environmental stability in the deep sea 2

• Shelf waters often have strong temperature fluctuations, especially on the east coast of North America, where climate is determined by weather systems coming from within the continent

Environmental stability in the deep sea 3

• Even in coasts that are affected by oceanic climate (e.g. west coast of U. S.), erratic events such as El Niños strongly change temperature in shallow waters

Environmental stability in the deep sea 4By contrast, deep sea environment is usually physically stable,

at least on the order of seasons, decades and perhaps even centuries. Temperature on abyssal bottoms varies throughout the year less than 1 degree C.

200m

30m

60m120m

1975 1976 1977

N J M M J S N J M M J S N

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Seasonal variation in bottom-water temperature at different depths

Deep-sea biodiversity changes

• Problem with sampling, great depths make it difficult to recover benthic samples

• Sanders and Hessler established transect from Gay Head (Martha’s Vineyard, Island, near Cape Cod) to Bermuda

• Used bottom sampler with closing device• Found that muddy sea floor biodiversity was

very high, in contrast to previous idea of low species numbers

• Concluded that deep sea is very diverse

Deep-sea biodiversity changes 2• Problem with sampling: when one collects more

specimens one tends to get more species, until some plateau of high sample size is reached. What if your sample size is on the ascending part of the curve?

Number of individuals collected

Num

ber

of

spec

ies

reco

vere

d

Deep-sea biodiversity changes 3

• Need method to estimate species numbers as function of sample size - rarefaction technique - estimate number of species you would have collected at a standard sample size and estimate the number of species for that sample size

Deep-sea biodiversity changes 4

• Results: Number of species in deep sea soft bottoms increases to maximum at 1500 - 2000 m depth, then increases with increasing depth to 4000m on abyssal bottoms

• In abyssal bottoms, carnivorous animals are conspicuously less frequent, which probably reflects the very low population sizes of potential prey species

Deep-sea biodiversity changes 5

Gastropods Polychaetes Protobranch bivalves

Invertebrate Fish megafauna Cumacea megafauna

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0 2000 4000 0 2000 4000 0 2000 4000Depth (m)

Deep-sea biodiversity changes. Why? 1

• Environmental stability hypothesis - deep sea more stable, less extinction, therefore more species

Deep-sea biodiversity changes. Why? 2

• Environmental stability hypothesis - deep sea more stable, less extinction, therefore more species

• Population size effect - in abyssal depths, food scarcity causes extraordinarily low population sizes, leads to extinction and lower diversity

Deep-sea biodiversity changes. Why? 3

• Environmental stability hypothesis - deep sea more stable, less extinction, therefore more species

• Population size effect - in abyssal depths, food scarcity causes extraordinarily low population sizes, leads to extinction and lower diversity

• Possible greater age of the deep sea, as opposed to constant fluctuations of shelf environments owing to sea level change in Pleistocene

Deep-sea biodiversity changes. Why? 4• Environmental stability hypothesis - deep sea more stable,

less extinction, therefore more species• Population size effect - in abyssal depths, food scarcity

causes extraordinarily low population sizes, leads to extinction and lower diversity

• Possible greater age of the deep sea, as opposed to constant fluctuations of shelf environments owing to sea level change in Pleistocene

• Particle size diversity greater at depths of ca. 1500m might create more habitat heterogeneity for particle feeding deposit feeders (but other groups also have a diversity maximum at this depth)

Deep Sea and LatitudeDeep-sea biodiversity also changes with Latitude - surprise because no great gradient:

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Gas

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Hot Vents - Deep Sea Trophic Islands

• Hot Vents - sites usually on oceanic ridges where hot water emerges from vents, associated with volcanic activity

• Sulfide emerges from vents, which supports large numbers of sulfide-oxidizing bacteria, which in turn support large scale animal community. Most animals live in cooler water just adjacent to hot vent source

Hot Vents - Deep Sea Trophic Islands -2

• Hot Vents - Animals near hot vents are uncharacteristically large and fast growing for deep sea

• Bivalves, also members of tube-worm group Vestimentifera - have symbiotic sulfide bacteria, which are used as a food source

Vestimentiferan tube worms at a hot vent

Population of hot-vent bivalve Calyptogena magnifica

Cold Seeps - Other Deep Sea Trophic Islands

• Deep sea escarpments may be sites for leaking of high concentrations of hydrocarbons

• These sites also have sulfide based trophic system with other bivalve and vestimentiferan species that depend upon sulfur bacterial symbionts

The End