Toxicological assessment of the effect of motorway runoff on stream macroinvertebrate community structure and function.

Road runoff contains a complex mixture of contaminants including metals, anions, and
hydrocarbons. This runoff discharges into natural water courses which are often small
streams. The concentration of these chemicals in the drainage water and receiving
stream depends on a number of site specific characteristics such as traffic volume, area
of road drained and size of the stream. It was postulated that these pollutants have a
deleterious affect on macroinvertebrate community structure which would result in
subsequent effects on macroinvertebrate function (i. e. litter processing). Further, it was
hypothesised that impacts would be greatest in small streams, receiving drainage waters
from large areas of heavily used motorway and that only a limited number of chemicals
would be responsible for any effects.
Field surveys demonstrated that macroinvertebrate community structure and function
was impacted at one of the three sites studied, namely Pigeon Bridge Brook. The
downstream station at this site received motorway runoff drainage from the largest area
of road surface, was the smallest stream and had the highest metal and hydrocarbon
concentrations in both stream water and sediments (Maltby et al., 1995a).
Macroinvertebrate species richness and diversity were significantly reduced below the
discharge. Species generally considered 'sensitive' to pollutants such as stoneflies,
gammarids, molluscs and trichopterans were reduced in relative abundance whilst more
'tolerant' opportunistic species such as chironomids and tubificid worms increased in
relative abundance downstream of the discharge. An assessment of the trophic
composition of the community (i. e. functional feeding groups) indicated that there was
a differential loss of functional groups, with significantly lower relative abundances of
shredders and scrapers and an increase in collectors downstream of the motorway
discharge. The changes in both the structure and trophic biology of the
macroinvertebrate community resulted in a significant reduction in macroinvertebratemediated
leaf processing downstream of the motorway discharge.
Although field surveys indicated macroinvertebrate community structure and function
were negatively impacted below the motorway discharge at Pigeon Bridge Brook they
cannot establish causal relationships. In-situ and laboratory studies were therefore
performed to address the mechanistic basis for the impact. In-situ and laboratory
lethality exposures did not fully explain the field distribution of the species used in
toxicological studies; Gammarus pulex (L. ), Nemoura cinerea (Retz. ), Potamopyrgus
jenkinsi (Smith), Chironomus riparius (Meigen) and Tubifex tubifex (Müller). In acute
lethality tests stream water from Pigeon Bridge Brook was not toxic to any of the
species. In contrast, G. pulex and N. cinerea showed slight, but significant mortality
when exposed to downstream sediment from this site. Sediment manipulation and
sediment solvent and acid extract exposures indicated that the solvent extractable
fraction of the sediment was responsible for this toxicity to G. pulex but not to N.
cinerea. These results indicated that aromatic hydrocarbons in the sediment may be
responsible for the toxicity and this has subsequently been shown to be the case (Maltby
et al., 1995b).
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Since lethality studies did not fully explain field -distributions of the animals sub-lethal
toxicity avoidance behaviour tests were employed using sediment, manipulated
sediments and sediment extracts. The sensitivity to downstream field sediment,
indicated by avoidance decreased in the order P. jenkinsi > G. pulex > C. riparius> T.
tubifex = N. cinerea and to a solvent extract of this sediment in the order G. pulex > P.
jenkinsi > C. riparius > N. cinerea > T. tubifex. Acid sediment extracts and solvent
extracted sediments induced no avoidance responses in these animals.
Gammarus pulex was thought to be the dominant shredding macroinvertebrate at
Pigeon Bridge Brook. Reductions in macroinvertebrate-mediated leaf processing could
therefore be the result of sub-lethal effects of motorway contamination on the feeding
activity of this species. In-situ exposures indicated that the consumption of leaf material
by G. pulex was reduced at the downstream station and laboratory exposures indicated
this was principally a result of sediment toxicity. Sediment extract exposures indicated
that the solvent extractable fraction was again responsible for the majority of this effect.
Accumulation of metals and aromatic hydrocarbons on the leaf material had very little
effect on leaf consumption or choice. However, reduced colonisation of leaf material by
aquatic hyphomycetes reduced both leaf choice and consumption when the material was
conditioned at the downstream station. The major uptake route of aromatic
hydrocarbons by G. pulex was via aqueous sources and not from food.
In conclusion motorway derived contamination in small streams has both lethal and sublethal
effects on some macroinvertebrates. This affects macroinvertebrate structural and
trophic characteristics which subsequently have a deleterious effect on important
ecosystem functions.