Pollution effects on body mass distributions of freshwater benthic communities

Theoretical studies of the resilience of ecological systems to environmental change predict that the size distributions of species in ecosystems should have discontinuities that reflect similar discontinuities in ecosystem processes. Body size distributions should have many peaks and troughs (modes) for natural, undisturbed ecosystems, but that as disturbances increases, so the number of modes declines. If so, this prediction has implications for assessing the quality of real ecological systems and has potential for environmental monitoring. This research was carried out to explore the relationship between water quality and body size patterns in benthic communities in order to establish the potential of size based indicators for assessing environmental condition, as well as testing Holling’s (1992) proposition that lumpiness occurs in body size distributions across a broad range of spatial and temporal scales. The invertebrate samples were collected from both lentic and lotic habitats including rivers, ponds and canals in Yorkshire, UK, known to experience different degree of pollution. Five stations on the River Ure system and nine sites on the River Aire were sampled for moving water while eight stations were sampled for static (canals and ponds) habitats in the area South East of York. Community Abundance, BMWP score, water conductivity and ASPT analysis confirmed varying water quality among these sites. Visual observation of body mass pattern showed skewed distributions towards smaller size classes and most had two very obvious modes at medium and large size classes except for the most polluted habitats. Similarly, statistically rigorous estimates using Kernel Density Analysis (KDE) revealed highly significant relationships between the number of modes and water quality for running water. However, this study was unable to firmly relate discontinuities in body mass distributions to water quality in static water bodies and in fact the relationship appear to be the opposite of those found for moving water. Analysis of the number of gaps, using Holling’s (1992) Body Mass Difference Index (BMDI), revealed wide variation in clean and intermediate water quality sites, though the most polluted site had the fewest gaps. However, other disturbed sites had more gaps and some clean sites had fewer gaps. It is clear that size distributions in benthic communities are lumpy, in the sense that most sites showed more than one mode or many gaps, but the number of gaps (discontinuities) is not always correlated with disturbance, at least for the water quality of freshwater systems.