Life in the Oceans

Exam Revision

What does the ocean sequester (store)?

The ocean sequesters carbon dioxide, helping to regulate the Earth's climate by absorbing excess atmospheric CO2.

What is the difference between continental crusts and ocean crusts?

Continental crusts are thicker and less dense, primarily composed of granitic rocks, while ocean crusts are thinner and denser, primarily composed of basaltic rocks. This difference affects their buoyancy and behavior at tectonic plate boundaries.

Tectonic Plates

Crust and upper part of the mantle (Earth’s Lithosphere) that move and interact with one another.

How does plate movement cause oceanic ridges and trenches?

Oceanic ridges are created due to the spreading of the sea-floor and trenches are formed when tectonic plates collide with each other.

Continental Margins of Ocean Basins

Continental shelf: 8% of ocean surface but biologically richest part of the ocean

Shelf break: Depths of 120-200m

Continental slope: Deep submarine canyons leading to ocean floor

Continental rise: Sediments building up at the base of the slope

The deep-sea floor (abyssal plain)

• 3000-5000 metres deep

• Relatively flat, with submarine channels, low abyssal hills, plateaus, rises, seamounts, volcanic island etc.

• Support a variety of life

Water Molecule

The oxygen atom is weakly negatively charged and the hydrogen atoms are weakly positively charged (polar molecule).

What are the 3 states of water?

1. Liquid

   • Speed of molecules depends on temperature → faster = higher temperature.

   • When molecule moves fast, it breaks free of all H bonds and will turn into the gaseous phase (evaporation).

2. Solid (ice)

   • Less dense

3. Gas (water vapour)

Why is the temperature of water relatively stable?

Because a lot of energy is needed to change the temperature of water, so water has a reasonably constant temperature.

The composition of seawater

Salinity

= The total amount of dissolved salt in water.

• Used to be measured as the amount of salt left behind after seawater evaporated.

• Today, salinity is measured via conductivity of seawater (ions increase the conductivity).

• Ocean water is generally ~35

• Saltier and colder water makes the water denser

Oxygen and Carbon Dioxide

• The amount of oxygen is affected by photosynthesis and respiration

• Co2 is more solvable than oxygen because it reacts chemically when it dissolves (carbonate/bicarbonate buffer system)

• The ocean stores 50 times more Co2 than the atmosphere.

Currents

• Strongest ocean current occur at the surface (caused by wind)

• Surface currents are driven by heat energy from the sun

• Currents and winds are influenced by the Coriolis Effect

What is the Coriolis effect?

Because the Earth rotates on its axis, circulating air is deflected toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere.

What does the Ekman transport cause?

The Ekman transport piles up surface water (at an angle of 45) in some areas of the ocean and removes water from other areas, producing variations in SSH. This causes the ocean surface to slope gradually.

Gyres

= a large system of rotating ocean currents.

• Warm western currents: Carry solar heat from the equator to higher latitudes

• Cold currents: Flow in the opposite direction into eastern sides

Ocean currents are like a thermostat where it warms the pools, cools the tropics and regulates the climate of our planet.

The layers of the ocean

Surface (mixed layer): 100-200m; mixed by wind, waves and currents.

Intermediate layer: 1000-1500m; site of permanent thermocline.

Deep layer: >1500 m; not in contact with seabed, cold (<4), dense.

Bottom layer: in contact with seabed, cold (<4), dense.

What are permanent thermoclines and seasonal thermoclines?

• Permanent thermoclines is a transition zone between warm surface water and cold water below.

• Seasonal thermoclines form in temperate and polar latitudes during spring and summer as a result of solar heating, and is destroyed by increased surface turbulence during the winter.

Thermohaline circulation

Thermohaline circulation, also known as 

the "global conveyor belt," is a 

large-scale ocean circulation driven by 

differences in water temperature (thermo) 

and salinity (haline).

• Temperature and Salinity: Cold, salty water is denser and sinks, while warm, 

  less salty water is less dense and rises.

• Global Impact: This circulation helps regulate Earth's climate by distributing

  heat and nutrients around the globe.

• Process: Cold, dense water sinks in polar regions and flows towards the equator,

  while warm water from the equator moves towards the poles.

Tides

Are caused by the gravitational forces exerted by the Moon and the Sun, as well as the rotation of the Earth.

• Gravitational Pull: The Moon's gravity pulls on the Earth's oceans, creating a bulge of water on the side of the Earth facing the Moon. A smaller bulge occurs on the opposite side due to the Earth's rotation.

• High and Low Tides: As the Earth rotates, different areas pass through these bulges, experiencing high tides. Areas between the bulges experience low tides.

• Sun's Influence: When the Sun, Moon, and Earth align, the combined gravitational pull creates higher high tides and lower low tides, known as spring tides. When the Sun and Moon are at right angles to each other, the gravitational forces partially cancel out, resulting in lower high tides and higher low tides, known as neap tides.

What 4 factors influence organisms?

1. Growth

2. Distribution

3. Abundance

4. Fecundity (fertility)

What is the difference between biotic and abiotic factors?

Biotic: Is how organisms affect each other e.g., predation, facilitation, mutualism and competition.

Abiotic: Physical and chemical factors e.g., temperature, salinity, waves, water and pressure.

K-selected species vs. R-selected species

K-selected species: Produce offspring that each have a higher probability of survival to maturity (e.g., rats).

R-selected species: Those that produce many offspring and contribute few resources to each individual offspring (e.g., elephants).

Major Marine Lifestyles

Neuston: Live in the water column and on, at or near the surface.

• Sometimes under their own movement and sometimes at the mercy of e.g. winds.

Plankton: Live in the water column and drift at the mercy of currents (Phytoplankton & Zooplankton).

Nekton: Live in the water column.

• Can swim against the current (but can be benthic = bottom living animals that may live on the seafloor or below the sediment).

Benthos: Live on, or buried in, the seafloor and might be sessile or mobile.

Continental margins of ocean basins (Pelagic)

The Neritic zone: Above the continental shelf.

The Oceanic Zone: Above the deep sea, beyond the shelf break. Is further divided by how much light can reach a certain depth.

• Epipelagic = photic zone

• Mesopelagic = light, but not enough to support photosynthesis

• Bathypelagic and below = constant darkness.

Continental margins of ocean basins (Benthic)

The intertidal zone (littoral zone): Exposed to air when the tides go out.

The subtidal zone (sublittoral zone): The continental shelf below the intertidal (always submerged).

+ the deep-sea floor.

Autotrops vs. Heterotrops

• Autotrophs: primary producers (they make the food) & usually use photosynthesis but some use chemosynthesis e.g., phytoplankton, algae.

• Heterotrophs: consumers (not all heterotrophs consume autotrophs) e.g., fish, molluscs, corals.

What are the 3 main components of the ocean basin?

1. Continental Shelf

2. Continental Slope

3. Abyssal Plain (Deep sea)

Pelagic Zone & the Neritic Zone

• The Pelagic zone is the ocean water away from the shores.

• The Neritic zone is found above the continental shelf in the subtotal zone and is always submerged.

Benthic & the subtidal zone

• The benthic zone is the region at or just below the sea floor and is always submerged.

Active vs. Passive Margins

Active Margins:

• Located at edges of tectonic plates where they interact

• Characterised by high tectonic activity e.g., earthquakes

• Little or no shelf, steep slope, trench.

Passive Margins:

• Located at edges of tectonic plater that are not interacting

• Characterised by low tectonic activity and stable conditions

• Wide shelf, gentle slope, continental rise.

Epipelagic Layer (Photic Zone)

Is the uppermost layer of the ocean, extending from the surface to a depth of circa. 200M.

• Light: Receives sufficient sunlight for photosynthesis, making it the primary layer for marine life.

• Biomass: Contains the highest biomass of all ocean layers due to abundant plankton, fish, and marine plants.

• Temperature: Generally warmer compared to deeper layers.

Why are continental shelves nutrient rich?

• Photosynthesis (euphotic zone)

• Contains dissolved organic matter and dissolved particulate matter (also from land sources → estuary, farm run-off and rain).

• Dissolved matter from the deep sea.

• Tides and currents mixing the water.

Ocean Fronts

Are boundaries between well-mixed nutrient-rich coastal water and stratified (not mixed) ocean water.

• Frontal system: These systems create sharp changes in water properties, leading to increased mixing and upwelling.

• Nutrients: The mixing at ocean fronts bring nutrient-rich water to the surface, supporting high concentrations of phytoplankton, which form the base of the marine food web.

How to classify organisms within benthic communities

1. Size

2. Location

3. Type of habitat

4. Feeding strategy (active or sessile?)

5. Lifestyle

Epifauna vs. Infauna

• Epifauna: Live on the seabed

• Infauna: Live below the surface (the majority).

Distribution of benthic organisms

• Distribution is usually patchy.

• Planktonic larvae select particular environment to settle on and undergo metamorphosis (change to become more mature)

• Recruitment = the arrival of new individuals to a population.

• Type of substrate is a main factor dictating which organism lives where (e.g., sandy, muddy or harder rock). → Particle size = soft-bottom and hard-bottom.

Soft-bottom va. Hard-bottom Sublittoral Communities

Soft-bottom:

• Habitat: Sandy, muddy, or silty substrates

• Organisms: Burrowing species like clams, worms, crabs and seagrasses.

• Nutrients: Often rich in organic material, supporting diverse benthic organisms.

• Dominate the continental shelves 45% of temperate & ~30% of tropical.

• Most are unvegetated.

  Hard-bottom:

• Habitat: Rocky substrates including reefs and boulders.

• Organisms: Sessile species e.g., mussels, barnacles and corals.

• Nutrients: Can vary, often support rich biodiversity due to complex structure.

• Can experience strong currents (laminar flow).

Methods for surveying benthic communities

Van Veen Grab → Can leave scars and can affect the results.

Bottom Trawl

Dredge

Box Corer

Underwater Camera or BRUVS

What are the 4 types of estuaries?

1. Drowned River Valley/Coastal Plain: Most common estuary, formed when rising sea levels flood existing river valleys.

2. Fjord: Created by glacial activity, where glaciers carve deep valleys that are later flooded by seawater.

3. Bar-Built Estuary: Formed by the or barrier island that partially enclose a body of water.

4. Tectonic Estuary: Rare, formed when land sank because tectonic plates were sinking and then filled with seawater.

Halocline vs. Isocline

Halocline: A layer in the ocean where the salinity changes rapidly with depth.

• Acts as a barrier to mixing between surface waters and deeper layers, affecting ocean circulation and marine life.

• Average seawater salinity ~35 - Freshwater salinity ~0.

Isocline: A line on a diagram or chart connecting points of equal value in a field.

• (Salty water is denser than fresh water).

What can influence salinity distribution in estuaries?

• The tidal regime

• The shape of the estuary e.g., how fast water flows

• Temperature → Rate of evaporation, salt gets left behind

• Seasonal variation → Temperature and height of tides

• Wind

Salt Wedge

A layer of seawater intrudes under a layer of freshwater due to differences in density and salinity.

Formation: A wedge shaped layer of dense, saltier seawater beneath the ligher, less salty river water.

Tides: Can influence the position and extent of the salt wedge, pushing it further upstream.

Salinity variation: The wedge created a boundary between fresh and salty water, resulting in significant variations in salinity.

Substrate of Estuaries and Anoxic Layer

Substrate:

• The composition of estuaries is sand or soft mud.

• Rich in organic material due to sediment deposition.

Anoxic Layer:

• A water layer devoid of oxygen, typically found in deeper regions (under mud) with poor water circulation.

• High organic matter decomposition and limited oxygen diffusion contribute to anoxic conditions.

• This is a challenging environment for organisms because of the lack of oxygen.

Euryhaline vs. Stenohaline vs. Brackish

Euryhaline: Can tolerate a wide range of salinity levels & most estuarine organisms live here.

Stenohaline: Organisms that can only tolerate a narrow range of salinity levels and are specialised (few organisms by comparison).

Brackish: Water that is saltier than fresh but not as salty as the sea (estuaries).

Osmoregulation vs. Osmoconformers

Osmoregulation: The process by which organisms regulate the balance of water and salts in their bodies to maintain constant.

Osmoconformers: Allow their body fluids to change with salinity.

• Some organisms will osmoregulate at certain salinities and osmoconform at other salinities e.g., crabs.

How do estuarine plants cope with salinity changes?

• Cordgrasses excrete salt through salt glands in their leaves.

• Pickleweed/glasswort absorb water to dilute the salts they take up.

Soft-Bottom Communities

Marine ecosystems found in areas with sandy, muddy, or silty substrates.

• Include species like clams, worms, crabs and types of seagrass (in estuaries, seaweeds struggle to stay in place because of softer sediments).

• Habitat depends on the kind of sediment, esp. the grain size.

• Plays crucial role in nutrient cycling, sediment stabilisation, and providing food sources for higher trophic levels.

Where are the Tropical Seas mostly found?

On the continental shelf

What counts as ‘Tropical’?

• They are the most Southern and Northern places on Earth where the sun is directly overhead.

• This area is less seasonly (warm).

• Organisms live in constant conditions.

Coral Reef Facts

• Within 30 degrees latitude of equator and bounded to a water temp. ~20C.

• Biodiversity hotspots: Lots of species on 1 patch of reef.

• Provide habitat structure & facilitate other species by providing range of niches (combination of conditions).

• Ecosystem Engineers → Alter their own habitats.

• Account for ~5% of world marine fisheries landings → 22 million artisanal - 6 million on coral reefs.

The benefits of coral reefs

Biodiversity: Most diverse ecosystem on the planet, supporting many marine species.

Habitat Structure: Provide habitat and shelter for many organisms.

Coastal Protection: Reducing the impact of waves and storms on coastlines.

Shoreline Maintenance: Provide nutrients by breaking down organic matter and recycling nutrients.

Tourism: Attracts tourists for activities.

Fisheries: Providing nutrients for humans.

The Coral Triangle

The global epicentre of marine biodiversity containing ~76% of the world’s coral species and ~37% of the world’s coral reef fish species.

• 6 out of 7 marine turtle species are found here.

What does ‘Coral’ mean?

A general term for several different groups of cnidarians → a phylum incl. jellyfish

(have stinging tentacles).

What are Hermatypic Corals?

= Reefs built by stony corals (Scleractinia) that produce calcium carbonate skeletons, forming the hard structure of coral reefs.

• Are made of polyps, which secrete the skeleton and host symbiotic algae (Zooxanthellae) for energy.

• Can reproduce sexually by releasing eggs/sperm (spawning), creating planktonic larvae that disperse and settle to form new colonies (depend on photosynthesis).

• Produce a-sexually through budding or fragmentation.

• Thrive in warm, shallow, sunlit waters.

Reef Zones

• Reef Crest: The shallowest part, exposed at low tide. Experiences strong wave action and is dominated by robust, wave resistant corals.

• Fore Reef: The seaward side of the reef, sloping down into deeper water. Is divided into the

  Upper Slope: Rich in coral diversity. Lower Slope: Less light penetration, with fewer but more specialised corals.

• Back Reef: The landward side of the reef, protected from strong waves. It often has seagrass beds, sand flats and patch reefs.

• Lagoon: The area between reef and shore, often with calm waters and diverse habitats like seagrass beds and small coral patches.

• Deep Reef: The steep edge of the reef that drops into the ocean depths, home to deep water corals and unique marine life.

Types of Reef

Fringing Reef: Along the coast, most common, susceptible to pollution and grow seaward.

Patch Reef: Small isolated reefs that grow up from the open bottom of the island platform or continental shelf.

Barrier Reef: Separated from land by lagoons, may develop from fringing reefs after sea level rise.

Atoll Reef (Volcanic Islands): Far from land, depths from ~1000m, ring of reefs/islands and sand cays, clear waters and low nutrients.

Reef Algae Types

Macro algae: Large and visible (e.g., seaweed). Competes with coral for space but provides habitat for marine life.

Coralline algae: Provides its own hard-body structure (slow growing), cements and stabilises.

Turf algae: Small and grass-like, fast growing, epilithic (grow on rock surface), with zooxanthellae.

Phase Shifts on Coral Reefs

Occur when coral reefs transition from coral-dominated to algae-dominated ecosystems due to stress such as:

• Coral bleaching

• Cyclones/hurricanes,

• Crown of Thorns Starfish (overgrazing)

• Eutrophication (high run-off of nutrients e.g., land).

Herbivorous Fishes

Browsers → Key for resilience, eat large fleshy macro algae.

Grazers/Croppers → Graze on algal turfs, can limit establishment and growth of macro algae.

Scrapers → Scrape coral surface and erode reefs, exposing new substrate for larvae settlement e.g., Parrotfish.

What are mangroves?

• Dicotyledonous woody shrubs.

• Confined to the tropics.

• Form dense forests - monospecific patches (uniform in their species in an area).

• Need soft sediments to get established and stabilise soil.

• Create structurally complex habitats above & below water.

• Example of convergent evolution.

Mangrove Environment

• Periodically inundates by the tides - extreme conditions.

• Soils permanently waterlogged, anoxic and are soft and unstable.

• Fluctuating salinity

Mangrove Adaptations to Waterlogged Soil

Mangroves adapt to waterlogged, oxygen-poor soils through specialised roots.

Buttress: Wide, shallow roots for stability in soft, muddy substrates.

Pencil: Emerge from the ground to absorb oxygen directly from the air.

Knee: Looped roots that rise above the water for gas exchange.

Prop: Aerial roots that grown downward from branches, for support and oxygen uptake.

Adaptation to Hypersalinity in Mangroves

Salt Exclusion: Specialised roots filter out salt during water uptake.

Salt Secretion: Salt glands in leaves excrete excess salt.

Salt Accumulation: Storing salt in older leaves, which later shed.

Osmotic Adjustment: Accumulating compatible solutes to balance internal salt leaves.

Pelagic zones of the open ocean

Epipelagic: From surface to 200m (Upper ocean zone)

• High biomass and diversity

• Enough sunlight to sustain photosynthesis.

• Most life in the ocean is reliant on the productivity within this zone.

Mesopelagic: Twilight zone, only 1% light reaches this zone.

• Most areas epilagic-mesolagic transition is marked by thermocline.

• Depth varies with seasonality, can restrict nutrient upwelling and limit productivity.

• Organisms have to deal with temperature change.

Bathypelagic: Midnight zone, no light, constant temp of 4C.

Abyssopelagic: ~40 of global surface, near constant conditions.

• Originate at air-sea interface in polar regions.

• Most life is benthic.

• Fuelled by marine snow (organic material).

Hadalpelagic: From 6000m to very bottom.

• Deepest point (11,022m) in Mariana Trench, Japan.

Depth presents variability in?

Light → Photosynthesis, UV, predation style.

Pressure

Oxygen → At certain depths depleted by bacteria breaking down sinking material.

What 2 basic groups of marine organisms live in the pelagic zone?

1. Plankton

   • Transported by currents

   • Plankton plants = phytoplankton

   • Plankton animals = zooplankton

2. Nekton

   • Free swimmers

   • Marine mammals, fish, invertebrates, reptiles and birds.

   • Most are carnivorous.

Zooplankton

• Microscopic, some are larger (e.g. Jellyfish)

• Heterotrophs (consumers)

• Link between primary production and larger organisms

• Holo-plankton = all of life cycle in the water column

• Mero-plankton = part of life cycle in water column

Diel Vertical Migration (DVM)

The upward migration of organisms towards the surface at night, and a downward movement to deeper waters in the daytime.

Staying afloat > increase buoyancy

Store lipids:

• Diatoms, fish larvae and eggs (oil droplets)

• Sharks (in liver)

• Whales, seals etc. have blubber.

Gas pockets:

• Swim bladders (most bony/teleost fish)

• Cyanobacteria can control gas in vacuoles.

Change composition of body fluids:

• SO4^-2 and Mg^+2 excreted (replace with NH^4+ and CI^-)

• Many planktonic species do this.

Epipelagic food webs

• Many epipelagic organisms consume across multiple tropic levels.

But is complicated by:

• Life stage → Different stages of development with predate on different organisms.

• Location → Tropical food webs have more steps than colder waters.

Why are polar seas so cold?

Earth’s tilt and orbit: Months of light (summer) or dark (winter).

Sun rays have to pass through more atmosphere: In colder climates there is a low angle of incoming sunlight.

Low angular height of the sun: At higher latitudes the lower sun reflects more than it absorbs.

High albedo: Reflectance of radiation from ice.

The Arctic Ocean

• Small and shallow (avg. depth ~1200m) and just beneath the twilight zone.

• Surrounded by land and river runoff contributes to nutrients (and pollution).

• The longest continental shelf (>900 miles) - Siberia.

• Temperatures are fairly stable → 1.8C.

• Sea ice is not permanent.

The Antarctic Ocean

• Larger and deeper than Arctic Ocean (~3200m deep).

• Steep, narrow continental shelf and the central meeting point of oceans.

• Sea ice varies year to year.

• Freshwater ice shelves feeding into the ocean due to climate change.

• Antarctic Circumpolar Current (ACC): A clockwise flow that leads to upwelling of cold, nutrient rich water.

Life in Ice

Ice provides a surface area that organisms and nutrients can adhere to (e.g. plankton, flatworms, crustaceans, bacteria).

→ The melting ice releases these nutrients.

• The most common life cycle is the krill and is the focal point of the Southern Oceans food web.

The Polar Benthos

Key challenge is scour - blocks of moving ice that scrape the sea floor.

Arctic: Has large amounts of decaying matter and has a mix of hard & soft sediments (more biodiversity).

Antarctic: Has a harder substrate and more scouring events.

• Slow paced life processes - slow metabolic rates → Gigantism.

Is a result of slow growth and increased dissolved oxygen concentrations in polar waters.

Diversity in Polar waters

• Polar water shows high endemism - large proportion of species that are not found anywhere else.

• Endemism is higher in Antarctic.

• Continental shelf differences lead to differences in accessibility (shallow shelf = easier to move to other locations).

What are Notothenioids?

They use an antifreeze glycoprotein to reduce the freezing point of their blood (Antarctic - 90% of the fish biomass on the continental shelf).