Species
Follow a species across the system
Use swordfish to explore how one organism connects distant ocean regions.
Ocean Literacy Principle 1
Earth has one big ocean with many features
The ocean covers about 70% of Earth’s surface and is the planet’s defining physical feature. It is one connected body of water: named oceans help us describe different regions, but they form one global system. This principle connects geologic features, ocean-water properties, circulation, and watersheds.
Guiding question: How can very different ocean places still belong to one connected system?
NMEA grade band: Grades 6–8
Other grade bands are in preparation.
At a glance
What students should take away
Students should come away understanding that the ocean is not a set of isolated facts or separate places. It is one global system made of many distinct regions that are linked by movement, exchange, and change.
Key ideas
- The ocean is one connected body of water.
- Moving tectonic plates shape ocean basins and seafloor features.
- Temperature and salinity affect seawater density.
- Wind, tides, Earth’s rotation, and density differences move ocean water.
- Currents transport heat, nutrients, and organisms.
- The water cycle and watersheds connect land, atmosphere, and ocean.
Teach this principle
Why this matters
This principle gives students an early introduction to systems thinking by showing that different places and features still belong to one connected ocean. It is foundational because later ideas about biodiversity, climate, migration, and human impact all depend on understanding connection.
Explore ocean features
- Student task
- Find a ridge, a trench, and a continental shelf on the map. Compare their shapes and locations.
- Ask the class
- What might explain these different seafloor features?
- Listen for
- Students begin to connect seafloor features with tectonic plate movement.
Teaching resources
Explore why water moves
- Student task
- Compare the temperature and salinity maps. Predict how colder or saltier water could behave differently from warmer or fresher water.
- Ask the class
- How could temperature and salinity differences help move ocean water?
- Listen for
- Students explain that colder or saltier seawater is generally denser and that density differences can help drive deep currents.
Follow what gets transported
- Student task
- Trace a current on the map. Use the upwelling card to explain how moving water carries heat, nutrients, and small organisms between ocean regions.
- Ask the class
- How could a change in one ocean region affect life somewhere else?
- Listen for
- Students use currents to explain how heat, nutrients, and small organisms move between connected ocean regions.
Teaching resources
Check understanding
If two ocean regions look very different, what evidence shows that they still belong to one connected system?
Explore the concepts
Geologic featuresThe movement of Earth’s tectonic plates shapes ocean basins and the varied features of the seafloor.
Ocean basins include major seafloor features.
Ocean basins contain ridges, trenches, rift valleys, seamounts, islands, continental shelves, and other features. Many of these features are created and changed by the movement of Earth’s tectonic plates.
Ocean basins change over geologic time.
The size and shape of ocean basins have changed over geologic time and continue to change. New ocean crust forms at spreading centers, while older, denser crust is recycled into Earth at subduction zones.
Earth’s internal heat contributes to plate movement. Continents that were once joined later separated along rift valleys, forming the continents and ocean basins we recognize today, and the continents are still moving.
Heat escaping from Earth drives slow convection within the largely solid mantle, which can deform and flow over geological time. Plate motion is part of this system, with forces including the pull of sinking plates; the mantle is not an ocean of liquid magma.
At several times in geological history, continents were assembled into supercontinents surrounded by a vast ocean. Their breakup changed ocean-basin geometry and circulation.
Connected concepts in other principles
Properties of ocean waterTemperature and salinity influence seawater density, connecting the properties of ocean water to circulation.
Temperature, salinity, and density are connected.
About 97% of Earth’s water is in the ocean. Temperature and salinity affect seawater density: colder or saltier seawater is generally denser.
Ocean temperature changes through warming and cooling, including heating by sunlight and cooling through contact with ice. Evaporation and sea-ice formation can raise salinity, while rainfall, river inflow, and melting ice can lower it. Differences in density help drive deep ocean currents.
Ocean salts come from several Earth systems.
Salts enter the ocean through erosion of land, volcanic emissions, reactions at the seafloor, and material deposited from the atmosphere.
More dissolved salt lowers seawater’s freezing point. Seawater is also generally slightly basic, although its pH varies across places and over time.
Ocean circulationOcean movement connects distant places and links the ocean to the water cycle, watersheds, atmosphere, and land.
Different forces move ocean water.
Wind, together with Earth’s rotation, helps organize surface currents. Differences in seawater density help drive deep currents, while the gravity of the Moon and Sun produces tides. The shapes of ocean basins and continents help guide circulation.
Currents transport heat, nutrients, and organisms throughout the ocean, linking distant regions and shaping marine ecosystems and climate.
Gyres, upwelling, and currents redistribute life.
Wind and Earth’s rotation organize many surface currents into large gyres. Large subtropical gyres generally rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Coastal upwelling is common along western coasts of continents, where it brings cold, nutrient-rich deep water into the sunlit surface ocean and supports primary producers. Currents also carry larvae and juveniles into new areas.
The deflection associated with Earth’s rotation is called the Coriolis effect.
The water cycle and watersheds connect land and ocean.
Water moves between ocean, land, and atmosphere through evaporation, condensation, precipitation, runoff, and groundwater flow.
Rivers and streams carry freshwater, salts, sediments, nutrients, and pollutants through watersheds to estuaries and the ocean. Where freshwater and seawater mix, they can create density differences that influence local circulation.
Sea level changes as ocean volume changes.
Global sea level rises when land-based glaciers and ice sheets melt and when warming seawater expands.
Connected concepts in other principles
Teaching resources
Start here
Ecoregion
Compare two regions
Compare one highly productive region with a contrasting basin or gyre to highlight both difference and connection.
Opportunity
Explore a current or process
Use deep currents to discuss what moves through the ocean and how distant places influence one another.
Explore this principle with the platform
WebGIS
Explore ocean basins and seafloor features, then compare temperature, salinity, and currents to investigate why ocean water moves.
Knowledge Graph
Trace how species, places, processes, threats, and opportunities are connected across the system.
Cards
Use cards to explore examples of movement, connection, and large-scale ocean processes.
Featured Species
Species
Copepod
Copepods show how currents transport small drifting organisms and help young marine life spread between connected ocean areas.
Species
Swordfish
Swordfish help explain the principle because they move through large parts of the ocean system rather than staying inside one region.
Species
Tiger shark
Tiger sharks show that wide-ranging predators depend on connections among coastal habitats, open water, and large-scale ocean movement.
Featured Ecoregions
Ecoregion
Arctic Ocean
Distinctive: This ocean is shaped by sea ice, cold water, and strong seasonal shifts in light and temperature.
Connected to the global system: It shows how ocean conditions help regulate climate and affect the limits of life-supporting environments.
Ecoregion
Mediterranean Sea
Distinctive: This semi-enclosed sea has high evaporation, relatively salty water, and restricted exchange through the Strait of Gibraltar.
Connected to the global system: Its salinity and density differences influence water exchange with the Atlantic, connecting ocean-water properties to circulation.
Ecoregion
Benguela Current
Distinctive: This eastern-boundary current brings cold, nutrient-rich water to the surface along southwest Africa.
Connected to the global system: It connects upwelling, productive food webs, and circulation between coastal waters and the wider Atlantic Ocean.
Featured Cards
Opportunity
Deep Currents
Deep Currents illustrate the principle by showing how water movement links distant parts of the ocean below the surface.
Opportunity
Upwelling
Upwelling shows how currents bring cold, nutrient-rich deep water into the sunlit surface ocean, supporting primary producers and connected food webs.
Sources and curriculum alignment
Grades 6–8 conceptual alignment: Adapted from NMEA Ocean Literacy Scope and Sequence under CC BY-NC-SA 4.0.
Adapted from the NMEA Grades 6–8 Principle 1 CFD (2021, handbook page 45). Source notes distinguish the original concepts from qualified adaptations. Classroom tasks and platform links are Blue Biome teaching suggestions.
Source concepts and connections
The notes below paraphrase the English NMEA source. They also explain qualified adaptations and corrected references.
A · Ocean basins change over geologic time.
- Ocean basin size and shape change through geological time.
A1 · Ocean basins include major seafloor features.
- Motion at plate margins creates features on the seafloor and land.
A2 · Ocean basins include major seafloor features.
- Continual plate movement shapes the seafloor.
A3 · Ocean basins change over geologic time.
- Spreading creates crust.
- subduction recycles older denser crust into Earth.
A4 · Ocean basins change over geologic time.
- The CFD attributes plate movement primarily to convection of hot fluids below the crust.
Adaptation notes (English)
Retain mantle convection while explaining slow deformation of largely solid rock and multiple plate-driving forces, rather than a liquid-magma mantle.
Read the explanationA5 · Ocean basins include major seafloor features.
- Features include trenches, rifts, ridges, seamounts, islands and shelves.
A7 · Ocean basins change over geologic time.
- Continents have assembled into supercontinents surrounded by an ocean.
A8 · Ocean basins change over geologic time.
- Rifting separated continents and formed new ocean basins.
B · Temperature, salinity, and density are connected.
- Most Earth water is ocean water, with distinctive chemical and physical properties.
B1 · Ocean salts come from several Earth systems.
- Salts enter through land erosion, volcanism, seafloor reactions and atmospheric deposition.
B2 · Ocean salts come from several Earth systems.
- Increasing salinity lowers freezing point.
- the CFD compares ocean pH as more basic than freshwater.
Adaptation notes (English)
Describe seawater as generally slightly basic; freshwater pH varies, so omit the universal comparison.
Read the explanationB3 · Temperature, salinity, and density are connected.
- Density differences can cause currents.
B4 · Temperature, salinity, and density are connected.
- Increasing salinity and decreasing temperature generally increase seawater density.
B5 · Temperature, salinity, and density are connected.
- Removing or adding water changes salinity through evaporation, ice changes, runoff and rain.
B6 · Temperature, salinity, and density are connected.
- Sunlight and contact with ice illustrate warming and cooling of ocean water.
C · Different forces move ocean water.
- One connected ocean circulates continually and participates in the water cycle.
C1 · Different forces move ocean water.
- Tides and currents contribute to ocean circulation.
C2 · Different forces move ocean water.
- Density differences help drive deep currents.
C3 · Gyres, upwelling, and currents redistribute life.
- Winds and Coriolis organize gyres with different rotation by hemisphere.
Adaptation notes (English)
Apply hemisphere rotation to major subtropical gyres, not every ocean gyre.
Read the explanationC4 · Different forces move ocean water.
- Earth, Moon and Sun interact gravitationally to produce tides.
C5 · Different forces move ocean water.
- Basins, continents and geological features guide circulation.
C6 · Different forces move ocean water.
- Currents transport heat, nutrients and organisms.
C7 · Gyres, upwelling, and currents redistribute life.
- Upwelling brings deep nutrients into sunlit water and supports producers, often on continental west coasts.
C8 · Gyres, upwelling, and currents redistribute life.
- Currents disperse larvae and juveniles to new areas.
C9 · The water cycle and watersheds connect land and ocean.
- Freshwater entering from watersheds changes density and circulation where it meets seawater.
C10 · The water cycle and watersheds connect land and ocean.
- Watersheds carry nutrients, salts, sediment and pollutants to the ocean.
Adaptation notes (English)
Printed P6 D17 concerns acidification; P6 D19 describes watershed pollutant transport.
Read the explanationC11 · Sea level changes as ocean volume changes.
- Melting land glaciers raises sea level.
Concept pathways (English)
Creation and recycling of crust; changing basins
Water properties, density and circulation
Forces and transport in circulation
Watershed inputs, density and sea level