natural selection - The resulting survival and reproduction of organisms due to the varying effects of environmental forces on genetically different members of a population; results in the preservation of favorable adaptations.

nematodes - Extremely diverse aquatic macroinvertebrates; roundworms; long, thin aquatic worms that writhe like snakes.

nephelometric turbidity unit (NTU) - A unit of measure for turbidity (as measured by a nephelometer). Turbidity is measured by determining the amount of light that is reflected off particles in the water. .

niche - The unique role of an organism in an ecosystem.

nitrate - One form of nitrogen plants use as a nutrient. One ion of nitrate is composed of one nitrogen atom and three oxygen atoms.

nitrogen - An non-metallic element designated with the chemical symbol N. All organisms need nitrogen to build protein.

non-native species - A species that has been imported or brought into an area.

non-point source pollution - Pollution discharged over a wide land area, not from one specific location. Diffuse water pollution caused by sediment, nutrients, and organic or inorganic toxic substances carried to lakes and streams by surface runoff.

northern water snake - A non-venomous snake of the genus Natrix that frequents streams and ponds.

noxious weed - A plant that is undesirable because it is harmful to other plants.

nutrient - A substance acquired from the environment that is needed for the survival, growth, and development of an organism.

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observation - The use of the five senses to note a phenomenon.

odor - The smell or scent of something. Chemicals from waste discharges, microbial activity, or natural sources may cause a body of water to have an odor.

omnivore - An organism that eats both plants and animals.

organic matter - Plant and animal residues; substances made by living organisms; contains carbon.

organism - An individual living thing.

oriental bittersweet - Celastrus orbiculatus; an exotic invasive plant, originally from Eastern Asia, which grows at the wood's edge and in intermittently flooded lowland forest. image

orthophosphate - Chemistry-based term that refers to an organic phosphate where the phosphate is attached on the ortho position in a benzene ring.

osprey - Pandion haliaetus; a fish eating hawk with plumage that is dark on the back and light below.

oxidation - Process by which an atom becomes more positively charged. Reactions with oxygen are the most common (e.g., formation of rust on iron).

oxygen demand - The amount of molecular oxygen required for biological and chemical processes in water.

oxygen saturation - The maximum amount of oxygen that will dissolve in water at a given temperature. Oxygen saturation is determined by pairing the temperature of the water with the dissolved oxygen value, after first correcting the dissolved oxygen measurement for the effects of atmospheric pressure.

oyster - Bivalve mollusks of the genus Ostrea. Oysters are chiefly found in shallow marine waters and have irregularly shaped shells.

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