Unit 03 · Marine Plants, Algae & Kelp Forests
Not everything green in the sea is a plant. This unit draws the line: seaweeds are algae — protists, not true plants — while seagrasses are flowering plants that returned to the ocean. You learn how algae are built (a holdfast, stipe, and blade, not roots, stems, and leaves), how the green, brown, and red groups differ, and why kelp forests and seagrass meadows rank among the ocean’s richest habitats and most productive carbon sinks. Mastery means you can key out what you pulled from the tide line and say what it is and why it matters.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Algae vs true marine plants | Calls everything attached to rock a “plant.” | Knows seaweed and seagrass differ but cannot say how. | Explains that seaweeds are algae (protists) while seagrasses are true flowering plants, and uses the right group for each specimen. |
| Algal structure & groups | Labels seaweed with “roots, stems, and leaves.” | Uses holdfast, stipe, and blade but cannot tell the algal groups apart. | Names the holdfast, stipe, and blade correctly and sorts specimens into green, brown, and red algae by their features. |
| Kelp forests & seagrass meadows | Thinks seaweed is just clutter in the water. | Knows kelp is large but not that it builds a habitat. | Explains kelp forests and seagrass meadows as three-dimensional habitats — shelter, nursery, and food for whole communities. |
| Photosynthesis, carbon & productivity | Does not connect algae to oxygen or carbon. | Knows algae photosynthesize but not at what scale. | Relates algal and seagrass photosynthesis to oxygen production, carbon storage, and some of the highest productivity in the sea. |
| Lab technique (algae & seaweed survey) | Collects specimens without pressing, keying, or labeling them. | Presses seaweed but keys it out inconsistently. | Runs a clean seaweed survey — presses, keys out, and classifies specimens into the correct algal group, recorded with care. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“This one’s a brown alga — you can see the holdfast, stipe, and blade, and it grips the rock instead of rooting into soil. Seaweeds are algae, not true plants; the seagrass over there actually flowers. A kelp forest like this shelters fish and stores a lot of carbon while it photosynthesizes.”
“It’s a sea plant. Those are its leaves and roots. Seaweed and seagrass are the same thing, basically.”
You demonstrate this unit through a seaweed and algae survey where you press, key out, and classify your specimens, then defend each identification aloud — not a multiple-choice test. A criterion counts as mastered only when you can both key the specimen and explain the biology that puts it in its group. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.