Phytoplankton are microscopic, photosynthetic organisms in aquatic ecosystems that serve as primary producers, generating oxygen and forming the base of the food web. On the AP Enviro exam, their explosive growth from excess nutrients drives algal blooms and eutrophication.
Phytoplankton are tiny floating organisms that photosynthesize, basically the ocean's and lake's version of plants. They're too small to see individually, but together they capture sunlight, pull in carbon dioxide, and pump out oxygen. As primary producers, they sit at the bottom of aquatic food webs, which means almost everything else in the water depends on them, directly or indirectly.
For AP Enviro, what matters most is how phytoplankton respond to nutrients. They need nitrogen and phosphorus to grow, the same nutrients found in fertilizers and old phosphate detergents. When a water body gets flooded with those nutrients (a process called eutrophication, STB-3.F.1), phytoplankton and algae multiply fast into an algal bloom. That bloom eventually dies, microbes decompose it, and that decomposition burns through the dissolved oxygen in the water (STB-3.F.2). The result can be a hypoxic dead zone where fish and other organisms suffocate.
Phytoplankton live in Topic 8.5 Eutrophication inside Unit 8: Aquatic and Terrestrial Pollution. They're the engine behind learning objective AP Enviro 8.5.A, which asks you to explain the environmental effects of excess fertilizers and detergents on aquatic ecosystems. The whole chain of essential knowledge (STB-3.F.1 through STB-3.F.4) only makes sense once you understand phytoplankton: nutrients feed them, their blooms crash, decomposers eat up the oxygen, and you end up with a hypoxic or dead-zone waterway. Connect that to bigger AP themes like energy transfer through food webs and the human impact of agricultural runoff, and you've got a term that ties pollution back to ecosystem function.
Eutrophication (Unit 8)
Eutrophication is what happens when phytoplankton get too much of a good thing. Extra nitrogen and phosphorus let them bloom out of control, and the die-off afterward strips oxygen from the water. Phytoplankton are the actor; eutrophication is the storyline.
Algal Bloom (Unit 8)
An algal bloom is just a phytoplankton population explosion you can see. The bloom looks like growth, but the real damage comes later when it dies and decomposers consume the dissolved oxygen, creating hypoxic water.
Primary Production (Units 1 & 8)
Phytoplankton are the ocean's primary producers, converting sunlight into energy that feeds everything above them. That's why a 2018 SAQ on an Arctic food web matters here, knock out the producers and the whole energy pyramid collapses.
Agricultural Runoff & Fertilizers (Unit 8)
Runoff carries nitrogen and phosphorus from farm fields into rivers, lakes, and estuaries. That's the nutrient delivery system that feeds phytoplankton blooms, linking land-use practices directly to aquatic dead zones.
Expect phytoplankton in both MCQs and FRQs about water pollution. MCQ stems often describe a lab setup, like tanks of lake water and phytoplankton dosed with phosphate detergent or nitrogen fertilizer, and ask you to predict growth or interpret a graph. A pristine lake with stable phytoplankton and high dissolved oxygen points you toward the term oligotrophic, while a nutrient-flooded one is eutrophic. The 2018 SAQ Q3 used an Arctic food web, so be ready to identify phytoplankton as the primary producer and trace energy flow upward. On FRQs about eutrophication, walk the full chain: excess nutrients, phytoplankton bloom, die-off, microbial decomposition, oxygen drop, fish die-off.
Phytoplankton are the organisms; an algal bloom is the event when those organisms multiply rapidly. Phytoplankton are present in healthy water at stable levels, but a bloom is the population spike caused by a nutrient surge. Saying "phytoplankton caused the dead zone" is sloppy; the bloom, then its decomposition and oxygen loss, caused it.
Phytoplankton are microscopic, photosynthetic primary producers that form the base of aquatic food webs and release oxygen.
They need nitrogen and phosphorus to grow, which is why fertilizers and phosphate detergents trigger their overgrowth.
A phytoplankton bloom isn't the problem itself; the oxygen crash after the bloom dies and decomposes is what kills fish.
Eutrophic waterways are nutrient-rich with big blooms and low oxygen, while oligotrophic waterways are nutrient-poor with stable phytoplankton and high oxygen.
On the AP exam, trace the full eutrophication chain: nutrients, bloom, die-off, decomposition, hypoxia, fish kill.
They're microscopic, photosynthetic organisms in water that act as primary producers, making oxygen and feeding the aquatic food web. On the exam they matter most because excess nutrients cause them to bloom, leading to eutrophication and oxygen-starved dead zones.
Both, depending on amount. At normal levels they're essential, producing oxygen and supporting nearly all aquatic life. But when fertilizer or detergent runoff over-feeds them, they bloom and crash, and decomposers use up the dissolved oxygen, which is the bad outcome the AP exam tests.
No, not directly. Living phytoplankton actually produce oxygen. The oxygen drop comes after the bloom dies, when microbes decompose all that dead material and consume the dissolved oxygen (STB-3.F.2), leaving hypoxic water.
Phytoplankton are the organisms themselves; an algal bloom is the rapid population explosion of those organisms after a nutrient surge. A healthy lake has stable phytoplankton, while a nutrient-polluted one experiences blooms.
Because they're the primary producers that capture sunlight and supply energy to everything above them, like in the 2018 Arctic food web SAQ. If you remove the phytoplankton, the energy supply for the whole web disappears.