parameter	A quantity which serves to define a certain item, the item itself, a numerical characteristic of a population of items. For physical and chemical properties of water, the term "water quality characteristics" should be used.
parasite	An animal or plant that lives in or on another (the host) from which it obtains nourishment.
pathogen	Any organism which in living on or within another organism (the host) causes disease in the host.
Pb (lead)	A heavy bluish-white metallic element that is highly malleable: used in alloys, accumulators, cable sheaths, paints etc. Symbol: Pb. Atomic number: 82.
pelagic	Relating to, living, or occurring in open water areas of lakes or oceans.
perturbation	A cause of disturbance.
pesticide	Artificial or natural substance used to kill pests.
pharynx	That part of the digestive tract situated immediately posterior to the mouth and anterior to the oesophagus: sometimes bears specialized structures (teeth, spikes, plates) for processing food. In fish, the gill arches surround the pharynx.
phosphate	1. Any salt or ester of any phosphoric acid 2. Any of several chemical fertilisers containing phosphorous compounds.
photoperiod	The duration of time in a given day during which organisms are exposed to light and dark. The light source can be natural or artificial.
photosynthesis	Synthesis by plant cells of organic compounds (mainly carbohydrates) in the presence of light, from carbon dioxide and water, with simultaneous production of water.
phytoplankton	Minute plants suspended in water with little or no capability of controlling their position in the water mass; frequently referred to as microalgae (the plant component of plankton).
plankton	Organisms living suspended in the water column and incapable of moving against water currents.
plastic	Any one of a large number of synthetic materials that have a polymeric structure and can be moulded when soft and then set. Plastics are used in the manufacture of many articles and in coatings and artificial fibres.
poison	Substance that when introduced into or absorbed by a living organism destroys life or injures health, esp. one that destroys life by rapid action and when taken in small quantity.
Polar region	Having to do with the regions of the north and or south poles of Earth. The poles are areas of high latitudes and typically are cold, icy regions. A polar climate is a cold climate, with temperatures less than 10 deg C (50 deg. F).
policy	A plan of action adopted by or pursued by an individual, government, or business etc.
pollutant	A pollutant is defined as any man-made or natural product unnaturally present in an environment, and usually with harmful effects.
pollution	Any anthropogenic or natural perturbation which is unnaturally present in the environment, and which may be harmful.
polychaete	A marine annelid worm with paired, flattened, bristle-tipped organs of locomotion. Some species are cultured as bait.
potassium	A light, silvery element of the alkali metal group that is highly reactive and rapidly oxidises in air. Symbol: K, atomic number: 19.
predation	Act of animal capturing or consuming another (prey) of a different and usually smaller species.
predator	An organism which hunts and eats other organisms. This includes both carnivores, which eat animals, and herbivores, which eat plants.
prehistoric	Of or relating to man's development before the appearance of the written word.
preliminary treatment (sewage)	Initial screening of urban sewerage for removal of large solids, maceration to produce a slurry, and extraction of grit.
pressure	The measurement of the amount of force per unit area on an object.
prey	An organism hunted and eaten by a predator.
primary production	The production of new organic matter by autotrophs
photosynthetic algae	This is very often the first step in a food chain. The total amount of organic matter produced per unit time by an individual, a population, an area or a water volume, is termed the gross primary production. When simultaneous losses of organic matter by catabolism (e.g. respiration) are subtracted from the gross primary production, a measure of net primary production is obtained. For photosynthetic algae, gross and net primary production are estimated by the light and dark bottle method.
primary treatment (sewage)	Waste from preliminary sewage treatment placed into settlement tanks. The supernatant liquid, which still has a high BOD, is discharged to the receiving waters. Sludge which has been settled out, is disposed of elsewhere.
producer	Any organism which brings energy into an ecosystem from inorganic sources. Most plants and many protists are producers.
productivity	Rate of production of biomass; expressed as production during a specific time interval. Gross: Rate of change in biomass including respiration and all losses during a specific time interval. et: Rate of accumulation or production of new organic matter or stored energy, less losses, during a specific time interval
protocol	Treaties sometimes provide for protocols and annexes. Protocols are subsequent agreements which amend, modify or supplement the main treaty
psychrophilic	Organisms preferring low temperatures
purify	1. To free (something) of contaminating or debasing matter. 2. To make clean (purification)