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Home > Special Topics > The Dead Zone > Glossary

The Dead Zone: Glossary

Algae – a group of marine or freshwater aquatic plants, many are microscopic. Examples of algae are phytoplankton, green and brown algae or “pond scum” and marine seaweeds.

Algal bloom – an explosive increase in the population of phytoplankton. Algal blooms are often associated with excess nutrients (eutrophic) conditions and can be composed of noxious algae species.

Anoxia – the absence of dissolved oxygen; 0 mg/L DO.

Anthropogenic – conditions generated by human activities.

Bacteria – single-cell organisms that use oxygen to decompose wastes and dead organisms.

Benthos – organisms living on the bottom of aquatic and marine ecosystems.

Continental shelf – the “shallow” part of seas near the continents where the bottom gradually slopes from shore to a depth of about 200 m.

Cross section – a diagram or drawing that shows features of a vertical section of something such as the earth or a water column.

Dead zone – hypoxic area of the Gulf of Mexico where limited life can survive.

Decomposition – the breakdown of organic matter by bacteria and other organisms. Decomposition of large amounts of organic matter depletes dissolved oxygen concentrations in the water column.

Density – mass per unit volume. In the Gulf, determined by temperature and salinity.

Diatom – a major phytoplankton group characterized by cells enclosed in a thin silicon shell.

Dissolved oxygen – DO is the concentration of oxygen dissolved in water, measured in milligrams per liter (mg/L).

Estuarine – of or pertaining to estuaries.

Estuary – a coastal area where saltwater from the ocean mixes with freshwater from rivers, rainfall and upland runoff.

Eutrophication – an increase in the organic production of an ecosystem, usually associated with very high rates of nutrient supply.

Fertilizer – plant nutrients added to the soil as commercial fertilizers, animal manure and other chemicals.

Flux – movement of water or material from one place to another.

Halocline – the zone in the ocean where salinity increases rapidly as depth increases.

Hydrologic – pertaining to the properties, distribution and flow of water.

Hypoxia – very low concentrations of dissolved oxygen, generally < 2mg/L.

Impoundment – a reservoir where water is held behind a control structure.

Jubilee – the crowding of fish, shrimp and crabs onto a beach from deeper hypoxic waters pushed onshore because of wind shifts. A jubilee is sometimes followed by a fish kill if the oxygen level is severely low, prolonged or located so that no escape is possible for the organisms.

Life cycle – the behavioral and biological details of the cycle of a species as it goes from birth to maturity, reproduction and death.

Marine – relating to the sea.

Nitrate – water soluble inorganic Nitrogen of the form NO3 .

Nitrogen – chemical element and nutrient needed for plant growth.

Nonpoint pollution – a source of chemical and/or nutrient inputs that is not from a single discharge. Agricultural runoff, urban runoff and atmospheric deposits are examples of nonpoint pollution.

Noxious algae – a form of algae that grow under specific conditions and become a nuisance, including types that pose a toxic risk to humans or marine animals.

Nutrient over-enrichment – condition where too many nutrients collect.

Nutrients – chemical elements, containing Nitrogen (N), Phosphorus (P) and Silica (Si), that are required for the growth of phytoplankton and other plants and animals.

Phosphate – water-soluble inorganic compound in the form of PO4.

Phosphorus – chemical element and nutrient needed for plant growth.

Phytoplankton – free-floating microscopic plants such as algae that are the foundation of the marine food chain.

Planktivorous – animals that feed on plankton.

Plankton – microscopic plants and animals that drift in water.

Point source pollution – pollution that is from a single source such as an outfall.

Primary productivity – the conversion of light energy and carbon dioxide into living organic material by phytoplankton and other plants.

Profile – vertical section of the water column from the surface to bottom showing specific data such as measures of temperature, salinity, density, dissolved oxygen, nutrients and other characteristics.

Pycnocline – the region of the water column characterized by the strongest change in density with depth. Temperature falls and salinity increases in this zone.

Respiration – the consumption of oxygen during energy use by cells and organisms.

Riparian – of, on or pertaining to the bank of a river or pond

River plume – the area where river water mixes with sea water at or near the mouth of the river.

Runoff – rain water that flows over land and enters streams rather than soaking into the soil.

Salinity – concentration of salts dissolved in water, expressed in parts per thousand (ppt).

Sediment – weathered rock that is transported and deposited by air, water or ice and forms layers on the Earth’s surface.

Silica – a white or colorless chemical compound and nutrient used by diatoms to form outer shell.

Station – a specific location along a transect line, often identified using latitude and longitude coordinates, where samples are taken.

Stratification – the separation of water masses into layers. Stratification keeps layers of water from mixing together and hinders the transport of dissolved gases and chemicals from one layer to another.

Surface water – water occurring on the surface of land, such as streams, rivers, ponds, lakes and oceans.

Thermocline – the zone of the ocean in which temperature decreases rapidly with depth.

Transect – a sampling line that crosses a research area and delineates the area to be sampled. A transect contains two or more stations.

Water column – a term applied to the vertical section of water from the surface to the bottom.

Watershed – the land area that is drained by a river, estuary or tributary; a drainage basin.

Wetland – land that is saturated with water all or part of the year. Wetlands support plants and animals that are adapted to living in a wet and sometimes salty environment. Bogs, marshes and swamps are examples of wetlands.