were located in close proximity to river mouths and cities, whereas samples located
more offshore showed lower concentrations, or pyrethroids were below detection
limits. This distribution supports urban pyrethroid emissions. In another area of
Southern California, sediments from the Monterey continental shelf were analyzed
together with suspended solids in the three rivers flowing into this marine region.
Whereas pyrethroids were found in almost all rivers particles (sampled after rain
events), with bifenthrin and permethrin as the dominant pyrethroids, they could not
be detected in the estuary nor in the deeper sediments of the Monterey canyons (from
100 to 300 m depth). A similar situation was observed in marine coastal waters off
Portugal, whereas no pyrethroid could be detected in sediments, while cypermethrin
was detected in the dissolved phase and tetramethrin, bifenthrin, cyhalothrin,
fenvalerate, and permethrin occurred at low concentrations in some samples of
oysters collected in the same area [33]. In marine sediments, contaminated river
particles are diluted by the autochthonous marine particles and by older riverine
particles in which pyrethroids have had the time to be degraded. As a consequence of
dilution, pyrethroids are often below detection limits in marine sediments (Table 2).
A recent review documented the occurrence of pyrethroids in sediments worldwide and showed significant correlations between pyrethroid occurrence and sediment toxicity [7]. The good correlations obtained proved that pyrethroids were the
main cause of toxicity and strongly suggested potential ecological risk to nontarget
aquatic species. Nevertheless, at some locations, such as in sediments from the Pearl
River Delta (China), other pollutants than pyrethroids likely contributed to the
overall toxicity of sediments. The authors concluded that the frequent occurrence
at high concentrations of pyrethroids in sediments from agricultural and residential
areas constitute a threat to freshwater ecosystems [7].
5 Pyrethroid Degradation
A characteristic feature of pyrethroid contamination in water and benthic ecosystems
is that a few compounds of the pyrethroid family may be present but not all the
series, in concentrations generally under the 100 ng/L range for water samples or
under the 100 ng/g range for sediments. Pyrethroid occurrence is highly variable in
time and space, so that samples from a given area may show detectable amounts of
one or several pyrethroids while others do not or comprise other active compounds.
This feature is much different from other ubiquitous pesticides classes and is a
consequence of their higher lability. The routes of degradation of pyrethroids may
be abiotic (hydrolysis, photolysis, and oxidation) or mediated by bacteria and fungi.
Pyrethroids degradation by microorganisms and fungi have been studied in soils
[63, 64]. Various carboxylesterases may induce the degradation of pyrethroids;
generally one gene exists in one pyrethroid-degrading microorganisms, with the
exception of Ochrobactrum anthropi, that possesses two pyrethroids degrading
genes [63]. Optimal conditions of pyrethroid biodegradation are between 30 and
35
C. Organic matter and clay content are also important parameters controlling
94
L. Méjanelle et al.
more offshore showed lower concentrations, or pyrethroids were below detection
limits. This distribution supports urban pyrethroid emissions. In another area of
Southern California, sediments from the Monterey continental shelf were analyzed
together with suspended solids in the three rivers flowing into this marine region.
Whereas pyrethroids were found in almost all rivers particles (sampled after rain
events), with bifenthrin and permethrin as the dominant pyrethroids, they could not
be detected in the estuary nor in the deeper sediments of the Monterey canyons (from
100 to 300 m depth). A similar situation was observed in marine coastal waters off
Portugal, whereas no pyrethroid could be detected in sediments, while cypermethrin
was detected in the dissolved phase and tetramethrin, bifenthrin, cyhalothrin,
fenvalerate, and permethrin occurred at low concentrations in some samples of
oysters collected in the same area [33]. In marine sediments, contaminated river
particles are diluted by the autochthonous marine particles and by older riverine
particles in which pyrethroids have had the time to be degraded. As a consequence of
dilution, pyrethroids are often below detection limits in marine sediments (Table 2).
A recent review documented the occurrence of pyrethroids in sediments worldwide and showed significant correlations between pyrethroid occurrence and sediment toxicity [7]. The good correlations obtained proved that pyrethroids were the
main cause of toxicity and strongly suggested potential ecological risk to nontarget
aquatic species. Nevertheless, at some locations, such as in sediments from the Pearl
River Delta (China), other pollutants than pyrethroids likely contributed to the
overall toxicity of sediments. The authors concluded that the frequent occurrence
at high concentrations of pyrethroids in sediments from agricultural and residential
areas constitute a threat to freshwater ecosystems [7].
5 Pyrethroid Degradation
A characteristic feature of pyrethroid contamination in water and benthic ecosystems
is that a few compounds of the pyrethroid family may be present but not all the
series, in concentrations generally under the 100 ng/L range for water samples or
under the 100 ng/g range for sediments. Pyrethroid occurrence is highly variable in
time and space, so that samples from a given area may show detectable amounts of
one or several pyrethroids while others do not or comprise other active compounds.
This feature is much different from other ubiquitous pesticides classes and is a
consequence of their higher lability. The routes of degradation of pyrethroids may
be abiotic (hydrolysis, photolysis, and oxidation) or mediated by bacteria and fungi.
Pyrethroids degradation by microorganisms and fungi have been studied in soils
[63, 64]. Various carboxylesterases may induce the degradation of pyrethroids;
generally one gene exists in one pyrethroid-degrading microorganisms, with the
exception of Ochrobactrum anthropi, that possesses two pyrethroids degrading
genes [63]. Optimal conditions of pyrethroid biodegradation are between 30 and
35
C. Organic matter and clay content are also important parameters controlling
94
L. Méjanelle et al.