Sign up to access the complete lesson and track your progress!
Unlock This CourseImagine taking a sip of seawater - it tastes salty, right? That saltiness is what we call salinity. Salinity is one of the most important properties of seawater, affecting everything from the types of fish that can live in different areas to how ocean currents flow around the planet. Understanding how to measure salinity helps marine scientists study ocean health, climate change and marine ecosystems.
Key Definitions:
Salinity affects water density, which drives ocean currents that regulate global climate. It also determines which marine organisms can survive in different areas - some fish thrive in salty water, whilst others need fresher conditions.
Scientists use several different units to measure salinity, each serving different purposes. Understanding these units helps us compare salinity measurements from different sources and time periods.
The most common unit for measuring salinity is parts per thousand, written as ppt or ‰. This tells us how many grams of salt are dissolved in 1000 grams (1 kilogram) of seawater. Average ocean salinity is about 35 ppt, meaning there are 35 grams of salt in every kilogram of seawater.
0-0.5 ppt - Rivers, lakes and streams typically have very low salinity.
0.5-30 ppt - Where rivers meet the sea, creating mixed salinity zones.
30-40 ppt - Open ocean water with typical marine salinity levels.
Modern oceanographers often use Practical Salinity Units (PSU), which are based on electrical conductivity measurements. PSU values are very close to ppt values - average seawater has a salinity of about 35 PSU. This unit is more precise because it's based on standardised electrical measurements rather than weighing actual salt content.
The Dead Sea has a salinity of about 340 ppt - nearly 10 times saltier than normal seawater! This extreme salinity makes it impossible for most marine life to survive, but allows people to float effortlessly on the surface.
Scientists use various tools and techniques to measure salinity, from simple handheld devices to sophisticated electronic instruments. Each method has its advantages depending on the situation and required accuracy.
A refractometer is a handheld optical instrument that measures how light bends when it passes through a water sample. Saltier water bends light differently than fresher water. Marine biologists often use refractometers because they're portable, quick to use and don't require batteries. You simply place a drop of seawater on the device and look through the eyepiece to read the salinity.
Quick, portable, no power needed, accurate for field work, easy to calibrate with distilled water.
These electronic devices measure how well water conducts electricity. Since dissolved salts make water more conductive, scientists can calculate salinity from conductivity measurements. Modern conductivity meters are very accurate and can also measure temperature, which affects conductivity readings.
The meter sends a small electrical current between two electrodes in the water. Saltier water allows more current to flow, giving higher conductivity readings that convert to salinity values.
CTD stands for Conductivity, Temperature and Depth. These sophisticated instruments are lowered from research ships to measure salinity at different depths in the ocean. They provide detailed profiles showing how salinity changes with depth, which helps scientists understand ocean circulation patterns and water masses.
The Mediterranean Sea has higher salinity (around 38-39 ppt) than the Atlantic Ocean (35 ppt) because it's nearly enclosed and loses lots of water through evaporation. Scientists use satellite measurements and CTD instruments to monitor how this affects water circulation between the Mediterranean and Atlantic through the Strait of Gibraltar.
Salinity isn't the same everywhere in the ocean. Several natural processes increase or decrease the salt content of seawater, creating the salinity patterns we observe around the world.
When seawater evaporates, only pure water vapour escapes, leaving the salt behind. This increases salinity. Conversely, when it rains or snows, fresh water dilutes the seawater and reduces salinity. Tropical areas with high evaporation often have higher salinity, whilst polar regions with melting ice have lower salinity.
Rivers carry fresh water into the ocean, reducing salinity near coastlines. The Amazon River, for example, creates a plume of low-salinity water that extends hundreds of kilometres into the Atlantic Ocean. Similarly, when glaciers and sea ice melt, they add fresh water that lowers local salinity levels.
Red Sea (40+ ppt), Persian Gulf (40+ ppt), Mediterranean Sea (38-39 ppt)
Open Atlantic (35 ppt), Open Pacific (34-35 ppt), Indian Ocean (34-36 ppt)
Baltic Sea (7-8 ppt), Arctic Ocean (30-32 ppt), Near river mouths (variable)
Precise salinity measurements are crucial for understanding ocean health, predicting weather patterns and protecting marine ecosystems. Scientists use this data to track climate change effects and manage marine resources sustainably.
Salinity affects water density, which drives deep ocean currents. The Global Conveyor Belt - a system of currents that circulates water around the planet - depends partly on salinity differences. Changes in salinity can alter these currents, affecting global climate patterns.
Different marine organisms have adapted to specific salinity ranges. Fish, corals and plankton all have optimal salinity levels for survival. By measuring salinity, scientists can predict where different species might be found and how they might respond to environmental changes.
Scientists use satellite measurements and automated buoys to monitor global salinity patterns. They've discovered that salty areas are getting saltier whilst fresh areas are getting fresher, suggesting that the water cycle is intensifying due to climate change. This information helps predict future changes in ocean circulation and regional climate patterns.
Salinity measurement isn't just for research - it has many practical applications that affect our daily lives and the marine industry.
Fish farmers must monitor salinity carefully to ensure their fish remain healthy. Different species require different salinity levels and sudden changes can stress or kill fish. Salmon farms, for example, need to gradually adjust salinity when moving fish from freshwater to seawater environments.
Facilities that remove salt from seawater to produce drinking water need accurate salinity measurements to operate efficiently. Higher salinity requires more energy to remove salt, affecting operating costs and environmental impact.
Environmental agencies measure salinity to detect pollution, monitor ecosystem health and ensure water quality standards. Sudden salinity changes can indicate problems like industrial discharge or unusual weather patterns.