Selection of an aquatic indicator species to monitor organic contaminants in trophically simple lotic food webs

A four-step procedure was used to identify an aquatic macroinvertebrate with which to monitor organic contaminant burdens in trophically simple lake food webs of Alberta, Canada. We identified leeches (Oligochaeta: Hirudinea) as the potential indicator assemblage (Step 1), and then documented their abundance and distribution in 16 lakes to identify a species-level bioindicator (Step 2). The latter two steps involved identifying the number and level of organic contaminants within the indicator species (Step 3) and a comparison with other taxa within the food web (Step 4). The majority of lakes within the watershed contained seven to nine leech species with individuals of Nephelopsis obscura being one of the most abundant and widely distributed species that is sufficiently large to allow for rapid collection of sufficient biomass for organic contaminant analyses. Concentrations of organic contaminants in leeches from 22 sites and in sediments (two lakes) within the watershed were low, and in the majority of cases less than analytical detection levels (i.e., 0.05 &mgr;g/kg wet weight). Nevertheless, leech tissues contained trace amounts of a fungicide (i.e., hexachlorobenzene) and four pesticides including: pp'-DDE, pp'-DDD, representing intermediate and final breakdown products of pp'-DDT, a-HCH (i.e., hexachlorocyclohexane) and g-HCH (Lindane). Concentrations of these contaminants did not differ significantly between leeches collected from lakes within a national park compared to areas outside the park that may have been more vulnerable to loadings of contaminants from industrial and agricultural sources. Lastly, we compared organic contaminant concentrations in N. obscura with Gammarus lacustris and Brook's stickleback, Culaea inconstans (Gasterosteidae). Organic contaminant concentrations in N. obscura were similar or higher than those present in G. lacustris and C. inconstans. Eight organic contaminants were found in N. obscura, whereas, C. inconstans and G. lacustris contained only three and four, respectively. The fact that N. obscura typically contained higher concentrations and greater numbers of detectable compounds suggest that it is a suitable indicator of organic contamination. While current levels of organic contaminants in the Beaver Hills watershed are low, despite the intense agriculture and industrial activities, they should be monitored to act as an early warning indicator of ecosystem degradation.