Ecotoxicology of oil derived pollutants in urban receiving waters

The Silk Stream in north London is one of two principal surface feeders into the Welsh Harp reservoir. The reservoir represents an important amenity and recreational site and is internationally recognized for its wintering wildfowl. However, the larger part of the catchment area lies within a highly urbanized zone and is therefore subjected to a variety of point and non-point pollution sources rendering the ecological balance considerably more delicate than corresponding rural areas. The construction of a litter screen and oil boom on the lowest reaches of the Silk Stream has highlighted the particular problem of oil inputs from urban runoff. In order to assess the ecotoxicological damage resulting from hydrocarbon pollution in the Silk Stream and at the receiving basin, two common, physiologically contrasting macroinvertebrates, Asellus aquaticus and Lymnaea peregra were used as caged biomonitors. Both organisms were found to be bioaccumulators of hydrocarbons when transferred from rural sites to the caged urban sites although tissue levels were substantially lower at the receiving basin site. Elevated mortality rates of both organisms were also observed at the three stream sites indicating the considerable recovery that occurs at the receiving basin. Hydrocarbon tissue concentrations were found to mirror those of the surrounding sediments in which the combustion derived compounds, fluoranthene and pyrene, were consistently the most abundant polycyclic aromatic hydrocarbons (PAHs). The alkane profiles were generally unimodal, peaking in the C20-C23 range. Inputs of biogenically derived hydrocarbons and of lubricating oils were also identified in the abiotic environment as well as in the organism tissues. The application of Principal Component Analysis revealed an association between a lubricating hydrocarbon source and mortality in L. peregra at two sites and with A. aquaticus at one site. Links between mortality and rainfall were also established at the Silk Stream for L. peregra and to a lesser extent for A. aquaticus. Neither hydrocarbon tissue burdens nor rainfall were linked to the low mortalities at the receiving basin. Laboratory toxicity tests showed that both organisms accumulate and depurate hydrocarbons rapidly and that lower mortalities were attained for similar tissue burdens compared with the field results. The depuration patterns indicated that a substantial proportion of the measured hydrocarbons were in non-assimilated forms.