Glossary - Freshwater Ecology: Concepts and Environmental Applications - page 496

Oxidation-reduction potential, see redox
Oxidized microzone, the small region in a sed-
iment between oxic water above and anoxic
water below
32 P, a radioisotope of phosphorus used as a
tracer to study phosphorus dynamics
Paleolimnology, the study of ancient lakes
Palynology, the study of pollen grains and
spores
PAR, see photosynthetically available radiation
Paradox of the plankton, the paradox that mul-
tiple competing phytoplankton species are
able to coexist without competitive exclu-
sion removing less competitive species
Parasitism, predation where the predator is
substantially smaller than the prey
Parthenogenesis, reproduction by development
of an unfertilized gamete
Particulate, large enough to be retained on a
filter (often 0.45

Pholeterous, living in crayfish burrows
Phosphatase, enzyme that cleaves organic phos-
phorus compounds to liberate phosphate
Phosphate, PO 4 3 , dominant ionic form of in-
organic phosphorus in natural waters
Photoautotrophs, photosynthetic organisms
Photoinhibition, the deleterious effects of

high light
Photophobic, repelled by light
Photosynthesis-irradiance (P-I), describes the
effect of light on photosynthetic rate
Photosynthetically available radiation (PAR),
light from 400 to 700 nm that is generally
available to cyanobacteria and eukaryotic
photosynthetic organisms; often expressed
in mole photons m 2 s 1
Photosynthetic photon flux density (PPFD),
photosynthetically available radiation
Phototactic, attracted toward light
Phreatic, from the zone below the water table
Phycobilins, protein pigments that collect light
for photosynthesis in the cyanobacteria
and red algae absorbing in the 550- to
650-nm range
Phytoperiphyton, the photosynthetic organisms
in periphyton, usually algae
Phytoplankton, suspended algae
Picophytoplankton, algal picoplankton
Picoplankton, plankton particles smaller than
3

m)
Particulate organic carbon (POC), organic car-
bon particles larger than 0.45 m

Particulate organic nitrogen (PON), organic N
retained on a filter
Paternoster, lakes formed by glacial scour in
mountain valleys
P:B, ratio of production to biomass
Peatland, any wetland that accumulates or-
ganic matter
Pelagic, in open water
Perched water table, a small pocket of ground-
water held above the main water table by
an impermeable layer
Percolate, when a solvent passes through a per-
meable substance
Periphyton, the mixed assemblage of organ-
isms attached to solid substrates in lighted
benthic habitats, including algae, bacte-
ria, protozoa, and invertebrates; a biofilm
containing algae; also called aufwuchs and
microphytobenthos
Permeability, ability of a substance to trans-
mit water
PH, activity of hydrogen ions; expressed as

m in diameter
Pieziometer, an instrument for measuring pres-
sure or compressibility; used in determining
groundwater flow patterns
Plankton, suspended organisms
Playa, a shallow marsh-like pond, not formed
glacially
Pleuston, macroorganisms living on the surface
of the water
Poikilothermy, having variable body tempera-
ture; also, complicated vertical temperature
profile
Point bar, a depositional area on the inside of
a river bend
Point source, a clearly defined source of pollu-
tion such as a sewage outfall (as opposed to
non-point source )

log 10 (moles H liter 1 )
Phobic, a response away from, such as a pho-
tophobic response in which light is avoided