413Operational Taxonomic Units (OTU) Operational defini-
tion of a species or group of species often used when only
DNA sequence data is available.
Operational taxonomic unit (OTU) a group of environ-
mental sequences sharing a given similarity level required
for quantitative and comparative sequence analyses.
Organohalide/Organohalogen Organic compounds con-
taining a C-halogen bond.
Osmotrophy Uptake of dissolved organic compounds by
transmission across the cell membrane.
Outcropping part of a geologic formation appearing at the
surface of the Earth.
Outlet a stream flowing out of a lake, pond, or other body
of standing water.
Oxidation The loss of one or more electrons by an atom,
molecule, or ion.
Oxycline a sharp gradient on oxygen concentrations.
Oxygen Index (OI) amount of oxygen content in the OM
released during pyrolysis in Rock-Eval analyzer, normal-
ized to TOC (expressed in mg CO 2 .g−^1 TOC).
Oxygen chemocline (so-called oxycline) strong dissolved
oxygen gradient in the water column, typically represent-
ing a redox boundary.
PCR Tag-sequencing NGS (next generation sequencing)
technique. Before sequencing, a PCR step of a hyper-
variable sequence is realized. A small oligonucleotide
sequence (tag sequence) specific to sample is added to
the primers used for PCRs, similarly to the technique of
barcoding.
PEG-Model Plankton Ecology Group (PEG) Model is a
standard template to describe factors driving the seasonal
wax and wane of phyto- and zooplankton in lakes.
Paleolimnology the study of ancient lakes from their sedi-
ments and fossils aiming at reconstructing the paleoen-
vironments of inland waters. Such studies are based on
analyses of sediment cores including the physical, chemi-
cal and mineralogical properties of sediments and divers
biological records.
Palynostratigraphy Determination of pollen grains is
commonly used as a stratigraphical tool in fossil terrains
ant more specially in peat-bog and lake deposits to iden-
tify various pollen spectrums (palynozones) correspond-
ing with palaeoenvironmental changes. Regarding the
recent volcanic activity in the Massif Central, the fol-
lowing palynozones are concerned from −14.000 y.BP:
Lateglacial; Allerød/Young Dryas/-11.500/Holocene:
Préboréal/Boréal/Atlantic/Subboreal/Subatlantic/
Present. Tephra layers, when clearly identified in vari-
ous pollen diagrams are used a linkage tool of precise
time markers.
Parasitism (Parasites) Non-mutual symbiotic relationship
between species, where one species, the parasite, benefits
at the expense of the other, the host. Parasites show a high
degree of specialization, and reproduce at a faster rate
than their hosts.
Partition (or distribution coefficients): concentration ratio
of an element in the solid over the melt at equilibrium.
Passive tracer A passive tracer is a chemical molecule or a
microorganism that is transported in the water body and
does not react or is transformed during its transport in the
water body.
Pavin event well dated and contextualized extra- ordinary
event of Pavin, diverging from a normal non-degassing
lake behavior. They are reported in Vassivière archives,
(e.g. in 1547, 1551), as a fantastic story, (1632) in an offi-
cial publication (1783), by a witness account (1936) and
possibly in a Gregorius text (Caluppa’s dragon encoun-
ter, around 590 AD). Events archived in Pavin sediments
complement these (Chap. 23 ).
Pavin experiments they include throwing a stone into the
lake to check its assumed immediate thunder, lightning
and storm, and shooting a gun to generate a thunder noise.
They were performed at Pavin since de Frétat’s visit in
1672 until the mid-nineteenth century.
Pavin legends ancient legends, describing pre or early
Christian representations of Pavin its surrounding and
Pavin events. So far few of them are found: Fairies gar-
den, the Fairies dance.
Pavin stories stories told in the eighteenth and nineteenth
century to Pavin visitors by their local guides, as the
Thrown stone and the Whirl and storm. They describe tra-
ditional Pavin (mis) behavior and are generally accompa-
nied with Pavin experiments.
Permafrost soil permanently frozen (or for at least 2 years)
and therefore impermeable.
Phagotrophy The process by which unicellular organisms
derive their food from engulfing and digesting other cells.
Phreatomagmatic a volcanic explosion that extrudes both
magmatic gases and steam caused by the contact of
magma with groundwater or shallow surface water.
Phylogenetic clade Set of related sequences that originate
from a single common ancestor.
Phylotype Observed similarity that classifies a group of
organisms by their phenetic relationship. This phenetic
similarity may reflect the evolutionary relationships.
Phytoplankton bloom a rapid increase or accumulation in
the population of algae in an aquatic ecosystem.
Picoplankton Heterogeneous assemblage of organisms 1–5
μm in size that live suspended in the water column.
Pierre au Trésor (Treasure stone) an enigmatic 250 kg
carved stone retrieved from Pavin waters in 1909. Its
antique origin has been confirmed as a Pompeian meta
millstone.
Piston cores sedimentary cores retrieved from a basin fill by
using a piston inside a cylinder which reduces friction by
creating suction.Glossary