Further investigations are needed to better determine the timeframe integrated by
contaminant accumulation in biofilms as a function of chemical, biological, and
environmental properties.
3.2 Contaminant Bioaccumulation in Sediment or Leaf Litter
Microbial Communities
Up to now, data on in situ bioaccumulation of metals and organic contaminants in
submerged microbial communities associated with sediments, leaves, or drift particulate matter has always included both biotic accumulation and abiotic sorption on
the substratum. As stated earlier, this is partly due to the fact that microbial
communities cannot be easily detached from these substrates and that the microbial
biomass obtained is still very limited (and not sufficient for chemical analyses).
The distribution of organic contaminants in the different river compartments
(water, sediment, and leaf litter) is influenced by the hydrology and geomorphology
of the system as well as by the physical and chemical properties of the contaminant.
For instance, suspended and bed sediments in the San Joaquin River and its
tributaries (in one of the most productive agricultural regions of the USA) serve as
a sink for hydrophobic contaminants (e.g., PAHs, DDT), whereas water-soluble
herbicides (e.g., atrazine, simazine, dimethyl tetrachloroterephthalate) are mostly
present in the dissolved phase of the water column (Pereira et al. 1996). A similar
pattern of pesticide distribution has been observed in rivers in Europe (e.g.,
Fernandez et al. 1999) and Asia (e.g., Chen et al. 2006). Pesticide dissipation in
water can be enhanced or reduced by the presence of sediments and according to the
properties of pesticide molecules (Laabs et al. 2007). Sediments can also accumulate
pharmaceuticals. An extensive study in four Spanish rivers (Ebro, Llobregat, Júcar,
and Guadalquivir) highlighted the presence of endocrine disruptors accumulated in
sediments at concentrations up to 7 ng g
À1 (Gorga et al. 2015). Similar levels of the
hormone β-estradiol were also quantified in sediments of the River Ouse
(UK) (Labadie and Hill 2007), whereas lower levels were reported in three rivers
in the Tianjin area (China) (Lei et al. 2009). Several antibiotics from urban sources
and aquaculture activities (e.g., sulfamethazine, sulfamethoxazole, norfloxacin,
among others) have been detected in the sediments of the Pearl River Estuary
(South China) at concentrations ranging from 1 to 8 ng g
À1 (Liang et al. 2013).
While contaminant accumulation in sediments contributes to the removal of toxic
substances from the surface, this apparent remediation is only temporary since those
contaminants can later be remobilized following changes in redox conditions leading
to the redissolution from sediment and diffusion from pore water and/or during
intense hydrological events, as shown by Domagalski et al. (2010) for pyrethroid
insecticides in different rivers. Flash-flood events in the Ebro river basin were also
found to mobilize huge amounts of hexachlorobenzene, DDT, and PCBs largely
exceeding existing regulatory reference values established for sediments (Quesada
et al. 2014).
Role of Biofilms in Contaminant Bioaccumulation and Trophic Transfer in Aquatic. . .
133
contaminant accumulation in biofilms as a function of chemical, biological, and
environmental properties.
3.2 Contaminant Bioaccumulation in Sediment or Leaf Litter
Microbial Communities
Up to now, data on in situ bioaccumulation of metals and organic contaminants in
submerged microbial communities associated with sediments, leaves, or drift particulate matter has always included both biotic accumulation and abiotic sorption on
the substratum. As stated earlier, this is partly due to the fact that microbial
communities cannot be easily detached from these substrates and that the microbial
biomass obtained is still very limited (and not sufficient for chemical analyses).
The distribution of organic contaminants in the different river compartments
(water, sediment, and leaf litter) is influenced by the hydrology and geomorphology
of the system as well as by the physical and chemical properties of the contaminant.
For instance, suspended and bed sediments in the San Joaquin River and its
tributaries (in one of the most productive agricultural regions of the USA) serve as
a sink for hydrophobic contaminants (e.g., PAHs, DDT), whereas water-soluble
herbicides (e.g., atrazine, simazine, dimethyl tetrachloroterephthalate) are mostly
present in the dissolved phase of the water column (Pereira et al. 1996). A similar
pattern of pesticide distribution has been observed in rivers in Europe (e.g.,
Fernandez et al. 1999) and Asia (e.g., Chen et al. 2006). Pesticide dissipation in
water can be enhanced or reduced by the presence of sediments and according to the
properties of pesticide molecules (Laabs et al. 2007). Sediments can also accumulate
pharmaceuticals. An extensive study in four Spanish rivers (Ebro, Llobregat, Júcar,
and Guadalquivir) highlighted the presence of endocrine disruptors accumulated in
sediments at concentrations up to 7 ng g
À1 (Gorga et al. 2015). Similar levels of the
hormone β-estradiol were also quantified in sediments of the River Ouse
(UK) (Labadie and Hill 2007), whereas lower levels were reported in three rivers
in the Tianjin area (China) (Lei et al. 2009). Several antibiotics from urban sources
and aquaculture activities (e.g., sulfamethazine, sulfamethoxazole, norfloxacin,
among others) have been detected in the sediments of the Pearl River Estuary
(South China) at concentrations ranging from 1 to 8 ng g
À1 (Liang et al. 2013).
While contaminant accumulation in sediments contributes to the removal of toxic
substances from the surface, this apparent remediation is only temporary since those
contaminants can later be remobilized following changes in redox conditions leading
to the redissolution from sediment and diffusion from pore water and/or during
intense hydrological events, as shown by Domagalski et al. (2010) for pyrethroid
insecticides in different rivers. Flash-flood events in the Ebro river basin were also
found to mobilize huge amounts of hexachlorobenzene, DDT, and PCBs largely
exceeding existing regulatory reference values established for sediments (Quesada
et al. 2014).
Role of Biofilms in Contaminant Bioaccumulation and Trophic Transfer in Aquatic. . .
133