The response of protist and metazoan communities in permeable pavement structures to high oil loadings.

Permeable pavement structures (PPS) have been demonstrated to provide an efficient and sustainable method of controlling urban derived hydrocarbon contamination. Until recently, laboratory PPS mesocosm models have used crushed granite as the load bearing sub-base material. However, the use of virgin stone may not be the optimal choice of substrate, as this is not necessarily sustainable or cost effective in the long term when compared to the use of recycled materials. However, recycled materials such as waste concrete may change the environmental conditions in PPS mesocosms, and the characteristics of the eukaryotic community may become different from those which have been previously described. In the current experiment, granite and recycled concrete sub-base materials were compared for their ability to retain 900 g/m2 of clean mineral oil applied to the mesocosm surface. It was observed that, even at this very high oil loading, 99.95% of the applied oil was retained within granite and concrete-based structures, but the effluent was two pH units more alkaline in concrete mesocosms than granite. The eukaryotic microfauna in the effluent from both mesocosm types showed a ten-fold increase in protist abundance, and a doubling in the number of protist genera, compared with earlier work using only 18 g/m2 of applied oil. Five genera of testate amoebae not previously recorded in PPS were identified, these included Arcella, Assulina, Cryptodifflugia, Cyclopyxis and Difflugia in addition to the three genera observed previously using the lower oil application. Metazoan abundances increased from 1.5 × 101 organisms per ml using the lower oil loadings to 2.0 × 103/ml in the current experiment. Rotifers and nematodes were the most numerous, but tardigrades were also observed in both concrete and granite-based mesocosms. Despite the differences in effluent pH, it was apparent that there were only marginal differences in the eukaryotic microbiology of the two mesocosm types. This was thought to be due to the layered structural arrangement of the pavement and the location of the highly oil-retentive polypropylene geotextile and extensive biofilm layer positioned above the concrete sub-base. Work is now underway to find oil loadings that will adversely affect the abundance and diversity of eukaryotic organisms in PPS mesocosms.