15.3 What communities exist on the shallow offshore ocean floor?

shallow offshore ocean floor extends from spring low-tide shoreline to seaward edge of cont. shelf

• mainly sediment covered, but rocky bototm exposures may occur locally near shore

  • rocky exposures have marine algae

moreate to low species diversity. diversity of benthic organisms is lowest beneath upwelling regions bc large amts of dead organic matter are produced —> when this matter falls down and decomposes, it consumes oxygen so oxygen supply can be locally depleted —> limiting benthic populations

Rocky Bottoms (Subtidal): Organisms and Their Adaptations

rocky bottom w/in shallow inner subtidal zone normally covered w/ times of macro algae

Kelp and Kelp Forests

Ex. giant brown bladder kelp uses root-like anchor called holdfast to attach to rocky bottoms

• holdfast so strong that only large storm waves can break it

• stipes and blades of the algae are supported by gas-filled floast called pneumatocysts allowing algae to grow upward and extend along surface to allow for good exposure to sunligth

form kelp beds = kelp forests along Pacific coast

smaller species of kelp = shrub kelp

• brown algae shrub kelp i.e. sargassum and rock weed

kelp forests = highly productive ecosystems providing shelter for variety of organisms living w/in or directly upon kelp as epifauna

• organisms = important food source for many animals living in and near kelp forest, i.e. mollusks, sea stars, fishes, octopuses, lobsters, and marine mammals

Lobsters

large crustaceans i.e. lobsters and crabs are common along rocky bottoms

Ex. spiny lobsters have two very large spiny antennae which serve as feelers and are equipped w/ noise-making devices near their base that are used for protection

• eat live animals i.e. mollusks, crustaceans and other lobsters

Oysters

thick-shelled organisms that grow best where there uis a steady flow of clean water to provide plankton and oxygen

• = food for sea stars, fishes, crabs, and snails that bore through the shell and rasp away soft tissue inside

• oyster beds composed of empty shells of previous generations that are cemented to a substrate or to one another, w/ living generation on top

• each female produces millions of eggs each year —> planktonic larvae then attach themselves to bottom

Coral Reefs: Organisms and Their Adaptations

coral reefs = wave-resistant structures produced by corals and other organisms

• composed of colonies of individual polyps = small benthic marine animals feeding w/ stinging tentacles, related to jellies

• corals construct hard CaCO3 structures for protection

• found throughout ocean but normally in shallow warmer water regions

Conditions Necessary for Coral Reef Development

corals very temp sensitive, require warm water to survive

• water that’s too warm kills them —> El Ninos stress them —> coral bleaching and other diseases

• water warm enough normally found in tropics

environmental conditions necessary for coral growth:

• strong sunlight = sunlight not required by coral, but required by zooxanthellae that live w/in coral’s tissues which have symbiotic relationship w/ coral, they’re dinoflagellates

• strong wave or current action - bring nutrients and oxygen

• lack of turbidity - suspended particles in water tend to absorb radiant energy, interfere w/ coral’s filter-feeding capability, and can bury corals

• salt water - corals die if water is too fresh

• hard substrate - corals can’t attach to a muddy bottom

once corals are established in an area that has the conditions necessary for their growth, they continue to grow upward layer by layer, with each new generation attached to the skeletons of its predecessors

• over millions of years, thick sequence of coral reef deposits may develop if conditions remain favorable

RECAP

corals = small colonial animals w/ stinging cells found in shallow tropical waters. to thrive, need strong sunlight, wave or current action, lack of turbidity, normal salinity seawater, and a hard substrate for attachment

Symbiosis of Coral and Algae

reef-building corals = hermatypic and normally have mutualistic relationship w/ zooxanthellae living w/in tissue of coral polyp

• these give coral their bright colors

zooxanthellae algae provide coral hosts w/ continual supply of food and corals provide zooxanthellae w/ nutrients

• coral polyps capture tiny planktonic food w/ stinging tentacles but most reef-building corals receive up to 90% of nutrition from symbiotic zooxanthellae algae

  • allows corals to survive in nutrient-poor waters of tropical waters

mutualistic relationship between coral and algae = sensitive to subtle changes in environment i.e. increased temp, salinity, and light

other reef animals have symbiotic relationship w/ types of marine algae:

• Ex. mixotrophs: derive part of nutrition from algae partners

  • i.e. coral, foraminifers, sponges, mollusks

  • algae nourish coral and contribute to its calcification by extracting CO2 from coral’s body fluids

coral reefs can contain up to 3x as much algal biomass as animal biomass

RECAP

corals able to survive in nutrient-depleted warm water by living symbiotically w/ zooxanthellae algae, which live w/in coral’s tissues, providing it w/ food and giving coral its color.

Coral Reef Development

origin of coral reefs depended on sinking of volcanic islands

3 stages of development in coral reefs:

• fringing

• barrier

• atoll

(1) fringing reefs - initially develop along margin of a landmass where temp, salinity, and turbidity of water are suitable for reef-building corals

• often associated w/ active volcanoes whose lava flows run down volcano and kill coral —> not very thick or well developed

• bc of close proximity to landmass, runoff can carry so much sediment that the reef is buried

• amt of living coral in a fringing reef at any given time = small w/ greatest concentration in areas protected from sediment and salinity changes

**if sea level doesn’t rise or land doesn’t subside, process stops at fringing reef stage

(2) barrier reef - after fringing, they’re linear or circular reefs separated from landmass by well-developed lagoon

• as landmass subsides, reefs maintsin position close to sea level by growing upward

(3) atoll reef - after barrier reef stage

• as barrier reef around a volcano continues to subside, coral builds up toward surface

• after millions of years, volcano becomes completely submerged but coral reef continues growing

• if rate of sinking is slow enough for coral to keep up, circular reef called an atoll is formed

  • atoll encloses a lagoon

• reef fenerally has many channels allowing circulation between lagoon and open ocean

RECAP

coral reefs grow over time and undergo 3 stages of development: fringing, barrier, and atoll

Coral Reef Zonation

many large coral reefs have well-developed vertical and horizontal zonation of the reef slope which is caused by changes in:

• sunlight

• wave energy

• salinity

• water depth

• temperature

• other factors

zones can be readily identified by types of coral present and assemblages of other organisms found in and near the reef

(1) reef slope = deepest water, active coral can only grow to a max depth of 150m bc algae w/in coral’s tissues need sunlight

• water motion less at these depths —> delicate plate corals can live on outer slope of reef

• from 50m to 20m below surface, water motion from breaking waves increases on the side of the reef facing into prevailing current flow —> mass of coral growth and strength of coral structure increase toward the top of this zone where light intensity is as low as 20% of surface value

(2) at surface on seaward side of coral reef, reef crest includes butress zone that protects the reef flat from incoming waves

• reef flat may have water depth of a few cm to a few m at low tide —> 60% of surface light intensity

• many species of colorful reef fish inhabit calm, shallow water also sea cucumbers, worms, and mollusks

• in protected water of reef lagoon live gorgonian coral, anemones, crustaceans, mollusks, and echinoderms

Importance of Coral Reefs

some of the largest structures created by living creatures on Earth

cover less than 0.5% of ocean’s surface area but are home to 25% of all marine species, including almost 1/3 of world’s estimated 20,000 species of marine fish

• provide shelter, food, and breeding grounds attracting many species:

  • sea anemones

  • sea stars

  • crabs

  • sea slugs

  • clams

  • sponges

  • sea turtles

  • mammals

  • sharks

coral reefs foster diversity of species surpassing that of tropical rainforests

coral reefs = most diverse comunities in marine environment

coral reefs provide many benefits to people

Coral Reefs and Nutrient Levels

when human populations increase on land adjacent coral reefs —> reefs deteriorate. fishing, trampling, boat collisions w/ reef sediment increase due to development, and removal of reef inhabitants by visitors all damage a reef

as nutrient levels increase in reef waters from sewage discharge and farm fertilizers, the benthic community changes:

• low nutrient levels - hermatypic corals and other animals that have algal symbiotic partners thrive

• moderate nutrient levels - fleshy benthic plants

• high nutrient levels, the phytoplankton mass exceeds the benthic algal mass, so benthic pops tied to phytoplankton food web dominate

  • high nutrient levels favor suspension feeders i.e. clams

increased phytoplankton biomass reduced clarity of water, which interferes w/ coral’s filter-feeding capability

• fast-growing members of phytoplankton based ecosystem destroy the reef structure by overgwoing the slow-growing coral and through bioerosion, which is erosion of reef by organisms

• bioerosion by sea urchins and sponges is particularly damaging to many

The Crown-of-Thorns Phenomenon

crown-of-thorns is a sea star that has greatly proliferated and destroyed living coral in many reefs

• sea star moves across reefs and eats coral polyps, devouring a lot of coral

• sea stars can suddenly multiply into millions causing major damage

• this issue is caused by human activities

  • overfishing removed their predators

Coral Bleaching and Other Diseases

coral bleaching = loss of color in corals that causes them to turn white, as if bleach has been poured on the reef

corals = very temp sensitive and coral bleaching is usually associated w/ elevated water temps that cause the coral’s colorful symbiotic partner, zooxanthellae algae to be expelled

• scientific studies suggest that reactive oxygen builds up in coral’s tissues and becomes toxic when temps are excessively warm

• causing the coral to shed its normally beneficial algae

• once bleached, coral no longer receives nourishment from algae and if coral does not regain its symbiotic algae w/in a matter of weeks, it’ll die

damaged corals are capable of regeneration if water temps return to normal and water quality remains good, but frequency and intensity of bleaching outbnreaks are now such that the percentahe of reef loss from coral bleaching has been increasing dramatically

• other factors can cause coral bleaching too:

  • elevated UV radiation

  • decrease in sunlight-blocking particles high in atmosphere

  • marine pollution

  • salinity changes

  • invasion of disease

other coral diseases:

• white plague disease

• white band disease

• white pox

• black band

• yellow band

• patchy necrosis

• rapid wasting

Coral Reefs in Decline

coral reefs in rapid decline bc of human caused factors

most serious threat = human activity

• overfishing - depleted populations of fish grazing on algae and if algae grows unchecked, it smothers reefs by preventing corals from being bathed in oxygenated seawater

• runoff w/ sediment and pollutants fuels algae and spreads harmful bacteria

• plastic trash entangling reefs, stressing corals through light deprivation, toxin buildup, and soffucation from oxygen depletion —> vulnerable to starvation and disease

• increased levels of human-caused atmospheric CO2 absorbed into oceans increasing ocean acidity, dissolving coral skeletons

• global warming = higher surface temps = more coral bleaching

CONCEPT CHECK 15.2

Discuss the dominant species of kelp, their epifauna, and animals that feed on kelp in Pacific coast kelp forests.

Describe the environmental conditions required for development of coral reefs.

Draw and describe each of the three stages of coral reef development. How does this sequence tie into plate tectonics?

Describe the zones of the reef slope, the characteristic coral types, and the physical factors related to its zonation.

What is coral bleaching? How does it occur? What other diseases affect corals?