<descriptor thcode="2"><thesaurus thcode="2" name="USGS Thesaurus" scope="Topics and methods of scientific study carried out by USGS, with product types, scientific disciplines, geologic time, and types of institutional structure and activities.  Broad and shallow, used to help people find scientific information." creator="U.S. Geological Survey" rights="Public domain" edition="2.0" date="2020-01-09" codetype="number" prefix="topic" uri="https://www2.usgs.gov/science/USGSThesaurus" tblname="term" nonpref="nonpref" relterm="relterm" root_code="1" contact="pschweitzer"><category><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/></category><altlabel><altlabel>USGSThesaurus</altlabel><altlabel>USGSThesaurus</altlabel></altlabel></thesaurus><term thcode="2" code="890" name="phycology" parent="127" scope="Branch of biology concerned with the study of algae."/><uf><usefor name="algology"/></uf><bt><term thcode="2" code="127" name="botany" parent="650" scope="The science of plants."/><term thcode="2" code="650" name="life sciences" parent="1019" scope="Branches of science that study living and fossil organisms."/><term thcode="2" code="1019" name="sciences" parent="1" scope="Major educational fields, fields of study, and professional groupings within USGS."/><term thcode="2" code="1" name="Categories" scope="Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."/></bt><rt><term thcode="2" code="29" name="algae" parent="843" scope="Chlorophyll-bearing primarily aquatic nonvascular species that have no true roots, stems, or leaves; most algae are microscopic, but some species can be as large as vascular plants."/><term thcode="2" code="30" name="algal blooms" parent="310" scope="Exceptional growth of algae and cyanobacteria in lakes, rivers or oceans due to excessive nutrients or climatic conditions."/><term thcode="2" code="138" name="calcareous nannoplankton" parent="29" scope="Organisms of the kingdom Protista that normally produce coccoliths, microscopic structures of calcite (calcium carbonate), during some phase of their life cycle."/><term thcode="2" code="250" name="diatoms" parent="29" scope="Microscopic, single-celled plants of the class Bacillariophyceae that have siliceous shells called frustules, and which grow in both marine and fresh water."/><term thcode="2" code="261" name="dinoflagellates" parent="29" scope="Single-celled planktonic organisms, chiefly marine, characterized by twirling motion and whip-like flagella with affinities to both plants and animals."/><term thcode="2" code="647" name="lichens" parent="843" scope="Plants that are composed of a fungus and an alga growing together symbiotically and often found growing on rocks or tree trunks."/><term thcode="2" code="903" name="plankton" parent="841" scope="Floating aquatic plants (phytoplankton) and animals (zooplankton) which are often microscopic and drift with the current in lakes, rivers, and oceans."/></rt></descriptor>