Unit 5

Name the world's five oceans in order from largest to smallest. (1)

• Pacific, Atlantic, Indian, Antarctic, Arctic

Identify and describe the five open ocean depth zones in terms of light penetration. (5)

• Epipelagic (0–200m) — sunlight zone, enough light for photosynthesis

• Mesopelagic (200–1000m) — twilight zone, some light but not enough for photosynthesis

• Bathypelagic (1000–4000m) — midnight zone, completely dark

• Abyssopelagic (4000–6000m) — abyss, no light, extreme pressure

• Benthic/Hadalpelagic (6000–10000m+) — deepest trenches, no light, crushing pressure

Explain the importance of oceans as carbon sinks. (4)

• Oceans absorb large amounts of CO₂ from the atmosphere

• CO₂ dissolves into seawater and is used by phytoplankton in photosynthesis

• Carbon is also stored in coral skeletons, shells, and deep sea sediments

• This reduces the amount of CO₂ in the atmosphere, slowing climate change

Explain the importance of oceans as sources of oxygen. (3)

• Phytoplankton in the surface ocean produce oxygen through photosynthesis

• Responsible for producing approximately 50% of Earth's oxygen

• Oxygen diffuses from the ocean into the atmosphere

Explain the importance of oceans in temperature buffering and global climate control. (4)

• Water has a high specific heat capacity so oceans absorb and release heat slowly

• This moderates temperatures in coastal regions and prevents extreme temperature swings

• Ocean currents (thermohaline circulation) distribute heat around the globe

• Warm currents (e.g. Gulf Stream) warm nearby land masses; cold currents cool them

State the conditions required for tropical coral reef formation. (5)

• Temperature: 16–35°C, optimal 23–25°C for proper enzyme function

• Clarity: non-turbid water — silt and algal blooms reduce light penetration and inhibit photosynthesis by zooxanthellae

• Depth: within 20m is best; must be in the photic zone

• Salinity: 30–40 ppt (optimal 35 ppt) for proper enzymatic function

• Substrate: hard basaltic rock for sessile coral polyps to attach to

Describe and compare the four types of tropical coral reef. (4)

• Fringing reef — attached directly to the coastline or island with no lagoon; closest to shore

• Barrier reef — separated from the mainland or island by a lagoon; runs parallel to the coast

• Atoll — ring of small coral islands surrounding a shallow central lagoon; formed over a submerged volcano

• Patch reef — small, isolated reefs within a lagoon or on a continental shelf; not attached to land

Describe corals in terms of their classification and basic biology. (4)

• In the phylum Cnidaria

• Form sessile colonies of individual units called polyps

• Polyps are the cup-like living stage that build the reef structure

• Many hard corals have a mutualistic relationship with zooxanthellae algae living in their tissues

Compare hard coral and soft coral. (2)

Hard coral: CaCO₃ exoskeleton; reef-building; many zooxanthellae
Soft coral: no CaCO₃ skeleton; not reef-building; few/none zooxanthellae

Describe the structure of a typical hard coral polyp and the function of each part. (7)

• Tentacles — capture prey at night and house zooxanthellae for photosynthesis

• Nematocyst — harpoon-like stinging organelle inside cnidocyte cells that contains toxin; used to capture prey

• Mouth — ingests captured prey and excretes waste

• Stomach — absorbs nutrients including oxygen which diffuses into the organism

• Calyx — the stony CaCO₃ cup in which the polyp lives

• Theca — the walls of the calyx

• Basal plate — the lower part of the calyx that separates the polyp from the substrate

Explain how corals obtain their nutrition. (3)

Corals are heterotrophs: use tentacles + nematocysts to catch zooplankton (mainly at night)
Mutualism with zooxanthellae: algae photosynthesise → give up to 90% energy
Exchange: coral provides shelter, CO₂, and nutrients

Discuss the importance of coral reefs. (5)

• Biodiversity: contain 25% of all marine species despite covering only 0.1% of the ocean floor

• Food source: support fisheries that feed millions of people globally

• Coastal protection: absorb wave energy, protecting coastlines, mangroves, seagrass beds, and coastal properties from erosion

• Medicines: source of anticancer drugs (KLH), antivirals, and pain medicines

• Tourism: generate significant economic revenue through diving, snorkelling, and ecotourism

Discuss the causes of reef erosion. (8)

Ocean acidification: CO₂ dissolves in seawater → forms carbonic acid → lowers pH → dissolves coral CaCO₃ skeletons.
Temperature rise: high temperatures cause coral bleaching → zooxanthellae expelled → coral turns white and weakened.
Bioerosion/predation: parrotfish, butterflyfish, crown-of-thorns starfish eat coral tissue and skeleton.
Physical damage: storms and hurricanes break coral structures and smother them with sediment.
Sediment/runoff: turbidity blocks sunlight → reduces zooxanthellae photosynthesis → coral growth slows.
Exposure to air: extreme low tides can desiccate coral tissue.
Coral disease: e.g., stony coral tissue loss disease damages coral health.
Human activity: destructive fishing (blast, cyanide, benthic trawling), careless tourism, and ship groundings physically damage reefs.

Explain what coral bleaching is and its effects. (5)

• Bleaching occurs when corals are stressed (usually by high temperature or low pH)

• Zooxanthellae are expelled from the coral tissue, leaving the coral white

• Without zooxanthellae, corals lose up to 90% of their energy supply

• Coral can recover if stress is short-lived, but prolonged bleaching leads to death

• Loss of coral means loss of habitat and biodiversity across the entire reef ecosystem

Discuss the use of artificial reefs. (4)

Substrate: provide hard surfaces for coral polyps and other organisms to attach
Materials: made from reef balls or sunken ships; holes create shelter and habitat
Ecological functions: mirror natural reefs → increase biodiversity, act as submerged breakwaters, support fisheries, promote tourism
Restoration: help restore degraded reefs and create habitat where natural reefs are lost

Describe the Darwin-Dana-Daly theory of atoll formation. (7)

• An oceanic volcano erupts and emerges above the sea surface, forming an island

• Coral colonises the edges of the island, forming a fringing reef

• The island slowly subsides (sinks) over geological time

• As the island sinks, coral continues to grow upward and outward, forming a barrier reef separated from the island by a lagoon

• Eventually the volcanic island disappears completely below the surface

• A ring-shaped atoll remains with a shallow central lagoon and patch reefs inside

• Fringing reefs take ~10,000 years to form; atolls can take up to 30 million years

What evidence supports the theory of atoll formation? (4)

• Coral cores from deep drilling: show growth bands like tree rings → indicate growth rate and past conditions

• Age-depth pattern: coral gets older with depth

• Carbon dating: C¹⁴/C¹² ratios date corals up to 60,000 years

• Subsidence evidence: shows land has sunk beneath coral → supports Darwin’s atoll theory

Identify the zones of a rocky shore and describe the changing abiotic factors across them during one tidal cycle. (5)

• Splash zone: above high tide mark, exposed almost all day, only wetted by spray; highest desiccation risk, highest salinity fluctuation

• Upper shore: only submerged at high tide; high desiccation and salinity change at low tide; high wave action

• Middle shore: exposed twice daily at low tide, submerged twice daily at high tide; moderate abiotic stress

• Lower shore: usually submerged; most stable temperature, salinity, and moisture; greatest biotic influence

• Subtidal zone: permanently submerged; most stable abiotic conditions of all zones

Explain how abiotic factors affect the distribution of organisms on a rocky shore. (5)

• Exposure to air during low tide causes desiccation risk — organisms higher on the shore must tolerate drying out more

• Salinity fluctuates in tide pools due to evaporation — organisms must osmoregulate or osmoconform

• Wave action is strongest on the upper shore — organisms must attach firmly to substrate

• Water temperature increases during low tide exposure — reduces dissolved oxygen

• Light availability increases with exposure — benefits photosynthetic organisms like algae

Explain how biotic factors affect the distribution of organisms on a rocky shore. (4)

• Competition for space and attachment sites drives vertical zonation — species occupy different zones to reduce competition

• Predation is a major influence in the lower shore where sea stars eat mussels and urchins graze algae

• Interspecific competition: barnacles and mussels compete for space on rock surfaces

• Lower shore has greater biodiversity because stable abiotic conditions allow more species to coexist, increasing competition and predation interactions

Describe the adaptations of organisms in the splash zone. (2)

• Periwinkles and limpets use a muscular foot to clamp tightly onto rock, preventing desiccation and being swept away by waves

• Can tolerate long periods of air exposure and high salinity fluctuation

Describe the adaptations of organisms in the upper shore. (3)

• Barnacles trap water inside their shell when the tide goes out, preventing desiccation

• Chitons trap water in their mantle cavity

• Strong attachment to substrate to withstand wave action

Describe the adaptations of organisms in the middle shore. (3)

• Mussels attach to rock using byssal threads and clamp their valves shut to trap water and avoid desiccation

• Brown algae such as kelp and wrack are flexible to withstand wave action

• Abalone clings tightly to rock using a powerful muscular foot

Describe the adaptations of organisms in the lower shore. (3)

• Fewer adaptations needed for desiccation as this zone is usually submerged

• Sea stars, sea urchins, and anemones live here — they tolerate wave action but cannot survive prolonged air exposure

• Greater biodiversity due to stable abiotic conditions

Describe the sandy shore as an ecosystem. (4)

• Unstable, constantly shifting substrate moved by wave action and wind

• Very porous — water passes through quickly, increasing desiccation risk

• Lack of attachment sites means low productivity, small food chains, and low biodiversity

• Fewer niches available so organisms tend to have generalised niches (detritivores and scavengers)

Explain how abiotic and biotic factors on a sandy shore lead to low biodiversity. (5)

• Constantly shifting substrate provides no stable attachment sites — macroalgae and sessile animals cannot establish

• High porosity means water drains rapidly at low tide, increasing desiccation and temperature extremes

• Low productivity because few producers can attach and grow

• Small food chains mean fewer trophic levels and fewer species can be supported

• Human impacts such as development, dredging, and recreation further reduce biodiversity

Explain the adaptations of organisms living on a sandy shore. (4)

• Burrowing — organisms like lugworms, razor clams, and ghost crabs burrow into the sand to avoid desiccation and wave action

• Infauna lifestyle — living within the sediment rather than on the surface provides protection from physical disturbance

• Ghost crabs are fast-moving and emerge only briefly, retreating to burrows to avoid desiccation

• Lugworms have a U-shaped burrow and feed on detritus within the sediment

Describe the mangrove forest as an ecosystem. (5)

• A tidal ecosystem featuring salt-tolerant trees and other plants

• Found in the littoral zone of tropical and subtropical coasts

• Grows on muddy shores with low dissolved oxygen in the substrate

• Characterised by a large tidal range and brackish to saline water conditions

• Supports diverse populations of fish, invertebrates, birds, and other organisms

State the conditions required for mangrove forest formation. (5)

• Tropical or subtropical location with warm temperatures

• Muddy substrate with low dissolved oxygen

• Sheltered from strong wave action — allows sedimentation

• Large tidal range providing regular inundation and exposure

• Brackish to saline water conditions

Explain the adaptations of the red mangrove (Rhizophora mangle). (3)

• Prop roots: tangled roots that grow down from the trunk into the water, providing stability in the unstable muddy substrate and supplementary oxygen uptake via lenticels in the roots

• Salt exclusion: root membranes filter out salt, allowing water uptake while preventing salt from entering the plant

• Viviparous reproduction: seeds germinate while still attached to the parent tree, growing into propagules; propagules detach and float until they find suitable substrate, then rapidly develop into young plants

How does the black mangrove (Avicennia) differ from the red mangrove in its adaptations? (5)

• Found further inland than red mangroves

• Roots grow upward out of the oxygen-deprived mud rather than downward

• These upward root projections are called pneumatophores

• Pneumatophores have lenticels that allow gas exchange in the anaerobic mud

• Some species excrete excess salt directly through their leaves, which can form visible salt crystals on the leaf surface

Explain the ecological importance of mangrove forests. (5)

• Nursery grounds: prop roots provide shelter and protection from predators for juveniles of many fish and invertebrate species, supporting commercial fisheries

• Sediment trapping: prop roots trap and slow sediment, stabilising and building new land, protecting coastlines from erosion, and preventing sediment from smothering nearby coral reefs and seagrass beds

• Carbon sink: store large amounts of carbon in their biomass and sediment (blue carbon)

• Filter pollutants from water before it reaches coral reefs and seagrass beds

• Provide dissolved oxygen and habitat for a wide range of species

Discuss the importance of mangrove forests to humans. (5)

• Coastal protection: reduce wave energy and erosion, protecting coastal communities and infrastructure

• Food source: support fisheries by acting as nursery grounds; some communities harvest mangrove species directly

• Timber and fuel: mangrove wood used for construction and as charcoal fuel source in some regions

• Tourism: ecotourism and recreational use generate economic revenue

• Biodiversity: support high species diversity, providing ecological services that benefit nearby ecosystems

Discuss the threats to mangrove forests. (4)

• Temperature change: climate change raises sea temperatures and increases storm intensity, stressing mangrove communities

• Over-harvesting: excessive logging for timber and charcoal reduces mangrove coverage

• Storm damage: hurricanes and cyclones physically destroy mangrove forests

• Change in coastal land use: conversion for shrimp farms, agriculture, urban development, and tourism infrastructure destroys large areas of mangrove habitat