Lecture Benthic Ecology

Benthos

• Unlike the drifting plankton and swimming nekton, benthic organisms – commonly referred to as benthos – live on or near the ocean bottom

• A benthic habitat may be shallow or deep• Most benthic organisms are sessile (immobile)

and anchored to the benthic environment; others crawl or swim over the ocean bottom

Benthos

• Of the 250,000 known species that inhabit the marine environment, >98% are benthos!

• The majority of benthic organisms live on the continental shelf; sunlit areas of relatively high primary productivity

• Benthos include animals, protists (algae) and even plants!

Why are benthic organisms important?

• Benthos link the primary producers (phytoplankton) with higher levels in the food web.

• Filter feeders such as clams and oysters consume plankton and organic particles.

• Many benthic creatures, particularly clams and worms, serve as food for larger, economically important species such as blue crabs, striped bass, spot, croaker and white perch.

Environmental snapshot

• Scientists study benthic organisms because they provide a good snapshot of environmental conditions in the Bay and its streams and rivers. Most benthic creatures cannot move very far, if at all, so they can’t avoid pollution or unhealthy water conditions.

• Benthic communities are exposed to many stressors, including low oxygen levels, excess sediment and chemical contaminants.

• In summer, high temperatures and nutrient pollution often lead to low-oxygen areas at the bottom of the Bay and its rivers.

Environmental snapshot

• Excess sediment suspended in the water can block sunlight from reaching bay grasses growing at the bottom. When sediment finally settles, it can bury oyster bars and other benthic species.

• Many chemical contaminants bind to bottom sediments, remaining there for years. Benthic species become contaminated when they feed and live in these toxic sediments.

Intertidal Zones

Intertidal Zones

• Rocky shorelines team with organisms that live on the ocean floor (epifauna)

• Typical rocky shores are divided into distinct zones, characterized by the height of the water (which is itself determined by the tides)

• So called intertidal zones are among the most densely-populated areas on Earth; hundreds of species crowd this junction of land and sea

Intertidal Zones

• Epifauna - live attached to hard surfaces such as rocks, shells and pilings. They also live directly on the surface of the Bay’s bottom. Epifauna include oysters, sponges, sea squirts, sea stars and barnacles. Ex)An oyster reef

• Infauna - burrow into bottom sediments. Worms, clams and other infauna form their own communities that are connected to the water by tubes and tunnels

Life in the intertidal zone is harsh!

• Adverse conditions require special adaptations of organisms to live both underwater (some of the time) and exposed to air (the rest of the time)!

• Strong wave activity, desiccation (drying out), limited space, rapid changes in temperature, salinity, pH, and oxygen content, and predation are just some of the challenges found here

Biotic factors affecting organisms living in the intertidal zone:

• Competition for space and food• Predation• Reproduction• Substrate settlement preference• Osmoregulation

Abiotic factors affecting organisms living in the intertidal zone:• Salinity• Temperature • Air and light exposure• Tidal flow• Waves and current action• Substrate• Wind direction and strength• Dissolved O2• Storms• Natural Disasters

Spray zone –region above the spring high tide line; covered by water only during storms

Intertidal zone – region between the high and low tidal extremes

Rocky shores: Intertidal zonation and organisms

Intertidal Zone

• High tide zone: relatively dry

• Middle tide zone: alternatively wet and dry

• Low tide zone: usually wet, but exposed during low tides

http://www.humboldt.edu/~intertid/zones.html

Mostly shelled organisms

Mostly soft-bodied organisms and algae

Life in the intertidal: High tide zone

• Most animals living in the high tide zone have protective coverings to prevent desiccation

• Seaweeds living in the high tide zone have thick cell walls to reduce water loss during low tides

• Many organisms in the high tide zone are sessile, and remain attached to bottom, competing with others for limited space

www.armofthesea.info/flora_faunaff_speciespp/moll.htm

Life in the Intertidal: Middle tide zone

• Seawater constantly bathes the middle tide zone, so a greater variety of seaweeds and soft-bodied organisms live here

• Greater biomass in middle tide zone, and so greater competition for space!

• Mussels and barnacles are common here –hard-bodied, filter-feeding organisms which require seawater to feed and to support planktonic larval stages

Crying cockles and mussels, alive, alive-O!

• The middle tide zone is also home to carnivorous snails and sea stars

• You, too, can be a sea sleuth….• Sea stars pry open clams and mussels with

hundreds of tube feet; Predatory snails bore holes with scraping tongues and suck out prey

• Hermit crabs, sea urchins, and sea anemones are also found here

Life in the Intertidal: Low tide zone

• The low tide zone is almost always submerged, so an abundance of algae (seaweed) is typically present

• Seaweeds are multi-cellular algae (protists)• Seaweeds attach themselves with a structure

known as a holdfast and use gas bladders to reach upward to sunlit surface water; photosynthetic; important source of habitat

Life in the Intertidal: Low tide zone

• Numerous crabs and shellfish live in the low tide zone

• Benthic fish swim through the low tide zone, along with larval nektonic forms which seek shelter and habitat in this protected area

Rocky shores

• Most organisms live on the surface (epifauna)• Zonation of rocky shores:

– Spray zone (rarely covered by water)– High tide zone– Middle tide zone– Low tide zone (rarely exposed)

• Upper zones have mostly shelled organisms• Lower zones have many soft-bodied organisms and

algae

Rocky shores: Intertidal zonation and organisms

Figure 15-2a

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Spray Zone

• Supratidal zone• Organisms

– Avoid drying out - eg. thick, impermeable shells to reduce water loss and radiation effects.

– Behavioural - eg. seeking small crevices for shelter to reduce the drying effects of the wind.

– Physiological - eg. an ability to lower metabolic rates during exposed periods to minimise oxygen consumption.

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Intertidal Zone Organisms

• Middle tide zone– Soft-bodied animals

• Low tide zone– Abundant algae– Many animals hidden by

sea weed and sea grass– Crabs abundant in all

intertidal zones

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Rocky Bottom Shallow Offshore Ocean Floor Communities

• Lobsters– Large, spiny antennae– Live in water deeper

than 20 meters (65 feet)

– Scavengers– Also feed on live animals

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Rocky Bottom Shallow Offshore Ocean Floor Communities

• Oysters– Sessile bivalve mollusks– Thick shell– Start life as plankton

Sea anemone

• A vicious predator cleverly disguised as a harmless flower but armed with stinging cells

Figure 15-4

Sediment-covered shores: Intertidal zonation and organisms

Figure 15-8

Sediment-covered shores: Modes of feeding

Figure 15-9

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Sandy Beach Organisms and Adaptations

• Crustaceans– Segmented body, hard

exoskeleton, paired jointed limbs

– Example: crabs, lobsters

Sand and Cobble Beach Communities• In fact, very few organisms survive in wave-

swept sandy beaches• Some larger crabs can outrun the crashing

waves and locate food within sand grains• Coquina shells and mole crabs are common

along Long Island sandy beaches

www.stripersonline.com/surftalkshowthread.php?t=417951

Sand and Cobble Beach Communities

• Not all intertidal areas are rocky or muddy; some are sandy or consist of gravel or cobbles

• As benign and peaceful as sandy beaches look, they are among the most hostile environments for small organisms

• Sand grains are abrasive and many organisms must have protective coatings and/or be able to burrow below the surface for protection

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Mud Flats

• Eelgrass and turtle grass common

• Bivalves and other mollusks

• Fiddler crabs