---
title: "Salinity — AP Environmental Science Definition & Exam Guide"
description: "Salinity is the concentration of dissolved salts in water, a key factor shaping aquatic biomes and ecological tolerance on the AP Enviro exam."
canonical: "https://fiveable.me/ap-enviro/key-terms/salinity"
type: "key-term"
subject: "AP Environmental Science"
unit: "Unit 1"
---

# Salinity — AP Environmental Science Definition & Exam Guide

## Definition

In AP Environmental Science, salinity is the concentration of dissolved salts in water, measured in parts per thousand (ppt). It helps determine where aquatic organisms live and is one of several factors defining aquatic biomes and ecological tolerance ranges.

## What It Is

Salinity is just how salty [water](/ap-enviro/unit-6/hydrogen-fuel-cell/study-guide/VBHYpOxkIwXQuPkI6px8 "fv-autolink") is, meaning the concentration of dissolved salts. It's usually measured in parts per thousand (ppt). Ocean water sits around 35 ppt, freshwater is close to 0, and places where rivers meet the sea ([estuaries](/ap-enviro/key-terms/estuaries "fv-autolink")) fall somewhere in between and change with the tides.

In the CED, salinity shows up in two spots. In **1.3 Aquatic Biomes**, it's one of the factors (along with depth, turbidity, [nutrients](/ap-enviro/unit-8/eutrophication/study-guide/pht3gvVqyWzrKeAwXm4F "fv-autolink"), and temperature) that determines which marine biome forms and where natural resources like fish are found. In **2.4 Ecological Tolerance**, salinity is listed as one of the conditions an organism can handle only within a certain range before injury or death. Push a freshwater fish into salt water, and you've shoved it past its salinity tolerance.

## Why It Matters

Salinity lives in [Unit 1](/ap-enviro/unit-1 "fv-autolink") (The Living World: Ecosystems) and Unit 2 (The Living World: Biodiversity). It supports **[AP Enviro](/ap-enviro "fv-autolink") 1.3.A** (EK ERT-1.C.4), which says the distribution of marine resources like fish varies because of a combination of salinity, depth, turbidity, nutrient availability, and temperature. It also supports **AP Enviro 2.4.A** (EK ERT-2.F.1), where salinity is one example of a condition that defines an organism's ecological tolerance range. The big-picture theme: living things are limited by their physical environment, and salinity is one of the dials that decides who survives where.

## Connections

### Ecological Tolerance (Unit 2)

Salinity is one of the textbook examples of an [ecological tolerance](/ap-enviro/unit-2/ecological-tolerance/study-guide/dLeq5qqhYeCboAOsuBiz "fv-autolink") factor. An organism survives only within a salinity range, so when conditions drift outside that range, you see injury, death, or a shift in where the species can live.

### Estuaries and Anadromous Species (Units 1-2)

Estuaries mix fresh and salt water, so salinity swings constantly. [Anadromous species](/ap-enviro/key-terms/anadromous-species "fv-autolink") like salmon are built to tolerate that change, migrating from salt water to freshwater to spawn, which is basically ecological tolerance in action.

### Turbidity, Depth, and Temperature (Unit 1)

Salinity rarely acts alone. EK ERT-1.C.4 bundles it with [turbidity](/ap-enviro/key-terms/turbidity "fv-autolink"), depth, nutrients, and temperature as the combined factors that decide where fish and other marine resources show up, so exam answers should treat them as a team, not a single cause.

### [Coral Bleaching (Unit 1)](/ap-enviro/key-terms/coral-bleaching)

Corals have a narrow tolerance for several factors. While bleaching is usually driven by temperature, salinity is the same kind of limiting factor, and both illustrate what happens when conditions exceed an organism's tolerance range.

## On the AP Exam

On the multiple-choice section, salinity shows up in questions about why a species lives where it does. Expect stems like "the global distribution of tuna is limited by which combination of factors?" where the right answer pairs salinity with temperature, depth, turbidity, and nutrients rather than picking just one. Another classic angle is an invasive fish thriving in a river with salinity swinging from 0.5 to 12 ppt, where the trick is recognizing that a wide salinity tolerance is what lets it spread. On FRQs, salinity tends to appear inside aquatic ecosystem prompts, like the 2024 stream-zone question, where you reason about what conditions different zones support. Your job is to connect a salinity value or change to which organisms can survive, not just define the word.

## salinity vs ecological tolerance

Salinity is a specific environmental factor (how salty the water is). Ecological tolerance is the broader concept, meaning the range of any condition (salinity, temperature, flow rate, sunlight) an organism can endure. So salinity is one example that fits inside ecological tolerance, not the same thing.

## Key Takeaways

- Salinity is the concentration of dissolved salts in water, measured in parts per thousand, with ocean water around 35 ppt and freshwater near 0.
- Salinity is one of the factors in EK ERT-1.C.4 (along with depth, turbidity, nutrients, and temperature) that determines where marine resources like fish are found.
- Salinity is a classic ecological tolerance factor, meaning organisms survive only within a certain salinity range before injury or death.
- Estuaries have constantly changing salinity, and species that handle that range, like anadromous salmon, have a wide salinity tolerance.
- On the exam, distribution questions usually want a combination of factors, not salinity alone, as the answer.

## FAQs

### What is salinity in AP Environmental Science?

Salinity is the concentration of dissolved salts in water, measured in parts per thousand (ppt). It helps define aquatic biomes (Topic 1.3) and is one example of an ecological tolerance factor that limits where organisms can live (Topic 2.4).

### Is salinity the same as ecological tolerance?

No. Salinity is a specific factor (saltiness of water), while ecological tolerance is the overall range of conditions an organism can endure. Salinity is just one of those conditions, alongside temperature, flow rate, and sunlight.

### Why does salinity matter for where fish live?

Each species can only survive within a certain salinity range. EK ERT-1.C.4 says the distribution of marine resources like fish depends on salinity combined with depth, turbidity, nutrients, and temperature, so salinity helps decide which fish thrive where.

### How does salinity affect estuaries and migrating fish?

Estuaries mix fresh and salt water, so salinity changes with the tides. Anadromous species like salmon tolerate that range because they move between salt water and freshwater to spawn, which shows a wide salinity tolerance.

### Does salinity cause coral bleaching?

Usually no, bleaching is mostly driven by water temperature exceeding the coral's tolerance. But salinity works the same way as a limiting factor, so both are examples of organisms pushed past their ecological tolerance range.

## Related Study Guides

- [1.3 Aquatic Biomes](/ap-enviro/unit-1/aquatic-biomes/study-guide/Ka0nsiIoWMSAbSKgVUqC)

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