Xenophobia: The fear of organisms belonging to a species other than one's own.

Year-Class Effect: The common domination of a species population by individuals recruited in one reproductive season.

Yeast and Molds: Yeast and molds are fungi. A fungus is a colorless (i.e., lacking chlorophyll) plant with practically no differentiation of cell structure. Yeast are small single-celled forms that reproduce by budding or spore formation. Molds produce spores for both asexual and sexual reproduction. Yeast and mold analysis is typically done on air-borne samples and surface wipes.

Yeast Method of Fertilization: A do-it-yourself method of supplying Co2 to fresh water aquariums. It involves yeast, sugar and two liter bottles.

Yellow Water: Name given to water with a slight yellow tinge. Can be caused by too much organic material in the water or to show that a water change is needed. Eliminated through carbon filtration and better water management.

Yolk Sac: An external pouch containing nutrients for the growing alevin. When the yolk sac is used up, the alevin is said to be "buttoned-up" and enters the fry stage.

Zeolite: A naturally occurring ore which will chelate (remove) ammonia and soften water. It is only effective in fresh water.

Zinc (Zn): The MCL is 5 mg/L, because of problems with the aesthetic quality due to the taste of zinc.

Zonation: Occurrence of single species or groups of species in recognizable bands that might delineate a range of water depth or a range of height in the intertidal zone.

Zooplankton: Microscopic aquatic organisms that drift in the ocean, including larvae, and are the first consumers in a marine food chain. See Plankton.

Zooxanthellae Algae: Tiny brown or yellow algae called dinoflagellates (single-celled microscopic organisms which belong to the Kingdom Protista) that live symbiotically with corals, tridacnid clams, and some sponges. The algae provides food for the host and in return gets the nitrogen, phosphorous, and carbon dioxide it needs to grow.