alkalinity: the concentration of negatively-charged ions that collectively raise the pH.

alternative stable states: Ponds are typically dominated either by phytoplankton or by rooted aquatic plants, and each of these two groups of primary producers, once established, tends to perpetuate their own existence while suppressing the other.

benthic: bottom-dwelling, typically associated with the sediments, rocks or plant surfaces.

blooms: masses of algae that usually accumulate at or near the surface.

blue-green algae: Algal species with very simple cell structure; also termed cyanobacteria, that often dominate hyper-eutrophic ponds.

carbon: The chemical “backbone” found in all organic molecules, and needed in large quantities during photosynthesis.

chlorophyll-a: photosynthetic pigment present in algae, and often used as a measure of primary producer biomass.

compensation depth: the depth below which respiration exceeds photosynthesis.

diatoms: algae with cell walls made of silica (glass), and often the most diverse and abundant members of the periphyton.

discharge: the flow rate into or out of a pond, often measured in cubic feet per second, or in liters per second.

emergent: plants with stems and leaves primarily above the water surface.

free-floating: Duckweeds and watermeal at the water surface, trapping light and often moving around in response to wind activity or water currents.

green algae: Members of the algal division Chlorophyta, appearing grass-green under the microscope. Filamentous green algae typically form the matrix of metaphyton mats.

hardness: The combined concentration of calcium and magnesium ions.

hydraulic retention time: The time (usually in days) that a drop of water remains in a pond before leaving via the outfall.

hyper-eutrophic: Highly productive trophic state, usually characterized by excessive nutrients and undesirable algal/plant growth

landscape position: The location of a pond in relation to streams. “Headwater” ponds are connected to first-order streams, “isolated” ponds have no stream connections, and “downstream impoundments” receive water from second- or higher-order streams (see stream order).

limiting nutrient: the critical nutrient (usually nitrogen or phosphorus) which is in least supply relative to need by primary producers, and whose concentration thus directly controls plant or algal growth.

macrophytes: primarily referring to flowering plants, but also including some ferns and large algae.

mean depth: an index of “shallowness”, determined as pond volume/area.

metaphyton: floating scums of filamentous algae, with associated smaller diatoms and invertebrates, that originate on the lake bottom and float to the surface under light- and nutrient-rich conditions.

nitrogen: Two forms of N (nitrate and ammonium) are important potentially limiting nutrient needed by aquatic organisms to produce proteins.

periphyton: algal biofilms associated with the pond bottom, plants or other benthic surfaces.

phosphorus: the nutrient thought to most frequently control the growth of primary producers, and needed for the construction of phospholipids, ATP, RNA and other organic molecules.

photosynthesis: The conversion of dissolved carbon dioxide and water, by primary producers in the presence of light energy, to form the organic molecule glucose.

phytoplankton: microscopic algae suspended in the water column.

primary producer: Photosynthetic plants or algae, capable of photosynthesis.

respiration: the process of breaking down stored chemical energy in organic molecules to simpler products in order to produce energy.

riparian vegetation: plants (e.g., trees, mowed grass) directly adjacent to a pond, often influencing pond water quality.

rooted, floating-leaved: aquatic plants extending from the pond sediments to the surface (e.g., water lilies).

Secchi depth: measure of water clarity or light penetration, requiring use of a Secchi disk.

specific conductance: the ability of a water sample to conduct an electric current, used to estimate the total ion content of pond water and often a good indicator of trophic state.

stratification: the formation of density layers within the water column, initiated by the conversion of light energy to heat near the surface, and often determining the vertical distribution of oxygen, nutrients and other materials.

stream order: classification system to approximate stream segment size and location. First-order streams have no tributaries, second-order streams have at least two first-order tributaries, etc.

submersed: rooted plants with leaves and stems primarily underwater.

trophic state: pond “productivity”, usually evaluated based on the extent of phytoplankton biomass (as chlorophyll-a), light penetration (as Secchi depth) and concentrations of total phosphorus.

zooplankton: a diverse assemblage of small invertebrates that spend their lives primarily in the water column, serving as important consumers of phytoplankton and bacteria, and in turn providing food for consumers such as fish.