2. biome

3. savanna

4. desert

5. tundra

6. plankton

7. estuary

8. Littoral zone

9. Open-water zone

10. Deep-water zone

11. wetland

12. deciduous

13. temperate

14. conifers

15. canopy

16. steppes

17. evergreen trees

18. prairies

19. permafrost

20. Marine

21. Intertidal zone

22. Coral reef

23. Benthic zone

24. Sargasso Sea

25. pollution

26. Hazardous waste

27. overpopulation

28. Renewable resource

29. Exotic species

30. Biodiversity *

31. Non-renewable resource

32. deforestation

33. Point source pollution

34. Recycle

35. conservation

36. biodegradable

37. Endangered species

38. Alternative energy sources