Marine Systems and the Ocean Environment
Core concepts
You must be able to explain how seawater properties drive ocean structure and movement. Key ideas: salinity, temperature and density (and how they combine into the TS-diagram and water masses); thermohaline circulation driven by density gradients; wind-driven surface circulation (gyres, Ekman transport, upwelling and downwelling); waves and tides (tidal range patterns, spring/neap cycles); and coastal and deep-ocean processes such as longshore drift, sediment transport, submarine canyons and turbidity currents. Link oceanography to biology: upwelling brings nutrients to the surface and fuels productivity; stratification limits mixing and nutrient supply.
Traps to watch
- Confusing cause and driver. Surface currents are wind-driven; deep circulation is density-driven. Do not attribute thermohaline circulation to tides.
- Mixing up salinity effects. Adding freshwater lowers salinity and density; evaporation raises both. Cold water is denser, but very salty warm water can also be dense.
- Tide terminology. Spring tides occur at new and full moon (aligned Sun–Earth–Moon), neap tides at quarter moons. Students often reverse these.
- Ekman transport direction. Net surface transport is about 90° to the wind (right in the Northern Hemisphere, left in the Southern), not directly downwind.
- Coriolis misconceptions. It deflects moving water, not the wind direction itself, and is zero at the equator.
Rule of thumb
When asked to predict circulation or stratification, ask: what is the density gradient, and what is forcing it? If the answer is heat or freshwater flux, expect thermohaline or stratification effects; if it is wind stress, expect surface currents, Ekman transport and upwelling.
Worked example
A coastal region experiences strong offshore winds. Predict the biological consequence. Offshore winds push surface water away (Ekman transport), causing upwelling of nutrient-rich deep water, raising primary productivity and supporting fisheries. If winds reverse, downwelling occurs and productivity falls.