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). ifi 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.