Source: Marine Biology Ch. 1.2, Intro to Marine Biology (Florida International University)
Difficulty: Introductory | Prerequisites: Chapter 1 notes on water properties, bonding, and polarity.
Now that you understand water's molecular structure and polarity, this section explains what that polarity does in practice. Water is the ocean's solvent, and the substances it dissolves (salts, gases) define the chemical environment marine organisms live in. Salinity, pH, and dissolved oxygen are three of the most important variables in marine biology, and they all trace back to water's ability to dissolve things. If the last section was "what water is," this one is "what water does."
Water's polarity makes it an excellent solvent. The ocean's salinity (averaging 35 ppt) rises with evaporation and salt-rich runoff, and falls with freshwater input. The pH scale measures hydrogen-ion concentration on a logarithmic scale; ocean pH has dropped from 8.2 to 8.1 due to rising atmospheric CO₂, representing a 25% increase in acidity. Gas solubility in seawater depends on temperature, pressure, and salinity, with cold, deep, low-salinity water holding the most dissolved gas.
Solvent
A substance that dissolves other substances. Water is the primary solvent in the ocean. In simple terms, the solvent is the thing doing the dissolving.
Solute
A substance that is dissolved by a solvent. Common solutes in seawater include sodium chloride, carbon dioxide, oxygen, and calcium carbonate. In simple terms, solutes are the things being dissolved.
Dissolution
The process by which a solute is dissolved into a solvent. Think of it as salt disappearing into water: the salt is still there as ions, just no longer visible as a solid.
Solubility
The extent to which a particular solute can be dissolved in a given solvent. Affected primarily by temperature. In simple terms, solubility tells you how much of something water can hold before it stops dissolving.
Salinity
A measure of the total dissolved solids in ocean water, expressed in parts per thousand (ppt). The average open-ocean salinity is 35 ppt. Think of it as: for every 1,000 grams of seawater, about 35 grams are dissolved salts.
Parts per thousand (ppt)
The unit used to express salinity. One ppt means one gram of dissolved substance per kilogram of water.
Hypersaline
A body of water with salinity greater than 40 ppt. Found in areas with high evaporation and limited freshwater inflow. In simple terms, it is water that is saltier than normal seawater.
Photic zone (surface layer)
The upper layer of the ocean (0 to 200 metres deep) where light penetrates and photosynthesis is possible. In simple terms, it is the sunlit layer where most marine plant life grows.
Fetch
The movement of water caused by wind blowing over the ocean surface, creating waves, currents, and mixing.
Halocline
A layer of water below the mixed surface layer where salinity changes rapidly with increasing depth. Think of it as a boundary zone where the salt concentration shifts.
Thermocline
A layer between two bodies of water with different temperatures, where temperature changes rapidly with depth. In simple terms, it is the boundary where warm surface water meets cold deep water.
pH (potential of hydrogen)
A logarithmic scale (0 to 14) measuring the concentration of hydrogen ions in a solution. Below 7 is acidic, 7 is neutral, above 7 is alkaline.
Alkaline (basic)
A solution with a pH above 7, meaning a low concentration of hydrogen ions. Normal ocean water is slightly alkaline (pH 8.1 to 8.2).
Carbonic acid
A weak acid (H₂CO₃) formed when carbon dioxide dissolves in water. This is the primary driver of ocean acidification.
Oxygen minimum layer
The zone between roughly 100 and 1,000 metres deep where dissolved oxygen concentration is at its lowest. Organisms here have adapted to survive on very little oxygen.
Water is one of the most effective solvents on Earth, owing to its polarity.
The partially positive hydrogen atoms in water surround negatively charged ions (e.g. Cl⁻)
The partially negative oxygen atoms surround positively charged ions (e.g. Na⁺)
This process pulls ionic compounds apart, dissolving them into the water
Temperature is the main factor affecting solubility: as water temperature rises, the dissolution rate of salts increases
Salinity measures the total dissolved solids in ocean water, expressed in parts per thousand (ppt). Open-ocean average is 35 ppt.
Factors that decrease salinity (all involve adding freshwater):
Melting glaciers
Precipitation (rain)
Freshwater runoff from rivers
Factors that increase salinity:
Salt-rich runoff: water flowing over land picks up minerals, nutrients, and other dissolved substances before entering the ocean
Evaporation: removes freshwater from the surface layer, concentrating the remaining salts
Freezing: removes water as ice, also concentrating dissolved salts (both evaporation and freezing have the same net effect on salinity)
Water bodies with salinity above 40 ppt are classified as hypersaline. These occur where evaporation rates are high and freshwater inflow is limited.
Surface layer (photic zone): 0 to 200 metres deep. Light penetrates, photosynthesis occurs. Wind-driven mixing (fetch) keeps this layer relatively uniform in temperature, salinity, and dissolved gases.
Density-driven mixing: When surface water cools, its density increases and it sinks, carrying nutrients and dissolved gases downward. As temperatures equalise, denser water from below rises, and gases and nutrients become evenly distributed.
Halocline: The layer below the mixed surface layer where salinity changes rapidly with depth.
Thermocline: The boundary layer where temperature drops sharply with depth. In the tropics, the gradient may run from 25°C at the surface to 1°C in the deep. In polar seas, the change is smaller (roughly 10°C to 1°C).
pH measures hydrogen-ion concentration on a logarithmic scale from 0 to 14
Below 7: acidic. At 7: neutral. Above 7: alkaline.
The ocean has historically been slightly alkaline, averaging pH 8.2
Rising atmospheric CO₂ has driven ocean pH down to approximately 8.1
Because the scale is logarithmic, a 0.1 decrease in pH represents roughly a 25% increase in acidity. Small numerical shifts mean large chemical changes for marine organisms.
Measuring pH:
Litmus indicator: paper changes colour to show acidic or alkaline
Universal indicator: similar to litmus but can indicate the strength of acidity or alkalinity
pH probes: measure hydrogen-ion concentration precisely and objectively
Gases in the atmosphere exist in equilibrium with gases dissolved in the ocean. When atmospheric gas concentrations rise, dissolved concentrations in the ocean also rise. Turbulence and wave action at the surface help maintain this equilibrium.
Four factors control gas concentration in seawater:
Gas solubility (intrinsic): CO₂ is highly soluble in water and forms carbonic acid (a weak acid) when dissolved. Oxygen is much less soluble. This is why there is more dissolved CO₂ than O₂ in the ocean.
Water temperature: Cold water dissolves more gas than warm water. As temperature rises, molecules move faster and dissolved gas escapes from the surface more readily. Polar waters hold more dissolved oxygen than tropical waters.
Pressure (atmospheric and water depth): Higher pressure increases gas solubility. As atmospheric pressure rises, more gas dissolves. Deeper water, with greater pressure from the water column above, holds dissolved gases more effectively. Conversely, low atmospheric pressure (e.g. during a tropical cyclone) allows dissolved gases to escape.
Salinity of the seawater: Gases dissolve more readily in fresh or estuarine water than in full-salinity seawater. The more dissolved salts already present, the less room there is for additional solutes. Salt reduces the solubility of most gases.
Surface layer (top 100 m): Highest dissolved oxygen concentrations, driven by photosynthesis (phytoplankton) and turbulence. Can reach supersaturation. Respiration by organisms depletes oxygen.
Oxygen minimum layer (100 to 1,000 m): The zone with the lowest dissolved oxygen. Organisms here have adapted by being relatively inactive, developing highly efficient gills, or possessing oxygen-efficient haemoglobin.
Below the oxygen minimum layer: Oxygen levels begin to rise again for three reasons:
Less food means fewer organisms respiring
Colder temperatures increase oxygen solubility
Greater pressure increases oxygen solubility
Salinity unit: Parts per thousand (ppt). Open-ocean average = 35 ppt. Hypersaline threshold = 40 ppt.
pH scale: 0 (strongly acidic) to 14 (strongly alkaline). Pure water = 7 (neutral). Ocean average = 8.1 (slightly alkaline).
Logarithmic relationship: Each whole-number step on the pH scale represents a tenfold change in hydrogen-ion concentration. A 0.1 pH drop ≈ 25% increase in acidity.
CO₂ dissolution: CO₂ + H₂O → H₂CO₃ (carbonic acid). This weak acid is the primary mechanism of ocean acidification.
Gas solubility relationships: Solubility increases with decreasing temperature, increasing pressure, and decreasing salinity.
Ocean acidification is one of the most pressing consequences of rising atmospheric CO₂. As more CO₂ dissolves into seawater and forms carbonic acid, the drop in pH threatens shell-building organisms like corals, molluscs, and certain plankton species. A shift from pH 8.2 to 8.1 sounds small, but that 25% increase in acidity is enough to weaken calcium carbonate structures.
The relationship between temperature and gas solubility explains why polar waters are richer in dissolved oxygen and support dense concentrations of marine life, while warm tropical surface waters hold comparatively less dissolved oxygen despite being more visibly productive.
Students often think a small pH change is insignificant. It is not. Because the pH scale is logarithmic, a 0.1 drop represents a roughly 25% increase in hydrogen-ion concentration. Small numbers, large chemical consequences.
Students sometimes believe evaporation adds salt to the ocean. It does not. Evaporation removes freshwater, which concentrates the salt already present. The total amount of salt stays the same.
A common error is assuming warm water holds more dissolved gas because warmer environments feel "more active." The opposite is true: cold water holds more dissolved gas because slower-moving molecules are less likely to escape the surface.
Students confuse the halocline and the thermocline. The halocline is defined by a rapid change in salinity with depth. The thermocline is defined by a rapid change in temperature with depth. They may overlap, but they measure different things.
⚠️ Know the four factors that affect gas solubility in seawater: gas type, temperature, pressure, and salinity. Be able to explain the direction of each relationship.
⚠️ Be prepared to explain why the ocean is becoming more acidic (rising CO₂ dissolves to form carbonic acid) and why a 0.1 pH drop matters (logarithmic scale, 25% acidity increase).
⚠️ Understand the three zones of dissolved oxygen: high at the surface, lowest in the oxygen minimum layer (100 to 1,000 m), rising again below that.
⚠️ Evaporation and freezing both increase salinity by the same mechanism (removing freshwater). This is a commonly tested equivalence.
⚠️ Exam terminology: use "organisms" rather than "animals" and "respiration" (cellular respiration) rather than "breathing."
True or false: Warm water can dissolve more gas than cold water.
Fill in the blank: The average salinity of the open ocean is ______ ppt.
True or false: A pH of 6 is alkaline.
Fill in the blank: CO₂ dissolved in water forms ______ acid.
True or false: Both evaporation and freezing increase salinity.
Answers: 1. False 2. 35 3. False (it is acidic) 4. Carbonic 5. True
Q: Name three factors that decrease ocean salinity.
A: Melting glaciers, precipitation, and freshwater runoff. All three add freshwater, diluting the dissolved salts.
Q: Explain why a 0.1 decrease in ocean pH is significant.
A: The pH scale is logarithmic. A 0.1 drop (from 8.2 to 8.1) represents approximately a 25% increase in hydrogen-ion concentration, which is enough to affect the ability of marine organisms to build and maintain calcium carbonate shells and skeletons.
Q: Why does cold water hold more dissolved oxygen than warm water?
A: In warmer water, molecules move faster and dissolved gas molecules escape from the surface more readily. In colder water, molecules move more slowly, so gases remain dissolved.
Q: What is the oxygen minimum layer, and how do organisms survive there?
A: The oxygen minimum layer is the zone between approximately 100 and 1,000 metres deep where dissolved oxygen is at its lowest concentration. Organisms adapt by being relatively inactive to conserve oxygen, developing specialised gills that extract O₂ efficiently, or having oxygen-efficient forms of haemoglobin.
Q: How does increased atmospheric pressure affect gas solubility in the ocean?
A: Higher atmospheric pressure increases the rate of gas dissolution into seawater. Conversely, when atmospheric pressure drops (e.g. during a tropical cyclone), dissolved gas molecules escape from the water more readily.
This section builds directly on Chapter 1's coverage of water polarity and hydrogen bonding, which explain why water is such an effective solvent. It connects forward to Chapter 1.3 (Density and Pressure), where temperature, salinity, and pressure all influence water density and ocean layering.
The concept of ocean acidification links to broader ecology and climate-change topics later in the course. Understanding how dissolved oxygen is distributed through the water column is foundational for studying marine habitats, food webs, and organism adaptations at different depths.
Solubility, solvent, solute, dissolution, salinity, parts per thousand, ppt, hypersaline, photic zone, surface layer, fetch, halocline, thermocline, pH scale, alkaline, acidic, neutral, ocean acidification, carbonic acid, hydrogen ions, logarithmic scale, dissolved oxygen, oxygen minimum layer, gas solubility, atmospheric pressure, water temperature, polar water, tropical water, CO₂, marine chemistry, BIO 101, Chapter 1.2